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Reaction dynamics and relative yields of the H- and CH3-displacement channels in the O + CH3CCH reaction
Abstract
The O + CH3CCH reaction has been investigated in crossed molecular beam experiments at a collision energy of 38.7 kJ mol (1). Product angular and translational energy distributions have been measured for the H- and CH3-displacement channels and their relative branching ratio has been determined. The CH3-displacement channel is more important (by a factor of 3) than the H-displacement channel. This is not explained by a preference for the addition on the substituted acetylenic carbon atom, but rather by the different tendencies of the two addition intermediates CH3CCOH and CH3COCH to undergo intersystem crossing to the underlying singlet potential energy surface.
... To solve this problem, soft photoionisation (PI) by tunable VUV synchrotron in CMB machines was first developed at the Advanced Light Source (ALS) of Berkeley (USA) [50][51][52][53] and, later, at the National Synchrotron Radiation Research Centre in Hsinchu (Taiwan) [54][55][56][57][58][59]. Table-top alternatives include VUV laser ionisation [41,[60][61][62][63] or, as pioneered in our laboratory, soft electron-ionisation (EI) by means of an ioniser with tuneable electron energy [16,17,[64][65][66][67][68][69][70][71][72][73]. Soft EI cannot compete with the high selectivity and resolution of the synchrotron or laser radiation. ...
... In this way we have been able to determine the product BRs and to explore the dynamics of polyatomic reactive systems thus achieving progress in the characterisation of the dynamics of complex reactions. In addition to the of O( 3 P) + unsaturated hydrocarbons, which are reviewed here [16,17,65,66,[69][70][71][72][73] we have also investigated a series of complex polyatomic reactive systems such as C( 3 P) + C 2 H 2 [67], C( 3 P) + C 2 H 4 [74], C 2 + C 2 H 2 [75], CN + C 2 H 2 [76], CN + C 2 H 4 [77], N( 2 D) + CH 4 [78,79] [85], and N( 2 D) + C 2 H 6 [86]. The present article surveys our recent progress in the investigation of the dynamics of the multichannel polyatomic reactions of ground state O( 3 P) atoms with unsaturated hydrocarbons, as made possible by the application of the soft EI detection method in CMB reactive scattering experiments. ...
... (for the values of the enthalpies of reaction, see Ref. [71]). The last four channels can only originate from the singlet PES. ...
We review the progress made in the understanding of the dynamics of the reactions of ground state oxygen atoms, O(3P), with unsaturated hydrocarbons (acetylene, ethylene, allene, propyne and propene) which are of great relevance, besides from a fundamental point of view, in combustion chemistry and of interest also in atmosphere- and astro-chemistry. Advances in this area have been made possible by an improved crossed molecular beams (CMBs) instrument with rotating mass spectrometric detection and time-of-flight analysis which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power. This apparatus offers the capability of identifying virtually all primary reaction channels, characterising the dynamics, and determining the branching ratios (BRs) for these polyatomic multichannel nonadiabatic reactions. The reactive scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces (PESs). For the simpler prototypical systems, such as O(3P) + ethylene, detailed comparisons with synergic state-of-the-art quasiclassical trajectory surface-hopping calculations on full dimensional ab initio coupled triplet and singlet PESs have recently been possible. For more complex systems, such as O(3P) + propene, comparisons with the results of synergic statistical calculations of BRs on ab initio coupled triplet and singlet PESs, have also been carried out very recently. The combined experimental/theoretical approach has allowed for a better understanding of the mechanism of these reactions. And overall has deepened considerably our understanding of chemical reactivity; in addition, these studies provide an important bridge between CMBs dynamics and thermal kinetics as well as valuable information for improving combustion and astrochemistry models.
... An account of two of the reaction channels of O( 3 P) with propyne (HCC−CH 3 , another isomer of C 3 H 4 ) has been recently reported. 10 These CMB experiments using soft electron-impact ionization are advantageous because they allow "universal" detection of nearly all products of these reactions. The measured product branching fractions, together with highlevel quantum chemical calculations 11−13 and more recently with quasi-classical trajectory calculations, 14−17 have shown that these reactions are subject to significant intersystem crossing (ISC) from the initial triplet PES of the adduct to the singlet PES en route to product formation. ...
... , and reported that R1b is favored over R1c by a factor of 3.1 ± 0.6. 10 Variations of channel R1d in which energetic forms of C 2 H 4 are produced (e.g., the 3 CH 3 CH ethylidene isomer) are energetically possible and are discussed in more detail later. ...
... We therefore conclude that C 2 H 2 O (identified as ketene, see Supporting Information) is mostly formed via secondary chemistry. The 10 and it is possible that the ketene signal observed here contains minor unresolvable contributions from primary production via R1g. Kinetics data for the C 3 H 2 O product that potentially comes from R1h could not be obtained with the signal-to-noise necessary to assign it as a primary or secondary product. ...
The reaction of O((3)P) + propyne (C3H4) was investigated at 298 K and 4 Torr using time-resolved multiplexed photoionization mass spectrometry and a synchrotron-generated tunable vacuum ultraviolet light source. The time-resolved mass spectra of the observed products suggest five major channels under our conditions: C2H3 + HCO, CH3 + HCCO, H + CH3CCO, C2H4 + CO, and C2H2 + H2 + CO. The relative branching ratios for these channels were found to be 1.00, (0.35 ± 0.11), (0.18 ± 0.10), (0.73 ± 0.27), and (1.31 ± 0.62). In addition, we observed signals consistent with minor production of C3H3 + OH and H2 + CH2CCO, although we cannot conclusively assign them as direct product channels from O((3)P) + propyne. The direct abstraction mechanism plays only a minor role (≤1%), and we estimate that O((3)P) addition to the central carbon of propyne accounts for 10% of products, with addition to the terminal carbon accounting for the remaining 89%. The isotopologues observed in experiments using d1-propyne (CH3CCD) and analysis of product branching in light of previously computed stationary points on the singlet and triplet potential energy surfaces (PESs) relevant to O((3)P) + propyne suggest that, under our conditions, (84 ± 14)% of the observed product channels from O((3)P) + propyne result from intersystem crossing from the initial triplet PES to the lower-lying singlet PES.
... In this respect, it came as a surprise that for the reaction O( 3 P) + C 2 H 4 ISC accounts for about 50% of the total reactivity 7,8 and even more for the reactions O( 3 P) + CH 2 CCH 2 13 and O( 3 P) + CH 3 CCH. 14 The role of ISC in the O( 3 P) + C 2 H 4 reaction has been confirmed by detailed theoretical work. 7,8,15 The central role of ISC in this family of reactions, specifically in the reactions O + ethylene and O + allene, was established experimentally by the pioneering CMB studies by Lee and coworkers in the late 1980s−early 1990s. ...
... The average fraction of energy in translation ⟨f T ⟩ for the H channels has a value (about 0.38 and 0.23 for the two isomeric channels) quite typical for reactions of this kind with an exit barrier and is in line with previous determinations on related systems, such as for instance those obtained for the corresponding channels in the O( 3 P) + propyne reaction where a fraction of 0.38 was derived. 14 The triplet intermediate CH 3 CHCH 2 O can also undergo competitively C−C bond rupture with formation of 3 CH 3 CH + H 2 CO via TS3 located only 1.1 kcal/mol with respect to the product asymptote. Because of the small exit barrier we would expect a small fraction of energy in translation: indeed the P(E′ T ) for the H 2 CO + 3 CH 3 CH channel peaks at only 2.5 kcal/mol but extends somewhat beyond the limits of energy conservation for the triplet channel. ...
Despite extensive kinetics/theoretical studies, information on the detailed mechanism (primary products, branching ratios (BRs)) for many important combustion reactions of O(3P) with unsaturated hydrocarbons is still lacking. We report synergic experimental/theoretical studies on the mechanism of the O(3P)+C3H6 (propene) reaction by combining crossed-molecular-beams experiments with mass-spectrometric detection at 9.3 kcal/mol collision energy (Ec) with high-level ab initio electronic structure calculations of underlying triplet/singlet potential energy surfaces (PESs) and statistical (RRKM/Master Equation) computations of BRs including intersystem-crossing (ISC). The reactive interaction of O(3P) with propene is found to mainly break apart the 3-carbon atom chain, producing the radical products methyl+vinoxy (32%), ethyl+formyl (9%), and molecular products ethylidene/ethylene+formaldehyde (44%). Two isomers, CH3CHCHO (7%) and CH3COCH2 (5%), are also observed from H atom elimination, reflecting O-atom attack to both terminal and central C-atoms of propene. Some methylketene (3%) is also formed following H2 elimination. As some of these products can only be formed via ISC from triplet to singlet PESs, from BRs an extent of ISC of about 20% is inferred. This value is significantly lower than recently observed in O(3P)+ethylene (~50%) and O(3P)+allene (~90%) at similar Ec, posing the question of how important it is to consider nonadiabatic effects for these and similar combustion reactions. Comparison of the derived BRs with those from recent kinetics studies at 300 K and statistical predictions provides information on the variation of BRs with Ec. ISC is estimated to decrease from 60% to 20% with increasing Ec. The present results lead to a detailed understanding of the complex reaction mechanism of O+propene and should facilitate the development of improved models of hydrocarbon combustion.
... In general, product detection in CMB experiments can be achieved through various techniques, including laser spectroscopic methods, but when the nature of the primary products is unknown, which is a common situation for polyatomic multi-channel reactions, or the product is a complex, sizeable radical or molecule, the use of universal MS detection is decisive, because it allows to probe all possible reaction products on the same footing, determining their mass and allowing to determine their relative importance (i.e., their BRs) (Schmoltner et al. 1989;Casavecchia et al. 2009Casavecchia et al. , 2015. This has been demonstrated in our laboratory by a series of studies on elementary reactions of atomic radicals (O( 3 P) (Capozza et al. 2004;Casavecchia et al. 2005Casavecchia et al. , 2009Leonori et al. 2012;Fu et al. 2012a, b;Leonori et al. 2014;Balucani et al. 2014;Cavallotti et al. 2014;Balucani et al. 2015;Leonori et al. 2015;Vanuzzo et al. 2016a, b;Gimondi et al. 2016;Caracciolo et al. 2017Caracciolo et al. , 2019aPratali Maffei et al. 2019); N( 2 D) (Balucani et al. 2010); C( 3 P) (Leonori et al. 2008), S( 1 D) (Berteloite et al. 2011) with saturated and unsaturated hydrocarbons. ...
The identification of the primary products and the determination of their branching ratios as a function of translational energy (temperature) for multi-channel elementary (bimolecular) reactions of importance in combustion flames and plasma-assisted combustion still represent a challenge for traditional kinetics experiments. On the other hand, this kind of information is central for the detailed modeling of combustion/plasma systems. In this short review, the significant contribution provided in this area by the crossed molecular beam (CMB) scattering method with “universal” mass-spectrometric detection and time-of-flight analysis is illustrated. In particular, we describe the basics of the CMB technique empowered with “soft” electron-impact ionization as recently implemented in our laboratory, and report on its application to the study of the multi-channel elementary reactions of ground state atomic oxygen, O(³P), with unsaturated hydrocarbons containing two carbon atoms (acetylene and ethylene), three carbon atoms (propyne, propene, and allene), and also four carbon atoms (1-butene, 1,2-butadiene, and 1,3-butadiene), which are of paramount interest in combustion flames and plasma-assisted combustion of hydrocarbons. These studies are usually complemented in a synergistic manner by high-level electronic structure calculations of the underlying potential energy surfaces and related statistical (and dynamical when feasible) calculations of product branching ratios. The complementarity to kinetics studies and the implications of the dynamics results for the modeling of combustion/plasma chemistry will be commented on.
... Considering the good overall agreement between detailed theoretical and experimental results for the O( 3 P) + C 2 H 4 reaction also at high E c , we can conclude that QCT surfacehopping calculations, using reliable coupled multidimensional PESs, can yield reliable dynamical information for polyatomic multichannel reactions in which ISC plays an important role. It is certainly desirable to extend this kind of dynamical calculations to also more complex O( 3 P) + unsaturated hydrocarbon reactions, such as those with three-carbon alkynes and alkenes, 7 such as propyne, 30 allene, 31 and propene, 32,33 for which reactive scattering data of similar quality to those reported here for ethylene have been recently obtained. ...
The combustion relevant O(3P) + C2H4 reaction stands out as a prototypical multichannel nonadiabatic reaction involving both triplet and singlet potential energy surfaces (PESs) which are strongly coupled. Crossed molecular beam (CMB) scattering experiments with universal soft electron ionization mass spectrometric detection have been used to characterize the dynamics of this reaction at the relatively high collision energy Ec of 13.7 kcal/mol, attained by crossing the reactant beams at an angle of 135°. This work is a full report of the data at the highest Ec investigated for this reaction. From laboratory product angular and velocity distribution measurements, angular and translational energy distributions in the center-of-mass system have been obtained for the five observed exothermic competing reaction channels leading to H + CH2CHO, H + CH3CO, CH3 + HCO, CH2 + H2CO, and H2 + CH2CO. The product branching ratios (BRs) have been derived. The elucidation of the reaction dynamics is assisted by synergic full-dimensional quasiclassical trajectory surface-hopping calculations of the reactive differential cross sections on coupled ab initio triplet/singlet PESs. This joint experimental/theoretical study extends and complements our previous combined CMB and theoretical work at the lower collision energy of 8.4 kcal/mol. The theoretically derived BRs and extent of intersystem crossing (ISC) are compared with experiment. In particular, the predictions of the QCT results for the three main channels (those leading to vinoxy + H, methyl + HCO and methylene + H2CO formation) are compared directly with the experimental data in the laboratory frame. Good overall agreement is noted between theory and experiment, although some small, yet significant shortcomings of the theoretical differential cross section are noted. Both experiment and theory find almost an equal contribution from the triplet and singlet surfaces to the reaction, with a clear tendency of the degree of ISC to decrease with increasing Ec and with theory slightly overestimating the extent of ISC.
Production of formyl radical, HCO, from reactions of O(³P) with alkynes (acetylene, propyne, 1-butyne, and 1-pentyne) has been investigated using cavity ringdown laser absorption spectroscopy (CRDLAS) and computational methods. No HCO was detected from reaction with acetylene, while the amount of HCO increased for propyne and 1-butyne, dropping off somewhat for 1-pentyne. These results differ from trends previously observed for reactions of O(³P) with alkenes, which exhibit the largest HCO production for the smallest alkene and drop off as the alkene size increases. Computational studies employing density functional and coupled cluster methods have been employed to investigate the triplet and singlet state pathways for HCO production. Because intersystem crossing (ISC) has been shown to be important in these processes, the minimum energy crossing point (MECP) between the triplet and singlet surfaces has been studied. We find the MECP for propyne to possess C1 symmetry and to lie lower in energy than previous studies have found. Natural Bond Orbital and Natural Resonance Theory analyses have been performed to investigate the changes in spin density and bond order along the reaction pathways for formation of HCO. Explanations are suggested for the trend in HCO formation observed for the alkynes. The trend in alkyne HCO yield also is compared and contrasted with the trend previously observed for the alkenes.
This work reports the detection and identification of volatile chemical compounds in fruit kernel of “Lodoicea maldivica” coco nucifera palm by gas chromatographic method. The analysis was performed by HS-SPME and GC/MS techniques to determine volatile aroma composition profiles in internal and external pulp. No qualitative differences in flavor composition were observed between two pulp parts, but there were notable variations in the abundance levels of the prominent compounds. Computational method was used to extract individual component mass spectra from GC/MS data files by using the AMDIS version 2.65 software. With such a procedure we are able to construct our own mass spectra library determining retention indices for chemical compounds of our interest in the used specific experimental conditions.
In the present study, we report the attempt to characterize the chemical composition of fruit kernel of "Lodoicea Maldivica" coco nucifera palm (commonly named as "Coco de mer") by gas chromatographic method. The analysis was performed by HS-SPME and GC/MS techniques to determine volatile aroma, sterol and fatty acid composition profiles in the internal and external pulp of two distinct coconuts. Despite no qualitative differences in flavour composition were observed between the two analysed coconuts and the relative two pulp parts, variations in the abundance levels of the prominent compounds have been recorded. The averaged quantity of total phytosterols, resulting from the two analysed "Coco de mer" samples, was almost constant in both kernels coconut, being 24.5 μg/g (of dry net matter) for the external, and 26.9 μg/g (of dry net matter) for the internal portion. In both coconuts, the fatty acid pattern composition was characterized by seven saturated acids ranged from C14:0 (myristic) to C20:0 (arachidic) and two monounsaturated acids, the palmitoleic (C16:1, ω7) and the oleic (C18:1, ω9). Palmitic acid (C16:0) was the predominant one with an average contribution of about 49.0%, followed by pentadecanoic 16.5%, stearic (C18:0) 11.6%, and myristic (C14:0) 9.9% acids in all two examined kernel portions. This article is protected by copyright. All rights reserved.
This work reports the first attempt for detection and identification of chemical compounds in fruit kernel of Lodoicea Maldivica coco nucifera palm. The analysis was performed by GC-MS technique to determine phytosterol and fatty acid composition profiles in internal and external pulp. Total phytosterol content was almost constant in both kernel coco-nut (24.6 μg/g for the external and 22.5 μg/g for the internal portion). The fatty acid pattern has been determined. The composition was characterized by seven saturated acids ranged from C14:0 (myristic) to C20:0 (arachidic) and two monounsaturated acids, the palmitoleic (C16:1, ω7) and the oleic (C18:1, ω9). Palmitic acid (C16:0) was the predominant one with contribution of about 49% followed by pentadecanoic, stearic (C18:0) and myristic acids (C14:0) in all two examined kernel parts. Despite its remarkable and widely known alimentary use and interesting peculiarities, Lodoicea Maldivica and its coconut (commonly named as “Coco de mer”) has not received sufficient scientific research attention. The analytical study here presented intends to fill this gap, with particular attention to highlight the health and food safety use of the fruit, and the possible presence of chemical compounds interesting from a nutritional and pharmacological point of view.
We have performed synergic experimental/theoretical studies on the mechanism of the O(3P)+CH3CCH (propyne) reaction by combining crossed molecular beams (CMB) experiments with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy (Ec) with ab initio electronic structure calculations at a high-level of theory of the relevant triplet and singlet potential energy surfaces (PESs) and statistical calculations of branching ratios taking into account intersystem-crossing (ISC). In this paper (I) we report the results of the experimental investigation, while in the accompanying paper (II) those of the theoretical investigation together with comparison to experimental results. By exploiting soft electron ionization detection to suppress/mitigate the effects of the dissociative ionization of reactants, products and background gases, product angular and velocity distributions at different charge-to-mass ratios have been measured. From the laboratory data angular and translational energy distributions in the center-of-mass system have been obtained for the five competing most important product channels and product branching ratios (BRs) have been derived. The reactive interaction of O(3P) with propyne under single collision conditions is mainly leading to the rupture of the three-carbon atom chain, with production of the radical products methylketenyl+atomic hydrogen (CH3CCO+H) (BR=0.04), methyl+ketenyl (CH3+HCCO) (BR=0.10), vinyl+formyl (C2H3+HCO) (BR=0.11), and molecular products ethylidene/ethylene+carbon monoxide (CH3CH/C2H4+CO) (BR=0.74) and propandienal+molecular hydrogen (CH2CCO+H2) (BR=0.01). Because some of the products can only be formed via ISC from the entrance triplet to the low-lying singlet PES, we infer from their BRs an amount of ISC larger than 80%. This value is dramatically large when compared to the negligible ISC reported for the O(3P) reaction with the simplest alkyne, HCCH (acetylene). At the same time, it is much larger than that (about 20%) recently observed in the related reaction of the three-carbon atom alkene, O(3P)+CH3CHCH2 (propene) at a comparable Ec. This poses the question of how important it is to consider nonadiabatic effects and their dependence on molecular structure for this kind of combustion reactions. The prevalence of the CH3 over the H displacement channels is not explained by invoking a preference for the addition on the methyl substituted acetylenic carbon atom, but rather it is believed to be due to the different tendencies of the two addition triplet intermediates CH3CCHO (preferentially leading to H elimination) and CH3COCH (preferentially leading to CH3 elimination) to undergo ISC to the underlying singlet PES. It is concluded that the main co-product of the CO forming channel is singlet ethylidene (1CH3CH) rather than ground state ethylene (CH2CH2). By comparing the derived BRs with those very recently derived from kinetics studies at room temperature and 4 torr we have obtained information on how BRs vary with collision energy. The extent of ISC is estimated to remain essentially constant (about 85%) with increasing Ec from about 1 kcal/mol to about 10 kcal/mol. The present experimental results shed light on the mechanism of the title reaction at energies comparable to those involved in combustion and when compared with the results from the statistical (RRKM/Master Equation) calculations on the ab initio coupled PESs (see accompanying paper II) lead to an in depth understanding of the rather complex reaction mechanism of O+propyne. The overall results are expected to contribute to the development of more refined models of hydrocarbon combustion.
The mechanism of the O(3P)+CH3CCH reaction was investigated using a combined experimental/theoretical approach. Experimentally the reaction dynamics was studied using crossed molecular beams (CMB) with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy. Theoretically master equation simulations were performed on a potential energy surface (PES) determined using high-level ab initio electronic structure calculations. In this paper (II) the theoretical results are described and compared with experiments, while in paper (I) are reported and discussed the results of the experimental study. The PES was investigated by determining structures and vibrational frequencies of wells and transition states at the CASPT2/aug-cc-pVTZ level using a minimal active space. Energies were then determined at the CASPT2 level increasing systematically the active space and at the CCSD(T) level extrapolating to the complete basis set limit. Two separate portions of the triplet PES were investigated, as O(3P) can add either on the terminal or the central carbon of the unsaturated propyne bond. Minimum energy crossing points (MECPs) between the triplet and singlet PESs were searched at the CASPT2 level. The calculated spin-orbit coupling constants between the T1 and S0 electronic surfaces were about 25 cm-1 for both PESs. The portions of the singlet PES that can be accessed from the MECPs were investigated at the same level of theory. The system reactivity was predicted integrating stochastically the 1D master equation (ME) using RRKM theory to determine rate constants on the full T1/S0 PESs, accounting explicitly for intersystem crossing (ISC) using the Landau Zener model. The computational results are compared both with the branching ratios (BRs) determined experimentally in the companion paper (I) and with those estimated in a recent kinetic study at 298 K. The ME results allow to interpret the main system reactivity: CH3CCO+H and CH3+HCCO are the major channels active on the triplet PES and are formed from the wells accessed after O addition to the terminal and central C, respectively; 1CH3CH+CO and C2H3+HCO are the major channels active on the singlet PES and are formed from the methylketene and acrolein wells after ISC. However, also a large number of minor channels (about 15) are active, so that the system reactivity is quite complicated. The comparison between computational and experimental BRs is quite good for the kinetic study, while some discrepancy with the CMB estimations suggests that dynamic non ergodic effects may influence the system reactivity. Channel specific rate constants are calculated in the 300-2250 K and 1-30 bar temperature and pressure ranges. It is found that as the temperature increases the H abstraction reaction, whose contribution is negligible in the experimental conditions, increases of relevance and the extent of ISC decreases from about 80% at 300K to less than 2% at 2250K.
We report direct experimental/theoretical evidence that, under single collision conditions, the dominant product channels of the O(3P)+propyne and O(3P)+allene isomeric reactions lead in both cases to CO formation, but the co-products are singlet ethylidene (1CH3CH) and singlet ethylene (CH2CH2), respectively. These data, which settle a long-standing issue on whether ethylidene is actually formed in the O(3P)+propyne reaction, suggest that formation of CO+alkylidene biradicals may be a common mechanism in O(3P)+alkyne reactions, in contrast to formation of CO+alkene molecular products in the corresponding isomeric O(3P)+diene reactions, either in combustion or other gaseous environments. These findings are of fundamental relevance and may have implications for improved combustion models. Moreover, we predict that the so far neglected 1CH3CH+CO channel is among the main reaction routes also when the C3H4O singlet PES is accessed from the OH+C3H3 (propargyl) entrance channel, which are radical species playing a key role in many combustion systems.
Comprehension of the detailed mechanism of O(3P) + unsaturated hydrocarbon reactions is complicated by the existence of many possible channels and intersystem crossing (ISC) between triplet and singlet potential energy surfaces (PESs). We report synergic experimental/theoretical studies of the O(3P) + propene reaction by combining crossed molecular beams experiments using mass spectrometric detection at 9.3 kcal/mol collision energy (Ec) with high-level ab initio electronic structure calculations of the triplet PES and RRKM/master equation computations of branching ratios (BRs) including ISC. At high Ec’s and temperatures higher than 1000 K, main products are found to be formaldehyde (H2CO) and triplet ethylidene (3CH3CH) formed in a reaction channel that has never been identified or considered significant in previous kinetics studies at 300 K and that, as such, is not included in combustion kinetics models. Global and channel-specific rate constants were computed and are reported as a function of temperature and pressure. This study shows that BRs of multichannel reactions useful for combustion modeling cannot be extrapolated from room-temperature kinetics studies.Keywords: O(3P) reaction dynamics with unsaturated hydrocarbons; crossed beams technique; potential energy surfaces; RRKM/master equation; intersystem crossing; ab initio quantum chemistry; CASPT2
In our laboratory a recent series of experiments by means of the crossed molecular beam (CMB) scattering technique with mass-spectrometric detection and time-of-flight analysis has been instrumental in fostering progress in the understanding of the dynamics of both simple triatomic insertion reactions and complex polyatomic addition–elimination reactions exhibiting competing channels. In the first part of this review we survey the advances made in the comprehension of the dynamics of the insertion reactions involving excited carbon, nitrogen and oxygen atoms – C( 1 D), N( 2 D), O( 1 D) – with H 2 (D 2 ), as made possible by synergistic comparisons of experimental reactive differential cross-sections with the results of exact quantum, quasiclassical trajectory and statistical calculations on reliable ab initio potential energy surfaces. Related experimental and theoretical work from other laboratories is noted throughout. In the second part, we review the progress made in the understanding of the dynamics of polyatomic multichannel reactions, such as those of ground state oxygen and carbon atoms, O( 3 P) and C( 3 P), with the simplest alkyne, acetylene, and alkene, ethylene, as made possible by the gained capability of identifying virtually all primary reaction channels, characterising their dynamics, and determining their branching ratios. Such a capability is based on an improved crossed molecular beam instrument which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power, and variable beam crossing angle for a larger collision energy range and increased angular and velocity resolution. The scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces. These detailed studies on polyatomic multichannel reactions provide an important bridge between crossed beam dynamics and thermal kinetics research.
The reaction between ground state oxygen atoms, O((3)P), and the acetylene molecule, C2H2, has been investigated in crossed molecular beam experiments with mass-spectrometric detection and time-of-flight analysis at three different collision energies, Ec = 34.4, 41.1 and 54.6 kJ mol(-1). From product angular and velocity distribution measurements of the HCCO and CH2 products in the laboratory frame, product angular and translational energy distributions in the center-of-mass frame were determined. Measurements on the CH2 product were made possible by employing for product detection the recently implemented soft electron-ionization (EI) technique with low-energy, tunable electrons, which has permitted suppressing interference coming from the dissociative ionization of reactants, products and background gases. It has been found that the title reaction leads only to two competing channels: H + HCCO (ketenyl) and CO + CH2 (triplet methylene). The branching ratio of cross sections between the two competing channels has been determined to be σ(HCCO)/[σ(HCCO) + σ(CH2)] = 0.79 ± 0.05, independent of collision energy within the experimental uncertainty. This value is in line with that obtained in the most recent and accurate kinetics determination at room temperature as well as with that predicted from recent theoretical calculations based on statistical rate theory and weak-collision master equation analysis and on dynamics surface-hopping quasiclassical trajectory calculations on-the-fly on coupled triplet/singlet ab initio potential energy surfaces. The firm assessment of the branching ratio as a function of translational energy for this important reaction, besides its fundamental significance, is of considerable relevance for the implementation of theoretical models of hydrocarbon combustion.
Recent laboratory experiments have demonstrated that even though contribution from other reaction channels cannot be neglected,
unsaturated hydrocarbons easily break their multiple C–C bonds to form CO after their interactions with atomic oxygen. Here,
we present an upgraded chemical modelling including a revision of the reactions between oxygen atoms and small unsaturated
hydrocarbons for different astrochemical environments. The first conclusion is that towards hot cores/corinos atomic oxygen
easily degrades unsaturated hydrocarbons directly to CO or to its precursor species (such as HCCO or HCO) and destroys the
double or triple bond of alkenes and alkynes. Therefore, environments rich in atomic oxygen at a relatively high temperature
are not expected to be rich in large unsaturated hydrocarbons or polycyclic aromatic hydrocarbons. In contrast, in O-poor
and C-rich objects, hydrocarbon growth can occur to a large extent. In addition, new radical species, namely ketyl and vinoxy
radicals, generated from other reaction channels can influence the abundances of hydrocarbons towards hot cores. We, therefore,
suggest that they should be included in the available databases. Hydrocarbon column densities are calculated in the 1013–1015 cm−2 range, in good agreement with their observed values, despite the small number of data currently published in the literature.
The reaction of O((3)P) with C(2)H(4), of importance in combustion and atmospheric chemistry, stands out as paradigm reaction involving not only the indicated triplet state potential energy surface (PES) but also an interleaved singlet PES that is coupled to the triplet surface. This reaction poses great challenges for theory and experiment, owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Crossed molecular beam (CMB) scattering experiments with soft electron ionization detection are used to disentangle the dynamics of this polyatomic multichannel reaction at a collision energy E(c) of 8.4 kcal∕mol. Five different primary products have been identified and characterized, which correspond to the five exothermic competing channels leading to H + CH(2)CHO, H + CH(3)CO, CH(3) + HCO, CH(2) + H(2)CO, and H(2) + CH(2)CO. These experiments extend our previous CMB work at higher collision energy (E(c) ∼ 13 kcal∕mol) and when the results are combined with the literature branching ratios from kinetics experiments at room temperature (E(c) ∼ 1 kcal∕mol), permit to explore the variation of the branching ratios over a wide range of collision energies. In a synergistic fashion, full-dimensional, QCT surface hopping calculations of the O((3)P) + C(2)H(4) reaction using ab initio PESs for the singlet and triplet states and their coupling, are reported at collision energies corresponding to the CMB and the kinetics ones. Both theory and experiment find almost an equal contribution from the triplet and singlet surfaces to the reaction, as seen from the collision energy dependence of branching ratios of product channels and extent of intersystem crossing (ISC). Further detailed comparisons at the level of angular distributions and translational energy distributions are made between theory and experiment for the three primary radical channel products, H + CH(2)CHO, CH(3) + HCO, and CH(2) + H(2)CO. The very good agreement between theory and experiment indicates that QCT surface-hopping calculations, using reliable coupled multidimensional PESs, can yield accurate dynamical information for polyatomic multichannel reactions in which ISC plays an important role.
Continuous supersonic beams of dicarbon (C2) and cyano (CN) radicals have been generated by a high-pressure radio-frequency discharge beam source starting from dilute mixtures in rare gases of suitable precursor molecules. Their internal quantum state distributions have been characterized by laser-induced-fluorescence (LIF) in a new crossed molecular beam-laser apparatus. These supersonic beams have been used to study the reactive scattering of C2 and CN radicals with unsaturated hydrocarbons. This paper reports here on the C2 and CN radical beam characterization by LIF and on dynamics studies of the reactions CN + C2H2 (acetylene) and CN + CH3CCH (methylacetylene) by the crossed molecular beam scattering technique with universal mass spectrometric detection and time-of-flight analysis. The role of CN rovibrational excitation on the dynamics of the CN + C2H2 reaction is discussed with reference to previous dynamics and kinetics studies. These reactions are of interest in the chemistry of planetary atmospheres (Titan) and the interstellar medium as well as in combustion.
The reaction of O((3)P) with propene (C(3)H(6)) has been examined using tunable vacuum ultraviolet radiation and time-resolved multiplexed photoionization mass spectrometry at 4 Torr and 298 K. The temporal and isomeric resolution of these experiments allow the separation of primary from secondary reaction products and determination of branching ratios of 1.00, 0.91 ± 0.30, and 0.05 ± 0.04 for the primary product channels CH(3) + CH(2)CHO, C(2)H(5) + HCO, and H(2) + CH(3)CHCO, respectively. The H + CH(3)CHCHO product channel was not observable for technical reasons in these experiments, so literature values for the branching fraction of this channel were used to convert the measured product branching ratios to branching fractions. The results of the present study, in combination with past experimental and theoretical studies of O((3)P) + C(3)H(6), identify important pathways leading to products on the C(3)H(6)O potential energy surface (PES). The present results suggest that up to 40% of the total product yield may require intersystem crossing from the initial triplet C(3)H(6)O PES to the lower-lying singlet PES.
The O((3)P) + C(2)H(4) reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H + CH(2)CHO, H + CH(3)CO, H(2) + CH(2)CO, CH(3) + HCO, CH(2) + CH(2)O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization mass-spectrometric detection at a collision energy of 8.4 kcal/mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential cross-sections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is it to consider the ISC as one of the nonadiabatic effects for this and similar systems involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH(3) + HCO and H + CH(2)CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.
The importance of intersystem crossing dynamics for the O((3)P)+C(2)H(2) reaction is demonstrated in this work. A direct dynamics trajectory surface hopping method has been employed to study the intersystem crossing effects. Our study reveals that there is a significant contribution from the spin nonconserving route to the chemical dynamics of the O((3)P)+C(2)H(2) reaction, despite small spin-orbit coupling constant values (<70 cm(-1)).
In this Perspective we highlight developments in the field of chemical reaction dynamics. Focus is on the advances recently made in the investigation of the dynamics of elementary multichannel radical-molecule and radical-radical reactions, as they have become possible using an improved crossed molecular beam scattering apparatus with universal electron-ionization mass spectrometric detection and time-of-flight analysis. These improvements consist in the implementation of (a) soft ionization detection by tunable low-energy electrons which has permitted us to reduce interfering signals originating from dissociative ionization processes, usually representing a major complication, (b) different beam crossing-angle set-ups which have permitted us to extend the range of collision energies over which a reaction can be studied, from very low (a few kJ mol(-1), as of interest in astrochemistry or planetary atmospheric chemistry) to quite high energies (several tens of kJ mol(-1), as of interest in high temperature combustion systems), and (c) continuous supersonic sources for producing a wide variety of atomic and molecular radical reactant beams. Exploiting these new features it has become possible to tackle the dynamics of a variety of polyatomic multichannel reactions, such as those occurring in many environments ranging from combustion and plasmas to terrestrial/planetary atmospheres and interstellar clouds. By measuring product angular and velocity distributions, after having suppressed or mitigated, when needed, the problem of dissociative ionization of interfering species (reactants, products, background gases) by soft ionization detection, essentially all primary reaction products can be identified, the dynamics of each reaction channel characterized, and the branching ratios determined as a function of collision energy. In general this information, besides being of fundamental relevance, is required for a predictive description of the chemistry of these environments via computer models. Examples are taken from recent on-going work (partly published) on the reactions of atomic oxygen with acetylene, ethylene and allyl radical, of great importance in combustion. A reaction of relevance in interstellar chemistry, as that of atomic carbon with acetylene, is also discussed briefly. Comparison with theoretical results is made wherever possible, both at the level of electronic structure calculations of the potential energy surfaces and dynamical computations. Recent complementary CMB work as well as kinetic work exploiting soft photo-ionization with synchrotron radiation are noted. The examples illustrated in this article demonstrate that the type of dynamical results now obtainable on polyatomic multichannel radical-molecule and radical-radical reactions might well complement reaction kinetics experiments and hence contribute to bridging the gap between microscopic reaction dynamics and thermal reaction kinetics, enhancing significantly our basic knowledge of chemical reactivity and understanding of the elementary reactions which occur in real-world environments.
The identification of the primary products and the determination of their branching ratios as a function of translational energy for multi-channel elementary reactions of importance in combustion chemistry still represent a challenge for traditional kinetics experiments. However, this kind of information is central for the detailed modeling of combustion systems. In this chapter, the significant contribution provided by the crossed molecular beam (CMB) method with “universal” mass spectrometric detection and time-of-flight analysis is illustrated. In particular, we describe the basics of the CMB method empowered with “soft” electron impact ionization as recently implemented in our laboratory and report on its application to the study of the multi-channel elementary reactions of atomic oxygen with unsaturated hydrocarbons (acetylene, ethylene, and allene) and hydrocarbon radicals (methyl and allyl), which are of paramount interest in the combustion of hydrocarbons. The complementarity to kinetics studies and implications of the dynamics
results for the modeling of combustion chemistry will be noted.
Supersonic beams of oxygen, nitrogen, and chlorine atoms and of metastable oxygen and nitrogen molecules produced from a high-pressure radiofrequency discharge beam source have been characterized by coupling velocity selection with magnetic analysis in the transmission mode. The present work leads to the determination of the relative populations of the electronic states of the species in the produced beams, showing that estimates of the populations from plasma temperatures or final translational temperatures could bring on incorrect conclusions.
Analysis of recent detections of water by Herschel/HIFI-PACS and Cassini/CIRS suggest for a steep gradient of the water profile in the lower stratosphere of Titan's atmosphere (Cottini2012, Moreno2012). This result provides a good opportunity to better understand the origin of oxygen compounds. However, the current photochemical models use an incomplete oxygen chemical scheme. In the present work, we improve the photochemistry of oxygen and introduce in particular a coupling between hydrocarbon, oxygen and nitrogen chemistries. Through the use of several different scenarios, we show that some oxygen compound abundances are sensitive to the nature of oxygen atoms (O+, OH and H2O) and the source of the flux (micrometeorites ablation or Enceladus' plume activity). Our model also predicts the presence of new and as yet undetected compounds such as NO (nitric oxide), HNO (nitrosyl hydride), HNCO (isocyanic acid) and N2O (nitrous oxide). Their future putative detection will give valuable constraints to discriminate between the different hypotheses for the nature and the source of oxygen compounds in the atmosphere of Titan. Through the use of a Monte Carlo-based uncertainty propagation study and global sensitivity analysis, we identify the key reactions that should be studied in priority to improve coupled photochemical models of Titan's atmosphere.
Rate coefficients for the reaction between ground-state O atoms and methylacetylene have been measured using the High-Temperature Photochemistry (HTP) technique at temperatures from 300 to 1350 K and pressures from 100 to 700 mbar. The oxygen atoms were generated by flash photolysis of O{sub 2} Or CO{sub 2}, and their relative concentrations were monitored by time-resolved resonance fluorescence. The data are independent of pressure and are well-fitted by the expression k(300-1350 K) = 2.9 x 10{sup -11} exp(-1134 K/T) cm{sup 3} molecule{sup -1} s{sup -1} with 2{sigma} precision limits of {+-}6% to {+-}12%, depending upon temperature, and corresponding 2{sigma} accuracy limits of about {+-}22%. An estimation for methyl group H-abstraction yielded k{sub H}(300-2500 K) 5.7 x 10{sup -20}(T/K){sup 2.16} exp(-2429 K/T) cm{sup 3} molecule{sup -1} s{sup -1}. This suggests that addition is the only significant initial O-atom attack step even at temperatures as high as those encountered in atmospheric-pressure methylacetylene-air flames. 38 refs., 2 figs., 2 tabs.
This is the second in a series of papers on the reaction of O(³{ital P}) with alkynes in which the internal state distribution of some products are studied. The first paper dealt with acetylene whose two product channels are CO+CHâ and H+HCCO. The present paper deals with the reactions of a series of higher alkynes; however, just the CO release and the H atom release channels were studied. The CO product was rotationally and vibrationally cold in every case. We therefore infer that, except possibly for acetylene, the initial ketocarbene undergoes intersystem crossing to a singlet state and isomerizes to a substituted ketene which then dissociates through a linear CâCâO transition state. The absence of CO vibration energy implies that the energy taken from the initially formed CâO bond to facilitate a 1,2 migration is not returned. The large H atom translational energy implies that the H atom is released simultaneously with the formation of a radical of high resonance energy. Finally, the CO and H atom yields decrease in the longer alkynes, presumably because the dominant reaction channel becomes CâC bond breaking leading to radical pair formation. {copyright} {ital 1996 American Institute of Physics.}
The crossed beam reaction of the d1-ethynyl radical C2D(X2Σ+), with methylacetylene, CH3CCH(X1A′1), was studied at an average collision energy of 39.8 kJ mol-1 in a crossed molecular beams experiment and electronic structure calculations. It was verified via experiments that the reduced cone of acceptance of the carbon atom adjacent to the methyl group favors a carbon-carbon σ bond formation at the C1 atom. A crossed beam experiment of C2D with partially deuterated methylacetylene, CD3CCH, explicitly showed that the reactive intermediates decompose via two distinct channels to form both methylacetylene and ethylallene isomers.
The chemical dynamics to form cyanopropyne, CH3CCCN (X 1A1), and cyanoallene, H2CCCHCN (X 1A′), via the neutral–neutral reaction of the cyano radical, CN (X 2Σ+), with methylacetylene, CH3CCH (X 1A1), is investigated under single collision conditions in a crossed molecular beam experiment at a collision energy of 24.7 kJ mol−1. The laboratory angular distribution and time-of-flight spectra of the C4H3N products are recorded at m/e=65, 64, 63, and 62. The reaction of d3-methylacetylene, CD3CCH (X 1A1), with CN radicals yields reactive scattering signal at m/e = 68 and m/e = 67 demonstrating that two distinct H(D) atom loss channels are open. Forward-convolution fitting of the laboratory data reveal that the reaction dynamics are indirect and governed by an initial attack of the CN radical to the π electron density of the β carbon atom of the methylacetylene molecule to form a long lived CH3CCHCN collision complex. The latter decomposes via two channels, i.e., H atom loss from the CH3 group to yield cyanoallene, and H atom loss from the acetylenic carbon atom to form cyanopropyne. The explicit identification of the CN vs H exchange channel and two distinct product isomers cyanoallene and cyanopropyne strongly suggests the title reaction as a potential route to form these isomers in dark molecular clouds, the outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan. © 1999 American Institute of Physics.
We report on the determination of primary products and their branching ratios for the combustion relevant O(3P)+allene reaction by the crossed molecular beams method with soft electron-ionization mass-spectrometric detection at a collision energy of 39.3 kJ/mol. We have explored the reaction dynamics of the open channels leading to C2H4+CO, C2H2+H2CO, C2H3+HCO, CH2CCHO+H, and CH2CO+CH2. Because some of the observed products can only be formed via intersystem crossing (ISC) from triplet to singlet potential energy surfaces, from the product branching ratios we have inferred the extent of ISC. The conclusion is that the O(3P)+allene reaction proceeds mostly (>90%) via ISC. This observation poses the question of how important it is to consider nonadiabatic effects for this and other similar systems involved in combustion chemistry. Another important conclusion is that the interaction of atomic oxygen with allene breaks apart the three-carbon atom chain, mostly producing CO and ethylene.Keywords: reaction dynamics; crossed molecular beam technique; multichannel reactions; combustion chemistry; atomic oxygen reactions; nonadiabatic effects
The appearance potentials and translational energies at onset of CH3+ from several hydrocarbons and amines have been determined by electron impact in a sector field mass spectrometer. The translational energy was determined by the deflection method and the total excess energy was computed from it. Very good agreement of the heats of formation of ethynyl, propargyl, allyl, amino, methylamino, and dimethylamino radicals with previous determinations was obtained. Reasonable values were also obtained for the dimethyl ally], dimethyl propargyl, and CH2NH2 radicals, but there were no previous values to which ours could be compared. Results for two more complex radicals appear to be somewhat in error, possibly because of the large correction factor resulting from the large number of vibrational modes.
Reactions of O atoms and OH radicals with acetylene and methylacetylene
were studied in high-intensity room-temperature crossed molecular beams.
Products were detected using photoionization mass spectrometry. Major and minor
observed reactive chnnnels were assigned on the basis of the identity of the
products observed and the magnitudes of the product ion signals. The numbers of
major observed reactive channels are two for O + C/sub 2/H/sub 2/, one for OH + C/
sub 2/H/sub 2/, seven for O + C/sub 3/H/sub 4/, and four for OH + C/sub 3/H/sub 4/
. Details on reaction dynamics were obtained using deuterated compounds and are
discussed. Evidence is presented for several types of mechanisms including
abstraction, displacement, and addition followed by decomposition. (auth) by f
cthe (thermal) cthe in the irradiated ture in irradiated fluorocarbon-hydrocarbon
mixtures. Addi
Dynamical studies of elementary gas-phase bimolecular reactions have progressed significantly during the last few years owing to advancements in molecular beam and laser techniques as well as in theoretical methodologies. In this article we give a brief overview of the recent progress in the field of reaction dynamics and then survey recent work from our laboratory on reactions of atoms and radicals with simple molecules by the crossed molecular beam scattering method using mass-spectrometric detection. Emphasis is on three-atom (A + BC) and four-atom (AB + CD) reactions for which the interplay between experiment and theory is the strongest and the most detailed. Reactive differential cross-sections for the three-atom Cl + H-2 and four-atom OH + H-2 and OH + CO reactions are presented and compared with the results of quasiclassical and quantum-mechanical scattering calculations on ab initio potential-energy surfaces in an effort to assess the status of theory versus experiment. The reaction dynamics of electronically excited atoms are discussed too; the effect of electronic excitation on the reaction dynamics of atomic oxygen is examined using the reaction O(P-3,D-1) + H2S as an example.
We investigated the dynamics of photodissociation of propenal (acrolein, CH(2)CHCHO) at 157 nm in a molecular beam and of migration and elimination of hydrogen atoms in systems C(3)H(4)O and C(3)H(3)O using quantum-chemical calculations. Compared with the previous results of photodissociation of propenal at 193 nm, the major difference is that the C(3)H(3)O fragment present at the 193-nm photolysis disappears at the 157-nm photolysis whereas the C(3)H(2)O fragment absent at 193 nm appears at 157 nm. Optimized structures and harmonic vibrational frequencies of molecular species with gross formula C(3)H(2-4)O were computed at the level of B3LYP/6-311G(d,p) and total energies of those molecules at optimized structures were computed at the level of CCSD(T)/6-311+G(3df,2p). Based on the calculated potential-energy surfaces, we deduce that the C(3)H(3)O fragment observed in the photolysis of propenal at 193 nm is probably CHCCHOH ((2)A") and/or CH(2)CCOH ((2)A") produced from an intermediate hydroxyl propadiene (CH(2)CCHOH) following isomerization. Adiabatic and vertical ionization potentials of eight isomers of C(3)H(3)O and two isomers of C(3)H(2)O were calculated; CHCCHOH ((2)A") and CH(2)CCOH ((2)A") have ionization potentials in good agreement with the experimental value of ∼7.4 eV. We also deduce that all the nascent C(3)H(3)O fragments from the photolysis of propenal at 157 nm spontaneously decompose mainly to C(2)H(3) + CO and C(3)H(2)O + H because of the large excitation energy. This work provides profound insight into the dynamics of migration and elimination of hydrogen atoms of propenal optically excited in the vacuum-ultraviolet region.
Biofuels, such as bio-ethanol, bio-butanol, and biodiesel, are of increasing interest as alternatives to petroleum-based transportation fuels because they offer the long-term promise of fuel-source regenerability and reduced climatic impact. Current discussions emphasize the processes to make such alternative fuels and fuel additives, the compatibility of these substances with current fuel-delivery infrastructure and engine performance, and the competition between biofuel and food production. However, the combustion chemistry of the compounds that constitute typical biofuels, including alcohols, ethers, and esters, has not received similar public attention. Herein we highlight some characteristic aspects of the chemical pathways in the combustion of prototypical representatives of potential biofuels. The discussion focuses on the decomposition and oxidation mechanisms and the formation of undesired, harmful, or toxic emissions, with an emphasis on transportation fuels. New insights into the vastly diverse and complex chemical reaction networks of biofuel combustion are enabled by recent experimental investigations and complementary combustion modeling. Understanding key elements of this chemistry is an important step towards the intelligent selection of next-generation alternative fuels.
For the reaction of O((3)P) with propyne, the product channels and mechanisms are investigated both theoretically and experimentally. Theoretically, the CCSD(T)//B3LYP/6-311G(d,p) level of calculations are performed for both the triplet and singlet potential energy surfaces and the minimum energy crossing point between the two surfaces are located with the Newton-Lagrange method. The theoretical calculations show that the reaction occurs dominantly via the O-addition rather than the H-abstraction mechanism. The reaction starts with the O-addition to either of the triple bond carbon atoms forming triplet ketocarbene (3)CH(3)CCHO or (3)CH(3)COCH which can undergo decomposition, H-atom migration or intersystem crossing from which a variety of channels are open, including the adiabatic channels of CH(3)CCO + H (CH(2)CCHO + H), CH(3) + HCCO, CH(2)CH + HCO, CH(2)CO + CH(2), CH(3)CH + CO, and the nonadiabatic channels of C(2)H(4) + CO, C(2)H(2) + H(2) + CO, H(2) + H(2)CCCO. Experimentally, the CO channel is investigated with TR-FTIR emission spectroscopy. A complete detection of the CO product at each vibrationally excited level up to v = 5 is fulfilled, from which the vibrational energy disposal of CO is determined and found to consist with the statistical partition of the singlet C(2)H(4) + CO channel, but not with the triplet CH(3)CH + CO channel. In combination with the present calculation results, it is concluded that CO arises mainly from the singlet methylketene ((1)CH(3)CHCO) dissociation following the intersystem crossing of the triplet ketocarbene adduct ((3)CH(3)CCHO). Fast intersystem crossing via the minimum energy crossing point of the triplet and singlet surfaces is shown to play significant roles resulting into nonadiabatic pathways for this reaction. Moreover, other interesting questions are explored as to the site selectivity of O((3)P) atom being added to which carbon atom of the triple bond and different types of internal H-atom migrations including 1,2-H shift, 3,2-H shift, and 3,1-H shift involved in the reaction.
The chemical dynamics to synthesize the 2,4-pentadiynyl-1 radical, HCCCCCH(2)(X(2)B(1)), via the neutral-neutral reaction of dicarbon with methylacetylene, was examined in a crossed molecular beams experiment at a collision energy of 37.6 kJ mol(-1). The laboratory angular distribution and time-of-flight spectra of the 2,4-pentadiynyl-1 radical and its fragmentation patterns were recorded at m/z = 63-60 and m/z = 51-48. Our findings suggest that the reaction dynamics are indirect and dictated by an initial attack of the dicarbon molecule to the pi electron density of the methylacetylene molecule to form cyclic collision complexes. The latter ultimately rearranged via ring opening to methyldiacetylene, CH(3)-C triple bond C-C triple bond C-H. This structure decomposed via atomic hydrogen emission to the 2,4-pentadiynyl-1 radical; here, the hydrogen atom was found to be emitted almost parallel to the total angular momentum as suggested by the experimentally observed sideways scattering. The overall reaction was strongly exoergic by 182 +/- 10 kJ mol(-1). The identification of the resonance-stabilized free 2,4-pentadiynyl-1 radical represents a solid background for the title reaction to be included into more refined reaction networks modeling the chemistry of circumstellar envelopes and also of sooting combustion flames.
The lowest-lying triplet and singlet potential energy surfaces for the O(3P) + CH2=C=CH2 reaction were theoretically characterized using the complete basis set model chemistry, CBS-QB3. The primary product distributions for the multistate multiwell reactions on the individual surfaces were then determined by RRKM statistical rate theory and weak-collision master equation analysis using the exact stochastic simulation method. The results predict that the electrophilic O-addition pathways on the central and terminal carbon atom are dominant up to combustion temperatures. Major predicted end-products for the addition routes include CO + C2H4, 3CH2 + H2CCO, and CH2=C*-CHO + H*, in agreement with experimental evidence. CO + C2H4 are mainly generated from the lowest-lying singlet surface after an intersystem crossing process from the initial triplet surface. Efficient H-abstraction pathways are newly identified and occur on two different electronic state surfaces, 3A'' and 3A', resulting in OH + propargyl radicals; they are predicted to play an important role at higher temperatures in hydrocarbon combustion chemistry and flames, with estimated contributions of ca. 35% at 2000 K. The overall thermal rate coefficient k(O + C3H4) at 200-1000 K was computed using multistate transition state theory: k(T) = 1.60 x 10(-17) x T (2.05) x exp(-90 K/T) cm3 molecule(-1) s(-1), in good agreement with experimental data available for the 300-600 K range.
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