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Plot of the observed 2nd order reaction rate coefficient vs bath gas pressure for the reaction of the methyl ion with hydrogen cyanide. Taken from Ref. 7. Using the drift-mode ICR experiment.
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The reaction of the methyl cation with hydrogen cyanide is revisited. We have confidence that we have resolved a long standing apparent contradiction of experimental results. A literature history is presented along with one new experiment and a re-examination of an old experiment. In this present work it is shown that all of the previous studies ha...
Citations
... This set of reactions was studied by Defrees et al. ( 1985 ), who found that its two products, CH 3 CNH + and CH 3 NCH + , are formed in a ratio of 85:15 due to unimolecular isomerization. Later, Anicich et al. ( 1995 ) examined the experimental literature on these reactions and determined a total radiative association rate coefficient of 2 × 10 −10 cm 3 s −1 for reactions (4) and (6). ...
... Although the rate coefficients of 1.7 × 10 −10 ( T /300 K) −3 for reactions (4) and (5) and 3.0 × 10 −11 ( T /300 K) −3 for reactions (6) and (7) from Anicich et al. ( 1995 ) and Defrees et al. ( 1985 ), respectively, are used throughout this work, we tested the effects of lowering the rate coefficients, as shown in Figs 4 and 5 . At ∼5 × 10 5 yr (Hincelin et al. 2011 ;Loomis et al. 2021 ), Fig. 4 shows that the observed abundances of CH 3 CN and CH 3 NC match the modelled abundance when the rate coefficients for this set of reactions (reactions 4-7 ) in TMC-1 conditions are between one and two orders of magnitude lower than previously modelled. ...
Two closely related isomeric pairs of cyanides, CH3[CN/NC] and H2C[CN/NC], are studied in cold, dark interstellar cloud conditions. In contrast to the diverse detections of methyl cyanide (CH3CN) in space, methyl isocyanide (CH3NC) has previously only been observed in warm and hot star-forming regions. We report the detection of CH3NC in the cold prestellar core TMC-1 using the Green Bank Telescope with a detection significance of 13.4σ. Hyperfine transitions in H3CCN and quadrupole interactions in CH3CN and CH3NC were matched to a spectral line survey from the GOTHAM large project on the Green Bank Telescope, resulting in abundances with respect to hydrogen of $1.92^{+0.13}_{-0.07} \times 10^{-9}$ for the cyanomethyl radical (H3CCN), $5.02^{+3.08}_{-2.06} \times 10^{-10}$ for CH3CN, and $2.97^{+2.10}_{-1.37} \times 10^{-11}$ for CH3NC. Efforts to model these molecules with the three-phase gas-grain code Nautilus in TMC-1 conditions overproduce both CH3CN and CH3NC, though the ratio of ∼5.9 % is consistent across observations and models of these species. This may point to missing destruction routes in the model. The models capture the larger abundance of H2CCN well. Dissociative recombination is found to be the primary production route for these molecules, and reactions with abundant ions are found to be the primary destruction routes. H + CH3NC is investigated with transition state theory as a potential destruction route, but found to be too slow in cold cloud conditions to account for the discrepancy in modelled and observed abundances of CH3NC.
... First detected toward Sgr B2 (Cernicharo et al. 1988;Remijan et al. 2005), CH 3 NC was also detected toward the Horsehead nebula (Gratier et al. 2013), Orion KL (López et al. 2014), and more recently toward the solar-type binary protostar IRAS16293-2422 (Calcutt et al. 2018). A few theoretical and experimental studies have investigated the isomers' chemistry and their abundance ratio (Huntress & Mitchell 1979;Defrees et al. 1985;Anicich et al. 1995), and converged on the same major gas-phase production pathways for both via the reaction: ...
... with k 2 and k 3 given in Table 2, followed by the dissociative recombinations of both protonated ions CH 3 NCH + and its isomer CH 3 CNH + to form CH 3 NC and CH 3 CN, respectively. However, the branching ratio is poorly constrained and depends on the stabilization processes of the intermediate complex (CH 3 NCH + ) * (e.g., Anicich et al. 1995). Due to its lower energy state, CH 3 CNH + is found to be the major product of the reaction CH 3 + +HCN(9). ...
Complex nitriles, such as HC₃N, and CH₃CN, are observed in a wide variety of astrophysical environments, including at relatively high abundances in photon-dominated regions (PDRs) and the ultraviolet exposed atmospheres of planet-forming disks. The latter have been inferred to be oxygen-poor, suggesting that these observations may be explained by organic chemistry in C-rich environments. In this study we first explore if the PDR complex nitrile observations can be explained by gas-phase PDR chemistry alone if the elemental C/O ratio is elevated. In the case of the Horsehead PDR, we find that gas-phase chemistry with C/O ≳ 0.9 can indeed explain the observed nitrile abundances, increasing predicted abundances by several orders of magnitude compared to standard C/O assumptions. We also find that the nitrile abundances are sensitive to the cosmic-ray ionization treatment, and provide constraints on the branching ratios between CH₃CN and CH₃NC productions. In a fiducial disk model, an elevated C/O ratio increases the CH₃CN and HC₃N productions by more than an order of magnitude, bringing abundance predictions within an order of magnitude to what has been inferred from observations. The C/O ratio appears to be a key variable in predicting and interpreting complex organic molecule abundances in PDRs across a range of scales.
... We ran nine models, which differed only in the rate coefficients used for the radiative association reactions CH 3 + + HCN CH 3 CNH + + hv and CH 3 + + HNC CH 3 CNH + + hv. The rate coefficient for the former reaction has been determined both experimentally and theoretically with results differing by a factor of 45 [120,121]. The latter reaction is not included in the original model, and was added to the other models since it could be important owing to the relatively high HNC abundance in TMC-1 [122]. ...
... In turn, CH 3 NCCN ϩ may methylate many organic molecules that have a higher PA than C 2 N 2 , and are therefore also expected to have a higher methyl cation affinity (MCA) than C 2 N 2 . These reactions would constitute low-energy alternatives to direct alkylation reactions of the type studied both by the JPL/McEwan [14,15] and the Cacace groups [16 -18], and would be similar to methyl cation transfer from CH 3 OH 2 ϩ to H 2 CO and CH 3 OH [19,20]. On the other hand, CH 3 NCCN ϩ may be strongly covalently bonded and may not transfer methyl cations. ...
The proton affinity (PA) of cyanogen (C2N2) was redetermined through selected ion flow tube (SIFT) measurement of the rate coefficients of the reaction C2H3+ + C2N2 = C2N2H+ + C2H2 in both directions. The observed ΔGo300 = −6.1 kJ mol−1 and derived ΔHo = −10.6 kJ mol−1, and previous equilibrium results with CH3Cl, give PA(C2N2) = 651.2 ± 2 kJ mol−1. The results are consistent with the recently revised PA(CH3Cl) = 647.3 kJ mol−1, and in good agreement with recent high-level theoretical values of PA(C2N2) = 655–657 kJ mol−1. We also observed that SO2H+ transfers a proton to C2N2 as well as to C2H2, and that in the reverse direction, the new reaction C2H3+ + SO2 → CH2SOH+ + CO occurs but no proton transfer, indicating that PA(SO2) < 641.1 kJ mol−1. The methylated species C2N2CH3+ does not transfer a methyl cation to HCN, CH3CN, (CH3)2CO and (CH3)3N. However, association is observed in these systems and in the reactions of C2N2H+ with C2H2 and SO2. These processes can contribute to the astrochemical synthesis of complex heteroatom containing organics. In particular, we observe the apparently covalent C2N2H+ · C2H2 adduct, an isomer of deprotonated 1,4-diazine, which suggests that similar reactions of other C2N2 containing ions and acetylenes can yield pyrimidine nucleic bases by simple ion-molecule processes.
Methyl cyanide (CH3CN) is one of the most abundant and widely spread interstellar complex organic molecules (iCOMs). Several studies found that, in hot corinos, methyl cyanide and methanol abundances are correlated suggesting a chemical link, often interpreted as a synthesis of them on the interstellar grain surfaces. In this article, we present a revised network of the reactions forming methyl cyanide in the gas-phase. We carried out an exhaustive review of the gas-phase CH3CN formation routes, propose two new reactions and performed new quantum mechanics computations of several reactions. We found that 13 of the 15 reactions reported in the databases KIDA and UDfA have incorrect products and/or rate constants. The new corrected reaction network contains 10 reactions leading to methyl cyanide. We tested the relative importance of those reactions in forming CH3CN using our astrochemical model. We confirm that the radiative association of CH3+ and HCN, forming CH3CNH+, followed by the electron recombination of CH3CNH+, is the most important CH3CN formation route in both cold and warm environments, notwithstanding that we significantly corrected the rate constants and products of both reactions. The two newly proposed reactions play an important role in warm environments. Finally, we found a very good agreement between the CH3CN predicted abundances with those measured in cold (∼10 K) and warm (∼90 K) objects. Unexpectedly, we also found a chemical link between methanol and methyl cyanide via the CH$_{3}^{+}$ ion, which can explain the observed correlation between the CH3OH and CH3CN abundances measured in hot corinos.
The chemistry that occurs in interstellar clouds consists of both gas-phase processes and reactions on the surfaces of dust grains, the latter particularly on and in water-dominated ice mantles in cold clouds. Some of these processes, especially at low temperature, are very unusual by terrestrial standards. For example, in the gas-phase, two-body association reactions form a metastable species known as a complex, which is then stabilized by the emission of radiation under low-density conditions, especially at low temperatures. In the solid phase, it has been thought that the major process for surface reactions is diffusive in nature, occurring when two species undergoing random walks collide with each other on a surface that has both potential wells and intermediate barriers. There is experimental evidence for this process, although very few rates at low interstellar temperatures are well measured. Moreover, since dust particles are discrete, modeling has to take account that reactant pairs are on the same grain, a problem that can be treated using stochastic approaches. In addition, it has been shown more recently that surface reactions can occur more rapidly if they undergo any of a number of non-diffusive processes including so-called three-body mechanisms. There is some experimental support for this hypothesis. These and other unusual gaseous and solid-state processes will be discussed from the theoretical and experimental points of view, and their possible role in the synthesis of organic molecules in interstellar clouds explained. In addition, their historical development will be reviewed.
This study provides the first evidence for the formation of the oxonium phenol ion and its hydrated clusters by the sequential addition of water molecules onto the phenylium ion in the gas phase. The oxonium phenol ion exhibits thermal stability at higher temperatures up to nearly 575 K while the hydrated phenyl cation dissociates by the loss of water at temperatures below 400 K. A second water molecule is attached reversibly to each of the oxonium phenol and the hydrated phenyl ions and equilibrium thermochemical measurements in the low temperature range yield an average −ΔH° of 15.5 kcal/mol reflecting 35% contribution from the hydrated oxonium phenol C6H5OH2⁺(H2O) and 65% contribution from the second hydration step of the phenyl cation C6H5⁺(H2O)2. In the high temperature range (373–423 K), the measured average −ΔH° of 18.7 kcal/mol reflects 45% and 55% contributions from the hydrated oxonium phenol and the second hydration step of the phenyl cation, respectively. DFT calculations at the B3LYP/6-311++G** level show that the sequential hydration of the oxonium phenol ion results in the formation of externally hydrated clusters C6H5OH2⁺(H2O)n where the incoming water molecules form a hydrogen bonding network attached to the oxonium site, and the sequential hydration energy decreases from 25.9 kcal/mol for n = 1 to 11.5 kcal/mol for n = 5. For the hydrated phenyl cation clusters C6H5⁺(H2O)n, the water molecules are attached at two CHδ+ sites of the phenyl cation, and the sequential hydration energy decreases slowly from 11.3 kcal/mol for n = 1 to 8.1 kcal/mol for n = 5.
The association of CH3+ with the three molecules C2N2, CH2CHCN, and HCCCN has been examined using ion cyclotron resonance (ICR) and selected ion flow tube (SIFT) techniques at room temperature. In each reaction, the mean lifetime of the complex (CH3·N⋮CR+)* formed in the association has a major influence on the outcome of the reaction and the product channels that are observed using ICR and SIFT. Termolecular rate coefficients are reported for the association of CH3+ + C2N2 for the bath gases M = He, Ar, N2, and C2N2. k3 = 8.2 × 10-24 cm6 s-1 (M = C2N2). In each system the association product channel occurs in competition with exothermic bimolecular channels. The complex lifetimes in all three systems are in the range 30−70 μs. Very rapid ion−molecule association reactions have been observed in several systems of hydrocarbons and nitriles, and the implications for Titan ion chemistry are discussed briefly.
The reactions of H2O+, H3O+, D2O+, and D3O+ with neutral H2O and D2O were studied by tandem mass spectrometry. The H2O+ and D2O+ ion reactions exhibited multiple channels, including charge transfer, proton transfer (or hydrogen atom abstraction), and isotopic exchange. The H3O+ and D3O+ ion reactions exhibited only isotope exchange. The variation in the abundances of all ions involved in the reactions was measured over a neutral pressure range from 0 to 2 × 10−5 Torr. A reaction scheme was chosen, which consisted of a sequence of charge transfer, proton transfer, and isotopic exchange reactions. Exact solutions to two groups of simultaneous differential equations were determined; one group started with the reaction of ionized water, and the other group started with the reactions of protonated water. A nonlinear least-squares regression technique was used to determine the rate coefficients of the individual reactions in the schemes from the ion abundance data. Branching ratios and relative rate coefficients were also determined in this manner.A delta chi-squared analysis of the results of the model fitted to the experimental data indicated that the kinetic information about the primary isotopic exchange processes is statistically the most significant. The errors in the derived values of the kinetic information of subsequent channels increased rapidly. Data from previously published selected ion flow tube (SIFT) study were analyzed in the same manner. Rigorous statistical analysis showed that the statistical isotope scrambling model was unable to explain either the SIFT or the tandem mass spectrometry data. This study shows that statistical analysis can be utilized to assess the validity of possible models in explaining experimentally observed kinetic behaviors.
Ion-molecule reactions of C2N2+ and C2N2 With several ions and neutrals observed or predicted to be present in the atmosphere of Titan have been examined in an ion cyclotron resonance spectrometer at room temperature and low pressures. Rate coefficients and branching ratios are reported for N+, N-2(+), HCN+, C2H2+, C2H4+, C3H5+, C4H2+, and C4H3+ With C2N2 and for C2N2+ With N-2 and C2H2. The association reaction between C2N2+ and C2N2 forming C4N4+ was examined in some detail. No evidence was found for radiative (bimolecular) association, but rapid collisional stabilization was observed. The termolecular association rate coefficient is k(3) = 1.7 x 10(-23) cm(6) s(-1)(M = C2N2), and the relative collisional stabilization efficiencies for cm the bath gases M = He, Ne, Ar, and N-2 were also measured. The mean lifetime of the (C4N4+)* complex that exhibited collisional stabilization was tau beta = 104 mu s where beta is the relative efficiency of collisional stabilization. A possible form of the C4N4+ product of association is the ion NCCNNCCN+ resulting from unrearranged addition. A mechanism is also presented showing how pressure saturation can occur when the rate coefficient for association is much less that the collision rate.
