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Comparison of PAs (in kJ mol −1 at 298 K) for selected nitriles and imino nitriles calculated with different levels of theory, together with experimental values when available.
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The gas-phase basicity of nitriles can be enhanced by a push–pull effect. The role of the intercalated scaffold between the pushing group (electron-donor) and the pulling (electron-acceptor) nitrile group is crucial in the basicity enhancement, simultaneously having a transmission function and an intrinsic contribution to the basicity. In this stud...
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Context 1
... articles, as well as others dealing with similar functionalities, refer to the corresponding families as N-cyano-imines or cyano-imines. On the other hand, the IUPAC nomenclature describes these compounds (Z: N) as cyanamide (H2N−CN) derivatives, with the series Z: CH being defined as acetonitrile derivatives (see Table S1 in the Supplementary Materials (SM) for IUPAC names for series 1-6). To avoid confusion, and to focus on our objective of comparing nitrile basicity values, we adopted names indicating a priority for the nitrile group. ...Context 2
... simple nitriles, the PAs calculated here for the cyano and imino N (Z) atoms, together with data for DFT, G2, and G4 from the literature, are summarized in Table 1. Generally, applications of the DFT methods lead to considerably higher PAs (and GBs) for the N-cyano site than those of the Gn theories (differences 10-20 kJ mol −1 ), with the exception of HCN. ...Context 3
... isomer b has higher G by 2 kJ mol −1 . For both derivatives (I.2 and II.2), the isomers c and d may be neglected in the isomeric mixtures owing to their high ΔGs (10 kJ mol −1 ); hence, their basicity data calculated at different levels of theory are not compared in Table 1. ...Context 4
... For a more limited series of substituents (X: H, NH2, NMe2, data from Table 1 and ref. [6]), the comparison of calculated PAs for the two disubstituted cyclopropene systems cyclo-X2C2C=Z-C≡N and X2C=Z-C≡N (substituted by 3 and 1, respectively) give a slope (Figure 8) of 0.7, which can be viewed as a transmission factor of substituent effects (see Figure S5 in SM). The cyclopropene scaffolds somewhat reduce the electron donor effect as compared to direct substitution in X-C≡N or to transmission by intercalating the C=Z double bond. ...Context 5
... 3. HOMA and NICS indices estimated at the DFT2 level for the rings in selected neutral and monoprotonated cyano (C≡NH + ) and imino (ZH + ) nitriles. See Table 1 ...Context 6
... unsubstituted derivatives (I.3-I.6 and II.3-II.6) already display higher PAs (Table 1) than the push-pull dimethylcyanamide Me2N-C≡N (DFT-calculated PA 868.7 kJ mol −1 [6]). Substitution of CPC and CPN parts by two NMe2 groups in I.9 and II.9 leads to derivatives with PAs (Table 1) higher than those of phosphazenenitrile (H2N)3P=N-C≡N (DFT calculated PA 978.8 kJ mol −1 [6]). ...Context 7
... display higher PAs (Table 1) than the push-pull dimethylcyanamide Me2N-C≡N (DFT-calculated PA 868.7 kJ mol −1 [6]). Substitution of CPC and CPN parts by two NMe2 groups in I.9 and II.9 leads to derivatives with PAs (Table 1) higher than those of phosphazenenitrile (H2N)3P=N-C≡N (DFT calculated PA 978.8 kJ mol −1 [6]). The positions of all these simple new π-π-conjugated nitrile bases (I.1-I.9 and II.1-II.9) on the DFT-calculated PA scale are given in Figure 11. ...Context 8
... also reveal some similarities; however, the imino N (Z) effect seems to be stronger for these larger scaffolds than that for the CPN derivatives ( Figure 6). Nitriles I.6 and II.6 (Table 1) exhibit the highest PAs in series of simple parent compounds (without pushing X groups). Their exceptionally high PAs (973 and 1004 kJ mol −1 , respectively) and the strong pushing effects of the two phosphazene groups in II.12 containing the CPN system when compared to the parent system II.3 (ca. ...Similar publications
This work extends our earlier quantum chemical studies on the gas-phase basicity of very strong N-bases to two series of nitriles containing the methylenecyclopropene and cyclopropenimine scaffolds with dissymmetrical substitution by one or two electron-donating substituents such as Me, NR2, N=C (NR2)2, and N=P (NR2)3, the last three being strong d...
Citations
... The relative quantities (ΔE, ΔH, TΔS, and ΔG) have been calculated separately for ionic isomers of mono-anions (ABH) − , di-anions (AB) 2− , mono-cations (AH2BH) + , and di-cations (AH2BH2) 2+ . The composition of the neutral and ionic isomeric mixtures of C and iC have be estimated on the basis of ΔGi calculated for individual neutral and ionic isomers using equation (1), where xi is the isomer mole-fraction [51][52][53][54]. Note that ΔGi can be different for neutral and ionic isomers, and thus the calculated xi cannot be the same for ionic and neutral species. ...
Inter- and intra-molecular proton-transfers between functional groups in nucleobases play a principal role in their interactions (pairing) in nucleic acids. Although prototropic rearrangements (intramolecular proton-transfers) for neutral pyrimidine bases are well documented, they have not always been considered for their protonated and deprotonated forms. The complete isomeric mixtures in acid-base equilibria and in acidity–basicity parameters have not yet been examined. Taking into account the lack of literature and data, research into the question of prototropy for the ionic (protonated and deprotonated) forms has been undertaken in this work. For the purposes of this investigation, two isomeric pyrimidine bases (C—cytosine and iC—isocytosine) were chosen. They exhibit analogous (symmetrical) general acid-base equilibria (intermolecular proton-transfers). Being similar polyfunctional tautomeric systems, C and iC possess two labile protons and five conjugated tautomeric sites. However, positions of exo groups are different. Consequently, structural conversions such as prototropy, rotational, and geometrical isomerism of exo groups (=O/−OH and =NH/−NH2) and their intramolecular interactions with endo groups (=N−/>NH) possible in neutral C and iC and in their ionic forms lead to some differences in compositions of isomeric mixtures. By application of quantum–chemical methods to the isolated (in vacuo) species, stability of all possible neutral and ionic isomers has been examined and the candidate isomers selected. The complete isomeric mixtures have been considered for the first time for di-deprotonated, mono-deprotonated, mono-protonated, and di-protonated forms. Protonation–deprotonation reactions have been analyzed in the gas phase that models non-polar environment. The gas-phase microscopic (kinetic) and macroscopic (thermodynamic) acidity–basicity parameters have been estimated for each step of acid-base equilibria. When proceeding from di-anion to di-cation in four steps of protonation–deprotonation reaction, the macroscopic proton affinities for C and iC differ by less than 10 kcal mol−1. Their DFT-calculated values are as follows: 451 and 457, 340 and 339, 228 and 224, and 100 and 104 kcal mol−1, respectively. Differences between the microscopic proton affinities for analogous isomers of C and iC seem to be larger for the exo than endo groups. Owing to variations of relative stabilities for neutral and ionic isomers, in some cases they are even larger than 10 kcal mol−1.
... This Special Issue entitled "Symmetry in Acid-Base Chemistry" belongs to the section of "Chemistry and Symmetry/Asymmetry". It comprises five original articles by 30 authors [1][2][3][4][5] (from Ukraine, Germany, Estonia, France, Portugal, Hungary, Turkey, Czech Republic, Slovakia, Denmark, Iran, and Poland) who discuss the most important questions of modern acid-base chemistry. Authors documented and analyzed only new original results both experimental [2] and theoretical [1,[3][4][5] that have been reported for the first time. ...
... It comprises five original articles by 30 authors [1][2][3][4][5] (from Ukraine, Germany, Estonia, France, Portugal, Hungary, Turkey, Czech Republic, Slovakia, Denmark, Iran, and Poland) who discuss the most important questions of modern acid-base chemistry. Authors documented and analyzed only new original results both experimental [2] and theoretical [1,[3][4][5] that have been reported for the first time. ...
... The last article accepted for the Special Issue presents a complete theoretical analysis of isomeric forms of neutral and protonated polyfunctional push-pull nitriles with various electron donor groups [5]. It has been submitted by Gal (France) and his co-workers (Poland, France, and Iran). ...
Most organic molecules, including natural products, drugs, and toxicants, contain functional groups that display acid-base properties [...]
... Comparison of pyrazole ring aromaticity index before and after protonation in these structures can support this claim that the Harmonic Oscillator Model of Aromaticity (HOMA) is used to evaluate pyrazole ring aromatics before and after protonation [32]. In this study, the HOMA index as a structural index was calculated using Formula (1) [33]. ...
The proton affinity (PA) of stable five-membered cyclic allenes 1–13 in the gas phase was studied by DFT/B3LYP computational method in conjunction with the 6-311+G(d,p) basis set. The PAs of the title compounds are large. Their protonation at the central carbon introduces super- and hyperbases as evidenced by their enthalpies of protonation ΔH = 272–315 kcal/mol, which are above the threshold of superbasicity and in some cases close to the hyperbasicity. The trends of PA correlate quite well with the energy level of the HOMO orbitals and the molecular electrostatic potential maps of the studied allenes. The Wiberg bond order analysis gives a reasonable explanation of the exocyclic delocalization involving the π-donor substituents with the allenic unit of the studied compounds. Protonation of central carbon in the cyclic allenes leads to a change in the structural arrangement and an increment in the aromaticity of the pyrazole ring, which was evaluated by the HOMA index. Furthermore, the B3LYP/6-311+G(d,p) calculated cation affinities for the cyclic allene 6 increases as K⁺<Na⁺<Li⁺<Cu⁺<Mg²⁺<Fe²⁺<Cu²⁺. Cation affinity is strongly dependent on the charge-to-size ratio of the Mⁿ⁺.
... The principle of the push-pull effect is to increase the electron density on an electron-pulling (or electron-withdrawing) part of a molecule by adding an electron-pushing (electron-donating) substituent on a scaffold capable of transmitting the resonance effect between the two entities and, at the same time, enhancing the functional group basicity. This approach can increase the basicity of a relatively weak functional group such as nitrile to the rank of a superbase [7,8], particularly when two pushing groups can be attached to a transmitting scaffold (Chart 1) [9]. ...
... In Part I of this study [9], we underlined the specificitie constituted by the unsaturated cyclic system made of three =CH-or =N-bond, namely the methylenecyclopropene and depicted in Figure 1. The small dimension of the resonanceduce the molecule size and molecular weight, which are amo tical utilization of superbases [5]. ...
... The small dimension of the resonanceduce the molecule size and molecular weight, which are amo tical utilization of superbases [5]. The optimization of the pr systems based on these two scaffolds necessitates the knowl of the two substituents that can be plugged to the transmi tivity (departure from additivity of the effect of each substitu steric/intramolecular interactions, and geometrical and/or These aspects are studied in this work by comparing results rically substituted nitriles [9] with new data obtained on no nitriles. X C N X π−π transmitter C N C N π−π transmitter Y X Chart 1. ...
This work extends our earlier quantum chemical studies on the gas-phase basicity of very strong N-bases to two series of nitriles containing the methylenecyclopropene and cyclopropenimine scaffolds with dissymmetrical substitution by one or two electron-donating substituents such as Me, NR2, N=C (NR2)2, and N=P (NR2)3, the last three being strong donors. For a proper prediction of their gas-phase base properties, all potential isomeric phenomena and reasonable potential protonation sites are considered to avoid possible inconsistencies when evaluating the energetic parameters and associated protonation or deprotonation equilibria B + H+ = BH+. More than 250 new isomeric structures for neutral and protonated forms are analyzed. The stable structures are selected and the favored ones identified. The microscopic (kinetic) gas-phase basicity parameters (PA and GB) corresponding to N sites (cyano and imino in the cyclopropenimine or in the substituents) in each isomer are calculated. The macroscopic (thermodynamic) PAs and GBs, referring to the isomeric mixtures of favored isomers, are also estimated. The total (pushing) substituent effects are analyzed for monosubstituted and disubstituted derivatives containing two identical or two different substituents. Electron delocalization is examined in the two π–π conjugated transmitters, the methylenecyclopropene and cyclopropenimine scaffolds. The aromatic character of the three-membered ring is also discussed.
... Though in some inorganic chemistry textbooks they are introduced as excellent donors [1], the authors of this paper share the more widespread view that organic nitriles are rather poor σ-donors and π-acceptors and so-called "temporary ligands" [2][3][4]. Nitriles are weak bases due to the sp hybridization of the nitrogen atom and the high s character, which does not favor the electron pair sharing with acidseither Bronsted (proton) or Lewis (metal cations) [5]. Gas phase basicities of nitriles (acetonitrile, 1,3-dicyanobenzene) are between those of phosphine PH 3 and benzyl alcohol [6] whereas the coordination ability of acetonitrile towards transition metal cations was placed between that of ethylene glycol and trifluoroacetic acid (neutral form); towards lanthanides it is even weaker complexing agent comparable to dichloromethane [7]. ...
The structures and luminescent properties of silver(I) complexes with weakly coordinating nitrile ligands: 1,4-dicyanobenzene (1,4-dcb) and 3-cyanopyridine (3-cpy) and nitrite or nitrate coordination are investigated. One molecular complex [Ag(3-cpy)2(NO2)], one 1-D coordination polymer [Ag(3-cpy)2NO3]∞ and two rare 2-D [Ag3(3-cpy)2(NO2)3]∞ and 3-D [Ag2(1,4-dcb)(NO3)2]∞ coordination polymers are described. Coordination of the luminescent ligands does not enhance their emission properties, however extension of coordination network may lead to new emission bands in the visible region.
In this review, the principles of gas-phase proton basicity measurements and theoretical calculations are recalled as a reminder of how the basicity PA/GB scale, based on Brønsted–Lowry theory, was constructed in the gas-phase (PA—proton affinity and/or GB—gas-phase basicity in the enthalpy and Gibbs energy scale, respectively). The origins of exceptionally strong gas-phase basicity of some organic nitrogen bases containing N-sp3 (amines), N-sp2 (imines, amidines, guanidines, polyguanides, phosphazenes), and N-sp (nitriles) are rationalized. In particular, the role of push–pull nitrogen bases in the development of the gas-phase basicity in the superbasicity region is emphasized. Some reasons for the difficulties in measurements for poly-functional nitrogen bases are highlighted. Various structural phenomena being in relation with gas-phase acid–base equilibria that should be considered in quantum-chemical calculations of PA/GB parameters are discussed. The preparation methods for strong organic push–pull bases containing a N-sp2 site of protonation are briefly reviewed. Finally, recent trends in research on neutral organic superbases, leaning toward catalytic and other remarkable applications, are underlined.
Proton affinities (PA) and gas phase basicities (GB) of novel structure of non-ionic organic superbases on the basis of guanidine embedded imidazole framework, were computed at B3LYP/6-311+G(d,p) level of theory. The used logic for designing these structures is synergistic effect of guanidine-imidazole twin in order to aromatization and subsequent stabilization of imidazole ring upon protonation which is strengthen with push-pull effect of variety electron-donating groups on the five-membered ring. This strategy caused to the design of organobases with considerable basicity in the range of 985.39–1177.68 kJ mol⁻¹. Majority of the designed structures show the PA values more than 1028 kJ mo1⁻¹, as the threshold of superbasicity. The HOMA/HOMED, and NICS(1)zz analyses were used for each structure and its conjugated acid in order to better understand and discuss the basicity values.
Microscopic basicity of some ketenimine derivatives 1‐12 and their isomers/tautomers, bearing an unsaturated ring, have been studied in the present work. Isomerization process for some ketenimine derivatives and their protonated forms was also investigated by DFT calculations. The protonation study in gas phase was performed by DFT‐B3LYP/6‐311+G(d, p) computational method. The results show that all the proposed ketenimines (less stable than their isomers/tautomers) have PA values that are more than at least 20 kJ mol‐1 greater than the PA of 1,8‐bis‐(dimethylamino)naphthalene. The primary strategy in this study for designing new exceptionally rare ketenimine forms possessing strong basicity is to link structurally an unsaturated ring to the molecular framework and to build conjugated systems, for which the positive charge of the conjugate acids is delocalized through resonance. PA values are enhanced by placing electron‐donating substituents on the unsaturated rings, stabilizing the conjugate acid. Two indices of aromaticity, the nucleus‐independent chemical shift and the harmonic oscillator model of aromaticity associated with the unsaturated ring, were calculated and compared for neutral and protonated forms. NBO‐Wiberg bond orders, HOMO and molecular electrostatic potential investigations were also applied to explain the π‐type interaction of the unsaturated ring with the ketenimine unit.
