Gas‐Phase Protonation and Deprotonation of Acrylonitrile Derivatives NCCHCHX (X=CH₃, NH₂, PH₂, SiH₃)

Abstract

A combined experimental and theoretical study on the gas-phase basicity and acidity of a series of cyanovinyl derivatives is presented. The gas-phase basicities and acidities of (N[triple chemical bond]C--CH==CH--X, X=CH(3), NH(2)) were obtained by means of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry techniques. The corresponding calculated values were obtained at the G3B3 level of theory. The effects of exchanging CH(3) for SiH(3), and NH(2) for PH(2), were analyzed at the same level of theory. For the neutral molecules, the Z isomer is always the dominant species under standard gas-phase conditions at 298 K. The loss of the proton from the substituent X was found systematically to be much more favorable than deprotonation of the HC==CH linking group. The corresponding isomeric E ion is much more stable than the Z ion, so that only the former should be found in the gas phase. The most significant structural changes upon deprotonation occur for the methyl and amino derivatives because, in both cases, deprotonation of X leads to a significant charge delocalization in the corresponding anion. Protonation takes place systematically at the cyano group, whereby the isomeric E ion is again more stable than the Z ion. Push-pull effects explain the preference of aminoacrylonitrile to be protonated at the cyano group, which also explains the high basicity of this derivative relative to other members of the analyzed series that present rather similar gas-phase basicities, GB approximately 780 kJ mol(-1), indicating that the different nature of the substituents has only a weak effect on the intrinsic basicity of the cyano group. The cyanovinyl derivatives have a significantly stronger gas-phase acidity than that of the corresponding vinyl compounds CH(2)==CH--X. This acidity-strengthening effect of the cyano group is attributed to the greater stabilization of the anion with respect to the corresponding neutral compound.

The Ever‐Surprising Chemistry of Boron: Enhanced Acidity of Phosphine⋅Boranes

“…Our results (see Table S5 of the Supporting Information) indicate that reaction (6) is 80 kJ mol À1 more exothermic than reaction (7). This means that the S N 2 nucleophilic substitution is thermodynamically favored over the deprotonation process.…”

“…As a consequence, a great deal of effort was concentrated on determining intrinsic reactivities, in particular intrinsic basicities and acidities. [2][3][4][5] The gas-phase acidity of phosphines has been reported [6,7] and the role played by the substituent on the acidity of the molecule clearly evidenced. In particular the presence of an a,b-unsaturated substituent [6] with or without an electronwithdrawing group [5,7] led to a huge increase in acidity.…”

Gas phase basicities of polyfunctional molecules. Part 6: Cyanides and isocyanides

“…By comparison the increase in PA from propionitrile to butyronitrile is equal to 4 kJ/mol only (Table ). It may be suggested that the large increase in PA value observed for CH 3 CH═CHCN is associated with an efficient hypercongugative effect of the methyl group with the C═C unsaturated moiety of the protonated form …”

“…Amino acrylonitrile itself, NH 2 CH=CHCN, has been the subject of several studies . This compound may be obtained by reaction of ammonia on cyanoacetylene as a Z:E mixture close to 1 in condensed phase .…”

Organic synthesis applied to space sciences

“…The corresponding E isomeric ion is much more stable than the Z-one, so that only the former should be found in the gas-phase. For the basicity, push-pull effects explain the preference of aminoacrylonitrile to protonate at the cyano group, which also explain the relatively high basicity of this derivative as compared to the other members of the set analyzed, which present rather similar gas-phase basicities: GB ≈ 780 kJ mol −1 , indicating that the different nature of the substituents has a weak effect on the intrinsic basicity of the cyano group [51]. This push-pull effect was also observed by photoelectron spectroscopy [52].…”