Relative stability and proton transfer reactions of unsaturated isocyanides and cyanides

Abstract

The typical Gibbs free energy difference between hydrocarbon substituted isocyanides and the corresponding cyanides is 25 to 28 kcal/mol in favor of the cyanides and is mostly independent of the substituent. Triple bonded species with a ―C ≡ C―RN,C (RN,C = CN, NC) structure can be considered as exceptions. Because isocyanide and cyanide species have very similar structures, the relative energy is independent of the pressure and temperature conditions. Theoretical and experimental gas‐phase investigations show that basicity of isocyanides ranges from 182.1 to 198.2 kcal/mol which is 14.0 to 19.7 kcal/mol higher than the basicity of respective cyanides. The most favored protonation centers are located on isocyanide or cyanide group depending on the species. The biggest increase of basicity was caused by bulkier substituents. The substitutions have greater influence on the basicity of cyanides than on the basicity of isocyanides. In regard to deprotonation, the cyanides are more acidic than the corresponding isocyanides. For most of the unsaturated cyanide and isocyanide species the (N,C)‐CHR′ hydrogen (the one connected to the carbon next to cyanide/isocyanide group) is the most acidic. Our work suggests that for derivatives bearing unsaturated substituent the favored deprotonation center may be different and some cyanides and isocyanides are unstable towards gas‐phase deprotonation equilibrium as the formed anion tends to isomerize. Copyright © 2016 John Wiley & Sons, Ltd.

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

“…The gas phase basicity of a large series of alkyl nitriles, from methyl‐ to octyl‐cyanide, have been experimentally determined using equilibrium and kinetic methods. Simultaneously, computational studies were performed to understand the protonation process and to refine the theoretical tools . Most of the relevant results are presented in Table .…”

“…Gas‐phase basicities of cyclopropyl‐, propadienyl‐, vinyl‐, allyl‐, and propargyl‐isocyanides have determined experimentally using FT‐ICR mass spectrometry . As noted by the authors, the GB values computed at the MP2/6‐31++G(d,p) level show a systematic overestimate of ca .…”

“…Simultaneously, computational studies were performed to understand the protonation process and to refine the theoretical tools. 14,15,40,67,[75][76][77] Most of the relevant results are presented in Table 6.…”

“…Recently, a set of unsaturated isocyanide has been experimentally and theoretically examined . Most of them are presented in Table .…”

“…This should be present in mind when a comparison between calculated values among different classes of compounds is done. It should be underlined here that single point calculations conducted at the MP2 or B3LYP levels, even with basis set as large as 6‐31++G(d,p), are not able to reproduce accurately PA or GB of cyanide and isocyanide molecules, systematic deviations of ±10 kJ/mol have been reported . Thus, for a number of representative molecules, we have undertaken high level computations using the composite methods G4MP2 and W1 which greatly enhance the accuracy of the results.…”