Abstract
An ab initio theoretical method based on a triadic formula has been employed to calculate changes in proton affinities (__PA__s) after sequential substitution of C–H by C–phenyl in methylamine and in the N‐methyl component of N‐methylacetamide. The overall objective was to investigate the cause of the recently reported unexpected basicity‐enhancing effect of the N‐trityl group in several amines and in acetamide. The triadic method indicates that the increase in PA from NH~3~ to CH~3~NH~2~ is principally due to destabilisation of the lone pair, compensated to some degree by a smaller bond energy term. The increasing __PA__s along the series CH~3~NH~2~, PhCH~2~NH~2~, Ph~2~CHNH~2~ and Ph~3~CNH~2~ are mainly due to an increasing relaxation energy effect following increasing phenyl substitution, supported by increasing bond energy terms. Introduction of para‐methoxy substituents in tritylamine would be calculated to result in small incremental increases in PA, in agreement with experimental findings. The PA of tricyclohexylmethylamine (233.6 kcal·mol^–1^) is calculated to be similar to that of trimethoxytritylamine (233.5 kcal·mol^–1^), a consequence of the destabilisation of the lone pair on nitrogen by the three bulky cyclohexyl groups. Results for acetamide indicate that protonation on oxygen is more favourable than on nitrogen, which supports the intuitive argument that conventional π‐electron resonance stabilisation of the O‐protonated cation should be appreciably greater than hyperconjugative stabilisation of the N‐protonated isomer. The increase in PA along the series MeCONHCH~3~, MeCONHCH~2~Ph, MeCONHCHPh~2~ and MeCONHCPh~3~ parallels the increase from CH~3~NH~2~ to Ph~3~CNH~2~ and is again principally a consequence of the increasing relaxation energies, which dominate the negative bond energy term contributions. Prior to protonation, the nitrogen atom of MeNHAc is perfectly planar, consistently with resonance delocalisation of the nitrogen lone pair into the carbonyl group. In contrast, there is a 6.5 % degree of pyramidalisation at the nitrogen in TrNHAc [calculated at the B3LYP/6‐31G(d) level], indicating a reduced resonance interaction due to steric crowding, in support of a previous suggestion. Finally, the computed __PA__s and predicted first adiabatic ionisation energies are in satisfactory agreement with experimental results where these are available.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)