engleski

Basicity of amines in the gas phase - Analysis of the base-strengthening effect of an N-trityl group by use of a triadic formula

A recently developed ab initio theoretical method based on a triadic formula (Z. B. Maksić, R. Vianello, J. Phys. Chem. A 2002, 106, 419-430) has been employed to calculate changes in proton affinities (PAs) following sequential substitution of C-H by C-phenyl in methylamine and in the N-methyl of N-methylacetamide. The overall objective was to investigate the cause of the unexpected base-enhancing effect of the N-trityl group in several amines and acetamide reported recently. The triadic method indicates that the increase in PA from NH3 to CH3NH2 is principally due to destabilisation of the lone pair compensated to some degree by a smaller bond energy term. The increasing PAs along the series CH3NH2, PhCH2NH2, Ph2CHNH2, and Ph3CNH2 are mainly due to an increasing relaxation energy effect following increasing phenyl substitution supported by increasing bond energy terms. Introduction of para-methoxy substituents of tritylamine are calculated to lead to small incremental increases in PA in accord 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 ; this supports the intuitive argument that conventional pi-electron resonance stabilisation of the O-protonated cation is appreciably greater than hyperconjugative stabilisation of the N-protonated isomer. The increase in PA along the series MeCONHCH3, MeCONHCH2Ph, MeCONHCHPh2, and MeCONHCPh3 is parallel with the increase from CH3NH2 to Ph3CNH2 and is again principally a consequence of the increasing relaxation energies which dominates the negative bond energy term contributions. Prior to protonation, the nitrogen atom of MeNHAc is perfectly planar in accord with resonance delocalisation of the nitrogen lone pair into the carbonyl group. In contrast, there is a 6.5% degree of pyramidalization 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 PAs and predicted first adiabatic ionization energies are in satisfactory agreement with experimental results where these are available.

base strength; proton affinity; triadic formula; ab initio MO calculation; tritylamine; steric hindrance

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