Quantumchemical Calculations on Nitrogen Bases. Part 6 *: Analysis of the Proton Affinities of Aliphatic Amines Using the Kitaura-Morokuma Scheme and the MINDO/3 Method

The energy partitioning scheme of Morokuma et al. has been applied to the proton affinities of fourteen aliphatic amines. The electrostatic, the polarization and the charge transfer component of the proton affinities have been calculated using the MINDO/3 method. Two different sets of structural parameters have been used for the complexes of the bases and the attacking proton. Within the framework of the semiempirical method employed here, the electrostatic interaction between an attacking proton and the base was found to be positive as a result of the relative magnitudes of the repulsion terms used in the proton-electron and the proton-core interactions.

In the case of the polarization term our findings roughly parallel those of Morokuma. For one of the two sets of structural parameters a good and almost group overlapping correlation was found between polarization energies and experimental proton affinities: the polarization part was found to decrease with increasing proton affinity. Independent of the choice of the amine geometries, the results for the charge transfer energy contradict the nonempirical data of Morokuma: Contrarily to his results we found values which increase in the order ammonia < methylamine < dimethylamine < trimethylamine.