Substituent effects on basicity in Group 15 compounds: an analysis based on proton affinities, charge distributions, and dipole polarizabilities

Ab initio calculations on a series of trisubstituted amines, phosphines, and arsines are presented at the MP2/6-311G(d,p) level. Specifically, the species studied are XH3, XF3, and X(Me)3, where X = N, P, or As. The influence of the substituents on the proton affinity is analyzed in terms of the charge distribution, its topology, some one-electron properties (dipole moments, electric field gradients), and dipole polarizabilities. An atoms-in-molecules. decomposition of the charge distribution, energetics, and polarizabilities also proves informative. There seems to be no straightforward way of rationalizing the basicities of these compounds on the basis of electrostatic properties (i.e., properties of the charge distribution in the unprotonated bases). However, substituent effects on basicities can be correlated with response properties, such as the molecular (and atoms-in-molecules) polarizability tensors, and the amount of charge that a substituent group or atom donates on protonation. Key words: quantum chemistry, proton affinity, electron density, lone pairs, polarizability, basicity, dipole moment, electric field gradient.