Ab initio and DFT calculation of molecular structure and proton affinity for selected chalcogenobispyridines

Abstract

The optimized minimum-energy geometries of different chalcogenobispyridines and of their protonated forms on every nitrogen site are determined by using ab initio and DFT calculations. Thorough geometry optimizations using B3LYP/6-31+G(2d,p) level, generally indicate for neutral molecules that the skew conformation is energetically preferred, however some non-symmetrical compounds rather assume an orthogonal orientation of the aromatic rings. This conformation is favoured by two stabilizing energies: the nucleophile–electrophile intramolecular interaction between an N atom and the opposite C atom, and the delocalization of the sulphur lone pair over the symmetry apt aromatic π-system. Because of the known interest of chalcogenobispyridines as ligands, the optimizations of three transition metal complexes are carefully evaluated and compared to relative experimental data. The molecular structure, vibrational frequencies and conformational changes caused by protonation of the same chalcogenopyridines are also investigated and discussed. On the basis of the gas-phase computed proton affinities (ZPE corrected) some of these substrates may be classified as superbases in the Jacob’s (unified) ladder of acids and bases.