Theoretical study of the interaction between pyridine derivatives and atomic chlorine. Substituent effect and nature of the bonding

The interaction of pyridine derivatives (H, 4-NH2, 4-CH3, 4-F, 4-CN, 4-NO2) with atomic chlorine is investigated theoretically by the density functional theory (DFT)-based LC-BLYP/aug-cc-pVDZ method. Pyridines and Cl degrees are held together by a (2c--3e) bond and the intermolecular distances range from 2.313 to 2.343 angstrom. The existence of a N...Cl bond is confirmed by the atom-in-molecule analysis of the systems. The binding energies of the adducts, ranging from -42.08 to -53.96 kJ mol(-1), are linearly correlated to the proton affinity of the pyridines. The charge transfer from pyridine to Cl degrees varies between 0.222 and 0.277 e. The spin density analysis shows that the strongest complex has the highest (2c-3e) character. The CH bonds are contracted and the (CH) vibrations are blueshifted owing to the decrease in sigma*(CH) occupation. This decrease results not only from the classical anomeric effect but also from the sigma(CC) and sigma(CN) sigma*(CH) delocalisation in the heteroaromatic ring.