Ab initio and B3LYP quantum-chemical study of donor-acceptor complexes of gallium halides with pyridine

Structural characteristics and vibration spectra of pyridine and its donor- acceptor complexes with gallium halides were calculated by ab, initio and d ensity-functional methods. Complexes GaX3Py are not rigid molecules with fl at potential-energy surface contour near the global maximum. Complex format ion increases the negative charge on the pyridine nitrogen atom due to addi tional stabilization of the system by strengthened electrostatic interactio ns. The charge increase is provided by the rearrangement (distortion) of th e pyridine structure, resulting in that most pi charge is concentrated in t he vicinity of the nitrogen atom. Standard thermodynamic characteristics of gas-phase dissociation of the complexes were calculated. Calculated Delta H-dis and Delta S-dis values quite well agree with experimental ones for th e chloride adducts and strongly depart from them for the bromide systems. T he short intramolecular X-H contacts suggest that the first stage of pyroly sis of the complexes involves elimination of hydrogen halide, while the gal lium-nitrogen bond remains intact.