AB-INITIO THEORETICAL AND MATRIX-ISOLATION EXPERIMENTAL STUDIES OF HYDROGEN-BONDING - VIBRATIONAL CONSEQUENCES OF PROTON POSITION IN 1 1 COMPLEXES OF HCL AND 4-X-PYRIDINES/

Ab initio calculations at the MP2/6-31 + G(d, p) level have been perfo rmed to determine the structures, binding energies, and vibrational sp ectra of a series of 1:1 HCl:4-X-pyridine complexes, with X = H, CH3, NH3, OH, F, Cl, CN, CHO, Li, Na, O- and S-. The substituents include b oth electron donating and electron withdrawing groups which systematic ally alter the basicity of the pyridine nitrogen. The hydrogen bonds i n these complexes span the range of proton positions, including Cl-H.. .N hydrogen bonds between neutral molecules, C1(-)...H-+-N hydrogen bo nds between ion pairs, and proton-shared Cl...H...N hydrogen bonds. Em phasis is placed on the relationship between proton position and vibra tional spectrum, particularly the vibrational bands associated with pr oton motion.