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 performed to determined the structures, binding energies, and vibrational spectra of a series of 1:1 HCl:4-X-pyridine complexes with X = H, Ch3, NH2, OH, F, Cl, CN, CHO, Li, Na, O− and S−. The substituents include both electron donating and electron withdrawing groups which systematically alter the basicity of the pyridine nitrogen. The hydrogen bonds in these complexes span the range of proton positions, including ClH…N hydrogen bonds between neutral molecules, Cl−…+HN hydrogen bonds between ion pairs, and proton-shared Cl…H…N hydrogen bonds. Emphasis is placed on the relationship between proton position and vibrational spectrum, particularly the vibrational bands associated with proton motion.