Matrix effects on hydrogen bonding and proton transfer in fluoropyridine - HCl complexes

We report an extensive computational and spectroscopic study of several fluoropyridine-HCl complexes, and the parent, pyridine-HCl system. Matrix-IR spectra for pentafluoropyridine-HCl, 2,6-difluororpyridine-HCl, and 3,5-difluororpyridine-HCl in solid neon exhibit shifts for the H-Cl stretching band that parallel the effects of fluorination on hydrogen-bond strength. Analogous spectral shifts observed across various host environments (solid neon, argon, and nitrogen) for pentafluoropyridine-HCl and 2,6-difluororpyridine-HCl convey a systematically varying degree of matrix stabilization on the hydrogen bonds in these complexes. An extended quantum-chemical study of pyridine-HCl and eight fluorinated analogs, including 2-, 3-, and 4-fluoropyridine-HCl, 2,6- and 3,5-difluororpyridine-HCl, 2,4,6- and 3,4,5-trifluropyridine-HCl, as well as pentafluoropyridine-HCl, was also performed. Equilibrium structures and binding energies for the gas-phase complexes illustrate two clear trends in how fluorine substitution affects hydrogen bond strength; increasing fluorination weakens these interactions, yet substitution at the 2- and 6-positions has the most pronounced effect. Bonding analyses for a select subset of these systems reveal shifts in electron density that accompany hydrogen bonding, and most notably, the values of the electron density at the N-H bond critical points among the stronger systems in this subset significantly exceed those typical for moderately strong hydrogen-bonds. We also explored the effects of dielectric media on the structural and bonding properties of these systems. For pyridine-HCl, 3-fluoropyridine-HCl, and 3,5-difluororpyridine-HCl, a transition to proton transfer-type structures is observed at ε-values of 1.2, 1.5, and 2.0, respectively. This is signaled by key structural changes, as well as an increase in the negative charge on the chorine, and dramatic shifts in topological properties of the H-Cl and N-H bonds. In the case of pentafluoropyridine-HCl, and 2,6-difluororpyridine-HCl, we do not predict proton transfer in dielectric media up to ε = 20.0. However, there are clear indications that the media enhance hydrogen-bond strength, and moreover, these observations are completely consistent with the experimental IR spectra.