C−H···O and O−H···O Hydrogen Bonding in Formic Acid Dimer Structures: A QM/MM Study Confirms the Common Origin of Their Different Spectroscopic Behavior

Abstract

Our previous analysis of the effect of a constant (Onsager reaction) electric field on a formic acid molecule showed that it produced an elongating force on the O−H bond resulting from the parallelism of field and dipole derivative, leading to a red-shifting of the frequency and an increase in infrared band intensity with the field, and a contracting force on the C−H bond resulting from the antiparallelism in the above quantities, leading to a blue-shifting of frequency and an initial decrease, followed by an increase, in band intensity with the field. In this paper we extend this analysis to the characterization of the O−H···O and C−H···O hydrogen bonds in the nonconstant fields present in the formic acid dimer. We use a QM/MM treatment that incorporates exchange-repulsive forces and we also include forces generated by intramolecular cross-term interactions. The excellent agreement with the ab initio bond length changes as a function mainly of the balance of the electrical and repulsive forces demonstrat...