Theoretical studies on dimerization vs. microhydration of carboxylic acids

Abstract

First principle based electronic structure calculations are carried out on two carboxylic acids (RCOOH: R=H, CH3); their dimers, (RCOOH)2; and hydrated clusters, RCOOH·nH2O It is predicted that carboxylic acid molecules form strong dimeric structures. In presence of sufficient number of water molecules, these dimeric structures break down and each monomeric acid molecule form hydrated clusters. Structures, energy parameters, rotational and vibrational spectral properties of these dimer as well as hydrated clusters of carboxylic acids are presented. Structures of acid dimers and hydrated clusters are calculated at DFT as well as MP2 level of theory and energy parameters of all these systems are further improved applying CCSD(T) level of theory. Minimum number of water molecules needed to break the dimeric structures of formic and acetic acids to form microhydrated structures is also examined by calculating free energy at low temperature (100K) and pressure (μTorr). Physical origin of molecular interactions between solvent water and solute acid molecules has been explored applying a scheme of energy decomposition analysis. Hydrogen bond strength of acid dimers and hydrated clusters of the acids is reported. Spectroscopic manifestation of formation of dimer and micro hydrated carboxylic acids in terms of IR bands is discussed.