Isomers of the acetic acid-water complex trapped in an argon matrix

The complexes of acetic acid (AcOH) with water have been studied using FTIR matrix isolation spectroscopy and DFT/B3LYP, DFT/B3LYP-D, and MP2 calculations with the aug-cc-pVTZ basis set. The AcOH/H2O/Ar matrices were prepared in two different ways. In one set of experiments, the vapor above a solid AcOH sample, cooled to 203 K, was diluted with H2O/Ar mixture in the vacuum chamber of the cryostat, and the mixture was solidified on the target. In the second set of experiments, the matrix was prepared by simultaneous deposition of AcOH/Ar and H2O/Ar mixtures at room temperature. The first method of matrix preparation strongly favors the formation of the "acyclic" higher energy AcOH-H2O complex I(B) compared to the second one. Warming of matrices containing the higher energy complex, I(B), from 11 to 39 K, results in the decrease of I(B) concentration and formation of the lowest energy cyclic complex I(A). The calculations indicate that I(B) is formed by an O-H···O hydrogen bond between the carbonyl oxygen and a water O-H group and, additionally, by a weak interaction between one of the methyl group hydrogen atoms and the water oxygen atom. The cyclic complex I(A) has a six-membered ring involving two O-H···O bonds. An activation energy of 0.94, 1.71, and 1.38 kcal mol(-1) was calculated for the I(B) → I(A) rearrangement at the B3LYP, B3LYP-D, and MP2 levels of theory, respectively. Van't Hoff plots for the association of H2O and AcOH leading to formation of the complexes I(A) and I(B) are presented and discussed. Evidence is also given for the formation of the AcOH-(H2O)2 and (AcOH)2-H2O complexes in the matrices. A potential atmospheric impact of the enhanced formation of the higher energy I(B) complex at low temperatures is discussed.