The DFT study of hydrogen bonding and thermodynamic parameters of (CH3OH)n(H2O)m (n, m = 1–8) clusters at different temperatures

Abstract

For the first time, the interaction of one molecule of water with up to 8 molecules of methanol, and one molecule of methanol with up to 8 molecules of water in different temperatures (273.15–403.15K) is investigated. The intermolecular hydrogen bonding and ΔG and ΔH of formation of (CH3OH)nH2O (n=1–8) and CH3OH(H2O)m (m=1–8) clusters is studied. The calculation is performed at the B3LYP/6-31G∗∗ level of theory. Similar to previous studies, herein a cyclic structure was optimized for (CH3OH)nH2O (n=2–4) clusters. In the case of (CH3OH)nH2O clusters with n>4, a bicyclic structure was optimized, in which the H2O molecule acts as a bridging group. The cyclic structures were also optimized for CH3OH(H2O)m clusters (m=2 and 3). However, for latter clusters where the number of water molecules was more than 3, a compact structure with maximum number of intermolecular hydrogen bonds was more stable than both the cyclic and bicyclic structures. It was shown that in all cases both the ΔH and ΔG of the formation of each cluster from the free molecules increase with increasing of the number of molecules in the cluster. The ΔH values of the formation of all clusters are negative in all temperatures but the corresponding ΔG values change to a positive number after a defined temperature, depending on the type and the size of the clusters.