Effect of Orientational Isomerism in Neutral Water Hexamers on Their Thermodynamic Properties and Concentrations in the Gas Phase

In order to estimate the effect of hydrogen bond network isomerism on the thermodynamic functions and concentrations of water clusters in the gas phase, the structural and thermodynamic parameters of all 133 possible isomeric structures of water hexamer (H2O)(6) in the initial conformations of the book, cage and prism types have been studied using the DFT (B3LYP/6-311++G(2d,2p)), DFT-D3 (B3LYP/6-311++G(2d,2p)), G4, W1BD, DFTB, and MB-pol calculations. It was found that accounting of the orientational isomerism leads to the values of water cluster gas-phase concentrations different by 1 or 3 orders of magnitude (depending on the method of energy calculation and averaging) from the results obtained when only single energetically favorable structure is considered. The absolute Boltzmann-averaged estimates of the hexamer binding enthalpy per monomer are of 5.50 (G4 results) and 5.77 (DFT results) kcal/mol. The total concentration of H2O6 including all isomers in the saturated water vapor at standard conditions are estimated as 1.61 . 10(3) (G4) and 8.17 . 10(5) (DFT) molecules/cm(3) with W1BD indirect estimates lying between these values.

Automated summary

The effect of hydrogen bond network isomerism on the thermodynamic functions and concentrations of water clusters in the gas phase was studied using various computational methods. The results showed that considering the orientational isomerism significantly affects the gas-phase concentrations of water clusters and the estimates of their binding enthalpy. This research highlights the importance of studying the structural and thermodynamic properties of water clusters for understanding the impact of hydrogen bond network isomerism.