Understanding the Effects of Hydrogen Bonding at the Vapor−Water Interface: Vibrational Sum Frequency Spectroscopy of H2O/HOD/D2O Mixtures Studied Using Molecular Dynamics Simulations

Molecular structure and bonding interactions at the vapor−water interface of varying H 2 O/HOD/D 2 O composition has been calculated using molecular dynamics simulations. From these simulations a surface vibrational sum frequency (VSF) spectrum of the OH stretch region has been generated and compared with experimental VSF results of similar isotopic mixtures. The peak frequency of the uncoupled, solvated OH stretch mode determined from the computational spectrum of the vapor−HOD interface shows excellent agreement with these experimental results. With the addition of H 2 O, the calculations performed in this work provide information as to how various OH stretch modes at the vapor−water interface are impacted by the coupling effects that are induced by hydrogen bonding to adjacent OH oscillators. The results of these calculations demonstrate the frequency shifting, spectral broadening, and changes in transition strength exhibited by the OH stretch modes of interfacial water species that occur with increased intermolecular and intramolecular coupling, providing an improved understanding of the different types of water species present at the vapor−water interface that have been difficult to assign in previous VSF experimental studies.