Local Mode Approach to OH Stretch Spectra of Benzene–(H2O)n Clusters, n = 2–7

Abstract

Isomer-specific resonant ion-dip infrared spectra are presented for benzene-water (Bz-(H2O)n) clusters with two to seven water molecules. Local mode Hamiltonians based on scaled M06-2X/6-311++G(2d,p) density functional calculations are presented that accurately model the spectra across the entire OH stretch region (3000-3750 cm(-1)). The model Hamiltonians include the contribution from the water bend overtone and an empirical parameter for the local OH stretch-bend Fermi coupling. The inclusion of this coupling is necessary for accurate modeling of the infrared spectra of clusters with more than three water molecules. For the cyclic water clusters (n = 3-5), the benzene molecule perturbs the system in a characteristic way, distorting the cycle, splitting degeneracies, and turning on previously forbidden transitions. The local OH stretch site frequencies and H···OH hydrogen bond lengths follow a pattern based on the each water monomer's proximity to benzene. The patterns observed for these cyclic water clusters provide insight into benzene's effects on the three-dimensional hydrogen-bonded networks present in water hexamer and heptamer structures, which also have their spectra dramatically altered from their pure water counterparts.