Calculation of OH-stretching vibrational properties of water clusters

Solar radiation is the main energy source for the Earth's climate system and determination of the amount of solar radiation that reaches the surface is extremely important. The solar spectrum is most intense in the ultraviolet-visible-near-infrared (UV/Vis/NIR) region of the electromagnetic spectrum where vibrational overtones are dominant. Energy budget models are used to estimate the attenuation of incident solar radiation by the atmosphere using empirically determined values for the concentrations and absorption cross sections of dominant species in the atmosphere such as water. Model predictions have been shown to overestimate the amount of solar radiation reaching the Earth's surface and it has been suggested that the excess solar absorption could be due to the presence of weakly bound van der Waals complexes of water in the atmosphere such as (H2O)n, H2O-N2 and H2O-O2. The formation of water clusters is suggested to change the absorption characteristics of the water units within the cluster, leading to new absorptions that are not included within the climate models. To successfully include the water clusters in the energy budget the fundamental and overtone vibrational frequencies and intensities are required. We have used computational methods to calculate the optimal structures and fundamental and overtone vibrational frequencies and intensities for the water clusters. The calculation of the fundamental and overtone OH-stretching spectra for (H2O)2, H2O-N2, H2O-O2 and the larger water clusters (H2O)n (n = 3-6) has been carried out using the harmonically coupled anharmonic oscillator (HCAO) local mode model. The calculated binding energies for the water-only clusters ((H2O)n n = 2-6) are proportional to the number of hydrogen bonds in the clusters and are considerably larger than the binding energies for H2O-N2 and H2O-O2. We find that the vibrational spectra of hydrogen bonded water clusters display two main regions arising from transitions involving the hydrogen bonded and free OH bonds, respectively. The bonded OH region has vibrational frequencies that are lower than those observed for water monomer (red-shifted). The hydrogen bonded vibrational bands display large fundamental intensities that almost completely disappear for the first overtone. The size of the red-shift in the vibrational frequency and the fundamental intensity for the bonded OH bond is proportional to the strength of the hydrogen bond. The free OH region has vibrational frequencies that are not significantly shifted relative to the vibrational frequencies of water monomer. The effect of the hydrogen bond appears to be limited to the bonded OH. For the higher overtones we observe that the total OH-stretching intensities are proportional to the number of water units in the cluster. We observe that density functional theory has significant problems with systems containing hydrogen bonds