HYDRATED PROTON CLUSTERS AND SOLVENT EFFECTS ON THE PROTON-TRANSFER BARRIER - A DENSITY-FUNCTIONAL STUDY

The density functional calculations using the Perdew nonlocal correcti ons to exchange and correlation have been carried out for a sequence o f hydrated proton clusters. The optimized structures were obtained up to H13O6+. It is found that H3O+ is indeed the central unit in all the lowest energy structures we found. Our results support the argument t hat the structure with a four-coordinate first solvation shell is very unlikely in small hydrated proton clusters. The density functional ca lculations with the Perdew nonlocal corrections to exchange and correl ation give somewhat shorter hydrogen bond lengths, but slightly longer chemical bond lengths as compared with the post-Hartree-Fock calculat ions. The harmonic vibrational frequencies and IR intensities of vario us vibrational modes have been generated for all the structures optimi zed. Results for small clusters are compared with the high resolution experimental spectroscopy studies of Yeh et al. and Begemann et al. Re sults for larger clusters are used to interpret the low resolution spe ctra of Schwartz. Very good accord with experimental results is obtain ed. The solvent effects on proton transfer energy barriers in clusters have been studied by designing a few model systems. The barrier is fo und to be very sensitive to the solvent configurations. When the solve nt water is replaced by the classical partial charge model, a signific ant change of the barrier is observed, indicating that a quantitative treatment will ultimately require a good pseudopotential to properly a ccount for the quantum nature of the solvent. A combined density funct ional and molecular dynamics simulation was used to calculate the prot on transfer energy and free energy barrier in aqueous solution. The ba rrier is found to be 3 kcal/mol higher than in gas phase. Very large s olvent fluctuation is observed which may have a significant influence on the reaction rate.