Ab initio study of an H24O12 zwitterion

An H24O12 zwitterionic water cluster based on the 4(4)5(4) cage geometry is described and studied at the B3LYP/ 6-311++G** level. The cluster's PES has two zwitterionic local minima, which are connected by low barrier pathways involving transfers of two protons, denoted H82 and H5U, along hydrogen bonds. The two zwitterionic minima sit in a broad megabasin that also contains two shallow saddles and a hilltop. All features lie within 0.5 kcal/mol of each other. The zwitterion converts to neutralized (H2O)(12) Clusters via proton transfers along any of eight embedded water wires. Optimized geometries for transition states and products for these neutralization reactions are computed. These neutralization pathways are endothermic at 77 K, suggesting that the zwitterion could be detected in a low temperature experimental system. Activation energies for neutralization range from 2.9 to 3.8 kcal/mol at 77 K and from 2.3 to 3.1 kcal/mol at 25 degreesC. Computed IR spectra for the zwitterion and its neutralized geometries are compared. The IR spectrum of the zwitterion has several modes involving coordinated vibration of H82 and H5U, which are not present in the spectra of the neutralized clusters. Benchmark comparisons against MP2/aug-cc-pVTZ show that B3LYP/6-311++G** may be superior to MP2/6-311++G** for proton transfer studies in water clusters.