Structure, spectra and dynamics of alkali cation microhydration clusters
The main focus of this work was the theoretical investigation of alkali cation microhydration clusters with sodium, potassium, and caesium as central ion and up to 24 water molecules per cluster. Structures were obtained applying global geometry optimisation, using a specialised version of genetic algorithms and the common TIP4P/OPLS model potential. The global and most important local minimum energy structures have been investigated and the results obtained constitute a first complete and systematic overview on structures of alkali cation microhydration clusters. By analysis of various interactions reasons for structural trends could be provided. A structural pattern all magic number cluster structures do have in common is that only three and/or four-coordinated water molecules can be observed. Molecular dynamics simulations were applied to all global minimum energy cluster structures at different temperatures (canonical ensemble). Entropy effects and free energy surfaces were obtained using constrained dynamics on dissociating magic number clusters. It was found that indeed cluster structures containing only three and/or four-coordinated water molecules are more stable than others thus arriving at a possible explanation for the existence of magic number clusters. General structural trends and building patterns already observed for the static picture obtained by the global geometry optimisation are consistent with the dynamical behaviour in a canonical ensemble. For a better understanding of these systems, and to provide data for direct comparison to the experiment, anharmonic infrared spectra for each global as well as some low-lying local minimum energy structures were calculated in the OH-stretch region within an empirical model. At the beginning of this investigation this model had been calibrated only for pure water clusters and ice, however, it was found to produce qualitatively correct data also for the present system. The spectra obtained with this model show characteristic correlations between cluster structures and certain spectral signatures which should make it possible to discern these cluster structures experimentally. It has been also shown that at temperatures up to 100 K global minimum structures (computed at 0 K) still dominate the spectra. Recently available experimental data, kindly provided by J.M. Lisy (University of Illinois) was used for a manual re-calibration of the empirical model. This provided almost perfect quantitative agreement with the experiment. Hence, it was possible for the first time to assign spectral peaks unambiguously to vibrational modes and deduce which isomers are present in the experiments. J.M. Lisy is currently extending this spectral measurements to larger clusters. In combination with our theoretical analysis, this will provide direct information on magic number cluster structures for the first time.