Ab initio study of cationic water cluster (H2O)9+ via particle swarm optimization algorithm

- Particle swarm optimization method was used to search isomers of (H2O)9+ cluster.
- Sixteen lower energy isomers of four types were obtained at the MP2 level.
- Simulated infrared spectra of (H2O)9+ of the lower energy isomer were experimentally verified.
- The interaction between the H3O+ core and water molecules is studied.

The 16 lowest energy isomers of cationic water cluster (H2O)9+ have been comprehensively searched by the particle swarm optimization algorithm combined with quantum chemistry calculations. Geometric optimization and vibration analysis of the interesting isomers were made in the MP2/aug-cc-pVDZ level. Our results show that cage structure is the most stable at low temperatures. The relative Gibbs free energy below 400 K was calculated, which indicates that the stability of the cage structure decreases as the temperature increases while the energy of ring is relatively slow. We also illustrate the computationally simulated IR spectra obtained at six different calculated levels and individual IR spectroscopy for seven isomers. The obtained molecular orbits of six representative isomers show good agreement with experiment. Finally, through the topological analysis and density gradient analysis, we compared the interaction between the H3O+ core and H2O as well as the OH radicals.

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