[Theory study of the structures and IR of (C5H5N)n(H2O), (n = 1-2, m = 1-4) clusters].

Using density functional theory, the possible geometrical structures of (C5H5N)n (H2O)m (n = 1-2, m = 1- 4) clusters were optimized at the B3LYP/6-311++G(d, p) level and the stable structures were attained. For the dimer formed between C5H5N and H2O, the calculation result shows that there is only one stable structure and the configuration of cluster formed through pi hydrogen bond (O-H...pi) interaction was not found. In order to study the stability of the clusters, the total energies and binding energies of the clusters were calculated at the same level of theory, and the result shows that the four-membered ring structures of water molecules in (C5H5N)n (H2O)4 (n = 1-2) clusters are more stable than structures of the triatomic ring of water molecules. The lowest energy structure of the C5H5N (H2O)4 cluster is more stable than the others and is probably a magic number structure by the analysis to the energy gap between HOMO and LUMO. At the end, the IR spectra of (C5H5N)n (H2O)m (n = 1-2, m = 1-4) clusters were analysed and the stronger peaks appearing in infrared spectra were assigned.