Structure and low-frequency vibrational properties of (H2O)10 composed of a ring form of (H2O)4 and a cage form of (H2O)6

One minimum energy structure of the water decamer composed of a ring form of the water tetramer and a cage form of the water hexamer is calculated at the Hartree–Fock, MP2, and density functional theory levels using the 6-31++G(d,p) and/or aug-cc-pVDZ basis sets. It has been shown that the basic structure of the isolated water tetramer and hexamer is retained in the decamer structure irrespective of the levels of theory. As a result of the intercluster hydrogen bonds between the water tetramer and hexamer, two types of four-coordinated water molecules are formed in the decamer. We have further carried out frequency calculations for the water decamer as well as the water tetramer and hexamer to investigate how the intermolecular dynamics of the water clusters are affected by their size and the coordination environments of the constituent water molecules. It has been found that a two-band feature around 60 and 170 cm-1 can be seen in the vibrational density of states of the water decamer and that a similar feature can also be seen in those of the water tetramer and hexamer. These intermolecular modes around 60 and 170 cm-1 are described as the O···O···O bending and the O···O···O stretching vibrations of hydrogen bonds, respectively. On the basis of these calculated results, we then discuss the low-frequency Raman spectra of liquid water, which is also characterized by two bands in this frequency range.