The potential energy surfaces of larger, neutral water clusters have several local minima for the different water cluster isomers. Some of the isomers have distinct geometries and include structural isomers like the water hexamer prism, cage, and book structures or have essentially the same structure but differ in their hydrogen bond networks as observed in the pentagonal prism water decamers. Over the past few years, we have used H218O water substitution to determine the structures of water clusters by molecular rotational spectroscopy. This presentation will focus on using isotopic substitution to more carefully explore the energy differences between the prism and cage cluster and to identify tunneling patterns in the water hexamer prism. In the case of the water hexamer, the energy difference between the cage and prism structures is calculated to be about 0.1 kcal/mol and this energy difference is on the order of the zero-point energy variation of the isomers. The accuracy of energies and vibrational frequencies from high-level quantum chemistry calculations is tested by the relative stability of the cage and prism geometries in (H2O)6, (D2O)6, and the 12 distinct (HOD)(H2O)5 isotopomers. In addition to minima with distinct structures, there are a large number of identical minima that result from permutation of the water molecules. In this case, cluster dynamics may cause measurable splitting from the tunneling between these equivalent minima and tunneling splittings are observed in the hexamer prism and decamer pentagonal prism rotational spectra. The analysis of these tunneling-rotation spectra show that there are two distinct tunneling paths that involve concerted motion of two water molecules. This concerted tunneling does not follow the minimum energy path. Instead the “deep tunneling” motion follows a path that minimizes oxygen atom motion leading to tunneling rates on the order of 1 MHz.

Isotopic and tunneling patterns in water clusters

251st National Meeting (2016)

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