Three-dimensional model treatment of the torsional levels of isotopic water trimers

A coupled three-dimensional model calculation of the low-frequency intermolecular torsional levels of the isotopically substituted water trimers (H2O)2 · D2O and (D2O)2 · H2O is presented, using the modEPEN torsional potential energy surface and a three-dimensional discrete variable representation approach. Relative to the pure trimers (D2O)3 and (H2O)3, the dynamical symmetry breaking leads to shifts of all torsional levels, as well as splittings of the degenerate eu and eg torsional levels of up to 320 cm−1. The lowest nine (eight) levels of (H2O)2 · D2O ((D2O)2 · H2O) are pseudorotational states. The lowest torsional excitation of (D2O)2 · H2O is predicted to lie at only 1.6 cm−1.