## Abstract

Quantum four-dimensional model calculations of the coupled intermolecular torsional vibrations of the cyclic homodromic water tetramers (H_{2}O)_{4} and (D_{2}O)_{4} are presented, based on the analytical modEPEN4B potential energy surface [S. Graf and S. Leutwyler, J. Chem. Phys. 109, 5393 (1998), preceding paper] and a four-dimensional discrete variable representation approach. The lowest 50 torsional levels were calculated up to 420 and 500cm^{-1} for (D_{2}O)_{4} and (H_{2}O)_{4}, respectively. For both clusters, the torsional ground state is split by a synchronous O-H torsional inversion process, similar to inversion tunneling in ammonia, with calculated tunnel splittings of 21.8 and 0.00012 MHz for (H_{2}O)_{4} and (D_{2}O)_{4}, respectively. As for the cyclic water trimer and pentamer, the four torsional fundamentals of the tetramer lie above the torsional interconversion barriers, between 185-200cm^{-1} for (D_{2}O)_{4} and 229-242cm^{-1} for (H_{2}O)_{4}, but also lie below the one-dimensional torsionally adiabatic barriers. The anharmonic fundamental frequencies lie both above and below the normal-mode frequencies, by up to 33%. Slightly above the fundamental torsional excitations, at 257-260 and 280-281cm^{-1} for (H_{2}O)_{4} and (D_{2}O)_{4}, respectively, lie four states corresponding to four versions of the uudd isomer, which form a pseudorotational manifold; the torsional interconversion occurs by a sequence of double O-H flips. Higher excited pseudorotational states are calculated up to a vibrational angular momentum of k=3. At ≈295 and ≈300cm^{-1}, a further group of eight states is found, corresponding to the eight permutationally equivalent versions of yet another isomer, the uuud structure. The four uudd and eight uuud states of (H_{2}O)_{4} exhibit inverse isotope effects, and lie at lower energy than their (D_{2}O)_{4} counterparts.

Original language | English (US) |
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Pages (from-to) | 5404-5419 |

Number of pages | 16 |

Journal | Journal of Chemical Physics |

Volume | 109 |

Issue number | 13 |

DOIs | |

State | Published - 1998 |

## ASJC Scopus subject areas

- General Physics and Astronomy
- Physical and Theoretical Chemistry