Quantum four-dimensional model calculations of the coupled intermolecular torsional vibrations of the cyclic homodromic water tetramers (H2O)4 and (D2O)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 (D2O)4 and (H2O)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 (H2O)4 and (D2O)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 (D2O)4 and 229-242cm-1 for (H2O)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 (H2O)4 and (D2O)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 (H2O)4 exhibit inverse isotope effects, and lie at lower energy than their (D2O)4 counterparts.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry