Structure and properties of the weakly bound trimer (H2O)(2)HCl. Theoretical predictions and comparison with high-resolution rotational spectroscopy

In this paper we report ab initio predictions of the minimum energy structu re of the (H2O2)(2)HCl cluster, its stationary points, low-energy tunneling pathways, the dipole moment, and the nuclear quadrupole coupling constants . Structures corresponding to the stationary points were optimized with the second-order Moller-Plesset theory, while the corresponding interaction en ergies and binding energies were computed using the coupled-cluster method restricted to single, double, and non-iterative triple excitations. It is s hown that the non-additive interactions play an important role. The contrib ution of the three-body term represents as much as 13-20% of the total inte raction energy. The nature of the intermolecular interactions in the cluste r was investigated by symmetry-adapted perturbation theory. As expected, th e complex is mostly stabilized by the electrostatic and induction interacti ons, but the dispersion term is far from negligible. It is found that the p otential energy surface of this cluster shows three low-energy pathways con necting two enantiomeric minimum energy structures. The height of the barri ers separating these minima suggests that it should be possible to observe spectroscopic transitions resulting from the tunneling between the equivale nt minima. From the study of these low-energy rearrangement processes we de termined the permutation-in version group of the complex, classified its vi bration-rotation-tunneling states, and determined the electric dipole selec tion rules and spin-statistical weights governing the intensity pattern in the spectra. The results of the theoretical predictions are compared with t he experimental data from the microwave measurements [Z. Kisiel et al., J. Chem. Phys. 112 (2000) 5767; Chem. Phys. Lett. 325 (2000) 523]. (C) 2001 El sevier Science BN. All rights reserved.