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

In this paper we report ab initio predictions of the minimum energy structure of the (H2O)2HCl 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 Møller–Plesset theory, while the corresponding interaction energies and binding energies were computed using the coupled-cluster method restricted to single, double, and non-iterative triple excitations. It is shown that the non-additive interactions play an important role. The contribution of the three-body term represents as much as 13–20% of the total interaction energy. The nature of the intermolecular interactions in the cluster was investigated by symmetry-adapted perturbation theory. As expected, the complex is mostly stabilized by the electrostatic and induction interactions, but the dispersion term is far from negligible. It is found that the potential energy surface of this cluster shows three low-energy pathways connecting two enantiomeric minimum energy structures. The height of the barriers separating these minima suggests that it should be possible to observe spectroscopic transitions resulting from the tunneling between the equivalent minima. From the study of these low-energy rearrangement processes we determined the permutation-inversion group of the complex, classified its vibration–rotation–tunneling states, and determined the electric dipole selection rules and spin-statistical weights governing the intensity pattern in the spectra. The results of the theoretical predictions are compared with the experimental data from the microwave measurements [Z. Kisiel et al., J. Chem. Phys. 112 (2000) 5767; Chem. Phys. Lett. 325 (2000) 523].