Ab initio study of (H2O)1,2•HCl:: accurate energetic and frequency shift of HCl

Spectroscopic properties of (H2O)(1,2).HCl have been re-investigated at various theoretical levels (second, third and fourth order Moller-Plesset Perturbation Theory and Coupled Cluster method). The calculated results have been compared to recent experimental data (Phys. Chem. Chem. Phys., 2002, 4, 3933-3937). A systematic study of H2O.HCl indicates that the third order Perturbation Theory (MP3) should be considered from a computational-cost/precision point of view as an optimal approach to well describe the experimental data for the H-bonded systems. At all the level of theory used here, the planar structure (C-2v) of H2O.HCl has been found to be actually a saddle point (with an imaginary frequency) corresponding to the transition state of the inversion pyramidal structure (C-s). Since the zero-point energy level of the out-of-plane bending mode of water in pyramidal C-s is located above the inversion barrier in the C-s potential at all levels of theory, it has been suggested that "averaged'' C-2v structure has to be considered as the observed structure rather than the C-s one. Particularly, it has been found that there is a significant splitting of the two V-alpha = 0 levels in the double-well of the out-of-plane bending mode which is consistent with a strong tunneling in the zero-point energy level. Moreover, for the planar structure the calculated rotational constant (14.4 cm(-1)) and frequency shift of the HCl moiety (158 cm(-1) corrected for anharmonicity and basis set superposition error) are in very good agreement with the experimental values (14-15 and 162.5 cm(-1)). Comparison between calculated and experimental values of the frequency shift of HCl in (H2O)(2).HCl (418.4 vs. 422 cm(-1)) allowed us to confirm the "tentative'' assignment of the experimental detection of the 2:1 complex.