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

Abstract

Spectroscopic properties of (H₂O)_1,2·HCl have been re-investigated at various theoretical levels (second, third and fourth order Møller–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_α = 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.