HCl–H2O dimer: an accurate full-dimensional potential energy surface and fully coupled quantum calculations of intra- and intermolecular vibrational states and frequency shifts

Abstract

The interaction between HCl and H₂O is of considerable theoretical and experimental interest due to its important role in atmospheric chemistry and understanding the onset of the dissociation of HCl in water. In this work, the HCl–H₂O complex is quantitatively characterized in two ways. First, we report a new full-dimensional potential energy surface (PES) for the HCl + H₂O system. The nine-dimensional (9D) PES is based on circa 43 000 ab initio points calculated at the level of CCSD(T)-F12a/AVTZ with the basis set superposition error correction using the permutation invariant polynomial-neural network method, which can accurately and efficiently reproduce the geometries, energies, frequencies of the complex of HCl with H₂O, as well as the relevant minimum energy path. Next, we present the results of the first fully coupled 9D quantum calculations of the intra- and intermolecular vibrational states of the HCl–H₂O dimer, performed on the new PES. They employ the highly efficient bound-state methodology previously used to compute accurately the rovibrational level structure of the H₂O/D₂O–CO and HDO–CO complexes [P. M. Felker and Z. Bačić, J. Chem. Phys., 2020, 153, 074107; J. Phys. Chem. A, 2021, 125, 980]. The 9D calculations characterize the vibrationally averaged nonplanar ground-state geometry of the HCl–H₂O complex, the intramolecular vibrational fundamentals of both H₂O and HCl moieties, and their frequency shifts, as well as the low-energy intermolecular vibrational states in each of the intramolecular vibrational manifolds and the effects of the coupling between the two sets of modes. The calculated properties of the HCl–H₂O dimer are in excellent agreement with the available spectroscopic data. The 9D computed dimer binding energy D₀ of 1334.63 cm⁻¹ agrees extremely well with the experimental D₀ equal to 1334 ± 10 cm⁻¹ [B. E. Casterline and A. K. Mollner and L. C. Ch'ng and H. Reisler, J. Chem. Phys., 2010, 114, 9774]. Moreover, the ground-state expectation value of the out-of-plane bend angle of H₂O, 33.80°, and the computed HCl stretch frequency shift, −157.9 cm⁻¹, both from the 9D calculations, are in very good accord with the corresponding experimental values.