Intermolecular potential energy surface of the proton-bound H2O+–He dimer: Ab initio calculations and IR spectra of the O–H stretch vibrations

Abstract

Rotationally resolved infrared photodissociation spectra of the O–H stretch fundamentals (ν1 and ν3) of the H2O+–He open shell ionic complex have been recorded in the doublet electronic ground state. The analysis of the complexation-induced frequency shifts (Δν1 = − 15 cm−1, Δν3 = − 5.1 cm−1) and the rotational structure is consistent with a planar, translinear proton-bound H–O–H–He equilibrium geometry. The derived intermolecular H–He separation and O–H–He bond angle are R0 = 1.756(4) A and φ0 = 175(5)° in the ground vibrational state. The experimental results are in good agreement with the ab initio calculations performed at the unrestricted MP2 level using a basis set of aug-cc-pVTZ quality. The global minimum on the calculated potential energy surface features a slightly translinear proton-bound equilibrium geometry, with an intermolecular separation of Re = 1.6990 A, a bond angle of φe = 173.2°, dissociation energies of De = 425.9 cm−1 and D0 = 158.6 cm−1, and frequency shifts of Δν1 = − 34.4 cm−1 and Δν3 = − 11.6 cm−1. Tunneling splittings in the perpendicular component of the ν3 band are attributed to hindered internal rotation exchanging the two equivalent proton-bound structures ia a planar transition state with C2v symmetry. The calculated barrier for this internal motion amounts to Vb = 202.9 cm−1 and the dimer is closer to the semirigid limit than to free internal rotation. The interpretation of the H2O+–He spectrum is supported by the corresponding spectrum of the monodeuterated HOD+–He complex.