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) Å 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 R_e = 1.6990 Å, a bond angle of φ_e = 173.2°, dissociation energies of D_e = 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 C_2v symmetry. The calculated barrier for this internal motion amounts to V_b = 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.