Internal rotation in NH4+–Rg dimers (Rg = He, Ne, Ar): Potential energy surfaces and IR spectra of the ν3 band

Abstract

The intermolecular potential energy surfaces for the electronic ground states of the ammonium ion–rare gas dimers NH₄⁺–He and NH₄⁺–Ne are calculated at the MP2 and CCSD(T)/aug-cc-pVXZ (X = D/T/Q) levels of theory. The global minima of both potentials correspond to proton (vertex)-bound structures, R_e = 3.13 Å, D_e = 171 cm⁻¹ (He) and R_e = 3.21 Å, D_e = 302 cm⁻¹ (Ne). The face- and edge-bound structures are local minima and transition states for the internal rotation dynamics, corresponding to barriers of ∽20 (He) and 50 cm⁻¹ (Ne). The ab initio potentials are employed in numerical solutions to the rotation–intermolecular vibration Hamiltonian to determine the term values and the rotational and distortion constants for the lowest bound levels in the intramolecular ground vibrational state of both complexes. The results are used to assess the accuracy of two-dimensional (fixed-R) representations of the potentials for determining the internal rotor levels in the ground and ν₃ vibrational states. This model is employed to produce simulations of the IR ν₃ transitions, which are compared to the experimental spectra recorded using photofragmentation spectroscopy. In the case of NH₄⁺–Ne the potential parameters are least-squares fitted to the experimental spectrum. The trends within the NH₄⁺–Rg series (Rg = He, Ne, Ar) revealed by both the IR spectra and theoretical calculations are discussed.