The Vibration-Rotation-Tunneling Levels of N2–H2O and N2–D2O
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In this paper, we report vibration-rotation-tunneling levels of the van der Waals clusters N2–H2O and N2–D2O computed from an ab initio potential energy surface. The only dynamical approximation is that the monomers are rigid. We use a symmetry adapted Lanczos algorithm and an uncoupled product basis set. The pattern of the cluster’s levels is complicated by splittings caused by H–H exchange tunneling (larger splitting) and N–N exchange tunneling (smaller splitting). An interesting result that emerges from our calculation is that whereas in N2–H2O, the symmetric H–H tunnelling state is below the anti-symmetric H–H tunnelling state for both K = 0 and K = 1, the order is reversed in N2–D2O for K = 1. The only experimental splitting measurements are the D–D exchange tunneling splittings reported by Zhu et al. [J. Chem. Phys. 139, 214309 (2013)] for N2–D2O in the v2 = 1 region of D2O. Due to the inverted order of the split levels, they measure the sum of the K = 0 and K = 1 tunneling splittings, which is in excellent agreement with our calculated result. Other splittings we predict, in particular those of N2–H2O, may guide future experiments.