Internal rotation in NH4+-Rg dimers (Rg = He, Ne, Ar): Potential energy surfaces and IR spectra of the nu(3) band

The intermolecular potential energy surfaces for the electronic ground stat es of the ammonium ion-rare gas dimers NH4+-He and NH4+-Ne are calculated a t the MP2 CCSD(T)/aug-cc-pVXZ (X = D/T/Q) levels of theory. The global mini ma of both potentials correspond to proton (vertex)-bound structures, R-e = 3.13 Angstrom, D-e = 171 cm(-1) (He) and R-e = 3.21 Angstrom, D-e = 302 cm (-1) (Ne). The face- and edge-bound structures are local minima and transit ion states for the internal rotation dynamics, corresponding to barriers of similar to 20 (He) and 50 cm(-1) (Ne). The ab initio potentials are employ ed in numerical solutions to the rotation-intermolecular vibration Hamilton ian to determine the term values and the rotational and distortion constant s for the lowest bound levels in the intramolecular ground vibrational stat e of both complexes. The results are used to assess the accuracy of two-dim ensional (fixed-R) representations of the potentials for determining the in ternal rotor levels in the ground and v(3) vibrational states. This model i s employed to produce simulations of the IR v(3) transitions, which are com pared to the experimental spectra recorded using photofragmentation spectro scopy. In the case of the NH4+-Ne potential parameters are least-squares fi tted 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.