Rotational and vibrational energy levels of methyl fluoride calculated from a new potential energy surface

A new potential energy surface of methyl fluoride is constructed using extended ab initio CCSD(T) calculations with the cc-pVQZ basis at 5100 nuclear configurations. Its analytical representation is determined through an expansion in symmetry adapted products of orthogonal coordinates involving 600 parameters up to 6th order. A good convergence for variational calculations of vibrational levels of the CH3F molecule was obtained with a RMS(obs.-calc.) deviation of less than 4 cm(-1) for fundamental band centers. The equilibrium geometry of the ab initio PES was empirically optimized using experimental J = 1 energy levels for four isotopologues (CH3F)-C-12, (CH3F)-C-13, (CD3F)-C-12, and (CD3F)-C-13. The resulting variational calculations with the full normal mode Hamiltonian in the irreducible tensor representation gave a RMS(obs.-calc) deviation of 0.00036 cm(-1) for rotational energies up to J = 5 for the major isotopologue. This represents a considerable improvement with respect to available global predictions of vibration and rotational levels of methyl fluoride. (c) 2012 Elsevier Inc. All rights reserved.