Accurate quantum dynamics calculations of vibrational spectrum of dideuteromethane CH2D2.

We report a rigorous variational study of the infrared (IR) vibrational spectra of both CH2D2 and (13)CH2D2 isotopomers using an exact molecular Hamiltonian. Calculations are carried out using a recently developed multi-layer Lanczos algorithm based on the accurate refined Wang and Carrington potential energy surface of methane and the low-order truncated ab initio dipole moment surface of Yurchenko et al. [J. Mol. Spectrosc. 291, 69 (2013)]. All well converged 357 vibrational energy levels up to 6100 cm(-1) of CH2D2 are obtained, together with a comparison to previous calculations and 91 experimental bands available. The calculated frequencies are in excellent agreement with the experimental results and give a root-mean-square error of 0.67 cm(-1). In particular, we also compute the transition intensities from the vibrational ground state for both isotopomers. Based on the theoretical results, 20 experimental bands are suggested to be re-assigned. Surprisingly, an anomalous C isotopic effect is discovered in the nν5 modes of CH2D2. The predicted IR spectra provide useful information for understanding those unknown bands.