Tunneling motion and splitting in the CH2OH radical: (Sub-)millimeter wave spectrum analysis

L. Coudert (1) , O. Chitarra (1) , J.-T. Spaniol (1) , J.-C. Loison (2) , M.-A. Martin-Drumel (1) , O. Pirali (1, 3)

1 ISMO - Institut des Sciences Moléculaires d'Orsay
2 ISM - Institut des Sciences Moléculaires
3 SSOLEIL - Synchrotron SOLEIL

The (sub-)millimeter wave spectrum of the non-rigid CH2OH radical is investigated both experimentally and theoretically. Ab initio calculations are carried out to quantitatively characterize its potential energy surface as a function of the two large amplitude ∠H1COH and ∠H2COH dihedral angles. It is shown that the radical displays a large amplitude torsional-like motion of its CH2 group with respect to the OH group. The rotation–torsion levels computed with the help of a 4D Hamiltonian accounting for this torsional-like motion and for the overall rotation exhibit a tunneling splitting, in agreement with recent experimental investigations, and a strong rotational dependence of this tunneling splitting on the rotational quantum number K a due to the rotation–torsion Coriolis coupling. Based on an internal axis method approach, a fitting Hamiltonian accounting for tunneling effects and for the fine and hyperfine structure is built and applied to the fitting of the new (sub)-millimeter wave transitions measured in this work along with previously available high-resolution data. 778 frequencies and wavenumbers are reproduced with a unitless standard deviation of 0.79 using 27 parameters. The N = 0 tunneling splitting, which could not be determined unambiguously in the previous high-resolution investigations, is determined based on its rotational dependence.

Domaines: Chimie-Physique [physics.chem-ph]

L. Coudert, O. Chitarra, J.-T. Spaniol, J.-C. Loison, M.-A. Martin-Drumel, et al.. Tunneling motion and splitting in the CH2OH radical: (Sub-)millimeter wave spectrum analysis. Journal of Chemical Physics, 2022, 156 (24), pp.244301. ⟨10.1063/5.0095242⟩. ⟨hal-03726378⟩