Rovibrational quantum dynamics of the vinyl radical and its deuterated isotopologues

Abstract

Rotational–vibrational states up to 3200 cm⁻¹, beyond the highest-lying stretching fundamental, are computed variationally for the vinyl radical (VR), H₂C_βC_αH, and the following deuterated isotopologues of VR: CH₂CD, CHDCH, and CD₂CD. The height of the C_αH tunneling rocking barrier of VR, partially responsible for the complex nuclear dynamics of VR and its isotopologues, is determined to be 1641 ± 25 cm⁻¹ by the focal-point analysis approach. The definitive nuclear-motion computations performed utilize two previously published potential energy hypersurfaces and reveal interesting energy-level and tunneling patterns characterizing the internal motions of the four isotopologues. A full assignment, including symmetry labels, of the vibrational states computed for CH₂CH is provided, whenever feasible, based on the analysis of wave functions and the related one- and two-mode reduced density matrices. The computed vibrational states of CH₂CD and CD₂CD are characterized up to slightly above the top of the barrier. Interestingly, it is the interplay of the ν₆ (formally CH₂ rock) and ν₇ (formally CH rock) modes that determines the tunneling dynamics; thus, the description of tunneling in VR needs, as a minimum, the consideration of two in-plane bending motions at the two ends of the molecule. When feasible, the computed results are compared to their experimental counterparts as well as to previous computational results. Corrections to the placement of the ν₄ and ν₆ fundamentals of VR are proposed. Tunneling switching, a unique phenomenon characterizing tunneling in slightly asymmetric effective double-well potentials, is observed and discussed for CHDCH. Despite the extensive tunneling dynamics, the rotational energy-level structure of VR exhibits rigid-rotor-type behavior.