Conformer specific nonadiabatic reaction dynamics in the photodissociation of partially deuterated thioanisoles (C6H5S-CH2D and C6H5S-CHD2)

Abstract

In this work, we have investigated nonadiabatic dynamics in the vicinity of conical intersections for predissociation reactions of partially deuterated thioanisole molecules: C₆H₅S-CH₂D and C₆H₅S-CHD₂. Each isotopomer has two distinct rotational conformers according to the geometrical position of D or H of the methyl moiety with respect to the molecular plane for C₆H₅S-CH₂D or C₆H₅S-CHD₂, respectively, as spectroscopically characterized in our earlier report [J. Lee, S.-Y. Kim and S. K. Kim, J. Phys. Chem. A, 2014, 118, 1850]. Since identification and separation of two different rotational conformers of each isotopomer have been unambiguously done, we could interrogate nonadiabatic dynamics of thioanisole in terms of both H/D substitutional and conformational structural effects. Nonadiabatic transition probability, estimated by the experimentally measured branching ratio of the nonadiabatically produced ground-state channel giving C₆H₅S·() versus the adiabatic excited-state channel leading to the C₆H₅S·(Ã) radical, shows resonance-like increases at symmetric (ν_s) or asymmetric (7a) S-CH₂D (or S-CHD₂) stretching mode excitation in S₁ for all conformational isomers of two isotopomers. However, absolute probabilistic value of the nonadiabatic transition is found to vary quite drastically depending on different conformers and isotopomers. The experimental finding that nonadiabatic transition dynamics are very sensitive to subtle changes in the nuclear configuration within the Franck–Condon region induced by the H/D substitution indicates that the S₁/S₂ conical intersection seam is quite narrowly defined in the multi-dimensional nuclear configurational space as far as the S-methyl predissociation reaction is concerned. In order to understand the relation between molecular structure and nonadiabaticity of reaction, potential energy surfaces near S₁/S₂ conical intersections have been theoretically calculated along ν_s and 7a normal mode coordinates for all conformational isomers. Slow-electron velocity map imaging (SEVI) spectroscopy is employed to unravel the extent of intramolecular vibrational redistribution (IVR) for particular mode excitations of S₁, providing insights into the dynamic interplay between IVR and nonadiabatic transition probability near the conical intersection seam.