Riding the conical intersection along to the dip: photodissociation of thiophenol
The Chemical Dynamics and Nanomaterials Lab has experimentally investigated the conical intersection seam generated by the bound S1(pp*) and continuum S2(ps*) states, encountered along an ultrafast S-D bond rupture of thiophenol-d1. Professor Sang Kyu Kim and coworkers have probed the nature of ultrafast bond dissociation and dynamic observables including speed and angular distribution of the D product using photofragment excitation spectroscopy and velocity-map ion imaging techniques. Precise experimental results and analysis are always important to obtain; when these are accompanied by theoretical calculations as well it leads to an important and complete study. Accompanying Ab initio calculations offered insights into experimental behavior and revealed new dynamics existing near the conical intersection which act as a dynamic funnel for the nonadiabatic reaction.
Organosulfur compounds are vitally important in nature and are used extensively in industrial processes. Thiophenol is a versatile and important model system and a very interesting analog to phenol and benzene. For chemical reactions on the electronically excited state, coupling of electronic and nuclear motions becomes nontrivial; significant nonadiabatic transitions occur in a number of important chemical and biological process. In this nonadiabatic chemistry, most dynamic outputs are dictated by the nature of the conical intersections (CI), generated by the intersection of two potential surfaces; it would be valuable if the structure and dynamic role of the CI can be elucidated.
A research team led by professor Sang Kyu Kim in the department of chemistry reports an example of the spectroscopic characterization of the conical intersection on the multidimensional seam from the photodissociation reaction of thiophenol-d1 (C6H5SD). In its planar geometry, two CIs are encountered along the S-D bond dissociation pathway. The S1/S2 CI opens the gate to the chemical reaction for the reactant, and the product C6H5S·()/ C6H5S·() state bifurcation takes place at S2/S0 CI.
The photofragment excitation spectra, which monitors the C6H5S· and D fragment for thiophenol-h1 and thiophenol-d1, respectively, indicate the prompt S-H(D) bond cleavage with an S1 lifetime of 1 origin band. Ultrafast S-D bond rupture should then be reflected in the angular distribution of fragments as the linearly polarized pump laser pulse interacts with the transition dipole moment (TDM). Here the direction is well-defined with respect to the dissociating S-D bond axis. Actually, the anisotropy parameter (β) is estimated to be positive (~+0.25) at the S1 zero-point level, consistent with the calculated S1-S0 TDM direction. Interestingly, however, an unanticipated variation of β, as a function of the excitation energy is found, giving a broad (ΔE ~200 cm-1) negative dip in β. This feature helps assign the coherent excitation of the bound S1 and unbound S2 in the conical intersection region. Theoretical considerations reveal that the peak in β is ascribed to the in-plane S-D bending mode excitation by which the nuclear configuration in the proximity of the S1/S2 conical intersection seam is accessed showing a mixed character. As a result, the conical intersection, which acts as a dynamic funnel for the nonadiabatic reaction, has been spectroscopically characterized through direct access to the nuclear configuration near the conical intersection seam, helping unraveling the nature of the complicated nonadiabatic surface crossing structures and dynamics of multidimensional polyatomic systems.
Sang Kyu Kim, his students Hyun Sik You, Songhee Han, and Jean Sun Lim were listed as the authors of this contribution published in The Journal of Physical Chemistry Letters in August, 2015 (J. Phys. Chem. Lett. 2015, 6, 3202-3208)
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