Abstract
Multidimensional conical intersection seam has been characterized by utilizing the dynamic resonances in the nonadiabatic transition probability experimentally observed in the predissociation of thioanisole isotopomers. The nonadiabatic bifurcation behavior of the reactive flux into either the Herzberg type-I (electronic) or type-II (vibrational) predissociation pathway is found to be strongly dependent on the quantum nature of the S1/S2 vibronic eigenstate, providing the essential information about structure and dynamic character of the conical intersection seam projected onto the normal mode space. By modifying the nature of the normal mode space through partial or full H/D substitution of the molecule, multiple aspects of the conical intersection seam could be characterized from different viewpoints set by the adjusted normal mode space. Theoretical calculations of potential energy curves along selected normal mode displacements support the experiment.
Highlights
IntroductionNonadiabatic transition occurs most efficiently when the reactive ux is prepared in the proximity of the conical intersection (CI) seam.[1,2] In the S–CH3 bond predissociation reaction of thioanisole (C6H5SCH3), the reactive ux prepared by the excitation to the rst electronically excited (S1; pp*) state bifurcates into either the adiabatic or nonadiabatic pathway at the rst S1/ S2 conical intersection where the upper-lying S2 (ps*) state is repulsive along the S–CH3 bond elongation coordinate.[3,4,5] The ux sliding on S2 undergoes another bifurcation at the second S0/S2 conical intersection to give either the excited (A~) or ground (X~) state of the C6H5Sc radical at the asymptotic limit as the former or latter is diabatically correlated from S0 or S2, respectively, Fig. 1
We have found that the 7a mode of C6D5S–CH3 shows the strongly enhanced X~/A~ product branching ratio, indicating that the nonadiabatic transition is highly facilitated by the 7a mode excitation
Quantum mechanical characterization of the multidimensional conical intersection seam has been demonstrated to be feasible in predissociation reaction of the excited thioanisole molecule
Summary
Nonadiabatic transition occurs most efficiently when the reactive ux is prepared in the proximity of the conical intersection (CI) seam.[1,2] In the S–CH3 bond predissociation reaction of thioanisole (C6H5SCH3), the reactive ux prepared by the excitation to the rst electronically excited (S1; pp*) state bifurcates into either the adiabatic or nonadiabatic pathway at the rst S1/ S2 conical intersection where the upper-lying S2 (ps*) state is repulsive along the S–CH3 bond elongation coordinate.[3,4,5] The ux sliding on S2 undergoes another bifurcation at the second S0/S2 conical intersection to give either the excited (A~) or ground (X~) state of the C6H5Sc radical at the asymptotic limit as the former or latter is diabatically correlated from S0 or S2, respectively, Fig. 1. The nonadiabatic transition efficiency is re ected in the X~/A~ product branching ratio, giving the quantitative estimation of the otherwise unambiguous nonadiabatic transition probability Both S1/S2 and S0/S2 conical intersections are located at the planar geometry and share the same branching plane in terms of its gradient and coupling vectors.[3,4,5] as the S1/S2 conical intersection is correlated to the S0/S2 conical intersection by the repulsive S2 state in the very short S–CH3 bond length region, the quantum mechanical character of the initial reactive ux especially along the degrees of freedom orthogonal to the reaction coordinate is not likely to be modi ed.[3,4,5,6,7,8] This has provided a great chance to characterize the S1/S2 conical intersection seam by monitoring the X~/A~ product branching ratio with varying the S1 vibronic mode excitation within the Franck–Condon region
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