Real-Time Tunneling Dynamics through Adiabatic Potential Energy Surfaces Shaped by a Conical Intersection

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Dynamics

Background:

• Conical intersections play a crucial role in the dynamics of polyatomic molecules.
• Adiabatic tunneling is a key process in chemical reactions and molecular rearrangements.
• Understanding vibrational mode effects on tunneling is essential for controlling reaction pathways.

Purpose of the Study:

• To investigate the influence of vibrational modes on S-H bond dissociation tunneling in ortho-substituted thiophenols.
• To explore the dynamic shaping of adiabatic potential energy surfaces by conical intersections.
• To elucidate the role of out-of-plane vibrational modes in controlling tunneling rates.

Main Methods:

• Experimental measurement of S-H predissociation tunneling rates (k) in the S1 state of 2-methoxythiophenol, 2-fluorothiophenol, and 2-chlorothiophenol.
• Excitation of specific low-frequency out-of-plane and in-plane vibrational modes.
• Analysis of tunneling rate dependence on internal energy and vibrational mode excitation.

Main Results:

• A resonance-like increase in the S-H tunneling rate was observed upon excitation of a 152 cm-1 out-of-plane vibrational mode in 2-methoxythiophenol.
• Excitation of an in-plane mode at 282 cm-1 led to a decrease in the tunneling rate.
• Similar phenomena were observed for 2-fluorothiophenol and 2-chlorothiophenol, indicating a general mechanism.

Conclusions:

• Low-frequency out-of-plane vibrational modes dynamically shape the adiabatic potential energy surfaces.
• These vibrations, parallel to the conical intersection coupling vector, significantly influence multidimensional tunneling dynamics.
• The findings provide direct evidence for the control of tunneling pathways by conical intersection geometry.