Phase or Faith: The Molecular Aharanov-Bohm Effect in Unimolecular Reactions

It is a standard practice in chemistry to consider reaction dynamics as nuclear motion on an adiabatic potential energy surface based on the Born-Oppenheimer approximation. This approximation, which forms a cornerstone of modern chemistry, rests on the large disparity between electronic and nuclear masses. However, it also engenders a geometric phase in electronic wavefunction when encircling a conical intersection. While non-adiabatic transitions are known to occur between different adiabatic states near a conical intersection in such processes as internal conversion, it is still commonly believed that the adiabatic dynamics can be reasonably approximated by this single-state adiabatic model. We demonstrate that adiabatic dynamics of photodissociation and unimolecular reactions can be strongly influenced by the geometric phase associated with a conical intersection along the reaction pathway, even when the energy is much lower than the crossing point. This so-called molecular Aharonov-Bohm effect can be thought as a quantum interference, which can significantly affect not only the tunneling lifetimes, but also product state distributions. These results suggest important caveats to adiabatic treatments of reaction dynamics.

References

1.           Xie C., Ma J., Zhu X., Yarkony D. R., Xie D., Guo H. Nonadiabatic tunneling in photodissociation of phenol. J. Am. Chem. Soc. 138, 7828-7831 (2016).
2.           Xie C., Malbon C. L., Yarkony D. R., Xie D., Guo H. Signatures of a conical intersection in adiabatic dissociation on the ground electronic state. J. Am. Chem. Soc. 140, 1986-1989 (2018).
3.           Xie C., Malbon C. L., Guo H., Yarkony D. R. Up to a sign. The insidious effects of energetically inaccessible conical intersections on unimolecular reactions. Acc. Chem. Res. 52, 501-509 (2019).