Successful attempts have been made in automated reaction discovery for ground state chemistry in the form of the ab initio nanoreactor. This versatile computational tool enables the detection of chemical species and reaction pathways by means of accelerated molecular dynamics followed by refinement procedures and determination of chemical reaction networks. Due to a multiplicity of technical and conceptual issues (e.g. the lack of a comprehensive nonadiabatic transition state theory), this framework does not yet exist for photochemistry. In this work, we present the nonadiabatic nanoreactor, an analogous framework for excited state reaction discovery. We automate the study of nonadiabatic decay mechanisms of molecules by probing the intersection seam between adiabatic electronic states with hyper-real meta-dynamics, sampling the branching plane for relevant conical intersections, and performing seam-constrained path searches to determine the reaction channels for nonadiabatic population transfer from the excited state. We illustrate the effectiveness of the nonadiabatic nanoreactor by applying it to benzene, a molecule with rich photochemistry and a wide array of photochemical products. Our study confirms the existence of several types of S0/S1 and S1/S2 conical intersections which mediate access to a variety of ground state stationary points. We elucidate the connections between conical intersection energy/topography and the resulting photoproduct distribution, which changes smoothly along seam space segments. The exploration is performed with minimal user input, and the protocol requires no previous knowledge of the photochemical behavior of a target molecule. We demonstrate that the nonadiabatic nanoreactor is a valuable tool for the automated exploration of photochemical reactions and their mechanisms. This work opens the way to many development lines, ranging from applications, such as the inclusion of bimolecular photoreactions or solvation treatment, to the implementation of a probabilistic nonadiabatic transition state theory.

Nonadiabatic nanoreactor: Towards the automated discovery of photochemistry

ACS Fall 2021

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