From roaming atoms to hopping surfaces: mapping out global reaction routes in photochemistry.

The photodissociation of small molecules occurs upon irradiation by ultraviolet or visible light, and it is a very important chemical process in Earth's atmosphere, in the atmospheres of other planets, and in interstellar media. Photodissociation is an important method used to thoroughly investigate the fundamental issues of chemical reactivity. Photodissociation involves molecules and reaction fragments moving over ground- and excited-state potential surfaces (PESs). Molecules can move on a single PES (adiabatic pathway) or can cross over from one PES to another (nonadiabatic pathways). For a full theoretical understanding of a photodissociation mechanism, all of the important nonadiabatic and adiabatic pathways must be determined. This is not an easy task. We have developed an efficient computational method, called the global reaction route mapping (GRRM) strategy, that allows a theoretical exploration of ground- and excited-state PESs and their crossing seams in an automatic manner. In this Perspective, we summarize our approaches and present examples of their application together with newly determined chemical insights. These include the complex photodissociation mechanism of the formaldehyde molecule, the exclusive excited-state roaming dynamics of the nitrate radical, and all product channels and conformational memory in the photodissociation of the formic acid molecule. Finally, perspectives for the theoretical design of photofunctional molecules are discussed.