Photochemical Pathways of Formaldonitrone: Quantum Chemical Calculations and Excited State Dynamics Simulations

The photochemistry of nitrones is extremely varied. It has been established that the photoconversion of nitrone to oxaziridine represents an important step in the formation of formamide. In this article, a detailed computational study of the photochemistry of the simplest nitrone, formaldonitrone (FN, CH₂NHO), is presented. Using multireference electronic structure calculations, we develop a detailed picture of molecular motions required for photodeactivation of FN. Ab initio multiple spawning (AIMS) direct dynamics simulations further support this and provide time-scales for these motions. The ground state reactivity of FN and its conformers is investigated by coupled-cluster methods. Energy barriers larger than 40 kcal/mol are found between FN and its conformers, thus ruling out cis-trans and [1,3]-electrocyclization reactions on the ground state surface. The nature and spectral position of vertical excited states of FN are characterized by a series of multireference methods. It is found that the energy ordering of vertical excited states is sensitive to dynamical correlation. Geometry optimizations computed the minimum energy conical intersection (MECI) on the seam of the S₁/S₀ CI, which are characterized by bond elongations and NHO pyramidalization. Minimum energy path (MEP) calculations show a barrierless pathway from the Franck-Condon region to MECI. The reaction coordinate is found to involve C-N and N-O bond stretching, pyramidalization of NHO, and CH₂ torsion. Furthermore, two energy valleys on the surface of the ground state are confirmed by intrinsic coordinate computations, leading to the predominant formation of oxaziridine and nitrone. AIMS dynamics simulations validate the ultrafast photorelaxation of FN on the S₁ state, with a decay time of 164 fs, and the initial molecular motions are consistent with those predicted by MEP calculations.