Once the carbene is formed in the S 1 state, it can experience excited state intramolecular proton transfer along a seam of crossing (ESIPT-SC) of the S 1 and S 0 states to yield the alkene derivative that is, the proton transfer reaction takes places on a degenerate potential energy surface where the two electronic states have equal energy. In contrast, the 350 nm radiation can directly decompose PDP into S 1 carbene (open shell) and N 2 when the conical intersection CI2 is avoided. Upon irradiation of PDP with the 520 nm light, the carbene is always generated in its ground state as closed-shell singlet carbene. It is demonstrated that the main intermediate of the photochemistry of the titled compounds is 1-ethyl-1-phenyl carbene ( EPC). It is found that the photochemistry of PDP is governed by nonradiative deactivation processes that can involve one or two S 1/S 0 conical intersections ( CI1 and CI2) depending on the wavelength of the radiation CI2 is only accessible at the shortest wavelength. In this work, we studied the wavelength (520 or 350 nm) dependence of the photochemical decomposition of 1-phenyl-1-diazopropane ( PDP) and 1-phenyl-1-propyl diazirine ( PED) by means of high-level ab initio quantum chemical calculations (CASSCF and MS-CASPT2) to obtain qualitative and quantitative results.
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