Mechanistic Insights into the Photochemical Formation of Carbenes from Phenyl-trifluoromethyl Diazirine and Its Diazo Isomer: An MS-CASPT2 Study

Abstract

The photochemical formation of carbenes from 3-phenyl-3-(trifluoromethyl)-3H-diazirine (1) and its diazo isomer, phenyl-trifluoromethyl-diazomethane (2), was investigated using high-level multiconfigurational wave function methods (MS-CASPT2) and density functional theory (DFT). Vertical excitation energies and the vibronic absorption spectrum of 1 were computed, confirming that the experimental signals reported in the literature originate from a single conformer. Our results demonstrate that S₁ excitation in both isomers leads to ultrafast nonradiative deactivation through a shared S₁/S₀ conical intersection (CI1). In the condensed phase, this relaxation triggers a branching mechanism: the system can either revert to the precursor, undergo diazirine–diazo isomerization, or fragment into N₂ and a closed-shell singlet carbene. Notably, the topology of the ground-state potential energy surface suggests that CI1 is a primary connection between the two isomers, relegating a conventional transition-state-mediated pathway to a less likely alternative for isomerization. Furthermore, a comparative assessment shows that both MS-CASPT2 and DFT provide consistent descriptions of the critical S₁/S₀ and S₀/T₀ crossing geometries. These findings provide a comprehensive mechanistic framework for the photolysis of aryl-trifluoromethyl diazirines.