Ultrafast Dynamics of Photochemical Nitrile Imine Formation

The chemical reactivity of nitrile imines is of great utility in organic synthesis with applications rapidly expanding into the materials and life sciences. Yet, our understanding of the electronic and molecular structures of nitrile imines remains incomplete and the elementary mechanism of their photoinduced generation is entirely unknown. Here, femtosecond infrared spectroscopy after 266 nm‐excitation of 2,5‐diphenyltetrazole has been carried out to temporally resolve the formation and structural relaxation dynamics of the nascent diphenylnitrile imine in liquid solution under ambient conditions. The infrared‐spectroscopic evolution is interpreted by an initial sequence of intersystem crossings within 250 fs followed by the cleavage of N2 with formation of a structurally relaxed nitrile imine on the adiabatic ground‐state singlet surface within a few tens of picoseconds. The infrared spectrum supports the notion of a “floppy” nitrile imine molecule whose equilibrium character ranges from fully propargylic to fully allenic in the room temperature liquid solution. Femtosecond UV‐pump/IR‐probe spectroscopy elucidates the molecular mechanism of the photo‐induced 1,3‐dipolar cycloelimination of N2 from a tetrazole. In liquid solution, the nitrile imine photoproduct is a very floppy molecule that can adopt a variety of different conformations ranging from propargylic all the way to allenic structures.