Ultrafast Dynamics of Photochemical Nitrile Imine Formation

Abstract

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.

Elucidating the N−N and C−N Bond‐breaking Mechanism in the Photoinduced Formation of Nitrile Imine

“…[3,6,7] In this context, Flesh and Vöhringer combined the femtosecond UV/ IR spectroscopy with TD-DFT calculations of the 2,5-diphenyl tetrazole (Dtz) system. [6] This study showed that the N 2 extrusion occurs in multiple adiabatic transitions from the S 1 excited state toward triplet states. Interestingly, the ultrafast dynamics which explain the NIm formation strongly suggest that the key bonding process resulting in the NÀ C and NÀ N bond cleavage takes place on the triplet potential energy surfaces.…”

“…In analogy with the timeresolved methods employed in ref [7], we analyzed the evolution of the electronic structure of the tetrazole ring along the intrinsic reaction coordinated (IRC) [15,16] connecting the most relevant energy stationary points implied in the NIm generation. [6,7] Our representation of electron reorganizations is based on the electron localization function (ELF � η(r)) analysis. [17][18][19][20] ELF constitutes a measure of electron pairs localization derived from the same-spin-pair conditional probability.…”

“…This study showed that the N 2 extrusion occurs in multiple adiabatic transitions from the S 1 excited state toward triplet states. Interestingly, the ultrafast dynamics which explain the NIm formation strongly suggest that the key bonding process resulting in the N−C and N−N bond cleavage takes place on the triplet potential energy surfaces [5–7] . Moreover, the real‐time IR analysis concerning the normal modes of Dtz enables identifying the vibrations of resonance structures responsible for the light‐induced mechanism of cycloelimination of N 2 [10–14] .…”

“…Interestingly, the ultrafast dynamics which explain the NIm formation strongly suggest that the key bonding process resulting in the NÀ C and NÀ N bond cleavage takes place on the triplet potential energy surfaces. [5][6][7] Moreover, the real-time IR analysis concerning the normal modes of Dtz enables identifying the vibrations of resonance structures responsible for the light-induced mechanism of cycloelimination of N 2 . [10][11][12][13][14] This fact is particularly relevant because changes in CÀ N and NÀ N stretching modes along the reaction process allow us to picture how the NIm species are formed.…”

“…This process requires a previous photochemical generation of the nitrile imine (NIm) compounds for its subsequent cycloaddition in the ground state [1] . The high reactivity of NIm species includes different valence structures generated mainly through N 2 elimination from 2,5‐tetrazoles [6–9] . The photochemical reaction mechanism of NIm production is the subject of ongoing research through experimental and theoretical methods [3,6,7] .…”