Trackinga Paternò–Büchi Reactionin Real Time Using Transient Electronic and Vibrational Spectroscopies.

A detailed mechanistic investigationof the early stages of thePaternò–Büchi reaction following 267 nm excitationof benzaldehyde in cyclohexene has been completed using ultrafast,broadband transient UV–visible and IR absorption spectroscopies.Absorption due to electronically excited triplet state benzaldehydedecays on a 80 ps time scale via reaction with cyclohexene. The growthand subsequent decay of the biradical intermediate produced followingC–O bond formation is followed by transient vibrational spectroscopy.The biradical decays by ring closure to an oxetane or by dissociating,reforming the ground state reactants. Detailed kinetic analysis allowedderivation of quantum yields and rate constants for these competingbiradical decay processes, ϕoxetane= 0.53, ϕdiss= 0.47, koxetane= 0.27 ±0.09 ns–1and kdiss=0.24 ± 0.09 ns–1. This study provides a strikingillustration of the ways in which contemporary ultrafast transientabsorption spectroscopy methods can be used to dissect the mechanismand kinetics of a classic photoreaction. [ABSTRACT FROM AUTHOR]