Pure and Applied Chemistry

Abstract: Time-resolved vibration spectroscopy is the only known way to directly observe reaction processes. In this work, we measure time-resolved vibration spectra of the Claisen rearrangement triggered and observed by few-optical-cycle pulses. Changes in molecular structure during the reaction, including its transition states (TSs), are elucidated by observing the transient changes of molecular vibration wavenumbers. We pump samples with visible ultrashort pulses of shorter duration than the molecular vibration period, and with photon energies much lower than the minimum excitation energy of the sample. The results indicate that the “nonthermal Claisen rearrangement” can be triggered by visible few-optical-cycle pulses exciting molecular vibrations in the electronic ground state of the sample, which replaces the typical thermal Claisen rearrangement.