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Pure Appl. Chem., 2013, Vol. 85, No. 10, pp. 1991-2004

Published online 2013-08-13

A new reaction mechanism of Claisen rearrangement induced by few-optical-cycle pulses: Demonstration of nonthermal chemistry by femtosecond vibrational spectroscopy

Izumi Iwakura1,2,3*, Atsushi Yabushita4, Jun Liu3, Kotaro Okamura3, Satoko Kezuka5 and Takayoshi Kobayashi3

1 Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Yokohama 221-8686, Japan
2 Innovative Use of Light and Materials/Life, PRESTO, JST, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan
3 University of Electro-Communications, 1‑5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
4 Department of Electrophysics, National Chiao-Tung University, Hsinchu 300, Taiwan
5 Department of Applied Chemistry, School of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292, Japan

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.