Development of novel reactions induced by coherent molecular vibrational excitation and direct observation of molecular structural change during "thermal" reactions

Femtochemistry began from the observation of transition states (TSs) in gas-phase photoreactions. Later developments in visible ultrashort laser pulses enabled visualization of real-time changes in the molecular vibrational modes during photoreactions, including those in the liquid phase. However, TSs in thermal reactions could still not be studied, because the conventional femtosecond laser pulse cannot trigger chemical reactions in the electronic ground state. In the present work, a visible ultrashort laser pulse was used to cause an induced Raman process, whereby Raman active molecular vibrational modes were excited in the electronic ground states, which triggers the reaction induced by coherent molecular vibrational excitation in the electronic ground state just after photoexcitation. Using the "reaction by coherent molecular vibration", we have successfully observed the TSs in a "thermal" reaction, thermal Claisen rearrangement.