IR multiphoton dissociation dynamics of 2,5-dihydrofuran. Time-resolved observation of concerted furan formation

On infrared multiphoton excitation by a pulsed CO2 laser, vibrationally excited 2,5-dihydrofuran (DHF) undergoes concerted dissociation generating hydrogen and furan as products. The real time evolution of nascent furan as a photoproduct is monitored. The formation rate of transient vibrationally hot furan, which has an absorption at 245 nm, fits a single exponential with a rate constant (1.8 ± 0.1) × 106 s−1. An RRKM calculation for the unimolecular dissociation of DHF gives an average energy of DHF as 70.1 ± 2 kcal/mol corresponding to the experimental rates. The nascent hot furan, with a vibrational temperature of 1816 ± 50 K, exibits a red-shift of 30 nm in the transient absorption spectrum.