Femtochemistry of Norrish type-I reactions: II. The anomalous predissociation dynamics of cyclobutanone on the S1...

The anomalous nonradiative dynamics for three cyclobutanone isotopomers ([D0 ]-, 3,3-[D2 ]-, and 2,2,4,4-[D4 ]cyclobutanone) have been investigated using femtosecond (fs) time-resolved mass spectrometry. We have found that the internal motions of the molecules in the S1 state above the dissociation threshold involve two time scales. The fast motion has a time constant of <50 fs, while the slow motion has a time constant of 5.0±1.0, 9.0±1.5, and 6.8±1.0 ps for the [D0 ], [D2 ], and [D4 ] species, respectively. Density functional theory and ab initio calculations have been performed to characterize the potential energy surfaces for the S0 , S1 (n,π*), and T1 (n,π*) states. The dynamic picture for bond breakage is the following: The fast motion represents the rapid dephasing of the initial wave packet out of the Franck-Condon region, whereas the slow motion reflects the α-cleavage dynamics of the Norrish type-I reaction. The redistribution of the internal energy from the initially activated out-of-plane bending modes into the in-plane ring-opening reaction coordinate defines the time scale for intramolecular vibrational energy redistribution (IVR), and the observed picosecond-scale (ps) decay gives the rate of IVR/bond cleavage across the barrier. The observed prominent isotope effect for both [D2 ] and [D4 ] isotopomers imply the significance of the ring-puckering and the CO out-of-plane wagging motions to the S1 α-cleavage dynamics. The ethylene and ketene (C2 products)-as well as CO and cyclopropane (C3 products)-product ratios can be understood by the involvement of an S0 /S1 conical intersection revealed in our calculations. This proposed dynamic picture for the photochemistry of cyclobutanone on the S1 surface can account not only for the abnormally sharp decrease in fluorescence quantum yield and lifetime but also for the dramatic change in the C3 :C2 product ratio as a function of increasing excitation energy, as reported by Lee and co-workers (J. C. Hemminger, E. K. C. Lee, J. Chem. Phys. 1972, 56, 5284-5295; K. Y. Tang, E. K. C. Lee, J. Phys. Chem. 1976, 80, 1833-1836).