Femtochemistry of Norrish Type-I Reactions: II. The Anomalous Predissociation Dynamics of Cyclobutanone on the S1 Surface (original) (raw)

The anomalous nonradiative dynamics for three cyclobutanone isotopomers ([D 0 ]-, 3,3-[D 2 ]-, and 2,2,4,4- [D 4 ]cyclobutanone) have been investigated using femtosecond (fs) time-resolved mass spectrometry. We have found that the internal motions of the molecules in the S 1 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 AE 1.0, 9.0 AE 1.5, and 6.8 AE 1.0 ps for the [D 0 ], [D 2 ], and [D 4 ] species, respectively. Density functional theory and ab initio calculations have been performed to characterize the potential energy surfaces for the S 0 , S 1 (n,p*), and T 1 (n,p*) 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 a-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 [D 2 ] and [D 4 ] isotopomers imply the significance of the ring-puckering and the CO out-of-plane wagging motions to the S 1 a-cleavage dynamics. The ethylene and ketene (C 2 products)Ðas well as CO and cyclopropane (C 3 products)Ðproduct ratios can be understood by the involvement of an S 0 /S 1 conical intersection revealed in our calculations. This proposed dynamic picture for the photochemistry of cyclobutanone on the S 1 surface can account not only for the abnormally sharp decrease in fluorescence quantum yield and lifetime but also for the dramatic change in the C 3 :C 2 product ratio as a function of increasing excitation energy, as reported by Lee and co-workers (