Femtochemistry of Norrish Type-I Reactions: II. The Anomalous Predissociation Dynamics of Cyclobutanone on the S_1 Surface

Abstract

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±1.0, 9.0±1.5, and 6.8±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,π^*), and T_1(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 [D_2] and [D_4] isotopomers imply the significance of the ring-puckering and the CO out-of-plane wagging motions to the S_1α-cleavage dynamics. The ethylene and ketene (C_2 products)—as well as CO and cyclopropane (C_3 products)—product 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 (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).

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