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Published May 18, 2001 | public
Journal Article

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


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).

Additional Information

© 2001 Wiley-VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany. Received: December 29, 2000. Version of Record online: 15 May 2001. This work was supported by the Office of Naval Research and the US Air Force Office of Scientific Research. C.K., a Feodor Lynen Fellow from the Alexander von Humboldt Foundation, acknowledges the Foundation and Caltech for support. We thank Prof. John E. Baldwin for many helpful discussions and Dr. Rajesh Shukla and Mr. Dharmesh B. Patel for preparation of the deuterium-labeled cyclobutanones; the relevance of the diradical work will be published separately. We are grateful to Prof. Joseph Casanova and Dr. Theis I. Sùlling for helpful discussions. E.W.-G.D. would like to acknowledge Prof. Ching-Han Hu for his many helpful suggestions and discussions.

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