Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA.
Chemphyschem. 2001 May 18;2(5):294-309. doi: 10.1002/1439-7641(20010518)2:5<294::AID-CPHC294>3.0.CO;2-5.
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).
使用飞秒（fs）时间分辨质谱研究了三种环丁酮同位素物（[D0]-、3,3-[D2]-和 2,2,4,4-[D4]-环丁酮）的异常非辐射动力学。我们发现，分子在 S1 态的内部运动在离解阈值以上涉及两个时间尺度。快速运动的时间常数小于 50fs，而对于[D0]、[D2]和[D4]物种，缓慢运动的时间常数分别为 5.0±1.0、9.0±1.5 和 6.8±1.0ps。进行了密度泛函理论和从头算计算，以表征 S0、S1（n，π*）和 T1（n，π*）态的势能面。键断裂的动力学图景如下：快速运动代表初始波包快速失相出 Franck-Condon 区域，而缓慢运动反映了 Norrish 型-I 反应的α断裂动力学。最初激活的面外弯曲模式的内部能量重新分配到面内开环反应坐标定义了分子内振动能量重新分配（IVR）的时间尺度，观察到的皮秒级（ps）衰减给出了穿过势垒的 IVR/键断裂的速率。[D2]和[D4]同位素物的明显同位素效应表明了环扭转和 CO 面外摆动运动对 S1α断裂动力学的重要性。乙烯和酮（C2 产物）以及 CO 和环丙烷（C3 产物）产物的比例可以通过我们计算中揭示的 S0/S1 锥形交叉来理解。这种在 S1 表面上的环丁酮光化学反应的提出的动力学图景不仅可以解释荧光量子产率和寿命的异常急剧下降，还可以解释 C3:C2 产物比作为激发能增加的函数的急剧变化，正如 Lee 及其同事所报道的那样（J.C.Hemminger，EKCLee，J.Chem.Phys.1972，56，5284-5295；KYTang，EKCLee，J.Phys.Chem.1976，80，1833-1836）。
