Photofragmentation mechanisms intermediate

Such characteristics led to the proposal (3,8) that the mechanism for the fragmentation pathway must involve the formation of a reactive intermediate, an isomer of RujCCO)] capable of first order return to the initial cluster or of capture by a two electron donor. Scheme 1 illustrates the proposed mechanism for photofragmentation. 

There is a difficulty with the mechanism of Scheme 3 in that the fragmentation of a triplet diradical should conserve spin, yet neither triplet Me2Si nor triplet tetraphenyl-naphthalene have been detected. The diradical pathway for the photofragmentation of silanorbornadienes confirms an earlier proposal by Barton and coworkers52. There is always the possibility that diradical intermediates such as the singlet and triplet 13 S and 13 T could function as silylenoids. Thus, the assumption that products from pyrolysis of 7-silanorbornadienes are formed from free silylenes must be treated with caution. 

The general theory for the absorption of light and its extension to photodissociation is outlined in Chapter 2. Chapters 3-5 summarize the basic theoretical tools, namely the time-independent and the time-dependent quantum mechanical theories as well as the classical trajectory picture of photodissociation. The two fundamental types of photofragmentation — direct and indirect photodissociation — will be elucidated in Chapters 6 and 7, and in Chapter 8 I will focus attention on some intermediate cases, which are neither truly direct nor indirect. Chapters 9-11 consider in detail the internal quantum state distributions of the fragment molecules which contain a wealth of information on the dissociation dynamics. Some related and more advanced topics such as the dissociation of van der Waals molecules, dissociation of vibrationally excited molecules, emission during dissociation, and nonadiabatic effects are discussed in Chapters 12-15. Finally, we consider briefly in Chapter 16 the most recent class of experiments, i.e., the photodissociation with laser pulses in the femtosecond range, which allows the study of the evolution of the molecular system in real time. 

Pyrazoles, on irradiation, are analogously converted into cyclopropenes or products derived therefrom. Much discussion has centred around the mechanism of this photoelimination, and evidence both for biradical intermediates and for vinyl azo intermediates has been reported. The dibenzo[2,4]spirenes (38) are obtained in good yield by irradiation of the spiropyrazoles (39) 24 pathways involving vinylcarbene and biradical intermediates have been proposed and are outlined in Scheme 1. The effect of substituents on the photofragmentation of other spiropyrazoles has been documented.26 Irradiation of the 317-pyrazoles... 

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