Photofragmentation mechanisms

Photofragmentation Mechanisms. Photolysis (A rr 405 nm) of Ru3(CO)y2 in hydrocarbon solvents under CO gave Ru(C0)5 as the sole product (Equation 1). The quantum yield proved markedly dependent on qq and on the solvent (Table I), with donor solvents such as THF giving much smaller values. Photofragmentation in octane (A rr 405 nm,... 

Ester derivatives of benzoin are known to display usually quite a low photoinitiation activity in the polymerization of vinyl monomers [85,104]. By contrast, ben2 in alkyl ethers are claimed to generate [17], by a photofragmentation mechanism, benzoyl and a-alkoxy benzyl radicals resulting in a much more active polymerization and crosslinking initiating species (Scheme 25). Thus, polymeric systems having the above moieties in the side chains have been prepared and their photoreactivity studied in more detail [105-107],... 

A detailed analysis of the kinetic data of Table 21 clearly shows that all the polymeric systems display higher photoinitiation activity than that of the corresponding low-molecular-weight analogues MBAc and MBEE [84]. The above results confirm that a positive polymer effect on activity is also present in polymeric systems working with a photofragmentation mechanism. 

A few benzoin derivatives decompose by photofragmentation mechanisms other than Norrish Type I. For example, a-halogen acetophenones [28], oxysulfonyl ketones [29], and sulfonyl ketones [30] sufficiently undergo p-cleavage upon UV irradiation and may be used for initiation, a-Dimethylamino-substituted benzoin undergoes both a-cleavage and P-pho-... 

Quack M, Sutcliffe E, Hackett P A and Rayner D M 1986 Molecular photofragmentation with many infrared photons. Absolute rate parameters from quantum dynamics, statistical mechanics, and direct measurement Faraday Discuss. Chem. Soc. 82 229-40... 

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. 

The mechanism for the oxidative photofragmentation of a,/ -amino alcohols is consistent with a preference for anti geometry in the cleavage step (Ci, X. Kellett, M. A. Whitten, D. G., J. Am. Chem. Soc., 1991, 113, 3893). Provide a rationalization based on the frontier orbitals of the system. 

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. 

Direct dissociation is the topic of this chapter while indirect photofragmentation will be discussed in the following chapter. Both categories are investigated with the same computational tools, namely the exact solution of the time-independent or the time-dependent Schrodinger equation. The underlying physics, however, differs drastically and requires different interpretation models. Direct dissociation is basically a classical process while indirect dissociation needs a fully quantum mechanical description. 

Roncero, O., Beswick, J.A., Halberstadt, N., Villarreal, P., and Delgado-Barrio, G. (1990). Photofragmentation of the Ne- ICL complex A three-dimensional quantum mechanical study, J. Chem. Phys. 92, 3348-3358. 

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. 

Transition metal carbonyl is important in laser chemistry as sources of metal atoms or as precursors for chemical vapor deposition (CVD) [104]. Ni(CO)4 shows a typical photofragmentation reaction initiated by the XeCl laser (308 nm, 4.03 eV), and the knowledge on the mechanism is valuable for the design and control of the laser-induced CVD. The SAC-Cl method was applied to the excitation spectrum and the potential energy curves relevant to the photofragmentation reaction [105]. 

Various kinds of the theoretical spectroscopies for the transition metal complexes were also reviewed. For the excitation spectrum of Cr02Cl2, the SAC-Cl method simulated accurate spectrum. For tetraoxo metal complexes, the systematic studies explained the spectral differences when the central metal was substituted. In the analysis of the NMR chemical shift, not only the optically allowed states but also the magnetically allowed states are important. In the molybdenum complexes, the inverse of the d-d excitation energy is proportional to the experimental chemical shift. The photofragmentation reaction of Ni(CO)4 was also studied and the reaction mechanism was clarified. 

A crucial difference between diatomic molecules and polyatomic molecules is that the density of nonfragmenting (bound) dark states can become so large that individual eigenstates can no longer be resolved. As a result, fast radiationless processes that do not involve photofragmentation (see Section 9.4.14) are the rule rather than the exception. However, most of the interpretive concepts, perturbation mechanisms, and dynamical models are well exemplified in the spectra and dynamics of diatomic molecules. 

No spectrum, no matter how simple, is dynamics-free. No dynamical process, no matter how complex, fails to reveal its essential characteristics in one or a series of well designed spectroscopic experiments. The essential unity of spectrum and dynamics, of patterns of eigenstates and dynamical mechanisms, of simple few-level perturbations and multi-continua photofragmentation processes is the subject of this book. 

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