Photofragmentation, direct

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. 

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... 

Doubt (75ZN(B)822) has been cast on a number of claims for the formation of 2-azetin-4-ones from cycloaddition of activated isocyanates to acetylenes (70TL119). The simple 2-azetin-4-one (246) was not isolated or even detected directly at -50 °C in the photofragmentation of compound (245), but indirect evidence for its formation was the isolation of adducts (248 X = MeO, MeNH) in the presence of methanol or methylamine (75TL1335). The most convincing evidence for an isolable 2-azetin-4-one involves treatment of the... 

The information available is discussed in light of the effects of excitation energy and the environment on the photofragmentation process of several transition metal cluster complexes. The photochemical information provides a data base directly relevant to electronic structure theories currently used to understand and predict properties of transition metal complexes (1,18,19). 

The most detailed possible photofragmentation cross section is the detailed final-state resolved differential photofragmentation cross section defined in Eq. (4.12), which measures the probability of the formation of a particular final state, vj,m.j scattered into a specified scattering direction, k = 0, 4>j. This cross section has been discussed in Ref. 80 in the context of time-independent... 

The direct determination of the rates of elementary photofragmentation reactions. These studies have provided information concerning the... 

Direct Determination of Rates of Elementary Photofragmentation Reactions. These studies have provided information concerning the molecular features that determine, for example, how energy is distributed among the products of a reaction, and they provide hints concerning when it is possible to achieve mode-selective dynamics in large molecules. 

Photofragmentation takes place either via a direct process... 

In indirect photofragmentation, on the other hand, a potential barrier or some other dynamical force hinders direct fragmentation of the excited complex and the lifetime amounts to at least several internal vibrational periods. The photodissociation of CH3ONO via the 51 state is a representative example. The middle part of Figure 1.11 shows the corresponding PES. Before CH30N0(5i) breaks apart it first performs several vibrations within the shallow well before a sufficient amount of energy is transferred from the N-0 vibrational bond to the O-N dissociation mode, which is necessary to surpass the small barrier. 

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. 

In contrast to indirect dissociation, which is the topic of Chapter 7, direct photodissociation is relatively simple to understand. The reflection principle describes qualitatively the fully state-resolved photofragmentation cross sections a E, n, j) as a multi-dimensional mapping of the initial coordinate distribution in the electronic ground state ... 

Fig. 7.7. Comparison of the different energy behavior of partial dissociation cross sections a(E,j) for the production of NO(j) in indirect, HONO(iS i), and in direct, ClNO(Si), photofragmentation. Note the quite different energy scales The results for HONO are obtained from a two-dimensional model (Schinke, Untch, Suter, and Huber 1991) and the cross sections for C1NO are taken from a three-dimensional wavepacket calculation (Untch, Weide, and Schinke 1991b).

There are two general classes of photoinitiators (1) those that undergo direct photofragmentation on exposure to uv or visible light irradiation and produce active free radical intermediates and (2) those that undergo electron transfer followed by proton transfer to form a free radical species. The choice of photoinitiator is determined by the radiation source, the film thickness, the pigmentation, and the types of base resin employed. Examples of typical photoinitiator systems used to cure reactive resins are shown in Table 14.2. Benzophenone is perhaps one of the most common photoinitiators. 

In direct photofragmentation, the thioxanthone derivative absorbs light at about 380 nm which results in homolytlc cleavage of the methylene-halogen or sul-fonyl-halogen bonds of the parent molecule to produce... 

Like alkenes (Section 6.1.1), chromophores containing the N=N (azo compounds) or C=N (imines, oximes, etc.) bonds can undergo E Z (or trans cis) photoisomerization (Scheme 6.156) and the resulting isomer concentration ratio in the photostationary state (PSS) reflects the absorption properties of the isomers and isomerization quantum yields (see Scheme 6.1 in Section 6.1.1). Since conventional (dark) synthesis generally provides access to more stable E-isomers, photochemistry is an exceptional tool for preparing sterically hindered Z-isomers.1061,1062 The photoisomerization reaction can be induced by a direct irradiation or by sensitization and it often competes with other phototransformations, such as photofragmentation or photorearrangement (Section 6.4.2). 

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