Photochemical reactions primary processes

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

In the mechanism of a photochemical reaction, at least one step involves photons. The most important such step is a reaction in which the absorption of light (ultraviolet or visible) provides a reactive intermediate by activating a molecule or atom. The mechanism is usually divided into primary photochemical steps and secondary processes that are initiated by the primary steps. 

It is the same rate as given in thermal reaction given by equation (3.31). Thus, both in thermal and photochemical reactions, the primary process is the dissociation of Br2 molecules. In spite of the chain mechanism, the quantum yield of H2 and Br2 reaction is very small, i.e. about 0.01 at ordinary temperature, although it increases as the temperature is raised. The reason is that the reaction immediately after initiation following the primary state, i.e. 

Describe the primary process in photochemical reactions with help of suitable examples. 

In special cases, other photochemical reactions can totally suppress an acyl group migration.76,117 The primary photochemical process of 195 is fast equilibration with trans-isomer 196. The crr-isomer 195 then cyclizes in 65% yield to dehydroaporphane (197) in a similar way as does stilbene and its derivatives.118 7>a j-isomer 196 cyclizes in 10-21% yield to dehydroproto-berberine (198).76 The latter reaction is analogous to the already discussed cyclization 110 -> 112 and 134 -> 131 of aromatic carbonates and carbamates. 

The net result of a photochemical redox reaction often gives very little information on the quantum yield of the primary electron transfer reaction since this is in many cases compensated by reverse electron transfer between the primary reaction products. This is equally so in homogeneous as well as in heterogeneous reactions. While the reverse process in homogeneous reactions can only by suppressed by consecutive irreversible chemical steps, one has a chance of preventing the reverse reaction in heterogeneous electron transfer processes by applying suitable electric fields. We shall see that this can best be done with semiconductor or insulator electrodes and that there it is possible to study photochemical primary processes with the help of such electrochemical techniques 5-G>7>. 

The triplet-triplet quenching mechanism implying as it does, the concentration of the energy from two separately absorbed quanta on a single molecular species, should also be considered as a possible participant in the primary processes of photosynthesis, where just such a concentration of quanta is necessary to provide the energy required for the photochemical reactions. 

These rules also predict the nature of photoproducts expected in a metal-sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation. 

Photochemical cis-trans isomerization in a conjugated polyene system is thought to be the crucial primary process in vision. The visual pigment (rhodopsin) is derived from 11 -crs-retinal by reaction of the aldehyde group with an amino substituent in a protein (opsin). There is considerable distortion in the geometry of this chromophoric group anyway, because of the spatial requirements of the protein... 

Photolysis at 1849 A produces H atoms with about 2.27 eV excess kinetic energy. The primary process is most likely the production of H + Cl with a quantum yield of unity since the absorption is continuous. The Cl atoms are in the 2P312 state [Mulliken (725)]. The photochemical reactions expected are similar to those of HI [Wilson and Armstrong (1051)]. 

In Figure 4.4 for example, the direct reaction from R to P would be a non-adiabatic process. Although there is no simple and general answer to this question, most primary photochemical reactions can be considered to be adiabatic when the primary photoproduct (PPP) retains a large part of the excitation energy. In some cases this is fairly obvious, when the photoproduct is formed in an excited state for instance in a reversible proton transfer reaction (see section 4.3). 

In a thermal reaction R—>TS—>P, as shown in Figure 4.4, the transition state TS is reached through thermal activation, so that the general observation is that the rates of thermal reactions increase with temperature. The same is in fact true of many photochemical reactions when they are essentially adiabatic, for the primary photochemical process is then a thermally activated reaction of the excited reactant R. A non-adiabatic reaction such as R - (TS) —> P is in principle temperature independent and can be considered as a type of non-radiative transition from a state R to a state P of lower energy, for example in some reactions of isomerization (see section 4.4.2). 

Stereo-isomerizations are quite common photochemical processes with unsaturated organic molecules (the primary photochemical reaction of vision is of this type). 

Photochemical substitution reactions can proceed through high-energy products such as radical ions, the primary process being a dissociation or an ionization of the excited molecule. Such processes do not have to follow the orientation rules dictated by the charge distribution of the excited molecule, and in many instances the product distribution is still little understood. 

A summary of the major chemical reactions of free radicals is given in Table 4.3. Broadly speaking these can be classified as unimolecular reactions of dissociations and isomerizations, and bimolecular reactions of additions, disproportionations, substitutions, etc. The complexity of many photochemical reactions stems in fact from these free radical reactions, for a single species formed in a simple primary process can lead to a variety of final products. 

Although the picture of the photochemical primary processes in cyclopentanone which has been presented seems self-consistent, a number of minor points still have to be explained. These are (a) the dependence of the ratio of ethylene to cyclobutane on the geometry of the system (6) the puzzling fact that a constant fraction, between 2/10 and 3/10, of the initially excited molecules seem to return to the ground state without decomposition, by a route that is virtually unaffected by pressure. Before this can be explained it is necessary to confirm the value for the quantum yield for decomposition and (c) the fact that 2.5 kcal./mole of energy affects the reaction path profoundly. In the ground state the enthalpies of 2 and 3 differ by 19 kcal./mole at 25° while 3 and 4 may be estimated to differ by 15 to 20 kcal./mole. This phenomenon may be explained when a clear understanding of the excited state of the molecule is obtained. 

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