Primary photochemical steps

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. 

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

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. 

The kinetics of the thermally induced homogeneous decomposition of phosphine (PH3) have not yet been studied. The species PH2, PH and P2 are formed on flash photolysis of PH3 and could be identified by their absorption spectra63. There are proposals as to the mechanism of the consecutive process after the photochemical primary step, but nothing is known about the kinetic parameters of these reactions. With arsine and antimony hydride only the heterogeneous decomposition has been studied64,65. 

Because of the paucity of rate data for the reactions of F and FO one can only speculate on the course of the reaction subsequent to the initial dissociation. The results of photochemical studies398,399 give some guidance. The quantum yield F2o of photodecomposition is 1.0 at 3650 A, independent of temperature in the range 15-45 °C, pressure of F20 and pressure of oxygen398 the primary step is almost certainly as in (2)398,3". Thus, at room temperature at least, any contribution from... 

In 1912, Einstein extended the concept of quantum theory of radiation to photochemical processes and stated that each quantum of radiation absorbed by molecule activates one molecule in the primary step of a photochemical process . This is known as Einstein law of photochemical equivalence. 

The results suggest the possible addition reaction of O2 to a diborane-type intermediate in the primary step. Despite the fact that B2H6 is the simplest boron-hydride, there are a number of unanswered questions regarding the photochemical behavior of this molecule. 

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

Further information on this system is available from studies directed at photochemical isotope enrichment (16). In this work a mercury resonance lamp containing only Hg19S was used as a source. A flowing mixture of natural mercury and water vapor exposed to the Hg198 fine structure component of the mercury resonance radiation (2537 A.) was found to result in HgO considerably enriched in Hg198. It was concluded that this could only occur if Hg(3Pj) atoms reacted in a primary step to form either a compound which is removed from further contact with the reaction or which itself may react further but must not regenerate free Hg. Either reaction (55) or (56) would satisfy these conditions. If reaction (55) is the primary reaction, the further reaction... 

A photochemical cyclization involving a phosphorus atom occurs in the reaction of diethyl 8-bromo-2, 3, 0-isopropylideneadenosine 5 -phosphite (212) upon irradiation in acetonitrile610. The primary step is homolysis of the C—Br bond and this is followed by intramolecular attack of the adeninyl 8-radical on the phosphite group with the formation... 

Photochemical initiation has often been used as an excellent method of studying radical and chain reactions.1 2 The primary step in many systems is followed by a sequence of steps, which may include conventional unimolecular processes of species having known or calculable energy. Examples are numerous and well known. In order to understand such systems, whether reaction is initiated photochemical ly or thermally, the typical characteristics of unimolecular reactions and their dependence on the energy parameters of the systems and on molecular structure must be clarified. This is the purpose of the present chapter, which will deal principally with the smaller hydrocarbon species below C6. 

Absorption of radiation energy may lead to dissociation of the absorbing molecule. In fact, in most of photochemical reactions involving molecules, the primary step is usually dissociation of some molecules into atoms, simple molecules or free radicals, which by further interaction either with each other or with different molecules continue the reaction sequence. The primary photochemical stage is dissociation. The secondary reaction proceeds by thermal means. 

Examples of well-known photochemical reactions which involve electron transfer include the primary step in plant and bacterial photosynthesis [2], the photoreduction of ketones by amines [3], a series of sensitized isomerizations of olefins and small ring compounds such as cyclopropanes or of strained polycyclics such as quadricyclane to norbornadiene or Dewar benzenes to benzenes [4], and the reactions of electron-rich substrates in the presence of oxygen which proceed via superoxide [5]. These reactions and others have proved valuable for synthetic applications in addition to their fundamental interest to photochemists. 

The absorption of light by a substance causes the formation of excited-state molecules. This excitation is followed by various elementary transformations which eventually lead to the deactivation or to the disappearance of those excited molecules. The absorption of light as well as each one of the elementary transformations of the original molecule in an excited state is a primary step. Specifically, a primary step may be (a) a transformation of the excited molecule into a different chemical species, as in steps 24, 15, and 14 of Figure 1, or (b) a radiative or nonradiative transition between different energy levels of the molecule, e.g., steps 02, 21, 22, 23, 13, 11, and 16 of Figure 1. Those corresponding to (a) are photochemical primary steps, while those of (b) are photophysical primary steps. 

Even so, the distinction between the two is sometimes a more subtle matter. Thus, in a photoisomerization a common excited state intermediate may undergo a transformation to either of the two isomeric cis-trans species of a planar ground-state molecule. These two transformations are virtually identical in nature, yet one leads back to the original species and is therefore a photophysical primary step, e.g., internal conversion or intersystem crossing, while the other leads to the chemically distinct isomer and should be called a photochemical primary step. As another example, the distinction between the formation of an excimer and of a photodimer lies in the instability and stability, respectively, of the dimeric species in the ground state. Excimer formation is usually considered as photophysical and photodimer formation as photochemical. These examples show that the classification of steps as photochemical and photophysical is in some cases arbitrary. 

Final photophysical primary steps and photochemical primary steps cannot be followed by any other primary steps, but only by physical and/or chemical transformations of molecules other than the original excited molecules, as will be discussed in Sections III.A.3. and III.C. 

As we have seen above, a photochemical primary step can itself be followed only by transformations of other than the original molecules. It follows, therefore, that a photochemical primary step can only be part of a primary process if it is the last step in the sequence. Such a primary process, ending on a photochemical primary step, is a photochemical primary process. A primary process which does not end with a photochemical primary step (and thus does not contain any such step in the sequence) is a photophysical primary process. Those which end with a final photophysical primary step are final photophysical primary processes. In Figure 1, sequences 02-24, 02-23-15, and 02-23-14 represent photochemical primary processes, while sequences 02 (absorption alone or absorption followed by vibrational relaxation as appropriate to the medium),... 

It is also possible for an excited molecule to be formed in a photochemical primary step. It could then also undergo subsequent physical transformations. In Figure 3, the excited molecule A dissociates to give two species, one a stable product, B, and the other an excited molecule, G, in the photochemical primary step 35. then may follow any of the paths b, c, and d, of which b and d are physical and c is chemical. These three steps are secondary steps, but not of the usual kind. Although examples of this behavior are rare, they may perhaps not remain so. It is, however, difficult to distinguish this situation from the case that G is first formed and then excited by energy transfer. 

PHOTOCHEMICAL PRIMARY PROCESS a primary process which ends with a photochemical primary step. 

Rosenbergdeduced Z)(CH2 -H)>71 kcal from a study of the mechanism of the photochemical decomposition of ketene, the primary step in which is the formation of methylene and carbon monoxide. They concluded that Z)(GH2-H) is probably about 80 kcal, and if they are correct, then Z)(GH -H) must be greater than 130 kcal, if Lq == 170 kcal, a result which of course is even more difficult to fit to the electron impact evidence. 

The photochemical oxidation of SO2 was studied by several workers in the 1950 s. As the work has been reviewed by Leighton, we will only summarize the more important results. The product of the photo-oxidation is SO3, or, if water is present, H2SO4. Various workers have obtained quantum yields of SO3 ranging from 0.3 to 0.003. As the photo-oxidation is initiated by light of insuflfi-cient energy to break the OS-0 bond, it seems certain that the primary step is the formation of excited SO2 molecules. Presumably these excited species then react with O2 to form a peroxide which subsequently decomposes by various steps, e.g. 

Diels-Alder reactions of Ceo are generally believed to proceed via a thermally allowed concerted (suprafacial) process or a photochemical concerted (antarafacial) process [283-286]. However, an alternative stepwise (open-shell) mechanism for the Diels-Alder reaction has recently merited increasing attention [287-294], Along this line several reports describe an electron transfer with the formation of radical ion pairs as primary step of the Diels-Alder reactions, followed by a stepwise bond formation [295-301], The photochemical Diels Alder reaction of Ceo with an-... 

Figure 36. The photochemically induced oxidative regeneration of NAD(P)+ cofactors (A) when the primary step involves the reductive quenching of the photoexcited dye by NAD(P)H, and (B) when the primary step involves the oxidative quenching of the photoexcited dye by an electron acceptor.

Nicholson was the first to observe a correlation between the cracking pattern and the photochemical behaviour of the aliphatic ketones. Sharkey et observed the rearrangement peaks of simple ketone molecule-ions in the electron impact investigation of ketones with a hydrogen atom on the y-carbon atom however, such peaks could not be detected when ketones having no y-hydrogens were irradiated. The same simple ketones were formed in primary step II of the photolysis. 

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