Photochemical transformation mechanism

It was mentioned earlier that the photochemical threshold for the radical decomposition process (eq. 22) is at 325 nm, 

A mechanism for the 325 to 360-nm region (up to the 2 41 transition) should be consistent with the following observations on H2CO (1) the value of rp decreases from 82 ns for the 41-level to 11 ns for the 2 4 level (164) (2) no laser-induced phosphorescence emission has been observed.by exciting into the A 2 X Ap system as well as into the aJA2 X A- system 

- Ti process. This does not rule out the possibility of the Ti- Sq process followed by the molecular elimination process (II) as proposed earlier (162) when Tp is produced at low energies by triplet benzene sensitization. 

Now it is possible to consider the mechanism of the molecular elimination process (eq. 23) in light of a new isomeric intermediate, hydroxymethylene (hCoH). In the low-temperature matrix photolysis of formaldehyde at 15 K, three sets of photochemical products were found (1) methanol and CO in solid formaldehyde (2) glycoaldehyde in a dilute Ar matrix and (3) hydroxyketene in a CO matrix (227). These observations were 

This carbene could give rise to the H2 and CO via a 1,2-elimination, 

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The formation of an olefin by the Norrish type II photoelimination was not much affected by the presence of an unconjugated olefinic group, but the formation of propylene (and acrolein) in 5-hexenal requires a new photochemical transformation mechanism. It has been suggested that an olefinic, triplet precursor which can undergo intramolecular H-atom abstraction reaction is responsible for this process. 

Interest in the mechanism and product distribution of thermal and photochemical transformations of aryl azides led to the isolation of some nitrogen-containing derivatives of heptafulvalene. Based on elemental analysis and spectroscopic data it has been suggested tentatively that the compound isolated following vapor-phase pyrolysis of azidopentafluoro-... 

The simple photochemical transformation of o-azidobiphenyls into carbazoles is preparatively useful. Considerable effort has been expended on study " of the mechanism of this process. Both carbazole and dimerized nitrene, for example 257, can be produced in the process in... 

In situ measurements of stratospheric reactive trace gas abundances provide an opportunity to test the fundamental photochemical mechanisms (3). The advantage of such measurements is that they are local, so the simultaneous measurements of trace gases place a true constraint on the possible photochemical mechanisms. These measurements are also able to resolve small-scale spatial and temporal structure in the trace constituent fields. The disadvantage of in situ measurements is that they do not capture the global or perhaps even seasonal view of photochemical transformations because they are seldom done frequently enough or in enough places to provide that information. Another disadvantage of in situ measurements is that they must be made from platforms in the stratosphere, and these remote observational outposts have their liabilities. 

The mechanism of photochemical transformations has been the subject of many articles and monographs and will be discussed only briefly. 

One way to generate surfaces is by explicit QM calculation of species as they are followed through some mechanism. SCF-CI calculations have proven of considerable value in the author s research. The philosophy here has been to include as basis orbitals only those atomic and hybrid orbitals which are part of chromo-phores or make up bonds which are altered, broken, formed or modified, during the photochemical transformation. Additionally, basis orbitals aimed along the directions of bonds are used, since then the SCF wavefunctions are linear combinations of recognizable orbitals of bonds rather then arbitrary vertically and horizontally oriented atomic orbitals. 

Photochromic l-acetoxy-2-piperidino- and l-acetoxy-2-dimethylamino-9,10-anthraquinone also manifested photodegradation.28 The mechanism of these irreversible photochemical transformations includes intermediate 1,10-anthraquinone, which exhibited the photochemical transformations well known for anthraquinones with dialkylamino substituents adjacent to the carbonyl group. 

Detailed analytical and mechanistic studies are useful for a better understanding of all the reported processes. They have been the subject of several recent publications, and the main results are presented in the subsequent sections. Knowledge of the mechanisms is useful to assess photochemical transformation under environmental conditions. 

Recent work on the mechanisms of the photohydrodehalogenation of haloarenes, with an emphasis on polyhaloarenes, and the related mechanisms of phototransformations of aliphatic halocom-pounds are reviewed. Attention is focused on the nature of the excimer in the photochemical transformations of haloarenes without additional electron transfer reagent and on the nature of the exciplex formed in phototransformations in the presence of electron transfer reagent. Applications of surface catalyzed photochemical transformations and photohydrodehalogenation of polyhaloarenes to toxic waste disposal is discussed. 

With the exception of photodissociation to radical ions observed for retinol in polar solvents (144), cis-trans isomerization is the major photochemical transformation undergone by all forms of the free retinyl-polyene chromophore. [Unidentified photochemical damage has been reported to occur with very low quantum yields, e.g., 0.04 in the case of all-trans retinal (177).] We shall subsequently see that critical comparisons between the photochemical behavior of the biopigments and that of the opsin-free chromophore have led to the conclusion that the protein moiety plays a major role in governing the photochemical mechanism in rhodopsin (176). It is, therefore, natural that in parallel to spectroscopic and theoretical investigations, considerable attention has been devoted to the photoisomerization of model compounds, particularly to that of retinal isomers. 

The excited states involved in the mechanisms of the photochemical transformations of the 2-alkylidenecyclo-butanones were elucidated without special difficulty (vide supra). These a, B-unsaturated ketones undergo only one of the reactions characteristic of cyclobutanones (i.e., ring expansion). In addition, the triplet energy of the enone chromophore is low enough that this excited state may be efficiently and selectively populated by standard carbonyl triplet sensitizers (e.g., acetophenone, xanthone, and benzophenone), thereby demonstrating that ring expansion occurs via the 2-alkylidenecyclobutanone state, while the isomerization process (i.e., [81]J[82]) occurs via T. ... 

Schuster, D. I. Mechanisms of photochemical transformations of cross-conjugated cyclohexadienones, Acc. Chem. Res. 1978,11, 65-73. 

Relevant to the mechanism of electroreduction of 5-halogenouracils are reported results on the photochemical transformation of these compounds in aqueous medium -124)... 

Acheson process, 566 Acid-initiated ring-opening polymerization comparison widi base-initiated process, 79 kinetics, 79-80 mechanism, 79-81 rate retardation by water, 81 relative reactivities of cyclosiloxanes, 79 step-growth process, 79 Acyclic oligosilanes, photochemical transformations, 432/-433/ Acylsilanes... 

The CIS stereochemistry of the products of the photochemical transformation is deduced from the relative inertness of the dienic system toward maleic anhydride. This cis stereochemistry would also be expected from the proposed mechanism which requires hydrogen transfer in the six-center transition state arising from the intermediate 7-cis isomer of the starting material. 

The mechanism of the photochemical transformation of compounds of the dehydro- 3-ionone group is explained in the same way as for compounds of the 8-ionone series itself. 

The photochemical transformation of 3,4-epoxy-2-methyl-l-phenylbutan-1-ones 142 proceeds by the same mechanism, but the configuration of the end-products is regulated by the spatial requirements of the substitutents on Ci and... 

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