Photochemistry competing reaction with

Electron-poor olefins with higher oxidation potentials may decrease the rate of electron transfer and other processes competing for deactivation of the iminium salt excited states may increase. Alternate reaction pathways involving olefin-arene 2 + 2 cycloaddition may take place in the photochemistry of 133 with electron-poor olefins (equation 62)120,121. 

In addition to the limitations enumerated above, which are inherent in the photochemistry, there are other side reactions of a free-radical nature which may compete seriously with the desired reaction. It is a simple matter to determine which of the products are derived from these reactions as they can be formed in the dark, using free-radical chain initiators. For example, chain reactions where the propagating steps are of the following type are fairly general. 

The secondary electron-transfer processes, often used in mechanistic in estiga-tions and in preparative applications of electron-transfer photochemistry, enhance the quantum yields of product formation [167], In fact, as we have already pointed out in a previous section, the efficiency of separation of the geminate pair is strictly dependent on the redox potentials (see also indirect photooxygenation processes) [43, 50, 80-83, 135], Anyway, although in the present case the subsequent electron-transfer from epoxide to biphenyl radical cation BP is endothermic enough, in the absence of very fast competing reactions this primary radical cation may still undergo an endothermic electron-transfer process with epoxides. 

As already mentioned in the previous section, also the frmda-mental laws of spin conservation may completely close or at least slow down certain reaction channels. ISC and spin inversion thus can strongly influence the balance between competing processes with a different regio- and stereoselectivity (5). While such effects are very common in metalloenz5une redox catalysis, their rational exploitation in bioinorganic photochemistry and photocatalysis is still in its infancy 3,6). 

The photochemistry of three thioxanthone-initiator dyads with carbonyl functions in the initiator moieties was compared by CIDNP and complementary techniques. Expectedly, the triplet-triplet energy gap determines the readiness of the molecule to undergo a-cleavage after intramolecular energy transfer. However, intramolecular electron transfer can also occur as a competing reaction and leads to dehydrogenation of the initiator part. 

To ascertain the magnitude and the mechanism of a photochemical reaction, it is necessary to determine the rate of the process (see eq. 1). In classical photochemistry, reactions which are very slow are usually considered to be unimportant and hence of little interest. In the marine environment very slow reactions can be significant if they are the only operating mechanism for a particular process or if they compete favorably with other abiotic or biotic mechanisms contributing to the same process. Remineralization of organic matter, for example, is generally attributed solely to biological routes, and for those compounds readily utilized by the biota... 

The BOVB method does not of course aim to compete with the standard ab initio methods. BOVB has its specific domain. It serves as an interface between the quantitative rigor of today s capabilities and the traditional qualitative matrix of concepts of chemistry. As such, it has been mainly devised as a tool for computing diabatic states, with applications to chemical dynamics, chemical reactivity with the VB correlation diagrams, photochemistry, resonance concepts in organic chemistry, reaction mechanisms, and more generally all cases where a valence bond reading of the wave function or the properties of one particular VB structure are desirable in order to understand better the nature of an electronic state. The method has passed its first tests of credibility and is now facing a wide field of future applications. 

When one takes up a polyfunctional molecule which contains at least one carbonyl group, one knows which reactions of that carbonyl group are possible. One does not know how well these reactions will compete with various physical and chemical interactions of that excited carbonyl group with other functional groups in the molecule. It is this aspect of carbonyl photochemistry which requires and deserves extensive future research. The uniquely well understood chemistry of the carbonyl group can serve as a monitor for studying interactions in electronically excited polyfunctional molecules. 

Atmospheric chemical processes are dominated by photochemistry biota, which compete with photochemistry as agents of chemical transformation in surface waters, are few in the air. Also, light intensities in the atmosphere are higher than those in surface waters, and the presence of ultraviolet light of shorter wavelengths contributes to an expanded suite of photochemical reactions that may occur (e.g., the photodegradation of CFCs in the stratosphere). 

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