Photochemical and thermal dissociation

The fraction of radical reactions in the cage is not necessarily the same for photochemical and thermal dissociation of the initiator. Photochemically generated radicals are usually excited and so they have less tendency to produce stable molecules. They may be flung away from each other by excess energy at the moment of generation, and the probability of mutual collision is reduced [124]. 

Photochemical and Thermal Dissociation Synthesis of Kiypton Difluoride... 

Especially detailed photoactivation studies have been carried out on the dissociation of NO2 and on the isomerization of cycloheptatriene. As well as the fluorescence experiments referred to in Section 1.3.4, the photodissociation of NO2 has been studied at selected wavelengths below the photochemical threshold at 397.9 nm. The photochemistry of NO2 is complicated by the profusion of low-lying excited states. It appears that it is a state that is primarily excited, but that these molecules rapidly decay into a dense manifold of levels which are strongly mixed with the ground state.Although this complicates some issues, it means that photochemical and thermal dissociation can be directly compared, and the system serves as a prototype of a small molecule decomposing into two free-radical fragments. 

Examples of silver(l) alkyl and alkenyl (including aryl) complexes have been known from as early as 1941 6-9 however, the number of examples is fairly limited with respect to that of the heavier congeners, copper(l) and gold(l). Such a phenomenon can readily be attributed to the relatively low stability of this class of complexes, both photochemically and thermally. Simple homoleptic alkyl and alkenyl complexes of silver(i) are known to be very unstable under ambient temperature and light, and successful isolation of this class is fairly limited and mainly confined to those involving perfluoroorganics.10 The structures and the metal-carbon bond-dissociation energies for... 

Comments on the thermal nitration of enol silyl ethers with TNM. The strikingly similar color changes that accompany the photochemical and thermal nitration of various enol silyl ethers in Table 2 indicates that the preequilibrium [D, A] complex in equation (15) is common to both processes. Moreover, the formation of the same a-nitroketones from the thermal and photochemical nitrations suggests that intermediates leading to thermal nitration are similar to those derived from photochemical nitration. Accordingly, the differences in the qualitative rates of thermal nitrations are best reconciled on the basis of the donor strengths of various ESEs toward TNM as a weak oxidant in the rate-limiting dissociative thermal electron transfer (kET), as described in Scheme 4.40... 

A volume profile for the reactions in Eq. (35) was constructed [119] with the aid of partial molar volume measurements on both the isomers, and is presented in Figure 27. The profile clearly demonstrates the dissociative nature of both the photochemical and thermal isomerization reactions, the large difference in the partial molar volume of the transition states being the effect of electrostriction and the difference in partial molar volume between Co11 and Co111. 

Although these compounds absorb more strongly in the near-UV than most of the other aromatic photoinitiators, their use as photoinitiators is limited, as they are thermally not very stable. The relatively weak N-O bond dissociates both photochemically and thermally at moderate temperatures. 

Photochemical and thermal conversions based on [Ru(CNMe)6] are summarized in Scheme 1. All the substitutions in this Scheme are believed to go dissociatively, through transient five-coordinate intermediates. Acetonitrile exchange occurs at [Ru(f) -CjH6)(MeCN)3] by an interchange mechanism, but at [Ru( -C5H5)(MeCN)3] by a limiting dis-... 

As an alternative to electrochemical or radiolytic initiation, homolytic dediazoniation reaction products can be obtained photolytically. The organic chemistry of such photolyses of arenediazonium salts will be discussed with regard to mechanisms, products, and applications in Section 10.13. In the present section photochemical investigations are only considered from the standpoint that the photolytic generation of aryldiazenyl radicals became the most effective method for investigating the mechanisms of all types of homolytic dediazoniations —thermal and photolytic —in particular for elucidating the structure and the dissociation of the diazenyl radicals. 

The concentration of bromine atoms in the dark reaction is known from the concentration of molecular bromine and the thermal dissociation constant of bromine, hence that prevailing at any stage of the photochemical reaction is found. 

Since the photochemical quantum yields for acetone (as well as for biacetyl) increase with increase in temperature it is logical to say that the triplet state because of its long lifetime is subject to a thermal dissociation with an activation energy. Due to the complexity of the mechanism an unambiguous determination of this activation energy is difficult. The value for biacetyl is about 16 2 kcal50 and the most recent determination for acetone indicates a probable similar value64. 

Since many substitution reactions of (1) are done under thermal or photolytic conditions where Re-Re bond rupture may occur, Re(CO)s was first regarded as an intermediate in both types of substitution reactions. However, the mechanistic studies of photochemically induced substitutions suggest that (1) photochemical substitution may not occur through the monomeric M(CO)5 but from the longer-lived primary photointermediate M2(CO)9 (equation 2) (2) both primary photointermediates may give the same reaction products and (3) substitution of CO in M(CO)5 is most probably associative and not dissociative as in most substitution reactions of 17-... 

The thermal, photochemical and Lewis acid catalysed dissociations of di-azabicyclo[2.2.1]heptenes with side chains of various lengths terminated by aldehyde groups 143 have also been examined [119] (Scheme 31). The course of the reaction depended on the length of the tether. For the precursor with n = 1, tetrahydroindenol 144 was the main isolated product. Thermolysis of... 

Hydrogen is also formed in large quantities as a byproduct in petrochemical processes, refineries, coking plants (coke oven gas) and in chemical and electrochemical processes e.g. chloralkali-electrolysis. Other processes such as the photochemical production of hydrogen or thermal dissociation of water are only used in special applications and are currently industrially unimportant. 

The various methods of generating o-quinone methides,4-5 including the thermal or (Lewis) acid-catalyzed elimination of a phenol Mannich base,149 150-160-161163 the thermal or (Lewis) acid-catalyzed dehydration of an o-hydroxybenzyl alcohol (ether),147-149-151-153-156-157-162-163-165-168 171-175-178-183 the thermal 1,5-hydride shift of an o-hydroxy styrene,171-173 175 178-183 the thermal dissociation of the corresponding spirochromane dimer,158 163-164,166 oxidation of substituted o-alkylphenols,152-170 and the thermal or photochemical-promoted cheletropic extrusion154-155 159 of carbon monoxide, carbon dioxide, or sulfur dioxide (Scheme 7-III), as well as their subsequent in situ participation in regiospecific, intermolecular [4 + 2] cycloadditions with simple olefins and acetylenes,147 149-151 152 153159 162-164... 

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