Reactions, fragmentation photochemical dissociation

For all investigated reactions, the photosubstitution quantum yield decreased significantly with increasing pressure. Under the assumption that nonradiative deactivation is relatively independent of pressure, the pressure dependence of /(l — < ) represents that of the photochemical reaction [Eq. (27)]. The positive volumes of activation fit well into the picture of a dissociative mechanism, that is, release of CO. This model cannot account for the observed trends in AF ( /(1 — < )) especially as a function of solvent. For this reason, a second way to account for the observed data was presented [100] according to which CO dissociation leads to a trigonal bipyramidal M(CO)5 fragment with dissociated CO within the solvent cage. The latter species can either recombine with CO, be trapped by solvent, or bind to the nucleophile L, which results in a competition between these reaction paths. The difference in the pressure dependence for the recombination with CO or combination with L can be used to account for the observed activation volumes. 

Reactions that proceed photochemically do not necessarily involve observations of an excited state. Long before observations are made, the excited state may have dissociated to other fragments, such as free radicals. That is, the lifetime of many excited states is shorter than the laser excitation pulse. This statement was implied, for example, by reactions (11-46) and (11-47). In these systems one can explore the kinetics of the subsequent reactions of iodine atoms and of Mn(CO)s, a 17-electron radical. For instance, one can study... 

Lastly, it is appropriate to comment on the relationships between the intermediates seen in photochemical studies and possible reactive intermediates along the reaction coordinates of related thermal transformations. Earlier kinetics studies (] 3) of the reactions of Ru3(CO)i2 with various phosphorous ligands PR3 have found evidence for both first order and second order pathways leading to substitution plus some cluster fragmentation. The first order path was proposed to proceed via reversible CO dissociation to give an intermediate analogous to II. 

Secondary rearrangements apparent isomerizations through radical recombination reactions. In the rearrangement reactions considered so far, the isomerization step is the primary photochemical process, except when a biradical is formed as an intermediate for in that case the primary photochemical process is really a dissociation, even though the fragments cannot separate. There are however cases of overall isomerizations which result from the recombinations of separated free radicals formed through a process of photodissociation. The photo-Fries reaction is an important example of this mechanism, and is illustrated in Figure 4.43. 

Because of the rapidity with which the first-formed excited states are usually converted to the Sx or Tx state, most photochemical reactions start from these states. There are exceptions. An obvious one is when an upper dissociative excited state, in which the molecule immediately fragments, is populated.49 Other exceptions occur when a molecule contains two different chromophores. For example, 20 reacts analogously to franj-stilbene (21) from its state but also undergoes photoreduction, a process typical of an n,n state (see p. 719).60... 

The first step in most photochemical reactions involves a transition of high probability (an optically allowed transition) from the ground state to an electronically excited state of a reactant species the excited state either initiates the observed chemical change or spontaneously dissociates into fragments which initiate those chemical changes. 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

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