Excited-state reactions reactivity

The basic theories of physics - classical mechanics and electromagnetism, relativity theory, quantum mechanics, statistical mechanics, quantum electrodynamics - support the theoretical apparatus which is used in molecular sciences. Quantum mechanics plays a particular role in theoretical chemistry, providing the basis for the valence theories which allow to interpret the structure of molecules and for the spectroscopic models employed in the determination of structural information from spectral patterns. Indeed, Quantum Chemistry often appears synonymous with Theoretical Chemistry it will, therefore, constitute a major part of this book series. However, the scope of the series will also include other areas of theoretical chemistry, such as mathematical chemistry (which involves the use of algebra and topology in the analysis of molecular structures and reactions) molecular mechanics, molecular dynamics and chemical thermodynamics, which play an important role in rationalizing the geometric and electronic structures of molecular assemblies and polymers, clusters and crystals surface, interface, solvent and solid-state effects excited-state dynamics, reactive collisions, and chemical reactions. 

A survey of many such reactions suggests that there is no single, simple pattern that can be used to predict the outcome of photochemical nucleophilic substitutions, but rather a situation in which oneof at least three mechanisms may operate, and this has been borne out by more detailed mechanistic studies. One approach to rationalizing the preferred orientation in the excited-state reactions is to calculate electron densities at the various ring carbon atoms for a particular pattern of substituents, and to assume that preferential attack by a nucleophile will take place at the position of lowest electron density. This static reactivity leads to the prediction that a nitro group is meta-directing for direct nucleophilic attack in the excited state,... 

Several examples of exoergic charge-transfer reactions that proceed at different rates with ground-state and electronically excited ions are listed in Table I. In some cases the cross section for the excited-state reaction may be smaller than that for the ground state, as is the case for the reactions Xe+(02, Xe)02+ Kr+(N20, Kr)N20+ Kr + (C02, Kr)COz+, whereas in other instances the excited state is more reactive, as for the processes N+(Kr,N)Kr+, N+(CO,N)CO+, 02+(Na,02)Na and 0/ (NO, 02)N0 +. The differences in reactivity are often more pronounced in the region of low ion translational energies1 lb (Fig. 10). The role of excited-state ions in charge-transfer reactions was reviewed by Hasted some time ago,175 but much more experimental data has been obtained recently, as indicated by the data shown in Table I. 

Most photochromic systems are not reversible indefinitely. However, very little careful analytical data have been accumulated to characterize the nature of the degradation products or to specify the degree of quantitative reversibility. The reasons for side reactions are inherent in the high photochemical reactivities of the systems. First of all, there must be an excited state formed by absorption this state is then transformed into other excited states or reactive species. The latter may include triplet states, carbonium ions, carbanions, free radicals, or other highly reactive intermediates. Certain of these are oxygen sensitive so that exclusion of the atmosphere and other potential sources of contaminants during irradiation is necessary. A second major route of degradation involves the excited state of the colored form which may already be... 

Different photoreactions can be initiated in structurally related complexes of a metal ion as a result of the intrinsic properties of the LMCT excited state and radical-ion pairs. The excited-state reactions of azido complexes of Co(III) are one example of this chemical diversity.106-109 Irradiation of Co///(NH3)5N2+ aqueous acidic solutions in the spectral region 214 nm < 2exc < 330 nm produces Coin(NH3)4(H20)N, 6 0.6, and Co(aq)2 +, molar ratio.93 The ammonia photoaquation has two sources that also account for the large quantum yield of the photoprocess. One source competes with the formation of Co(aq)2 + from radical-ion pairs. These pairs must be produced with a quantum yield 0.5. The second source is a process unrelated to the Co(aq)2 + production and it has a quantum yield excited state where a Co-NH3 bond has been considerably elongated and where the electronic relaxation of the excited state has been coupled with aquation. A second rationale for the large aquation quantum yield is that a reactive LF excited state is populated by the LMCT excited state. 

Photoproducts consistent with cationic reactive intermediates are also formed in the singlet excited state reaction of the allylic iodides geranyl and neryl iodide in n-hexane (equation 24)127. The favourable 2-Z geometry in neryl iodide leads to a larger proportion of the intramolecular alkylation product compared to the 1,4-HI elimination. Use of tetrahydrofuran instead of n-hexane promotes the formation of the cyclization products127, and so does the presence of Cu(I)57, which probably acts as a template. 

The reactivity of the triplet state in photoreduction reactions was used by Nakamaru et al. (1969) to investigate the triplet state basicity of acridine. It should be possible to extend this method to any compounds in which an excited state reaction is affected by protonation. 

Hydrogen abstraction by triplet carbonyl compounds has been the most widely studied excited state reaction in terms of structural variations in reactants. Consequently, the most detailed structure-reactivity relationships in photochemistry have been developed for hydrogen abstraction. These correlations derive from studies of both bimolecular reaction and intramolecular reactions. The effects of C—H bond strength and the inductive and steric effects of substituents have been analyzed. The only really quantitative comparisons between singlets and triplets and between n,n and 71,71 states have been provided by studies of photoinduced hydrogen abstractions. 

From an examination of products alone, therefore, it is not always possible to quantitatively assess the relative reactivity of the various electronic states. But when photochemical products differ from those obtained by thermal activation alone, profound effects from the redistribution of electron density can be inferred. In some cases the excited state may simply give the same product as from thermal activation of the ground state. However, when photoreaction occurs it should be realized that the conversion to product occurs within the lifetime of the excited state. Even the longest-lived excited metal complexes ate of the order of 10 3 s ( 5, j6) in lifetime and the longest-lived metal-metal bonded complex in 298 K fluid solution is of the order of -10 6 s in lifetime ( 7). Thus, excited state reactions of any kind must... 

At present it seems that before such theoretical considerations may be used in a truly predictive manner, further development and refinement of the methods are necessary. This development may be derived from consideration of (i) the reactivity of other types of states, for example, IL and CT excited state compounds (34,48) (ii) theoretical models for radiationless transitions of reactive excited state compounds (60-66) (iii) the symmetry requirements of excited state reactions (67) and (iv) correlations of photoreactivity with detailed calculations of excited state bonding properties, geometry, and so on (56, 58,68,69). The accumulation of data regarding primary photoprocesses should provide a solid basis for this maturation and also a means of evaluating more highly developed models. 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

The potential energy snrfaces for excited-state reactions are extensions of the reaction profiles nsually fonnd in the ground state (two different minima connected by a transition structure). In photochemistry, several reaction profiles are connected by a state crossing. In Fignre 2.4 we ontline two reaction profiles to introduce some important concepts in the analysis of photochemical reactivity. We also give an overview of the conclusions that can be drawn from these calculations, together with the more difficult problems that must be addressed with dynamics. 

In some instances extrusion of S02 does not occur, as with the ketosulphone (74) which fragments to yield ketene and the sulphene (75). Both the singlet and the triplet excited states are reactive. There is some doubt as to the concertedness of the process and a 1,4-biradical is proposed as an intermediate formed by C—S bond fission82. Irradiation of the sulphone (76) in acetonitrile or dichloromethane also results in the formation of a sulphene (77) which loses SO to afford the ketone (78). An alternative reaction path, that of S02 extrusion, affords a biradical from which 1,3,4,6-tetraphenylcyclohexa-l,4-diene is formed83. 

Salem5 has reported detailed calculations analysing several photochemical reactions with special reference to the way by which the excited state of the molecule decays back to the ground state. Another publication has dealt with the classification of photochemical reactions and is in part an elaboration of an earlier paper.7 Further attention has been directed at the Stern-Volmer analysis of photochemical reactions dealing with non-linearity when two excited states are reactive and one or both are quenched.8 9 A generalized treatment of the equation has resulted.10... 

Gnmwald-Winstein analysis, 505 Chromium(III) complexes doublet excited states reactions, 400 emission rules, 395 ligand field states reactivity, 397 magnetic behavior, 272 mixed-ligand reactivity, 398 nitrito... 

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