Reactions of excited molecules

Weller A 1961 Fast reactions of excited molecules Progress in Reaction Kinetics (Oxford Pergamon) pp 187-214... 

Weller A (1961) Fast reactions of excited molecules. Prog React Kinet 1 189-214... 

Photolysis of CO has been conducted18-20 only at wavelengths greater than the threshold for dissociation ( 1100 A) where reactions of excited molecules... 

Weller A. (1961) Fast Reactions of Excited Molecules, Prog. React. Kinetics 1, 189-214. 

In the following we shall first discuss the theoretical basis for redox reactions of excited molecules at electrodes in comparison with such reactions in the ground state. We shall then present and discuss some typical examples for the best studied types of reactions in the case of semiconductor and insulator electrodes. Since the properties of these two materials are sufficiently different to need different techniques for the investigation and rather distinct models for the theoretical interpretation, we shall deal with reactions in the case of semiconductors and insulators in separate chapters. 

Electrochemistry is the study of chemical reactions of molecules at electrodes. Photoelectrochemistry is the study of the reactions of excited molecules or photoproducts at an electrode, or the reactions of ground state molecules at electronically excited electrodes. 

These reactions have a characteristic free energy which implies the minimal voltage required. As discussed in section 4.1, an excited molecule is at the same time more easily oxidized and reduced than the ground state species. Reactions of excited molecules at electrodes are however practically unknown because their short lifetimes preclude the contact with the electrode when irradiation takes place in the bulk of the liquid. In practice the photoelectro-chemical reactions at non-excited electrodes are simply the thermal reactions of photoproducts. We shall give here two examples of such reactions. 

Weller, A., "Study of Fast Reactions of Excited Molecules by a Fluorescence Technique," Z. Elektrochem., 1960,64, 55. 

It was not necessary to include reactions of excited molecules. A similar correlation between mass spectral and radiolytic results was obtained for n-hexane22. If these correlations are significant it would indicate that, in these systems at least, excited species that may be produced decay without chemical decomposition. 

Tunneling in Electron Transfer Reactions of Excited Molecules... 

In most cases in the range of conditions of interest to us (moderate temperatures and elevated—not less than ambient atmospheric—pressure), reactions of excited molecules can be excluded. The only exception will be discussed in detail in Section III.D the group of total pressure-dependent reactions, which proceed via the formation and deactivation of excited molecules (compounds). 

It would appear, therefore, that all of the products identified in the vapor-phase radiolyses could be formed from excited molecules. Some products, however, are so much more abundant in vapor-phase radiolysis than in photolysis or liquid-phase radiolysis as to suggest the likelihood of additional precursors. In particular, the formation of acetylene, the isomerization of the xylenes, and the replacement of aromatic hydrogen by methyl groups are difficult to explain solely in terms of reactions of excited molecules. 

In other alkanes the reaction of excited molecules with nitrous oxide is less important, and in some cases it may not occur at all. In n-hexane the N2 yields are about one-half what they are in cyclohexane. In other alkanes such as 2,2,4-trimethylpentane the yields of N2 were quite low. A small yield could be attributed to one of the other effects discussed, but if it is attributed to excited alkane molecules, then energy transfer to nitrous oxide is much less important in 2,2,4-trimethylpentane than in cyclohexane. 

Radiolysis. The photochemical experiments suggest that in the radiolysis a reaction of nitrous oxide with excited molecules would be expected in cyclohexane but should be less important in 2,2,4-trimethylpentane. The radiolysis results (Figure 3 and Table III) show that at nitrous concentrations less than 10 mM, where reactions of excited molecules are unimportant, G(N2) is the same for cyclohexane and 2,2,4-trimethylpentane solutions. At concentrations of nitrous oxide from 20 to 160 mM, G(No) from cyclohexane solutions is greater than G(N2) from 2,2,4-trimethylpentane solutions, and the excess yield increases with the concentration of nitrous oxide. [The nitrogen yields reported here for the concentration range 5-200 mM are in good agreement with those reported by Sherman (20)] Nitrous oxide reduces G(H2) from cyclohexane (16, 17, 18, 20, and Table III), but it has little effect on G(H2) and G(CH4) from 2,2,4-trimethylpentane. 

ELEMENTARY CHEMICAL REACTIONS OF EXCITED MOLECULES FRIDMAN-MACHERET a-MODEL... 

Association reactions of excited molecules, made possible by the fact that such a system contains two unpaired electrons. The best example... 

These methods are obviously applicable only to the reactions of excited molecules. Fluorescence and flash-photolysis experiments may give complementary information on a given system for example, fluorescence quenching can be used to investigate the singlet excited states of systems whose triplet states are studied by the flash-photolysis technique (Section 4.3.1). In recent years fluorescence techniques have found fruitful application in the study of biological systems many proteins, for example, are fluorescent, and others... 

Reactions of excited molecules. Here the solvent would be expected to take part in deactivation (cf. Section 4.3.2.3), as well as in electron transfer. Evidence that this can occur comes from experiments on flash excitation of betaines, producing zwitterions (such... 

It is difficult to experimentally detect simultaneously reactants and products in definite quantum states. Either the quantum state of products or the quantum state of reactants is usually detected. In the first case, we have to speak about the eneigy distribution in the reaction products, and in the second case, about the reaction of excited molecules. These approaches are equivalent because the corresponding rate constants are related by the principle of microscopic reversibility. 

Table 4.10. Ratios of rate constants Hn- ) of reactions of excited molecules to rate constant of thermal reactions...

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