Solvent KIE

Solvent kies are also of interest. Here several factors may come into play that are not always easy to sort out. For example, let us imagine that H20 is replaced by D20. These are the effects one may see. There is a small difference in polarity (e = 78.5... 

At its best, the study of solvent kies by the formalism given can be used to learn about proton content and activation in the transition state. For this reason it is known as the proton inventory technique. The kinetics of decay of the lowest-energy electronic excited state of 7-azaindole illustrates the technique.25 Laser flash photolysis techniques (Section 11.6) were used to evaluate the rate constant for this very fast reaction. From the results it was suggested that, in alcohol, a double-proton tautomerism was mediated by a single molecule of solvent such that only two protons are involved in the transition state. In water, on the other hand, the excited state tautomerism is frustrated such that two water molecules may play separate roles. Diagrams for possible transition states that can be suggested from the data are shown, where of course any of the H s might be D s. 

Further evidence for the Aa11 mechanism was obtained from a solvent kinetic isotope study. The theoretical kinetic isotope effects for intermediates in the three reaction pathways as derived from fractionation factors are indicated in parentheses in Scheme 6.143,144 For the Aa11 mechanism (pathway (iii)) a solvent KIE (/ch2o A d2o) between 0.48 and 0.33 is predicted while both bimolecular processes (pathways (i) and (ii)) would have greater values of between 0.48 and 0.69. Acid-catalysed hydrolysis of ethylene oxide derivatives and acetals, which follow an A1 mechanism, display KIEs in the region of 0.5 or less while normal acid-catalysed ester hydrolyses (AAc2 mechanism) have values between 0.6 and 0.7.145,146... 

From rates of the solvolysis of 25c at different sulfuric acid concentrations in D20/CD3CN and H20/CD3CN (Fig. 13), the observed solvent KIE was found to be 0.44 (+0.02) confirming, therefore, that the transition state for solvolysis lies along... 

The plateau region corresponds to effectively complete N-protonation. The pK value measured spectrophotometrically (1.90) agreed with that derived from the kinetic measurements. Similar good agreement was obtained for the N-Et and 4-Me reactants and also for the unsubstituted phenylhydroxylamine in D2O. The measured solvent KIE was also in agreement with the mechanism in Scheme 7. Acid catalysis at high acidity is believed... 

Solvent KIEs for MeCl + Cl in the presence of 1—4 molecules of water were examined by ab initio calculations.70 The ratio h/ d was <1 for the monohydrated system and > 1 for the dihydrated system it increased with the number of micro solvating water molecules, owing to breakage of hydrogen bonds in attaining the transition state. 

Other theoretical studies discussed above include investigations of the potential energy profiles of 18 gas-phase identity S 2 reactions of methyl substrates using G2 quantum-chemical calculations," the transition structures, and secondary a-deuterium and solvent KIEs for the S 2 reaction between microsolvated fluoride ion and methyl halides,66 the S 2 reaction between ethylene oxide and guanine,37 the complexes formed between BF3 and MeOH, HOAc, dimethyl ether, diethyl ether, and ethylene oxide,38 the testing of a new nucleophilicity scale,98 the potential energy surfaces for the Sn2 reactions at carbon, silicon, and phosphorus,74 and a natural bond orbital-based CI/MP through-space/bond interaction analysis of the S 2 reaction between allyl bromide and ammonia.17... 

AG values of adduct III and TV with that of adduct II, sufficient data for adduct II is not available. Yet the similar solvent KIE is a strong evidence to say that the a,p-dihydroxyethyl radical (I lO CI ICH,OH) is a good model for the 6-methyluracil-6-yl and 6-methyl-i-cytosine-6-yl radices when their adducts III and IV lead to similar transition states on their way to the products. And adducts II, III and IV differ with adducts I and V where former set have an OH group in p position as shown in their structures above. The strong difference in KIE between the two sets of adducts I and... 

A detailed kinetic study, a Hammett plot with /0 = h-1.98 using para-substituted styrene oxides, an inverse solvent KIE (fcjjpjp/A jjp[p.(i2) = 0.86, and studies on nonlinear effects" indicating the mechanism of the (salen)Co- and amine-co-catalysed... 

Computational methods for treating organic reactions in solution have been reviewed in Japanese. Examples quoted include solvolysis, the Finkelstein reaction and the Menshutkin reaction. The same author carried out ab initio MO calculations at various levels on the hydrolysis of methyl chloride, in which up to 13 solvent water molecules were explicitly considered.It was found that the attacking H2O molecule kept two hydrogen atoms at the transition state, and the proton transfer from the attacking water to the water cluster began to occur after the transition state. Solute and solvent KIEs were calculated and compared with experimental results. The calculations for the system with 13 solvent water molecules reproduced the experimental energetics and deuterium KIEs fairly well. 

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