Swain relation

An additional category related to the above was suggested by A. D. Swain ... 

Another criterion that has been used for detecting the presence of tunnelling is that the Swain-Schaad relation (55)... 

Relative Values for Deuterium and Tritium Isotope Effects The Swain-Schaad Relation... 

For further important work on this and related concepts, see Rucker, J. and Kliman, J.P. (1999). Computational study of tunneling and coupled motion in alcohol dehydrogenase-catalyzed reactions Implication for measured hydrogen and carbon isotope effects. J. Am. Chem. Soc. 121, 1997 -2006, and Kohen, A. and Jensen, J.H. (2002). Boundary conditions for the Swain-Schaad relationship as a criterion for hydrogen tunneling. J. Am. Chem. Soc. April 17, 124(15), 3858-3864. 

The observation of a primary tritium isotope effect (H/T) that is substantially larger than the value predicted on the basis of the semiclassical Swain-Schaad relation (Chart 3) from a heavy-hydrogen (DAT) isotope effect. The same information can be expressed in terms of a Swain-Schaad exponent required to relate the two isotope effects that is substantially larger than the semiclassical value of 3.26. 

Albery, W.J. and Knowles, J.R. (1977). The determination of the rate-limiting step in a proton-transfer reaction from the breakdown of the Swain-Schaad relation. J. Am. Chem. Soc. 99, 637-638... 

Cooper-Driver, G. A. and Swain, T. (1976). Cyanogenic polymorphism in bracken (Pterid-ium) in relation to herbivore predation Ifchistocercagregarid). Nature 260,604. 

RERSISTENCE TIME SWAIN-LUPTON RELATION SWAIN RELATIONSHIP... 

The method described by Swain and Hillis (41) applied to wine (40) gives V /LA which is related to the state of polymerization in condensed tannins. The tannin sample is put through two tests whose reactions depend on the state of condensation. These two reactions are... 

Several approaches have been made to calculate 13C chemical shifts of coumarins by MO methods. Good correlations were found between the 13C chemical shift values of coumarin (also protonated) and the n charge densities calculated by the CNDO/2 method [962], and of coumarins with it charge densities calculated by the Hiickel MO method (which, however, fails for methoxylated coumarins) [965]. Chemical shifts of mono- and dimethoxycoumarins have been correlated with parameters determined by refined INDO MO calculations, in which n bond orders, atom-atom polarizabilities, excitation energies and electron-nucleus distances were taken into consideration [966], In 3-substituted 4-hydroxy and 4-hydroxy-7-methoxycoumarins chemical shifts were found to be related to Swain and Lupton s parameters iF and M [388], according to equation 5.4 (SE = Substitution Effect) ... 

The Hammett and Taft equations are not the only linear free-energy relationships known. We shall encounter others—for example, the Bronsted relations, and the Grunwald-Winstein and Swain-Scott equations later in this book. 

Solvolysis of the R,R and R,S isomers of 2-bromo-9-(l-X-ethyl)fluorenes, X = Cl, Br, I, or OBs, in 25% (v/v) acetonitrile in water has been studied with respect to rates of formation of elimination products and of substitution products (X = OH or NHCOMe).142 The parent 9-(l-X-ethyl)fluorenes and the 2,2/-dibromo-9-(l-X-ethyl)-fluorenes were also studied. Various effects of leaving group and of the presence of nucleophiles on the competition between the reactions were observed and the Bronsted equation was applied to the results for the elimination reactions. A related study of solvolysis of 9-(X-methyl)fluorenes, X = I, Br, or Bs, was also carried out, in which the Swain-Scott equation was applied to nucleophilic selectivities in the S 2 reactions.143... 

The simplest linear free energy relation that has been used to describe the attack of a series of nucleophiles in an SN2 reaction is the Swain-Scott relation (Swain and Scott, 1953). In our notation the relation becomes (45), where the nucleophilicity parameter for X, x, is defined by (46) or its equivalent (47)... 

In general this analysis shows that the behaviour which the Swain-Scott relation attempts to describe is governed by both kinetic and thermodynamic contributions. The mix of the two contributions (Table 6 and Fig. 11) is different for different nucleophiles. This is probably the reason why the Swain-Scott relation does not hold for a large number of reactions (Leffler and Grunwald, 1963b Koskikallio, 1972) and why more complicated expressions, such as the Edwards relation (Edwards, 1954), have been proposed. 

In this section we examine the evidence about transition state structure that can be obtained from Hammett op relations. We start by considering substitution in the attacking nucleophile X or in the leaving group, Y. From the general relation (36) we deduce (117). As with the Swain-Scott n and the m Y ... 

Finally, we would like to emphasize that, besides obtaining values of ccfrom the Marcus analysis, another great advantage is that one can separate kinetic and thermodynamic contributions to the parameters in linear free-energy relations. In this article we have done this for the Swain-Scott nucleophilicity parameter n, for m describing the change of reaction rate with solvent, and for the Hammett p-values. 

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