Transition state properties

Durant, J. L., 1996, Evaluation of Transition State Properties by Density Functional Theory , Chem. Phys. Lett., 256, 595. 

Fig. 18 B3LYP/6-31G -derived energetics, ground and transition state properties for the HERON reaction of iV-methoxy-N-dimethylaminoformamide to 1,1-dimethyldiazene and methyl formate.

These transition state properties have been modelled computationally at the HF/6-31G level.45 The transition state geometry and properties for the reaction of 7V-formyloxy-/V-methoxyformamide 40 with ammonia giving 69 (Scheme 13), is illustrated in Fig. 20. In the transition state the nucleophile and leaving group,... 

Relatively few data are available (Table H) for reactions involving intramolecular general acid catalysis, but in most cases the EM s fall in the same range as those for general base catalysis (Tables E-G). This is expected if EM is a characteristic transition-state property, because a general acid catalysed reaction is always the microscopic reverse of a general base catalysed process as shown in equation (5), although in no case has the EM been measured in both directions. 

The activation energy for a reaction is affected by the exothermicity. As suggested long ago by Bell, Evans, and Polanyi (15, 16), the transition state properties should re-... 

Phase space theory can be thought of as, in effect, considering a loose or, as it is sometimes called, orbiting [333] transition state regardless of the nature of the reaction. The need to select transition state properties for each individual reaction considered is avoided and it has been argued that a virtue of the theory is that it gives definite predictions [452]. 

Symmetry factors, o, do not appear in eqn. (28) because the numbers of equivalent pathways have been allowed for in the definition of the kinetic isotope effect. F0(d is the critical energy of the decomposition involving the lighter isotope and F0(II) that of the decomposition involving the heavier isotope. The density of states, N(E), of the reactant ion is, of course, common to both decompositions and does not affect the intramolecular kinetic isotope effect. The intramolecular kinetic isotope effect is, therefore, dependent only upon transition state properties. 

The important conclusion is that intermolecular kinetic isotope effects are not a sensitive probe of transition state properties in the way that intramolecular kinetic isotope effects are, indeed the intermolecular effects can be effectively independent of transition state characteristics. 

Gonzalez-Lafont A, Troung TN, Truhlar DG (1991) Interpolated variational transition-state theory practical methods for estimating variational transition-state properties and tunneling contributions to chemical reaction rates from electronic structure calculations, J Phys Chem 95 8875-8894... 

To be able to utilize this formula a great deal of information concerning molecular parameters is required. To calculate N E) rotational constants and vibrational frequencies of internal motion are required and in many case these are available from spectroscopic studies of the stable molecule. Unfortunately the same cannot be said for the parameters required to calculate G E) because, by definition, the transition state is a very short lived species and is therefore not amenable to spectroscopic analysis. The situation is aggravated still further by the fact that many unimolecular dissociation processes do not have a well defined transition state on the reaction coordinate. It is precisely these difficulties that make ILT an attractive alternative as it does not require a detailed knowledge of transition state properties. 

Table 6.3. Numerical constants characterizing the energy surface V = k (Er] + r /2 + k hrihr2 + k Er + + + calculated transition-state properties...

Since then, galloping improvements in computer power, combined with the development of new algorithms, have enabled us to address many problems with quantitative accuracy. Gradient techniques permit the determination of equilibrium structures, vibrational frequencies and transition state properties ... 

Interpolated Variational Transition-State Theory Practical Methods for Estimating Variational Transition-State Properties and Tunneling Contributions to Chemical Reaaion Rates from Electronic Structure Calculations. 

The advantage of using solvent activity coefficients in a discussion of the transition state for non-solvolytic reactions in mixed solvent media, is that the solvent activity coefficient is a function of state of the system and can be used to infer transition state properties without cognisance of, or concern for, the structure or detailed identity of the solvation sphere. If on the other hand the mechanism of a bimolecular reaction is clearly defined on other grounds, then the solvent activity coefficients should lead to understanding of the solvent reorganisation upon activation. At this stage very few results have been accumulated in binary solvent mixtures. ... 

Soucy P, Lacoste L, Luu-The V (2003) Assessment of porcine and human 16-ene-synthase, a third activity of P450cl7, in the formation of an andro-stenol precursor. Eur J Biochem 270 1349-1355 Fluck CE, Miller WL, Auchus RJ (2003) The 17, 20-lyase activity of cytochrome P450cl7 from human fetal testis favors the D steroidogenic pathway. J Clin Endocrinol Metab 88 3762-3766 Yoshimoto FK, Zhou Y, Peng HM, Stidd D, Yo-shimoto JA, Sharma KK, Matthew S, Auchus RJ (2012) Minor activities and transition state properties of the human steroid hydroxylases cytochromes P450cl7 and P450c21, from reactions observed with deuterium-labeled substrates. Biochemistry 51 7064-7077... 

Yoshimoto FK, Zhou Y, Peng HM, Stidd D, Yoshi-moto JA, Sharma KK, Matthew S, Auchus RJ (2012) Minor activities and transition state properties of the human steroid hydroxylases cytochromes P450cl7 and P450c21, from reactions observed with deuterium-labeled substrates. Biochemistry 51 7064-7077... 

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