Secondary isotope effects reactions

An inverse isotope effect will occur if coordination at the reaction center increases in the transition state. The bending vibration will become more restricted. Entry 4 in Scheme 4.2 exenqilifies such a case involving conversion of a tricoordinate carbonyl group to a letravalent cyanohydrin. In this case the secondary isotope effect is 0.73. 

Secondary isotope effects at the position have been especially thoroughly studied in nucleophilic substitution reactions. When carbocations are involved as intermediates, substantial /9-isotope effects are observed. This is because the hyperconjugative stabliliza-... 

Deuterium isotope effects have been found even where it is certain that the C—H bond does not break at all in the reaction. Such effects are called secondary isotope effectsf" the term primary isotope effect being reserved for the type discussed previously. Secondary isotope effects can be divided into a and P effects. In a P secondary isotope effect, substitution of deuterium for hydrogen p to the position of bond breaking slows the reaction. An example is solvolysis of isopropyl bromide ... 

The extent or nature of solvent-solute interactions may be different in the deuterated and nondeuterated solvents this may change the energies of the transition state, and hence the activation energy of the reaction. These are secondary isotope effects. Two physical models for this third factor have been constructed. ... 

Deuterium Substitution. The a and P secondary isotope effects affect the rate in various ways (p. 298). The measurement of a secondary isotope effects provides a means of distinguishing between SnI and Sn2 mechanisms, since for Sn2 reactions the values range from 0.95 to 1.06 per a D, while for S l reactions the values are higher. This method is especially good because it provides the minimum of perturbation of the system under study changing from a H to a D hardly affects the reaction, while other probes, such as changing a substituent or the polarity of the solvent, may have a much more complex effect. 

When reactions were run with substrate deuterated in the ortho position, isotope effects of 1.22 were obtained. It is difficult to account for such high secondary isotope effects in any other way except that an incipient... 

The consequences on the magnitude of the secondary a-deuterium KIE of coupling the motion of the nontransferring a-hydrogen into the reaction coordinate motion, as suggested by Kurz and Frieden, was investigated in some model calculations by Huskey and Schowen (1983). They used two different models to calculate the secondary isotope effects for the hydride transfer reaction (45). 

Secondary isotope effects are small. In fact, most of the secondary deuterium KIEs that have been reported are less than 20% and many of them are only a few per cent. In spite of the small size, the same techniques that are used for other kinetic measurements are usually satisfactory for measuring these KIEs. Both competitive methods where both isotopic compounds are present in the same reaction mixture (Westaway and Ali, 1979) and absolute rate measurements, i.e. the separate determination of the rate constant for the single isotopic species (Fang and Westaway, 1991), are employed (Parkin, 1991). Most competitive methods (Melander and Saunders, 1980e) utilize isotope ratio measurements based on mass spectrometry (Shine et al., 1984) or radioactivity measurements by liquid scintillation (Ando et al., 1984 Axelsson et al., 1991). However, some special methods, which are particularly useful for the accurate determination of secondary KIEs, have been developed. These newer methods, which are based on polarimetry, nmr spectroscopy, chromatographic isotopic separation and liquid scintillation, respectively, are described in this section. The accurate measurement of small heavy-atom KIEs is discussed in a recent review by Paneth (1992). 

Abstract This chapter describes a number of examples of kinetic isotope effects on chemical reactions of different types (simple gas phase reactions, Sn2 and E reactions in solution and in the gas phase, a and 3 secondary isotope effects, etc.). These examples are used to illustrate many aspects of the measurement, interpretation, and theoretical calculation of KIE s. The chapter concludes with an example of an harmonic semiclassical calculation of a kinetic isotope effect. 

In reaction (10.11) the deuterium isotope effect is a secondary isotope effect, that is one in which the bonding to the isotopically substituted atom is not broken or formed during the course of the reaction. Secondary deuterium isotope effects are generally much smaller than primary ones. 

In reactions 14.32 and 14.33 the hydrogen atoms are not involved in any bonds that are being made or being broken in the reaction. The isotope effect is therefore referred to as a secondary a-deuterium isotope effect since the position of isotopic substitution is a to the bond being broken in the rate limiting step (see Chapter 10 for discussion of secondary isotope effects). 

Secondary isotope effects measure transition-state structure 37 Quantum tunneling in enzyme-catalyzed reactions breakthroughs 42 Experimental phenomenology of quantum tunneling in enzyme-catalyzed... 

Just as in the preceding examples, early indications of tunneling in enzyme-catalyzed reactions depended on the failure of experiments to conform to the traditional expectations for kinetic isotope effects (Chart 3). Table 1 describes experimental determinations of -secondary isotope effects for redox reactions of the cofactors NADH and NAD. The two hydrogenic positions at C4 of NADH are stereochemically distinct and can be labeled individually by synthetic use of enzyme-catalyzed reactions. In reactions where the deuterium label is not transferred (see below), an... 

If secondary isotope effects arise strictly from changes in force constants at the position of substitution, with none of the vibrations of the isotopic atom being coupled into the reaction coordinate, then a secondary isotope effect will vary from 1.00 when the transition state exactly resembles the reactant state (thus no change in force constants when reactant state is converted to transition state) to the value of the equilibrium isotope effect when the transition state exactly resembles the product state (so that conversion of reactant state to transition state produces the same change in force constants as conversion of reactant state to product state). For example in the hydride-transfer reaction shown under point 1 above, the equilibrium secondary isotope effect on conversion of NADH to NAD is 1.13. The kinetic secondary isotope effect is expected to vary from 1.00 (reactant-like transition state), through (1.13)° when the stmctural changes from reactant state to transition state are 50% advanced toward the product state, to 1.13 (product-like transition state). That this naive expectation... 

The exalted secondary isotope effect was suggested to originate in reaction-coordinate motion of the secondary center. 

Yeast alcohol dehydrogenase (YADH), catalysis of reduction by NADH of acetone formate dehydrogenase (FDH), oxidation by NAD of formate horse-liver alcohol dehydrogenase (HLAD), catalysis of reduction by NADH of cyclohexanone With label in NADH, the secondary KIE is 1.38 for reduction of acetone (YADH) with label in NAD, the secondary KIE is 1.22 for oxidation of formate (FDH) with label in NADH, the secondary KIE is 1.50 for reduction of cyclohexanone (HLAD). The exalted secondary isotope effects were suggested to originate in reaction-coordinate motion of the secondary center. 

In the following year, Cleland and his coworkers reported further and more emphatic examples of the phenomenon of exaltation of the a-secondary isotope effects in enzymic hydride-transfer reactions. The cases shown in Table 1 for their studies of yeast alcohol dehydrogenase and horse-liver alcohol dehydrogenase would have been expected on traditional grounds to show kinetic isotope effects between 1.00 and 1.13 but in fact values of 1.38 and 1.50 were found. Even more impressively, the oxidation of formate by NAD was expected to exhibit an isotope effect between 1.00 and 1/1.13 = 0.89 - an inverse isotope effect because NAD" was being converted to NADH. The observed value was 1.22, normal rather than inverse. Again the model of coupled motion, with a citation to Kurz and Frieden, was invoked to interpret the findings. 

In 1983, Huskey and Schowen tested the coupled-motion hypothesis and showed it to be inadequate in its purest form to account for the results. If, however, tunneling along the reaction coordinate were included along with coupled motion, then not only was the exaltation of the secondary isotope effects explained but also several other unusual feamres of the data as well. Fig. 4 shows the model used and the results. The calculated equilibrium isotope effect for the NCMH model (the models employed are defined in Fig. 4) was 1.069 (this value fails to agree with the measured value of 1.13 because of the general simplicity of the model and particularly defects in the force field). If the coupled-motion hypothesis were correct, then sufficient coupling, as measured by the secondary/primary reaction-coordinate amplimde ratio should generate secondary isotope effects that... 

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