Bond breaking kinetic isotope effect

Gives bond breaking kinetic isotope effect (BBKIE)... 

Equilibrinm isotope effects are normal (>1.0) if the substrate is more stiffly bonded and inverse (<1.0) if the product is more stiffly bonded. Primary kinetic isotope effects are almost always normal, as the atom being transferred is more weakly bonded in the transition state. Isotope effects in positions not subject to bond breaking or making are secondary and are good tools for determining transition state stmctnre. They may be either normal or inverse. 

A kinetic isotope effect that is a result of the breaking of the bond to the isotopic atom is called a primary kinetic isotope effect. Equation (6-88) is, therefore, a very simple and approximate relationship for the maximum primary kinetic isotope effect in a reaction in which only bond cleavage occurs. Table 6-5 shows the results obtained when typical vibrational frequencies are used in Eq. (6-88). Evidently the maximum isotope effect is predicted to be very substantial. 

The nitrogen kinetic isotope effect for this reaction is 1.034 0.002, which indicates that breaking of the N-O bond occurs in the slow step. ... 

A. Alkaline Hydrolysis. -The low kinetic isotope effect observed in the protonation of carbanions formed in phosphonium salt hydrolysis leads to the idea that there is little breaking of the phosphorus-carbon bond and correspondingly little transfer of a proton to the incipient carbanion in the transition state (87) of the rate-determining step. ... 

Mechanistic studies have been designed to determine if the concerted cyclic TS provides a good representation of the reaction. A systematic study of all the E- and Z-decene isomers with maleic anhydride showed that the stereochemistry of the reaction could be accounted for by a concerted cyclic mechanism.19 The reaction is only moderately sensitive to electronic effects or solvent polarity. The p value for reaction of diethyl oxomalonate with a series of 1-arylcyclopentenes is —1.2, which would indicate that there is little charge development in the TS.20 The reaction shows a primary kinetic isotope effect indicative of C—H bond breaking in the rate-determining step.21 There is good agreement between measured isotope effects and those calculated on the basis of TS structure.22 These observations are consistent with a concerted process. 

It should be emphasised that primary kinetic isotope effects are observed experimentally with values intermediate between the maximum calculated value and unity (i.e. no isotope effect) these too can be useful, as they may supply important information about the breaking of particular bonds in the transition state. 

In some cases the coupling reaction is found to be base-catalysed, and this is found to be accompanied by a kinetic isotope effect, i.e. /c, > k2> and the breaking of the C—H bond in (35) is now involved in the rate-limiting step of the reaction. 

Figures 7-9 show the fractional conversion of methanol in the pulse as a function of temperature for the three catalysts and the three methanol feeds. Evidently the kinetic isotope effect is present on all three catalysts and over the complete temperature range, indicating that the rate limiting step is the breaking of a carbon-hydrogen bond under all conditions. From these experiments, the effect cannot be determined quantitatively as in the case of the continuous flow experiments, but to obtain the same conversion of CD,0D, the temperature needs to be 50-60° higher. This corresponds to a factor of about three in reaction rate. The difference in activity between PfoCL and Fe.(MoO.), is larger in the pulse experiments compared to tHe steady stateJ results.

Methanol still proceeds through an initial C H bond scission, but reacts with water before the OH bond breaks. Alternatively, formaldehyde formation likely occurs along the same pathway as CO formation. This is true if HCO is an intermediate in the decomposition pathway. Furthermore, the lack of a kinetic isotope effect for CH3OD indicates that formaldehyde is not the product of an initial O-H scission.94 Because formaldehyde and formic acid are not the thermodynamically favored products of methanol oxidation, they must be the result of kinetic limitations preventing the full oxidation to C02, analogous to the production of H202 for the reduction of oxygen (see next section). 

Isotope effects on both the carbon and hydrogen of the breaking C-H bond have been measured. However, for this reaction both forward and reverse commitments are sizable so the three equations corresponding to Equation 11.48 have four unknowns the forward and reverse commitments and two intrinsic isotope effects. Measurements of the secondary deuterium kinetic isotope effect (at position 4 of nicotinamide ring of NADP+) and the carbon kinetic isotope effect with the secondary position deuterated introduce two additional equations, but only one more unknown ... 

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