Electronic isotope effect

Much attention has been paid to the chemistry and biological chemistry of singlet dioxygen and superoxide ion. The lifetime of the former species, known to be solvent-dependent, is much increased for deuterated acetone, acetonitrile, benzene and chloroform compared to the undeuterated solvent. This is a remarkable solvent deuterium isotope effect. Electron transfer to 02 from tetramethylphenylenediamine to give superoxide has been confirmed, the rate of reaction being close to the diffusion-controlled limit. Kinetic studies on the photochemical formation of superoxide from jU-superoxodecacyanodicobalt(III) ions and oxygenated ethanol solutions have been reported. 

Consider a reactant molecule in which one atom is replaced by its isotope, for example, protium (H) by deuterium (D) or tritium (T), C by C, etc. The only change that has been made is in the mass of the nucleus, so that to a very good approximation the electronic structures of the two molecules are the same. This means that reaction will take place on the same potential energy surface for both molecules. Nevertheless, isotopic substitution can result in a rate change as a consequence of quantum effects. A rate change resulting from an isotopic substitution is called a kinetic isotope effect. Such effects can provide valuable insights into reaction mechanism. 

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

This effect is observable within a series of very similar electrophiles. Zollinger27 found that in reactions of diazonium ions substituted with 4-C1, 3-C1, and 3-N02 substituents (i.e. the reactivity and electron-withdrawing power of the ion increased along the series) the respective kinetic isotope effects were 6.55, 5.48 and 4.78. 

Kinetic studies of mercuration have also been used as a test for hyperconjugation. Toluene and toluene-aaa-mercuric acetate (0.5 M) in acetic acid containing water (0.25 M) and perchloric acid (0.050 M) and an isotope effect, ArH/ArD = 1.00 0.03, obtained. This insignificant effect was considered as evidence against the participation of hyperconjugation in electron supply by a methyl group449. 

White et al.1A have obtained similar kinetic results for the acid-catalysed rearrangement of N-nitro-N-methylaniline, i.e. a first-order dependence on the nitroamine with a linear H0 plot of slope 1.19 for phosphoric acid, and a deuterium solvent isotope effect of about three, although the results have only been presented in preliminary form. Further, an excellent Hammett a+ correlation was claimed for thirteen para substituted nitroamines which gave a p value of —3.9. Since it is expected that the rate coefficients would correlate with a (rather than different basicities of the amines, the a+ correlation implies that the amino nitrogen is electron-deficient in the transition state,... 

Mechanism Kinetic" Order P-Hydrogen Exchange Faster Than Elimination General or Specific Base Catalysis hAd Electron Withdrawal atCp Electron Release at C Leaving- Group Isotope Effect or Element Effect... 

The electronic, rotational and translational properties of the H, D and T atoms are identical. However, by virtue of the larger mass of T compared with D and H, the vibrational energy of C-H> C-D > C-T. In the transition state, one vibrational degree of freedom is lost, which leads to differences between isotopes in activation energy. This leads in turn to an isotope-dependent difference in rate - the lower the mass of the isotope, the lower the activation energy and thus the faster the rate. The kinetic isotope effects therefore have different values depending on the isotopes being compared - (rate of H-transfer) (rate of D-transfer) = 7 1 (rate of H-transfer) (rate of T-transfer) 15 1 at 25 °C. 

The lack of steric effects in oxidations of hydrocarbons by Cr(VI) renders D and E unacceptable. The activated complex of scheme C is non-linear and hence does not comply with the magnitude of the observed isotope effect. Two pieces of evidence are quoted which indicate A to be the more probable of the remaining two. Firstly, the p constant of —1.17 is more in agreement with that obtained for bromine atom abstraction from toluenes (—1.369 to —1.806) than those found for solvolyses involving electron-deficient carbon ( — 2.57 to —4.67) . Secondly, the correlation between the relative rates of oxidation of the series... 

The effect of solvent upon k2 has been reported , and it was concluded that the activated complex is not sufficiently polar to be called ionic . The oxidations of toluene and triphenylmethane exhibit primary kinetic deuterium isotope effects of 2.4 and ca. 4 respectively. No isotopic mixing occurred during formation of the Etard complex from a mixture of normal and deuterated o-nitrotoluene . The chromyl chloride oxidation of a series of substituted diphenylmethanes revealed that electron-withdrawing substituents slow reaction while electronreleasing groups have the opposite effect, the values ofp andp being —2.28 + 0.08 and —2.20 + 0.07 respectively . ... 

Rosenblatt etal have examined the effect of structure and isotopic substitution upon the permanganate oxidation of some alky famines (Table 4). The isotope effect of 1.84 is considered to be sufficiently low to be compatible with aminium radical-cation formation, and it is felt that, while C-H cleavage is significant for oxidation of primary amines, the dominant mode of oxidation of tertiary amines is electron-transfer, e.g. 

This is in accordance with the primary kinetic isotope effect for Mn(III) sulphate With Co(III) electron abstraction may occur to give a radical-cation which suffers further oxidation. The alternative explanation of the lack of an isotope effect is that formation of the Co(III)-ketone complex is rate-determining this lacks, however, other kinetic support . ... 

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. 

---