Secondary hydrogen isotope effects

In studies of metastable ions [388, 671], H- losses from (CH4)t, (CH3D)t, (CH2D2)t and (CHD3) have been compared and the abundances (corrected for the number of H atoms available) found to be in the respective ratios 1 2.2 5.9 17 [388] and 1 2.2 3.5 11 [671]. The corresponding figures obtained with a tandem magnetic deflection/ICR instrument in which the observation window extended to a few milliseconds (ti ps t2 ms) were 1 2.2 4.4 12 [777]. The isotope effects are 

Intramolecular isotope effects on the loss of methyl have been reported in the El mass spectra of CH3CH2CH2D [795] and CD3CH2CH2CH2CH3 [681]. However, these reactions have been shown to occur by a combination of direct cleavage and extrusion reactions [928], which would complicate any interpretation of the isotope effects. 

Strong secondary deuterium isotope effects have been reported in the loss of methane from metastable propane ions [see Sect. 7.5.3(a)]. 

The intermolecular isotope effect on loss of methane from 2-methylpropane has been the subject of an FIK study. It was found that at 10 ps, loss of CH4 from (CH3)3CD was faster by a factor of 2 than loss of CD4 from (CD3)3CH. At Ins, the rates became equal while at lps, loss of CD4 is twice as fast. On the other hand, at times less than 1 ns, losses of CH3 and CD3 from respective compounds were found to be equal [231]. 

By working at very high mass resolution, it was possible to study isotope effects on methyl loss from toluene and cycloheptatriene following El [32]. In the El mass spectrum, the intermolecular isotope effect ZcHj/ cd, was 1.12 in both cases (comparisons of C7H8 and C7D8). 

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Not only primary but also secondary hydrogen isotope effects can be indicative of tunneling. The most frequently employed criteria of tunneling are the temperature dependence of kinetic isotope effects and the isotopic ratio of the pre-exponential factors in Arrhenius plots, but the pre-exponential criterion has been shown to be invalid for small secondary isotope effects. 

Welsh, K.M., Creighton, D.J. and Khnman, J.P. (1980). Transition-state structure in the yeast alcohol dehydrogenase reaction the magnitude of solvent and alpha-secondary hydrogen isotope effects. Biochemistry 19, 2005-2016... 

Ingemann, S. Hammerum, S. Derrick, P.J. Secondary Hydrogen Isotope Effects on Simple Cleavage Reactions in the Gas Phase The a-Cleavage of Tertiary Amine Cation Radicals. J. Am. Chem. Soc. 1988, 770,3869-3873. 

A physical implication of this assumption is that the occurrence of isotopic substitution in one of the positions of XLm has no effect on the exchange equilibrium of any of the other positions, i.e. the exchange behaviour of the molecule XLm is equivalent to m molecules of a hypothetical solute X L containing one hydrogen nucleus per molecule (Block and Gold, 1959). In other words, thermodynamic secondary hydrogen isotope effects are assumed to be absent. 

Kinetic and product isotope effect discrepancies 95 Oxidations by inorganic oxidants and quinones 95 Substituent effects on rates and equilibria 98 Miscellaneous reactions of dihydropyridines 101 Secondary hydrogen isotope effects 102 Theoretical studies 103 References 105... 

A secondary hydrogen isotope effect has been observed in the loss of methyl radical from the -butylbenzene ion. In the El mass spectrum, the isotope effect IchJIct>3 was 1.1 and for metastable ions the isotope effect ranged from 1.5 to 1.9 [642], The isotope effect in the mass spectrum is equal to the ratio of the stretching frequencies of the dissociating bonds. The isotope effect on the metastable ions has been discussed in terms of an excited electronic state and specific radiationless transitions. The isotope effect is, however, explicable within QET. If the critical energy for CHj loss is 3.4 kJ mole-1 less than that for CDj loss, an isotope effect of about 2 is predicted by QET for metastable ions [853]. 

SECONDARY HYDROGEN ISOTOPE EFFECTS FOR SOME E2 REACTIONS... 

Isotope studies have been reported for acetate pyrolyses but the results are complicated as they represent a combination of primary and secondary hydrogen isotope effects. There is undoubtedly a need to separate these effects and measurements of oxygen and carbon isotope effects might prove instructive. However, it may be easier to measure isotope effects for the xanthate reactions as the lower reaction temperature will afford greater numerical ratios. 

In their discussion, the authors used a mathematical analysis to address the C5 covalent mechanism using the available isotope effect data. The secondary hydrogen isotope effect was an effect on the kinetic constant V/K, in-... 

R.P. Bell and J.E. Crooks, Secondary hydrogen isotope effect in the dissociation constant of formic acid, Trans. Faraday Soc. 58 (1962), pp. 1409-1411. 

Alvarez FJ, Ermer J, Hiibner G et al (1991) Catal5rtic power of pyruvate decarboxylase. Rate-limiting events and microscopic rate constants from primary carbon and secondary hydrogen isotope effects. J Am Chem Soc 113 8402-8409... 

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