Deuterium isotope effect, methane

Ion abundances for methane loss from metastable (CH3CD2CHD2)t ions have been found to be in the ratios 87 1 12 (CH4 CH3D CH2D2) after correction for the numbers of equivalent pathways [250]. The reaction is a 1, 2 elimination, so these results indicate both a large primary and a large secondary deuterium isotope effect (see Sect. 7.5.4). On the basis of these intramolecular isotope effects, a mechanism involving a non-classical transition state with a CCH three-centre bond has been proposed for this methane elimination [250]. 

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

Laufer, A.M., and J.R. McNesby, Deuterium isotope effect in vacuum ultraviolet absorption coefficients of water and methane. Canad J Chem f3, 3487, 1965. 

The lack of a large intramolecular deuterium isotope effect in the reaction of hot tritium with partially deuteriated methanes has been explained by Rowland and coworkers by postulating the existence of a very large secondary deuterium isotope effect which counterbalances the normal primary replacement isotope effect. 

In terms of nonequilibrium interaction conditions the lack of the intramolecular deuterium isotope effect in the hot tritium atoms reaction with partial deuteriated methane, implies that the greater chance of encounter between fast moving... 

Sauer, S. P. A. and Provasi, P. F. (2008). The anomalous deuterium isotope effect in the NMR spectrum of methane An analysis in localized molecular orbitals. ChernPhysChem, 9, 1259-1261. 

Wilkins, P. C., Dalton, H., Samuel, C. J., and Green, J., 1994, Further evidence for multiple pathways in soluble methane-monooxygenase-catalysed oxidations from the measurement of deuterium kinetic isotope effects, Eur. J. Biochem. 226 5559560. 

We have mentioned above some attempts to explain these facts theoretically. Preliminary calculations show that Cu insertion of a C-H bond in methane is competitive with the abstraction reaction in H2, so that the activation of hydrogen (specially when the deuterium isotope is used) is less effective in the presence of methane. Also CIPSI calculations on the Cu -f N2 interactions shows the formation of the Cu(N2)n complexes which were first proposed by Gracie (Gracie, C., M.Sc. Thesis, University of Toronto, 1985). Such complexes make the interaction of Cu with D2 much less effective, raising the activation barriers and explaining the scarcity of the CuD and D subproducts (Sanchez-Zamora, M. Novaro, 0. and Ruiz, M.E., submitted for publication). These modest attempts however only make it more imperative to carry out more theoretical studies both to obtain better potential energy surfaces and to do trajectory calculations for Cu -fD2 reactions. 

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