Methane isotope effects

Table 7.7 Isotope effects on NMR spectra of isotopically substituted methane... 

A slightly more complicated example, yet one which is useful to illustrate a problem which can occur when analyzing isotope effects is the reaction between hydroxyl radical and methane ... 

Compare that figure with the result expected when enriched perdeuterated methane is mixed with unlabeled hydroxyl. The observed isotope effect corresponds to k(io.9)/k(io.i3) where ... 

Table 13.1). In the solid P(CH4) > P(CD4) but the curves cross below the melting point and the vapor pressure IE for the liquids is inverse (Pd > Ph). For water and methane Tc > Tc, but for water Pc > Pc and for methane Pc < Pc- As always, the primes designate the lighter isotopomer. At LV coexistence pliq(D20) < Pliq(H20) at all temperatures (remember the p s are molar, not mass, densities). For methane pliq(CD4) < pLiq(CH4) only at high temperature. At lower temperatures Pliq(CH4) < pliq(CD4). The critical density of H20 is greater than D20, but for methane pc(CH4) < pc(CD4). Isotope effects are large in the hydrogen and helium systems and pLIQ/ < pLiQ and P > P across the liquid range. Pc < Pc and pc < pc for both pairs. Vapor pressure and molar volume IE s are discussed in the context of the statistical theory of isotope effects in condensed phases in Chapters 5 and 12, respectively. The CS treatment in this chapter offers an alternative description.

Methane is slightly soluble in HF-SbF5 even at atmospheric pressure (0.005 M), which facilitates direct kinetic studies by NMR. Thus the transition states for methane activation in this medium have been studied experimentally by Ahlberg et al.49 The first-order rate constants [Eqs. (5.9) and (5.10)], determined experimentally on the basis of2H -decoupled 600-MHz 1H NMR time-dependent spectra (Figure 5.2), are on the order of 3.2 x 10 1 s 1 at —20°C and show a secondary kinetic isotope effect (SKIE) of 1 0 02. 

Reactions may exhibit kinetic isotope effects as a result of the lighter isotope reacting faster. To illustrate this type of isotope effect, consider the oxidation of methane in the atmosphere (22). The oxidation is initiated by a reaction with the hydroxyl free radical (OH) in which OH irreversibly abstracts a hydrogen atom from the carbon. Methane molecules containing the lighter carbon isotope react faster, and as a consequence, the 613C value of the product is lower. 

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