C-12 C-13 KINETIC ISOTOPE EFFECTS IN THE REACTIONS OF CH4 WITH OH ANDCL/

In qualitative agreement with previous MP2 level calculations of the C -12/C-13 kinetic isotope effects (KIEs) for H-abstraction of CH4 by OH and Cl, higher level calculations yield KIEs at 300 K that differ fro m the corresponding experimental values, but in different directions. At QCISD(T)/6-311 + G(2df,p), the theoretical KIE for CH4 + Cl is 32 p arts per thousand less than the measured value, while for CH4 + OH, th e theoretical KIE is greater than the measured value, but only by 5 pa rts per thousand. Noticeable differences in atmospheric model simulati on studies can occur if the measured KIEs are replaced by the analogou s theoretical values. For example, the effect of carbon isotopic fract ionation from the CH4 + Cl reaction on stratospheric delta(13)CH(4) wi ll be significantly lower than that obtained by Bergamaschi et al. [19 96]. It is suggested, for the CH4 + OH reaction and especially for the CH4 + Cl reaction, that additional KIE measurements are needed.