Secondary Hydrogen Isotope Effect in the Unimolecular Decomposition of 2-Methylpropane Radical Cations

Article abstract of DOI:10.1021/ja00530a011

The intensities of the mass spectral metastable peaks for loss of methane from 2-methylpropane-1,1,1,3,3,3-d6 reveal a strong secondary hydrogen isotope effect, in that the rate of loss of CH3D is approximately an order of magnitude greater than the rate of loss of CD3H, which in turn is several times greater than the rate of loss of CD4.This isotope effect is interpreted in terms of a nonclassical transition state involving a three-center bond.This nonclassical structure is analogous to the transition state containing a three-center two-electron bond, belived to be involved in the reaction of 2-methylpropane with superacids.Photoionization appearance energies, determined for the molecular and fragment ions from 2-methylpropane, 2-methylpropane-2-d1, and 2-methylpropane-1,1,1,3,3,3-d6, and detailed rate calculations provide support for the nonclassical structure.