Matrix effect on hydrogen atom tunneling from alkane to free deuterium atoms in cryogenic solid

ESR study on alkyl radicals generated in deuterated organic matrixes at 77 K by hydrogen atom tunneling from alkane molecules to free deuterium atoms has been carried out for elucidating control actors for the tunneling. For small alkane molecules, the tunneling rate is determined by the height of t he potential energy barrier for the tunneling. For larger molecules in glas sy matrixes, the rate decreases with increasing number and length of alkyl chains bonded to a carbon atom to be hydrogen abstracted. The tunneling rat e from antepenultimate tertiary carbon is much slower than that from penult imate secondary carbon, even the potential energy barrier is lower. This ab normal effect is explained as being due to the steric hindrance by matrix m olecules to the deformation of alkane molecules during the reaction. The al kyl chains surrounded by matrix molecules prevent the deformation of the ch emical bonds of the carbon atom from the initial sp(3) to the final sp(2) c onfiguration, which may cause the increase of the thickness of the potentia l energy barrier and thereby decrease of the tunneling rate. Free deuterium atoms in a crystalline adamantane matrix selectively abstract hydrogen ato ms from the antepenultimate tertiary carbon, probably due to less steric hi ndrance to the deformation.