Control factors for hydrogen-atom abstraction from organic molecules in cryogenic solids

Hydrogen-atom transfer from organic molecules to free deuterium atoms and to methyl radicals in cryogenic organic solids due to quantum-mechanical tunneling of C-H hydrogen was studied for elucidating the control factors for the reaction. The following differences were found in comparison with regular thermal reactions: (1) The rate of hydrogen-atom tunneling decreases with increasing number and length of alkyl chains attaching to C-H carbon to be hydrogen-abstracted. This is due to the increase of steric hindrance to the deformation of the chemical bonds of the C-H carbon. The abstraction accompanies the change of the chemical bonds for the C-H carbon from sp(3) to sp(2), SO that prevention of the deformation by the alkyl chains causes the increase of the thickness of the potential barrier for the tunneling. The tunneling rate therefore decreases. (2) The rate of abstraction increases with the energy released by the reaction. The energy is used to excite the vibrational states of product molecules, which accelerates the tunneling from the initial reactant state to the final product state. (C) 1999 Elsevier Science Ltd. All rights reserved.