Energy, phase, and temperature effects on hydrogen atom abstraction reaction from isobutane by hydrogen atoms at 4 and 77 K

Abstract

Hydrogen atom abstraction reaction from isobutanes, such as HC(CH/sub 3/)/sub 3/, DC(CH/sub 3/)/sub 3/, or HC(CD/sub 3/)/sub 3/, by H atoms has been studied in the solid phase at 4 and 77/sup 0/K by EST spectroscopy. In order to obtain H atoms with different initial kinetic energies, we performed the photolysis of HI with 254 and 229 nm, and the dissociative electron attachment to HI in radiolysis was also utilized. The relative rate constants for the abstraction reaction from a primary H atom (k/sub 1H/) and that from a tertiary H atom (k/sub 3H/) were estimated from the yields of isobutyl and tert-butyl radicals produced. The H atom with high energy abstracts preferentially the primary H atom of isobutane, while that with low energy abstracts the tertiary H atom. The ratio k/sub 1H//k/sub 3H/ depends k/sub 3H/ depends remarkably upon phase and temperature. For example, k/sub 1H//k/sub 3H/ for the reaction of H atoms, produced the 254-nm photolysis of HI at 77/sup 0/K, exceeds 11 in the crystalline state, while k/sub 1H//k/sub 3H/ is only 0.15 in the glassy state. The ratio k/sub 1H//k/sub 3H/ for the reaction of H atoms, produced by the 229-nm photolysis of HI in themore » glassy state, exceeds 11 at 4/sup 0/K, while the ratio amounts only to 0.21 at 77/sup 0/K. The phase and temperature effects on the abstraction reaction by the H atoms are interpreted in terms of the different efficiencies of the energy loss of H atoms with excess kinetic energies. The energy loss efficiency decreases in the following order: glass at 77/sup 0/K > crystal at 77/sup 0/K > glass at 4/sup 0/K > crystal at 4/sup 0/K.« less