Radiolysis of alkanes and olefines in xenon matrices at 4.2 K as studied by ESR: Formation and trapping of hydrogen atoms and their subsequent reactions at cryogenic temperatures

Abstract

Radiolysis of CH 4, CH 3CH 3, CH 3CD 3, C 3H 8, iso-C 4H 10, and CH 2CH 2 in Xe matrices at 4.2 K gave trapped H(D) atoms with a comparable yield of radicals formed from scission of a C-H(D) bond through energy transfer from Xe to the hydrocarbon additives. H tr and >.;CH 2CD 3 are preferentially formed from CH 3CD 3 with the H tr/D tr ratio of 2.1 and the >.;CH 2CD 3/CH 3>.;CD 2 ratio of 2.3 H tr and CH 2>.;CH are formed from CH 2CH 2 by a rapture of olefinic C-H bond. Upon annealing at around 45 K, H tr is detrapped and mobilized to react with C 2H 6 and C 2H 4 giving >.;C 2H 5 by H abstraction and addition, respectively. Preferential abstraction of H from CH 3CD 3 took place at around 45 K showing a large isotope effect with k H / k D > 9. The reactions of thermal H atoms produced by radiolysis of CH 4 with CO to form H>.;CO have also been observed at around 45 and 18 K in Xe and CH 4 matrices, respectively. These results suggest that methane may not be an exceptional compound to produce hydrogen atoms in the radiolysis of neat alkane at low temperature. Absence of trapped H atoms in irradiated neat alkanes except CH 4 may imply that all the H atoms produced react with alkanes except CH 4. The reactivity of thermal H atoms at cryogenic temperature suggests that not only hot but also thermal H atom reactions are involved in the solid phase radiolysis below 77 K.