Fracture patterns and alkyl radical yields in irradiated crystalline alkanes

Abstract

Relative yields, migration characteristics, and decay mechanisms are described for various alkyl radicals produced by gammas in crystalline alkanes. ESR measurements at low power levels (<10microW) were made on single crystals of n-eicosane ( n-C 20H 42) which were irradiated with gammas at 77 K. The relative concentrations of the three possible types of alkyl radicals were examined, first at 77 K and then after warming the crystals to various temperatures for several time intervals and cooling them to 77 K for measurement. Also the relative radical concentrations were measured at room temperature as a function of time for several days. Following are the two principal results: 1. The alkyl radicals found in samples with an absorbed dose of 250 krad were almost entirely of the interior type (-CH 2CHCH 2-). Samples given higher doses showed an increasing concentration of the penultimate (-CHCH 3). The primary terminal radical (-CH 2CH 2CH 2) was not observed. 2. There was a slow conversion of the interior alkyl radical to the penultimate form particularly at higher temperatures. These results were used as the basis for explaining the apparent contradictions between Gillbro and Lund, (1) and Kimura et al (2) regarding the relative yields of the penultimate and interior radicals in irradiated eicosane. In agreement with Kimura et al., the results suggest that the fracture of C-H bonds in irradiated eicosane favors the yield of the interior secondary alkyl radical far more than is to be expected from a random fracture pattern. The results of the ESR studies also demonstrate that the intramolecular migration of the alkyl radical site is more rapid than intermolecular migration. This confirms the conclusions of Hama et al. (3) obtained by studies on alkane diols but conflicts with conclusions of Shimada et al. (4) on irradiated urea-polyethylene samples.