The Trapping by Nitric Oxide of Initially‐Formed Free Radicals in Photochemical Reactions: 1. The mercury‐6(³P₁)‐photosensitized decomposition of propane and the deuterated propanes

Abstract

A study has been made of the relative primary yields of n-propyl and i-propyl radicals in the reactions of Hg6(3P1) atoms with C3H8, CsDe, CHsCDzCHs and CDsCH2CDs. A fast flow system was used, at 250, and a pressure of 11-12 mm, with (i) pure NO and (ii) a mixture of NO and l-GHs, added as inhibitors.With pure NO, the propyl radicals formed the corresponding oximes, nitropropanes, nitrites and nitrates. Low flow rates and low % (NO) favoured oxime formation. From the products, the relative percentage of those of n-propyl origin (%(n-pr)) could be determined. With pure NO, %(n-pr)rose to a maximum with %(NO) in the 15-30 range. The high NO levels were required to scavenge primary H-atoms. The same effect could be achieved by using 2% NO, and 4.8% 1-butene. This dual-inhibitor method gave the following results with the %(n-pr) values in parentheses : CsHe(42); CsDa(49); CHsCDzCHa(77); CDsCHsCDs(25). It is shown that the results can be explained in terms of a cyclic quenching complex involving quasi-bivalent bonding of H g 6(3P~) simultaneously to a primary and secondary C-H (or C-D) bond. The C3H8 and C3De cyclic complexes symmetrically cleave forming nearly equal yields of the two propyl radicals. With C-H and C-D bonds in the complex, as in CH3CD2CHs and CDsCH2CD3, a D/H isotope effect of ca three results, favouring C-H cleavage.