Radical pair formation in n -decane and n -decane-d22 single crystals by γ irradiation. Evidence by electron spin resonance for pairwise trapping at six different distances

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Abstract
Radicals and radical pairs in γ‐irradiated single crystals of n ‐decane, n ‐decane‐d 22, and n ‐octane‐d 22 have been studied by means of electron spin resonance. A simple device was constructed to permit orientation of single crystals in the magnetic field at subambient temperature. In n ‐decane two different radical species, CH3CH(CH2)7CH3, R(I), and −CH2CHCH2−, R(II), are produced. The hyperfine coupling tensor for the α proton of the R(I) radical has the principal elements Axx = 11.7 G, Ayy = 35.2 G, and Azz = 19.6 G. In n ‐decane‐d 22 only one species of radical, −CD2CDCD2−, RD(II), is formed. The radical pairs in n ‐decane and n ‐decane‐d 22 consist of two R(II) or RD(II) radicals. In n ‐decane‐d 22, six different radical pairs, differing in their fine‐structure splitting, are observed, and of the total amount of radicals about 3% are paired at 77 K. Each radical pair is characterized by a certain temperature above which it is unstable. A short distance between the radicals constituting the pair implies a low degree of thermal stability. The radical pairs are believed to disappear by recombining to form dimers. The mechanism of radical pair formation is discussed.

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