Proton Hyperfine Splittings in the ESR of Some Alkylphosphine Dimeric Radicals

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The triplet splittings due to protons were observed for the ESR of the dimeric cation radicals of P(CH3)3, P(CH2CH3)3, and PH(CH3)2; they were formed by the γ irradiation of the rigid CH2Cl2 solutions of these alkylphosphines at 77 K. The observed proton hyperfine splittings for the dimer-cation radicals, [(CH3)3P–P(CH3)3]+ and [(CH3CH2)3P–P(CH2CH3)3]+, were assigned to the hyperfine interaction with two of the methyl and with two of the methylene protons respectively, while those for the [(CH3)2HP–PH(CH3)2]+ radical were assigned to the hyperfine interaction with the two P–H protons. In the former two radicals, the rotations of the methyl and the ethyl groups are restricted. The conformations of the methyl and the ethyl groups have been estimated on the basis of the observed proton hyperfine splittings. The P–H protons of the [(CH3)2HP–PH(CH3)2]+ radical have an extremely large isotropic coupling constant of 58 G; it may be attributed to the direct interaction with the unpaired electron in the anti-bonding σ* orbital of the P–P bond.
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