The electron spin resonance spectrum of X-irradiated single crystals of diphenylphosphine oxide

X-irradiation of single crystals of yields the (C6H5)2PO radical, the phosphorus analogue of diphenyl nitroxide. The principal values of the 31P hyperfine coupling constants are 1439 MHz, 929 MHz and 908 MHz, which may be resolved into an isotropic component of 1092 MHz, and anisotropic components of 347 MHz, -163 MHz and -184 MHz. This implies that unlike diphenyl nitroxide, this radical is nonplanar (s/p ratio = 0·18, s-character = 0·11, p-character = 0·60).