E.S.R. studies of 31P hyperfine anisotropy in phosphabenzene radical anions

E.S.R. studies have been performed on the fluid and glass solutions of phosphabenzene radical anion (I⊝) and three of its 2,4,6-substituted derivatives (II⊝, III⊝ and IV⊝). The spectra of fluid solutions are characterized by a dominant doublet splitting due to the 31P nucleus (a p = +26 to +36 gauss†) and an unusually large line-width of the order 1 gauss. The spectra of glassy solutions show that this line-broadening arises mainly from a marked 31P hyperfine anisotropy (A p xx , A p yy = -11 to -16; A p zz = +116 to +132 gauss) which in turn is a consequence of a high π-spin population at the phosphorus centre (ρ p = +0·42 to +0·47). The spin distribution in the radical anions I⊝ to IV⊝ is excellently accounted for by a simple HMO-McLachlan model of a planar cyclic π-system (α p = α + 0·3β; β cp = 0·7β). Rotational correlation times for I⊝ and II⊝ in fluid solutions (τ r = 25 to 300 picosec) are evaluated and the corresponding ‘effective’ molecular radii (r = 2·8 to 5·0 A‡) discussed.