Electron spin resonance studies. Part XXVII. The geometry of oxygen-substituted alkyl radicals

The isotropic e.s.r. parameters, including in some cases a(¹³C), are reported for a number of cyclic and acyclic radicals in which the tervalent carbon atom is bonded to one or two oxygen substituents. The information which these data, and those for related radicals, give about the geometry at the tervalent carbon atom is discussed. Not only a(α-¹³C) but also a(α-H) is found to be especially significant; although the sign of the latter cannot normally be measured, evidence is adduced that it changes from negative to positive as the tervalent carbon atom changes from a planar towards a tetrahedral configuration, and in some cases the sign can be deduced with assurance.

Correlations of a(α-H) and a(α-¹³C) for the fragment H–C· with J(¹³CH) for the corresponding fragment H–C–H and of a(α-H) with J(HH_gem), are noted and their origins are examined. The correlations for a(α-H) not only help in the assignment of the sign of this parameter in ambiguous cases, but also point to the importance of two factors in determining the degree of bending at the tervalent carbon atom. These are, first, increase in the p-character of the carbon bonds, as induced for example by constraining these bonds within a small ring, and secondly, attachment to the tervalent carbon of a conjugatively electron-releasing group which serves to increase the electron density at the carbon atom; in the latter respect, two oxygen substituents have a far greater influence than one, and the changes in geometry which accompany the replacement of a hydrogen atom at the tervalent carbon atom by a methyl or trifluoromethyl group can also be understood. The a(β-H) values and the g-factors vary with radical geometry in a way which can be rationalised although, for a reason which is described, a(β-H) is far less sensitive to bending than is a(α-H). Finally, the results of INDO calculations for two of the smaller radicals are in agreement with the conclusions reached from the experimental data.