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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 ( a -I * C ) but also a ( a -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( a -H ) and a( a-ISC) for the fragment H-C* with J ( I S C H ) for the corresponding fragment H-C-H and of a(cc-H) with J ( H H u m ) , are noted and their origins are examined. The correlations for a ( a -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 a t 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 a t the tervalent carbon atom by a methyl or trifluoromethyl group can also be understood. The a ( P -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(P-H) is far less sensitive to bending than is a(a-H). Finally, the results of INDO calculations t for two of the smaller radicals are in agreement with the conclusions reached from the experimental data.Heslington, York YO1 5DD THIS paper describes approaches to the determination of the geometry at the tervalent carbon atom of organic radicals in solution and the elucidation of the factors which govern it; we have been concerned particularly with cyclic radicals in which the tervalent carbon is bonded to one or two oxygen atoms.2 Our principal source of information has been the e.s.r. data for these species; we have found that not only a(a-13C) but also a(ct-H) provides important evidence about radical shape, and that a( a-H) and the g-factor are informative.$We have also explored the n.m.r. data for the parent t INDO method : semi-empirical molecular orbital calcula-$ a, / I , y, 6 refer respectively to the tervalent carbon atom and l Part XXVI, D. J. Edge, R.

A previous study of the relationship between the hyperfine splitting parameters of an aliphatic radical and the geometry at its tervalent carbon atom has been extended to a wider range of radicals. In particular, the significance of a(%) is discussed, data for some simple ketyls *CR1R2*0-are reported and interpreted, and it is suggested that useful information is available from the ratio a(Me) :a(H) both for radicals *CHMe-X and for the pairs of radicals *CMeXY and *CHXY. It is noted that, in radicals of the type *CHY*CH,X (X = an electronegative substituent, Y = OH or OR),a(CH,) is unusually low; it is argued that this is a manifestation of an interaction which is associated with an eclipsed conformation of the C-X bond and the half-filled orbital and also with less deformation from coplanarity a t the tervalent carbon atom than in the absence of X. Finally, INDO calculations for some small cyclic radicals support the conclusion from the experimental data that the species are not coplanar a t the tervalent carbon atom.WE have previously presented evidence that, in organic radicals in which the tervalent carbon atom is attached to at least one hydroxy-or alkoxy-substituent, the three bonds to that carbon atom are not coplanar.2 (For simplicity, we refer to such species as ' bent '.)The degree of bending increases with the number of such substituents and is also dependent, for cyclic radicals, on the size of the ring. It was suggested that the capacity of the oxygen-containing substituents to induce bending is related to their +M effects; thus, +-conjugation leads to an increase in the electron density a t the tervalent carbon atom, and bending occurs to reduce the resulting inter-electronic repulsions. The dependence of bending on ring size was attributed to increase in the 9 : s ratio of the ring bonds involving the tervalent carbon atom as the strain in the ring is increased. The results of INDO calculations for two of the simpler radicals were in reasonable agreement with the conclusions from the experimental data,We have now extended our study of radical geometry to a much wider range of substituted alkyl radicals. In particular, first, we have examined the significance of a(a-13C) in the light of data for a more extensive list of radicals than was available before. Secondly, we have explored in greater detail the possibility, previously hinted at,2 that for radicals of the type -CHMe*X the ratio of the hyperfine splitting constants for the (p) methyl protons and the single ( a ) proton can give information about the hybridisation of the tervalent carbon atom; 7 we have concluded that it can do so, and this in turn has enabled us to discuss further the factors that determine the hybridisation of this carbon atom, and hence the shape of the radical, and to attempt to separate the dependence of proton-splitting constants on delocalisation within the radical from their t a, p, y Refer respectively t o the tervalent carbon atom and