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

Abstract

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.

An INDO study of the hyperfine coupling constants and equilibrium geometries of some tetra-atomic radicals

“…The poorest fit occurs for CH2F which is almost certainly less pyramidal than we calculated. We note that the minimum energy configuration predicted by the INDO method for CH2OH (isoelectronic with CHEF) also corresponds to proton splittings much smaller than the experimental values [16]. In fact there seems to be a general tendency for the INDO method to overemphasize the pyramidal character of tetra-atomic radicals.…”

Mechanisms of Mutagenic DNA Nucleobase Damages and Their Chemical and Enzymatic Repairs Investigated by Quantum Chemical Methods

“…(Sanderud & Sagstuen 1996) It is noted that the C 3 centre and adjacent oxygen have spin densities of 0.84 and 0.11 respectively, suggesting slight delocalization of the SOMO. Dobbs et al (Dobbs et al 1971;Dobbs et al 1972) have suggested that this is due to incomplete rehybridization of the carbon-centered radical and is common when the carbon is bonded to an oxygen. Indeed, in radical-cationic III in which the phosphate group on C 3 has been lost, i.e., there is no C 3 -O bond, the radical is now fully localized on C 3 .…”

Photochemistry of cyclic α-hydroxy ketones. I. The nature and genesis of the photoproducts from steroidal 5-hydroxy 6-keto steroids and related compounds

“…Simple alkyl radicals (26) (with the possible exception of the tert-butyl radical (27)) and cyclohexyl 3~h e s e factors are considered to account also for the stereoselective nature of the photolyses of the 5a-methoxy ketone 37 For personal use only. radicals (28) are generally considered to have a planar radical geometry.4 Introduction of oxygen functionality at the carbon radical center induces deviation from planarity (30,31). This deviation from planarity has been attributed primarily to delocalization involving a lone pair on the oxygen atom and the unpaired electron (30,32).…”

“…radicals (28) are generally considered to have a planar radical geometry.4 Introduction of oxygen functionality at the carbon radical center induces deviation from planarity (30,31). This deviation from planarity has been attributed primarily to delocalization involving a lone pair on the oxygen atom and the unpaired electron (30,32).…”