Electron spin resonance studies. Part XXXII. Information from hyperfine splittings for aliphatic radicals about shape and conformation

Abstract

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

Electron Spin Resonance Studies in Aqueous Solution: Fragmentation of Radical Intermediates Derived from β-Amino Alcohols

“…constants are observed; for example aC,,,(2H) = 8.5 G for the choline derived radical + (CH,),NCH2C~OH. Further lowering beyond that predicted by 19 is attributed to the electron delocalization shown in structures 20 and 21, a model that suggests planarity at the a-carbon and a distortion from tetrahedral geometry at the P-carbon (20). Furthermore, the relatively large P-nitrogen h.f.s.…”

“…constants that are observed for organic n radicals of the type YCH,CHX have been analyzed by Norman and co-workers (20) in a wide-ranging general treatment. Observing that the phenomenon was restricted to cases where Y was an electron acceptor and X had an available electron pair, he proposed a preferred conformation of the radical that gained stabilization as a result of this sub-PCH, h.f.s.…”

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).…”

“…Crystallization from methanol afforded lactone 24 as colorless plates, mp 159.5-160.5"C (lit. (40) Irradiation of 3~-benzylox~~-5-hydroq-5~-cholestar1-6-011e (22 Irradiation of 3P-acetoxy-5-deutero,ry-5a-cholestan-6-one (31 ).…”