Delocalisation steric inhibition

The hetero radicals that have already been referred to—(9, p. 301), (10, p. 302), (14, p. 302) and (15, p. 302)—owe their relative stability [with respect to their dimers—apart from l,l-diphenyl-2-picrylhydrazyl (10)] to a variety of factors (a) the relative weakness of N—N, S—S and 0—0 bonds, (b) the delocalisation through the agency of aromatic nuclei, and (c) steric inhibition of access to the atom with the unpaired electron, or to an aryl p-position, cf. (50). The latter factor bulks large (in addition to the weakness of O—O bonds) in the great stability of (15, cf. p. 302) and all three factors operate to stabilise (51), which is wholly dissociated in solution ... 

Interestingly enough, some biradical character has been observed in systems similar to (136) even when there are no bulky chlorine atoms present to inhibit delocalisation sterically. Thus with the system (138) = (139),... 

Adjacent substituents should inhibit formation of planar, or even partially planar conformations. Thus 1,2,3,4- and 1,2,3,8-tetra-methylcyclooctatetraenes have been isolated as separate compounds which do not interconvert at room temperature [12,13]. The 1,2,3,8--isomer can be converted slowly into the 1,2,3,4-isomer at temperatures above 350 but the reverse isomerisation was not observed even at 420 [12]. In the case of 1,2,5,6- and 1,4,5,8-tetramethylcyclo-octatetraenes bond shift is somewhat inhibited but not sufficiently so to give the separate isomers shelf life [13]. It has been pointed out that steric interactions between the methyl groups will in fact be less in the delocalised planar form than in a localised planar form [14] and for 1,2,3,4-tetramethylcyclooctatetraene AG j for bond shift and ring inversion are equal [15]. 

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