The radical anions of 5H-dibenzo [a,d]cycloheptene (9), 5H-dibenzo[c,fl[ 1,2]diazepine (lo), 5,6-dihydrodibenzo[a,e]cyclooctene (1 l), 5,6-dihydrodibenzo[c,g][ 1,2]diazocine (It), and (E)-2,2,5,5-tetramethyl-3,4-diphenylhex-3-ene (13) were characterized by ESR and ENDOR spectroscopy. Their hyperfine data were compared with those previously reported for radical anions also containing the stilbene or the azobenzene n-system. Whereas the n-spin distribution in the radical anions of the stilbene series is only moderately sensitive to deviations of the n-system from planarity, the radical anions of the azobenzene series respond to steric strain by shifting the n-spin population from the benzene rings to the azo group. This finding is impressively demonstrated by the similar hyperfine data for 9'-and 11'-which contrast with the strongly differing ones for their azo counterparts 10'-and 12'-, as well as by the corresponding values for sterically highly hindered 13'-. A plausible interpretation is readily provided by the electron affinities of the constituent n-moieties in stilbene and azobenzene. While those of benzene and ethene are both comparatively low, the azo group has a considerably higher electron affinity.Introduction. -The isoelectronic (E)-stilbene (1) and (E)-azobenzene (2) belong to the basic organic compounds of which radical anions were first investigated by ESR spectroscopy 30 years ago [l-31, and which have since been repeatedly studied by ESR and ENDOR techniques [4][5][6]. Due to the pronounced electron affinity of the azo group, the half-wave potential of 2 (-1.38 V vs. SCE [6]) is markedly less negative than that of 1 (-2.22 V [7]). Despite this difference, the radical anions 1'-and 2'-have a similar 71-spin distribution which is evidently governed by the same topology of their 71 -systems. It may, however, be argued that this distribution will be affected differently by deviations of the 71-system from planarity.Conversion of 1 and 2 into their corresponding (2)-isomers 3 and 4 is a straightforward way to increase such deviations. Both radical anions, 3'-and 4'-, are not persistent because they rapidly isomerize to 1'-and 2'-, respectively. Nevertheless, 3'-could be
