Effect of non‐planarity on the spin distribution in the radical anions containing the stilbene or the azobenzene π‐system: An ESR and ENDOR Study

Abstract

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

Electronic Structure of the Neutral Tyrosine Radical in Frozen Solution. Selective 2H-, 13C-, and 17O-Isotope Labeling and EPR Spectroscopy at 9 and 35 GHz

“…Larger out-of-plane orientations cause direct overlap of the proton s orbital with the p z orbital of the neighboring carbon and will drastically increase the magnitude of the isotropic proton hfi . Deviations of the whole π-system from planarity have only a moderate effect on the coupling constants of the α-protons …”

Effects of Steric Congestion on Stilbene Radical Anions and Dianions: DFT Calculations in the Interpretation of Stilbene Radical Anion ESR Spectra

“…12 The dianion of 3 (or 4) is reported to form when 3 (or 4) is treated with potassium in dimethoxyethane at 193-213 K. These solutions show no ESR signal, but both gave the same ESR spectrum when the dianion solution was irradiated with light. 12 The species suggested for this signal is 3 1-. The reported a H values for 3 1are given in Table 1 along with experimental data on a series of related compounds, 7-9.…”

Electron‐transfer Reactions of Aromatic Compounds

“…This is impressively demonstrated by similar hyperfine data for 110'-and 111'-which contrast with the strongly differing one for their azo counterparts 112'-and 113.-, as well as by the corresponding values for sterically highly hindered 114'- [330].…”