ENDOR studies of the (E)-stilbene anion radical and its methylated derivatives

Higuchi, Jiro; Ishizu, Kazuhiko; Nemoto, Fujito; Tajima, Kunihiko; Suzuki, Hiroshi; Ogawa, K.

doi:10.1021/ja00331a003

Cited by 21 publications

(9 citation statements)

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“…), proved to be the radical anion of 7, as confirmed by the reduction of an authentic sample of 7. The hyperfine data determined for 7: in the present work closely agree with those reported for this radical anion in [16]. Both radical anions 1 ; and 7; were also obtained with 4 as the starting material.…”

supporting

confidence: 89%

The Radical Anions of [2.2]Paracyclophane‐1,9‐diene and Some of its Derivatives. An Unexpected Conformational Interconversion

Bruhin

Gerson

Möckel

et al. 1985

Helvetica Chimica Acta

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Dedicated to Prof. Virgil Boekelheide on the occasion of his 65th birthday (27.XII.84) ~The radical anions of [2.2]paracyclophane-1,9-diene (2) and its 1,10,12,13,15,16-hexadeuterio derivative 2-D6, as well as those of 4,5,7,8-tetramethyl[2.2]paracyclophane-l,9-diene (3) and its 12,13,15,16-tetradeuterio derivative 3-D4, have been studied by ESR spectroscopy. The coupling constants for 2: at 178 K are 0.422 mT for four equivalent olefinic protons and 0.046 and 0.020 mT, each for a set of four equivalent aromatic protons. This hyperfine pattern is consistent with either benzene ring bearing two pairs of equivalent protons and it points to a lowering of the anticipated D,, symmetry. The ESR spectra of 2: are strongly temperature dependent, due to modulation of the two coupling constants of 0.046 and 0.020 mT; these have opposite signs and average to 0.013 mT at 273 K. The experimental findings are interpreted in terms of a transition state of D2h symmetry, 33 kJ/mol above two interconverting equivalent conformations of lower symmetry. Several pieces of evidence suggest that this symmetry is D,, i.e., the benzene rings in 2: are twisted in opposite directions about the vertical axis. Temperature dependence of the ESR spectra, resulting from modulation of the hyperfine interactions with the aromatic protons, is also observed for 2-D6; and 37. In the case of 3', the olefinic protons are, as expected, only equivalent in pairs, the pertinent coupling constants being 0.560 and 0.325 mT. Upon standing at low temperatures, 2; and 3: gradually convert into the radical anions of [2.2]paracyclophane (1) and its 4,5,7,8-tetramethyl derivative, respectively. At higher temperatures, cleavage of one bridging chain in 2: also occurs, with the formation of the radical anion of (E)-4,4'-dimethylstilbene (7). Both reactions of 27 must involve the transient radical anion of [2.2]paracyclophane-l-ene (4) as proved by the observation of the spectra of 1 : and 7: with 4 as the starting material.Introduction. -Replacement of the single by the double bonds in the two bridging alkyl chains of [2.2]paracyclophane (l), to yield the 1,9-diene 2, has only a small effect on the geometry and electronic structure. As shown by the X-ray analyses of 1 [I] and 2 [2],

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supporting

confidence: 89%

The Radical Anions of [2.2]Paracyclophane‐1,9‐diene and Some of its Derivatives. An Unexpected Conformational Interconversion

Bruhin

Gerson

Möckel

et al. 1985

Helvetica Chimica Acta

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“…Tables 1 and 2 For the radical anions of (E)-stilbene (1) and (E)-azobenzene (2), the assignments in Tables I and 2 were confirmed by specific deuterations [2] [3] [6] and alkyl substitutions [4][5][6]. In particular, ESR and ENDOR studies on the radical anions of 2,2'-and 3,3'-di- [6].…”

mentioning

confidence: 72%

“…-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]).…”

mentioning

confidence: 99%

“…-5H-Dibenzor a,d]cycloheptene (9). 5H-Dibenzo[ c,f/[ 1,2]diazepine (lo), 5,6-Dihydrodibenzo[ a,e]cyclooctene (1 l), and 5,6-Dihydrodibenzo[ c,g//l ,Z/diazocine (12). The radical anions 9'--12'-were generated from their neutral precursors with K in 1,2-dimethoxyethane (DME).…”

mentioning

confidence: 99%

“…alkyl derivatives of 1 [5] and 2 [4] [6] allowed one to distinguish between the two pairs of nonequivalent protons in the ortho-positions (o and o') and, similarly, between two such pairs in the meta-positions (m and rn'). The coupling constants of the ortho-protons with the larger absolute value (1'-: -0.302 mT; 2'-: -0.294 mT) could be unequivocally assigned to the sterically more congested o '-positions; likewise, those of the rneta -protons with the larger laHl value (l'-: +0.082 mT; 2'-: +0.089 mT) were attributed to the adjacent rn'-positions.…”

mentioning

confidence: 99%

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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

Gerson¹,

Lamprecht²,

Scholz³

et al. 1996

Helvetica Chimica Acta

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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

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Electron‐transfer Reactions of Aromatic Compounds

Gescheidt¹,

Khan²

2001

Electron Transfer in Chemistry

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Aromatic molecules are inclined to undergo electron-transfer reactions. The additional charges introduced by the electron-transfer process are efficiently stabilized by delocalization. Nevertheless rearrangements of the molecular skeleton or followup reactions can occur. The course of the electron-transfer reactions and the properties of the species thus formed are the subject of this section.In particular, we concentrate on the species formed after an one-electron transfer reaction. Starting from closed-shell diamagnetic precursors, paramagnetic stages, i.e. radical cations or radical anions are formed in this first step. To understand the properties and reactivity of these species, detailed knowledge in terms of charge and spin delocalization and electronic structure is an important prerequisite. This information is preferably derived from electronic and paramagnetic resonance (EPR and electron-nuclear multiple resonance-techniques such as ENDOR or Triple spectroscopy). It has recently been shown that the experimental results can be substantiated by the use of quantum-chemical calculations. In addition to Hartree-Fock-based ab initio procedures, calculations on the density-functional level of theory have been established as rather efficient tools. Therefore, before representing examples of electron-transfer-generated radicals a short survey of the computational methods is given.Another substantial factor directing the kinetic and thermodynamic stability of charged radicals is ion pairing. This phenomenon, although well established for many years, is often not directly distinct in experiments. To establish the importance of ion pairing, or, in other words, supramolecular interactions, a separate introductory chapter is dedicated to these aspects.The references are selected particularly from recent publications, without, however, ignoring some 'classical' contributions. In some sections topics are included which do not strictly represent aromatic molecules but involve interactions of ztype orbitals.

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ENDOR studies of the (E)-stilbene anion radical and its methylated derivatives

Cited by 21 publications

References 0 publications

The Radical Anions of [2.2]Paracyclophane‐1,9‐diene and Some of its Derivatives. An Unexpected Conformational Interconversion

The Radical Anions of [2.2]Paracyclophane‐1,9‐diene and Some of its Derivatives. An Unexpected Conformational Interconversion

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

Electron‐transfer Reactions of Aromatic Compounds

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