ESR spectra of endohedral metallofullerene Ce@C82 radical anions in dimethylformamide and pyridine

Endohedral metallofullerene Ce@C₈₂ dissolved in dimethylformamide or pyridine is reduced to the radical anion. Analysis of hyperfine coupling with the topologically different ¹³C nuclei indicates the electronic structure with bivalent cerium and the paramagnetic carbon framework Ce²⁺@C₈₂ ^·3–. The ESR spectra of the radical anions of the functionalized Ce@C₈₂ derivatives are detected.     

ESR.-Spektren des Radikal-Anions des syn-1, 6; 8, 13-Bis-oxido-[14]annulens

The ESR.-spectra of the radical anion of syn-1, 6; 8, 13-bis-oxido-[14]annulene have been recorded. The hyperfine structure of the electrolytically generated anion (solvent: N, N-dimethylformamide; gegenion: Et₄N^⊕) is that of an unassociated species; on the other hand, evidence of strong ion-pairing can be derived from the spectra of chemically prepared anions (solvent: 1,2-dimethoxyethane; gegenion: K⊕ or Na⊕). The distribution of the n-spin population confirms the conclusion previously drawn for the radical anion of 1,6-oxido-[70]annulene that the overall effect of the oxygen bridging is electron repelling.     

Isotropic and anisotropic chlorine hyperfine coupling of dichloro-dicyano benzoquinone anion radical

The ESR spectrum of 2,3-dichloro-5,6-dicyano benzoquinone anion has been recorded in the isotropic and in the nematic phase of liquid crystal solvents. In the isotropic phase it shows splitting only by two ¹⁴N nuclei, while in the nematic phase at high orientation degree, the pattern is that expected for hyperfine coupling with two chlorine nuclei. The orientation of the free radical in the nematic solvent is inferred from the variation of the ¹⁴N splitting.The chlorine anisotropic coupling is used to get the unpaired electron spin density on the chlorine π orbital and the isotropic coupling is discussed in terms of a relationship analogous to that proposed by Karplus and Fraenkel for ¹³C.     

ESR.-Untersuchungen an Radikal-Anionen des 1, 6-Imino-[10]annulens und seines N-Methyl-Derivats

The ESR.-spectrum of the radical anion of 1,6-imino-[10]annulene (II) has been recorded. Its hyperfine structure reflects the reduced symmetry (C_s) of the molecule, as compared with that (C_2v) of 1,6-methano- and 1,6-oxido-[10]annulenes (I and III, resp.). The coupling constants of the ring protons in II^? are intermediate between the corresponding values of I^? and III^?. The ESR.-spectrum of the radical anion of 1,6-methylimino-[10]annulene (IV) has also been obtained, but not analysed in detail. The relative stabilities of the radical anions of the four bridged [10]annulenes are: I^??II^? > III^? > IV^?. The main secondary product identified by ESR.-spectroscopy after the decay of II^?, III^? and IV^? is the naphthalene radical anion. A remarkable exception is IV, when reduced with sodium in 1,2-dimethoxyethane: in this case the ESR.-spectrum of the azulene radical anion is observed.     

Notiz über das ESR.-Spektrum des Phenalenyl-Radikals

The ESR. spectrum of the phenalenyl radical has been reexamined under increased resolution. The proton coupling constants (a₁ = 6,29 and a₂ = 1,805 gauss) are in agreement with the values reported by BENNETT [3]. Two ¹³C coupling constants have been determined (a_C1 = 9,66 and a_C2 ≈? a_C10 = 7,84 gauss) and their assignment has been discussed.     

Fulvalene-Embedded Perylene Diimide and Its Stable Radical Anion

The first example of a two-state (neutral and reduced), stable electron-accepting material and its radical anion is presented. FV-PDI, generated from cyclocarbonylation and then a carbonyl coupling reaction, shows a largely degenerate LUMO of −4.38 eV based on the delocalization of π-electrons across the whole molecular skeleton through a fulvalene bridge. The stability and electron affinity allow spontaneous electron transfer to afford a stable radical anion. Spectroscopic characterization and structural elucidation showed that the radical anion [FV-PDI]^.− has remarkable stability and near-infrared absorptions extending to 1200 nm. Single-crystal X-ray diffraction analyses revealed significant changes in the molecular shape and packing arrangement of the formed radical anion. The central C−C bond linking the two PDI halves is lengthened from approximately 1.33 to 1.43 Å, and the alternating arrangement of positively and negatively charged units favor the stable charge-transfer complex.     

The isotropic and anisotropic EPR spectra of the tetrafluoroethylene radical anion

The tetrafluoroethylene radical anion has been generated in solid solutions by electron addition to the parent molecule. Both its isotropic and anisotropic EPR spectra have been observed, including the isotropic ¹³C satellite lines in natural abundance. The isotropic EPR parameters are a_F = 94.3 G, a_C = 48.7 G, and g = 2.0027. Two possible geometries, planar and chair, are discussed for this radical anion. The magnitude of the ¹⁹F and ¹³C hyperfine coupling constants are consistent with a planar (D_2h) structure provided that the unpaired electron occupies the 5b_1u (σ*) rather than the 2b_1g (π*) molecular orbital which is predicted by ab initio calculations to be the LUMO of the parent molecule. On the other hand, the EPR parameters do not rule out a chair (C_2h) structure if the bending of the CF₂ groups introduces only a small distortion from planarity.     

The Radical Ions of Dipleiadiene (Dicyclohepta[de,ij]naphthalene) and Its 12b,12c-Homo Derivative. An ESR and ENDOR study

The radical anions and the radical cations of dipleiadiene (dicyclohepta[de,ij]naphthalene; 1 ) and its 12b, 12c-homo derivative 2 were characterized by ESR and ENDOR spectroscopy. Their singly occupied orbitals are related to the degenerate nonbonding MOs of a 16-membered π-perimeter. The π-spin distribution over the perimeter is similar in the radical cations 1 ^.+ and 2 ^.+, and an analogous statement holds for the radical anions 1 ^.? and 2 ^.?. However, deviations of the π-system from planarity lead to a decrease in the absolute values of the negative coupling constants of the perimeter protons in 2 ^.+ and 2 ^.? relative to those in 1 ^.+ and 1 ^.?. The hyperfine data for the perimeter protons in the radical ions correlate with the changes in ¹³C chemical shifts on passing from the neutral compounds to the corresponding diions. It is concluded from the coupling constants of the CH₂ protons in the radical ions of 2 that the cation 2 ^.+ exists in the methano-bridged form ( A ) of the neutral 2 (and, presumably, also of the dication 2 ²⁺), whereas the anion 2 ^.? adopts the bisnorcaradiene form ( B ) of the dianion 2 ^2?.     

13C hyperfine coupling constants in o-benzosemiquinones

The ¹³C hyperfine splitting constants of the tetrachloro-o-benzosemiquinone radical anion are measured for naturally occurring ¹³C nuclei in all the possible positions in the molecule. Their assignment is based on the linewidth analysis of the single hyperfine components and this is discussed with reference to the reported spin density distribution in o-semiquinones.     

Fulvalene‐Embedded Perylene Diimide and Its Stable Radical Anion

The first example of a two‐state (neutral and reduced), stable electron‐accepting material and its radical anion is presented. FV‐PDI, generated from cyclocarbonylation and then a carbonyl coupling reaction, shows a largely degenerate LUMO of ?4.38 eV based on the delocalization of π‐electrons across the whole molecular skeleton through a fulvalene bridge. The stability and electron affinity allow spontaneous electron transfer to afford a stable radical anion. Spectroscopic characterization and structural elucidation showed that the radical anion [FV‐PDI]^.? has remarkable stability and near‐infrared absorptions extending to 1200 nm. Single‐crystal X‐ray diffraction analyses revealed significant changes in the molecular shape and packing arrangement of the formed radical anion. The central C?C bond linking the two PDI halves is lengthened from approximately 1.33 to 1.43 Å, and the alternating arrangement of positively and negatively charged units favor the stable charge‐transfer complex.     

Synthesis of new derivatives of a representative o-quinone scaffold by reduction at the electrode

The concomitant electrochemical reduction of 9,10-phenanthrenequinone and benzenediazonium tetrafluoroborate, under constant potential conditions (?0.5 V vs Ag/Ag⁺), has been performed in different organic solvents. An interesting entry to new C–C radical coupling products is here described. Another cathodic pathway where the anion radical intermediate acts as an EGB is also described. The electrochemical behaviour of both systems and mechanistic proposals are given.     

Etude RMN 13C d'un Dérivé Uniformément Enrichi en 13C. Signe des Valeurs des Constantes de Couplage Observées

The ¹³C NMR parameters of 3-O-acetyl-1,2:5,6-di-O-isopropylidène-α-D-[U-¹³C] glucofuranose, used as a sample for analysis in double labelling biosynthetic experiments, have been measured. Homonuclear double resonance experiments ¹³C? {¹³C} at 62.8 MHz have permitted the determination of all the ¹³C? ¹³C coupling constants. By theoretical computation of spectra, in connection with the second order effects existing at 25.2 MHz and 15.08 MHz, the sign of the coupling constants has been determined. The theoretical computation of spectra took into account all the isotopomers and was calculated with the help of a program (adapted from the LAOCOON program) allowing for the weighted addition of the spectra.     

ESR. Spectra and Structures of Radical Anions in the Dibenzo[a,e]cyclooctene Series

ESR. studies are reported for the radical anions of 5,6-didehydro- and 5,6,11,12-tetradehydro-dibenzo[a,e]cyclooctene (III and IV, resp.), in addition to that of dibenzo[a,e]cyclooctene (II) itself, the spectrum of which has been reexamined. Comparison of the proton and ¹³C coupling constants for II·?, III·? and IV·? indicates that the three radical anions do not differ greatly in their electronic and molecular structures. This statement implies that II·? should also be substantially planar, i.e., the tub-shaped eight-membered ring in II is expected to flatten on passing from the neutral molecule to its radical anion. Support for postulating such a change in geometry, analogous to that encountered with the parent cyclooctatetraene (I), is provided by INDO calculations.     

Pyramidal distortion of the NO<Subscript>2</Subscript> group in the radical anion of 2,4,6-trimethoxynitrobenzene

Electrochemical reduction in DMF is used to produce the radical anion of 2,4,6-trimethoxynitrobenzene with an EPR spectrum that shows hyperfine splitting for ¹⁵N nuclei of the nitro group and ¹³C nuclei of the benzene ring given the natural abundance of the isotopes. According to UB3LYP/6-31+G* quantum chemical calculations considering solvation in the PCM model, the nitro group in the equilibrium conformation of the RA of 2,4,6-trimethoxynitrobenzene is rotated relative to the benzene ring plane by an angle close to 90° and has a pyramidal structure. The calculated isotropic hyperfine coupling constants for this conformation are closest to the experimental results.     

The structure of thiophene sesquioxide: Syn,endo-3a,4,7,7a-tetrahydro-4,7-epithiobenzo[b] thiophene 1,1,8-trioxide

Thiophene sesquioxide is shown to be syn, endo-3a,4,7,7a-tetrahydro-4,7-epithiobenzo[b]-thiophene 1,1,8-trioxide (I) by ¹ H and ¹³ C nmr evidence. Assignment of the ¹³ C spectrum was facilitated by a cross-ring long-range ¹³ C-¹ H coupling. The mass spectrum of I is dominated by an unusual break-down to give benzene radical cation.     

Electroreduction of 9-fluoro-10-cyanoanthracene

The electroreduction of 9-fluoro-10-cyanoanthracene (1) was voltammetrically studied. In contrast to expectations, no halide elimination was observed at the radical anion level but after a radical ion coupling the dimer 9,9^′-bianthryl-10,10^′-dicarbonitrile (2) was formed in quantitative yield. The mechanism was studied using digital simulation. Thermodynamic and kinetic parameters were determined. The low activation energy of the dimerization step indicates a diffusion-controlled radical ion coupling, which produces a σ-dimer but not a π-dimer as often suggested in the literature.     

The E.S.R. Spectrum of the tetra(1-adamantyl)cyclobutadiene radical cation

A revised interpretation is proposed for the ¹H and ¹³C hyperfine coupling in the e.s.r. spectrum of the tetra-1-adamantylcyclobutadiene radical cation, and assumes that free rotation occurs about the 1-adamantyl bond.     

13C?13C spin coupling constants in tetrahydronaphthylene,hexahydrobenzanthracene and benzo[a]pyrene derivatives

The ¹³C? ¹³C spin coupling constants have been determined in substituted [1-¹³C]tetrahydronaphthylenes, [5-¹³C]hexahydrobenzanthracenes and [5-¹³C]benzanthracene. In addition, the ¹³C? ¹³C spin coupling constants for 7-hydroxy[7-¹³C]benzopyrene, trans-7,8-dihydro[7-¹³C]benzopyrene-7,8-diol and trans-7,8-dihydro[10-¹³C]benzopyrene-7,8-diol are reported, together with the one-bond carbon-carbon coupling constants between C-4 and C-5 in selected 4,5-disubstituted benzopyrenes. Values for the directly bonded coupling constants and long-range coupling constants are similar to those reported previously for other aromatic and aliphatic systems. Substituent effects on carbon-carbon coupling are compared for similarly substituted cyclic and acyclic systems.     

1H-,2H-,13C- and 14N-endor studies of coppinger's radicals in liquid solutions and in liquid crystals

Proton and non-proton ENDOR studies are reported on isotopically labelled Coppinger's radical (galvinoxyl) and on N-Coppinger's radical in isotropic solutions and in liquid-crystalline mesophases. The hyperfine coupling constant shifts have been measured over the temperature range of the nematic mesophase and an analysis of the shifts in terms of anisotropic hyperfine tensors and ordering parameters is given. A short phenomenological discussion of the ¹³C and¹⁴N ENDOR response is presented. Several methods for the sign determination of hyperfine coupling constants are discussed. Finally, a survey of the ENDOR spectroscopy on galvinoxyl type biradicals is given.     

13C dynamic nuclear polarization using isotopically enriched 4‐oxo‐TEMPO free radicals

The nitroxide‐based free radical 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) is a widely used polarizing agent in NMR signal amplification via dissolution dynamic nuclear polarization (DNP). In this study, we have thoroughly investigated the effects of ¹⁵N and/or ²H isotopic labeling of 4‐oxo‐TEMPO free radical on ¹³C DNP of 3 M [1‐¹³C] sodium acetate samples in 1 : 1 v/v glycerol : water at 3.35 T and 1.2 K. Four variants of this free radical were used for ¹³C DNP: 4‐oxo‐TEMPO, 4‐oxo‐TEMPO‐¹⁵N, 4‐oxo‐TEMPO‐d₁₆ and 4‐oxo‐TEMPO‐¹⁵N,d₁₆. Our results indicate that, despite the striking differences seen in the electron spin resonance (ESR) spectral features, the ¹³C DNP efficiency of these ¹⁵N and/or ²H‐enriched 4‐oxo‐TEMPO free radicals are relatively the same compared with ¹³C DNP performance of the regular 4‐oxo‐TEMPO. Furthermore, when fully deuterated glassing solvents were used, the ¹³C DNP signals of these samples all doubled in the same manner, and the ¹³C polarization buildup was faster by a factor of 2 for all samples. The data here suggest that the hyperfine coupling contributions of these isotopically enriched 4‐oxo‐TEMPO free radicals have negligible effects on the ¹³C DNP efficiency at 3.35 T and 1.2 K. These results are discussed in light of the spin temperature model of DNP. Copyright © 2016 John Wiley & Sons, Ltd.