Radical Anions of Cyclic Azoalkanes: An ESR,ENDOR, and TRIPLE-Resonance Study

Radical anions often monocyclic and bicyclic azoalkanes containing the azo group in (Z)-conformation, have been fully characterized by their hyperfine data with the use of ESR, ENDOR, and general-TRIPLE-resonance spectroscopy. These azoalkanes are represented by 3,3,5,5-tetramethyl-1-pyrazoline ( 1 ), 2,3-diaza-bicyclo[2.2.1]hept-2-ene ( 4 ), and 2,3-diazabicyclo[2.2.2]oct-2-ene ( 9 ), as well as by their derivatives 2 , 3 , 5 – 8 , and 10 . For all radical anions 1 ″– 10 ″, the ¹⁴N-coupling constant, a_N, is in the range of +0. 83 to +0. 97 mT; this finding indicates that the spin population is essentially restricted to the π system of the azo group. The ¹⁴N-hyperfine anisotropy largely affects the width of ESR lines, particularly at low temperatures. Substantial coupling constants of ⁷Li-, ²³Na-, ³⁹K-, and ¹³³Cs-nuclei point to a close association of the radical anions with their alkali-metal counterions. With the exception of ³⁹K, these nuclei give rise to readily observable ENDOR signals which appear along with those stemming from protons. The prominent hyperfine features of 1 ″– 10 ″ are discussed.     

Radical Anions of Polyalkylazulenes: An ESR and ENDOR Study

Proton-hyperfine data are reported for the radical anions generated from azulene ( 1 ) and its alkyl derivatives 2 – 11 in 1,2-dimethoxyethane both ‘chemically’ with K and electrolytically. The alkyl derivatives are 1,3-dimethyl- ( 2 ), 5,7-dimethyl- ( 3 ), 1,3,5,7-tetramethyl- ( 4 ), 2-methyl- ( 5 ), 4,6,8-trimethyl- ( 6 ), 2,4,6,8-tetramethyl-( 7 ), 1,3,4,6,8-pentamethyl- ( 8 ), 1,3,4,8-tetramethyl-6-propyl- ( 9 ), 6-(tert-butyl)-1,3,4,8-tetramethyl- ( 10 ), and 1,2,3,4,6,8-hexamethylazulene ( 11 ). Alkyl substituents at the odd-numbered centers μ = 1, 3, 5, and 7 partly shift the π-spin population from the seven- to the five-membered ring, whereas those at the even-numbered centers μ = 4, 6, and 8 exert an opposite effect on the π-spin distribution.     

Radical Anions of Sterically Protected Polyenes: An ESR and ENDOR Study

Owing to the steric protection by four bulky substituents in the terminal positions 1 and n, several conjugated polyenes could be reduced with K or Cs metal in 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF) to fairly persistent radical anions. These compounds, denoted here as 2 , 3 ,…︁ 7 (which corresponds to the number, \2 n=2, 3, …︁7, of their formal double bonds) are 1,1,n,n-tetra(tert-butyl) derivatives of buta-1,3-diene, hexa-1,3,5-triene, octa-1,3,5,7-tetraene, deca-1,3,5,7,9-pentaene, dodeca-1,3,5,7,9,11-hexaene, and tetradeca-1,3,5,7,9,11,13-heptaene. In addition to the six polyenes 2 – 7 with all-trans-configuration, the studies comprised an isomer of 3 , the trans,cis,trans-triene, c -3 . The radical anions 2 ^.− – 7 ^.− and c -3 ^.− were characterized by their hyperfine data acquired with ESR, ENDOR, and TRIPLE-resonance spectroscopy. The ¹H-coupling constants comply with the spin distribution predicted for the radical anions of such `linear' π-systems by simple MO models. Ion pairs formed with K⁺ in DME were loose but became tighter with Cs⁺ in THF. Propensity to ion pairing decreased with the lengthening of the π-system on going from 2 ^.− to 3 ^.− – 7 ^.−. Hyperfine data are likewise reported for the radical anions of all-trans-polyenes 8 and 9 , in which two tert-butyl substituents in one terminal position of 2 and 3 , respectively, were replaced by CN groups.     

An ESR. and ENDOR. Study of the Radical Anions of Cyano- and Nitro-Monosubstituted Naphthalenes

¹H- and ¹⁴N-coupling constants have been determined by ESR. and ENDOR. spectroscopy for the radical anions of 1- and 2-cyano-, and 1- and 2-nitronaphthalene.     

Radical Cations of Tetrazinodi(heteroarenes): An ESR and ENDOR Study

¹⁴N- and ¹H-Coupling constants, determined by ESR, ENDOR, and general-TRIPLE-resonance spectroscopy, are reported for the radical cations of tetrazinodi(heteroarenes) 1–8 . The results comply with the expectation that donor properties of these compounds are mainly due to the electron-rich dihydrotetrazine ring.     

The Radical Anions of 1,2-diphenylcyclohexene and Structurally Related Compounds. Conformational ESR and ENDOR studies

ESR, ENDOR, and TRIPLE resonance studies have been performed on the radical anions of 1,2-diphenylcyclohex-1-ene ( 4 ), 1,2-di(perdeuteriophenyl)cyclohex-1-ene ((D₁₀) 4 ) the trans-configurated 3,4-diphenyl-8-oxabicyclo[4.3.0]non-3-ene ( 5 ) and its 2,2,5,5-tetradeuterio derivative (D₄) 5 , and 2,3-diphenyl-8,9,10-trinorborn-2-ene ( 6 ). The spectra of \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document} exhibit strong temperature dependence along with a specific broadening of ESR hyperfine lines and proton ENDOR signals. The coupling constant, which bears the main responsibility for these features, is that of the β-protons in the quasi-equatorial positions of the cyclohexene ring, and the experimental findings are readily rationlized in terms of relatively modest conformational changes without invoking the inversion of the half-chair form. The hyperfine data for the β-protons in \documentclass{article}\pagestyle{empty}\begin{document}$ 5^{- \atop \dot{}} $\end{document} closely resemble the corresponding low-temperature values for \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document}, However, the ‘unusual’ features observed for \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document} are absent in the ESR and ENDOR spectra of \documentclass{article}\pagestyle{empty}\begin{document}$ 5^{- \atop \dot{}} $\end{document}, because the half-chair conformation of the cyclohexene ring in \documentclass{article}\pagestyle{empty}\begin{document}$ 5^{- \atop \dot{}} $\end{document} is deprived of its flexibility. Although the boat form of this ring in \documentclass{article}\pagestyle{empty}\begin{document}$ 6^{- \atop \dot{}} $\end{document} is also rigid, the spectra of \documentclass{article}\pagestyle{empty}\begin{document}$ 6^{- \atop \dot{}} $\end{document} are temperature-dependent, due to an interconversion between two propeller-like conformations of the phenyl groups. The pertinent barrier is 30 ± 5 kJ ·mol^?1. An analogous interconversion presumably takes place in \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ 5^{- \atop \dot{}} $\end{document} as well, but, unlike \documentclass{article}\pagestyle{empty}\begin{document}$ 6^{- \atop \dot{}} $\end{document}, it is not amenable to experimental study.     

Radical Cations and Radical Anions of Three Bridgehead Azopolycycloalkanes: A Fluid-Solution ESR Study

The radical mono-ions of three azoalkanes in which the azo group is connected to the polycyclic alkane moieties at the bridgehead C-atoms, i. e. 1,1′-azonorbornane ( 1 ), 1,1′-azotwistane ( 2 ), and 1,1′-azobi-cyclo[3.2.1]octane ( 3 ), were studied in fluid solution by ESR spectroscopy. According to the ESR parameters and MO models, the radical cations of 1 – 3 should be considered as σ radicals, whereas the corresponding radical anions are π radicals. INDO calculations point to a remarkable dependence of the ^l4N-coupling constants on the geometry at the N-atoms in the radical cations of aliphatic azo compounds.     

Radical Cations and Radical Anions of Three Bridgehead Azopolycycloalkanes: A fluid-solution ESR study

The radical mono-ions of three azoalkanes in which the azo group is connected to the polycyclic alkane moieties at the bridgehead C-atoms, i.e. 1,1′-azonorbornane ( 1 ), 1,1′-azotwistane ( 2 ), and 1,1′-azobicyclo[3.2.1]octane ( 3 ), were studied in fluid solution by ESR spectroscopy. According to the ESR parameters and MO models, the radical cations of 1–3 should be considered as σ radicals, whereas the corresponding radical anions are π radicals. INDO calculations point to a a remarkable dependence of the ¹⁴N-coupling constants on the geometry at the N-atoms in the radical cations of aliphatic azo compounds.     

ESR. Spectra of the Radical Anions of Cycloalkylbenzenes

Radical anions of five cycloalkylbenzenes (alkyl = propyl, butyl, pentyl, hexyl, or heptyl) have been studied by ESR. spectroscopy at ?90°C. In the case of cyclopropylbenzene, no reliable experimental data could be obtained, because of the instability of its radical anion. The spectra of the radical anions of the four higher homologues have been analysed by means of a computer program. The assignment of the coupling constants to the α-protons in the para position of the benzene ring and to the cycloalkyl ß-protons has been based on the spectra of specifically deuterated derivatives. The experimental data of the radical anion of cycloalkylbenzenes have been compared with those of the radical anions of five alkylbenzenes (alkyl = methyl, ethyl, n-propyl, isopropyl, or t-butyl), the spectra of the latter being reexamined at ?90°C. In the cycloalkyl series the degeneracy of the two benzene-type lowest antibonding orbitals is the more effectively removed the larger the substituent, whereas the reverse relation holds for the non-cyclic series. The preferred conformation of the substituents is that in which the alkyl or cycloalkyl ß-proton is near to the nodal plane of the benzene π-system, and this preference is accentuated with the increasing size of the substituent group.     

The Radical Cation of Azatriquinane: An ESR Study

Azatriquinane (=10-azatricyclo[5.2.1.0^1,10]decane; 1 ) was oxidized to its radical cation 1 ^.+ by γ- irradiation in a CF₂ClCFCl₂ matrix at 77 K. A prominent feature of the ESR spectrum of 1 ^.+ is the ¹⁴N-hyperfine anisotropy which broadens the components with M_I(¹⁴N)=±1. The observed coupling constants are a_N=+ 2.5 and a_H=+ 4.0 mT for the ¹⁴N nucleus and the three methine β-protons, respectively. While the a_N value points to pyramidalization at the N-atom comparable to that in the radical cation of quinuclidine (=1-azabicyclo[2.2.2]octane; 4 ), an eclipsing of the singly occupied orbital at this atom by the three C−H_β bonds is indicated in view of the large a_H value. Theoretical calculations on the geometry of 1 ^.+ are in accord with the conclusions drawn from the experimental findings.     

Conformational Study by ESR. of Some Alkyl Substituted 6a-Thiathiophthene Radical Anions

ESR. data are reported for the radical anions (II^? to VI^?) of five alkyl substituted 6a-thiathiophthenes. Rates and activation parameters for the inversion of the 3,4-trimethylene chain in IV^?, V^? and VI^? have been obtained by means of an iterative least squares computer program ESRCEX. Preferential conformations of the alkyl substituents are discussed in terms of the 〈cos²θ〉 dependence of the β-proton coupling constants and with the aid of molecular models. Experimental evidence strongly suggests that the partial rotation of the ethyl and isopropyl groups in V^? and VI^? is correlated with the inversion of the 3,4-trimethylene chain.     

ESR. Spectra of Radical Anions in the Cyclazine Series

The ESR. spectra of the radical anions of three cyclazines are described. Their π-spin distributions are discussed in terms of simple MO theory and compared with those of structurally related species.     

An ESR and ENDOR study of irradiated 6Li-formate

Lithium formate ((6)LiOOCH.H(2)O), 95% (6)Li enrichment, combined with an exchange of crystallization water with D(2)O was investigated. The ESR spectrum of the radiation induced free radicals stable at room temperature consists of a singlet with a narrow line width, 0.92mT. (6)Li has smaller magnetic moment and nuclear spin, which resulted in the narrower line width accompanied with an increase in peak amplitude. In comparison with lithium formate with natural isotopic composition, (6)Li (7.5%, I=1) and (7)Li (92.5%, I=3/2), the sensitivity was increased by a factor of two. With optimised spectrometer settings (6)Li formate had seven times higher sensitivity compared to alanine. Therefore this material is proposed as a dosimeter material in a dose range down to 0.1Gy. The g and the (13)C-hyperfine (hf) tensors of the CO(2)(-) radical anion, major paramagnetic products, were evaluated to be g=(2.0037, 1.9975, 2.0017), and A((13)C)=(465.5, 447.5, 581.3) MHz for polycrystalline samples at room temperature. Furthermore, the (1)H-hf and (6)Li-hf tensors observed for the surroundings of CO(2)(-) by ENDOR technique were in fairly good agreement with DFT calculations. The CO(2)(-) radicals are found to be so stable that the formate is applicable to the ESR dosimetry, because of fully relaxing in a fully relaxed geometrical structure of the CO(2)(-) component and remaining tight binding with the surroundings after the H atom detachment from HCO(2)(-).     

Radical Ions of a Bishomobinaphthylene Containing two 1,6-methano[10]annulene moieties: An ESR and ENDOR study

The radical cation and the radical anion of ‘syn’-cyclobuta[1,2-c:3,4-c′]di-1,6-methano[10]annulene (‘syn’-4a,12a:6a, 10a-bishomobinaphthylene; 3 ) have been characterized by their hyperfine data. The highly resolved ESR spectrum of $ 3^{+ \atop \dot{}} $ is dominated by a triplet splitting from the outer pair of methano β-protons (H_o). In contrast, the ESR spectrum of $ 3^{- \atop \dot{}} $ is poorly resolved with the largest coupling constants arising from perimeter α-protons. The different hyperfine features of $ 3^{+ \atop \dot{}} $ and $ 3^{- \atop \dot{}} $ are rationalized by MO models. The SOMO of $ 3^{+ \atop \dot{}} $ ψ_SA(b₁), has substantial LCAO coefficients of the same sign at the bridged atoms C(1), C(6), C(11), and C(16), whereas in the SOMO of $ 3^{- \atop \dot{}} $, ψ_SS(a₁), the four atoms lie in the vertical nodal planes. The large width and the reluctance to saturation of the lines in the ESR spectrum of $ 3^{- \atop \dot{}} $ are attributed to the near-degeneracy of the lowest antibonding MO's. Due to their similar nodal properties, the SOMO's of $ 3^{- \atop \dot{}} $ and the radical anions of binaphthylene ( 4 ), 1,6-methano[10]annulene ( 1 ), and naphthalene ( 2 ) are interrelated. Moreover, because the cyclic π-systems in 3 and 1 deviate in the same way from planarity, the effect of such distortions on the coupling constants, a_Hμ, of the perimeter α-protons in $ 3^{- \atop \dot{}} $ and $ 1^{- \atop \dot{}} $ should be comparable. Indeed, on going from $ 4^{- \atop \dot{}} $ to $ 3^{- \atop \dot{}} $, the |a_Hμ| values are reduced exactaly by half as much as the corresponding values on passing from $ 2^{- \atop \dot{}} $ to $ 3^{- \atop \dot{}} $, of which the cyclic π-systems are twice contained in $ 4^{- \atop \dot{}} $ and $ 3^{- \atop \dot{}} $ respectively.     

Multifrequency ESR and ENDOR studies of ionomers

We describe methods for determining the local environment of cations and the process of ionic clustering in ionomers, using electron magnetic resonance spectroscopy. The distance between Cu²⁺ cations in perfluorinated membranes (Nafion) containing terminal sulfonic groups and swollen by water has been deduced from an analysis of ESR spectra at L (1.25 GHz), S (2.36 GHz) and X (9.36 GHz) bands, in membranes containing cupric ion concentrations in the range 1–30 percent of the total amount needed to fully neutralize the pendant acid groups. At higher cation concentrations ESR spectra indicate the presence of aggregated cations. The intercation distance determination is based on the simulation of spectra from isolated cations using distribution widths δg₁₁ and δA₁₁ and extraction of the residual width ΔH^R, which is due to dipolar interactions. No aggregation is detected in membranes swollen by less polar solvents such as methanol, dimethylformamide (DMF) and tetrahydrofuran (THF); these results are in contrast to SAXS experiments in membranes swollen by methanol, which exhibit the “ionic peak”. Cu²⁺-Cu²⁺ and Ti³⁺-Ti³⁺ dimers have been detected in Nafion swollen by water, methanol, DMF and THF, and have been characterized by an analysis of the spin-forbidden half-field Δm_s=2 transition, and by computer simulations. The intercation distance in the cupric dimers, deduced from the intensity ratio of the Δm_s=2 and Δm_s=1 dimer transitions, is 5.0±0.2 Å. A model for the dimer has been proposed, which explains the crosslinking of the polymer chains by the metal cations. ENDOR signals from ¹H, ²H and ¹⁹F nuclei have been detected in Nafion neutralized by Ti³⁺. The ENDOR results allow determination of the local environment of the paramagnetic cations, to a distance of ∼10 Å.