Line-width Alternation Research Articles

In situ simultaneous electrochemical ESR study of radicals generated from 2,2-dinitroethene-1,1-diamine (FOX-7). Intramolecular chemical exchange resulting in an alternation line-width effect

The molecule of 2,2-dinitroethene-1,1-diamine (FOX-7) is one of the most interesting molecules with multiple redox centres stabilized by push-pull effect. To reveal the detailed mechanism of its electrochemical process radical intermediates formed in the course of its electrochemical reduction in organic aprotic media have been studied by in situ simultaneous electrochemical ESR measurements (SEESR). The radical generated on the second reduction step possesses an alternating line-width (AL) effect in the ESR spectra as a result of intramolecular dynamic processes in the timescale of ESR splitting constants. The spectra measured at different temperatures (230–335 K) were analysed with the help of a fitting program which includes a molecular dynamics. Observed dynamics describes well an asymmetric 2-site exchange model for the whole temperature range. With help of the optimized parameters and quantum chemical calculations this radical has been identified as 2,2-dinitroethane-1-amine-1-imine radical dianion, [(H2N)(HN)C=C(NO2)2]2−. The dynamic process responsible for the AL effect consists of mutual turning (changing of dihedral angle) of the both nitro groups, resulting in an intramolecular spin-density (electron) transfer. The dynamic parameters of the process have been established.

Formation of a long-lived electron-transfer state of a naphthalene–quinolinium ion dyad and the π-dimer radical cation

An electron donor-quinolinium ion dyad, 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+-NA), has been synthesized based on a rational design. The X-ray crystal structure of QuPh+-NA indicates that the dihedral angle between the NA and QuPh+ moieties of QuPh+-NA is nearly perpendicular. The one-electron reduction potential (E(red)) was observed as a well-defined reversible wave at -0.90 V versus SCE. The one-electron reduced species (QuPh*-NA) was detected by ESR. The electron self-exchange rate constant (k(ex)) between QuPh+-NA and QuPh*-NA has been determined from the ESR linewidth alternation. The reorganization energy (lambda) of the electron self-exchange was determined to be 0.42 eV from the k(ex) value. Femtosecond laser irradiation of QuPh+-NA at 355 nm results in formation of the ET state (QuPh*-NA*+) within 0.5 ps via photoinduced ET from NA to the singlet-excited state of QuPh+. The transient absorption bands at 420 nm and 700 nm are assigned to the QuPh and NA*+ moieties, respectively. The nanosecond laser excitation of QuPh+-NA affords the broad absorption band at 1000 nm and is due to the pi-dimer radical cation formed between QuPh*-NA*+ and QuPh+-NA. The intramolecular back electron-transfer process was too slow to compete with the intermolecular back electron-transfer reaction judging from the decay time profile of QuPh*-NA*+, which obeyed second-order kinetics.

Isocyanurate Anion Radicals via Electron-Initiated Cycloaddition of Isocyanates

Room temperature sodium metal reductions of alkyl isocyanates lead to the rapid electron-initiated formation of alkyl isocyanurate anion radicals, which exhibit EPR coupling to only two equivalent nitrogens. Reduction of (13)C-enriched ethyl isocyanate reveals that the odd electron localizes in the pi system of one carbonyl in the isocyanurate ring. EPR line-width alternation effects indicate that at least two stable conformers are in rapid equilibrium undergoing fast exchange.

Combined ESR and Thermodynamic Studies of the Superoxide Adduct of 5-(Diethoxyphosphoryl)-5-Methyl-1-Pyrroline N-Oxide (DEPMPO): Hindered Rotation around the O−O Bond Evidenced by Two-Dimensional Simulation of Temperature-Dependent Spectra

Experiments were performed to elucidate the origin of the superhyperfine structure and line width alternation (LWA) seen in the ESR spectrum of the major diastereoisomer (1) of DEPMPO-OOH, the remarkably persistent superoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO). Using selectively deuterated DEPMPO derivatives, we demonstrated that the superhyperfine pattern can be unambiguously attributed to long-range couplings. The recording in pyridine of highly resolved spectra in a wide temperature range, combined with two-dimensional simulation, allowed us to characterize an inverted LWA in 1 and revealed a uniform line broadening in the spectrum of the minor DEPMPO-OOH diastereoisomer (2), with both effects originating from a chemical exchange between conformers. When the individual spectra of 1 presenting LWA in the fast-exchange regime were simulated, four equally good fits were obtained and this ambiguity could be resolved by using a two-dimensional simulation technique. The thermodynamic and kinetic constants of this exchange were consistent with a rotation around the O-O bond. We propose that line broadening effects in 1 and 2 result from this O-O rotation concerted with the pseudo-rotation of the pyrrolidine ring.

Ligand Induced Stereoisomers Revealed in Copper(II) Complex of Nitrolawsone Oxime: EPR and Electronic Spectral Studies

Amphi‐syn stereoisomers with 62:38 compositional ratio existing in 3‐nitrolawsoneoxime [S.Y. Rane et al., 1990] leads to two configurational forms in its copper (II) complex 1 viz. [Cu(NO2‐LwOx)2] · 3H2O. The D4h:D2h geometries of 1 have been detected in 70:30 ratio from solution EPR spectra. Line width alternation effect with superimposition of metal‐ligand hyperfine splits lead to anisotropic magnetic parameters. Best fitted bonding parameters of 1 are compared with O∧O and N∧O donor complexes.

Time-Resolved EPR Studies of Main Chain Radicals from Acrylic Polymers. Dynamic Effects Due to Conformational Motion

The temperature dependencies of the time-resolved electron paramagnetic resonance (TREPR) spectra of five main chain acrylic free radicals are presented and discussed in terms of conformational dynamics. The radicals are produced in liquid solution at temperatures ranging from 25 °C to over 100 °C by direct excitation (248 nm) of the ester group in the polymers leading to Norrish I α-cleavage of the side chain ester moiety. Restricted rotational motion near the radical center leads to modulation of the β-hyperfine coupling constants, which manifests itself in some of the spectra as an alternating line width effect near room temperature. A standard hyperfine modulation model (two-site exchange) is proposed and has been added to the simulation routine. The model works especially well for two of the polymers (poly(ethyl acrylate) and poly(methyl d3-methacrylate), for which activation barriers for rotation are extracted from Arrhenius plots. The model is somewhat successful for poly(ethyl cyanoacrylate) but fails for poly(ethyl methacrylate) and poly(methyl methacrylate). This failure is discussed in terms of radical structure and dynamics and the possibility that jumping between more than two low-energy conformational sites is involved. Disruption of the symmetry of the hyperfine couplings at intermediate temperatures supports this explanation.

Diastereoselective Synthesis and ESR Study of 4-PhenylDEPMPO Spin Traps

[reaction: see text] The cis and trans diastereoisomers of 5-(diethoxyphosphoryl)-5-methyl-4-phenylpyrroline N-oxide (4-PhDEPMPOt 8 and 4-PhDEPMPOc 9) were prepared stereoselectively and used as spin traps for hydroxyl and superoxide radicals. The spin adduct formed by reaction of the cis stereoisomer 9with superoxide radical anion exhibited an 8-line ESR spectrum, showing only a reduced alternating line width phenomenon. This spectrum is simpler than the 12-line spectrum of DEPMPO-OOH, which exhibits a strong alternating line width phenomenon. The half-life times of the 4-PhDEPMPOc-OOH and DEPMPO-OOH adducts were of the same order: 14.5 and 15.5 min, respectively.

Synthesis of the cis diastereoisomer of 5-diethoxyphosphoryl-5-methyl-3-phenyl-1-pyrroline N-oxide (DEPMPPO c) and ESR study of its superoxide spin adduct

The cis and trans diastereoisomers of 5-diethoxyphosphoryl-5-methyl-3-phenyl-1-pyrroline N-oxide (DEPMPPO), the C(3)-phenyl analogue of DEPMPO, were prepared in three steps from phenylacetaldehyde and used in ESR-spin trapping of various carbon-, oxygen- and sulfur-centred radicals. In the case of the cis-isomer, the presence of the phenyl group cancels the alternating line width phenomenon observed for the DEPMPO–OOR (R = H, Bu t ) spin adducts. The ESR spectra of the DEPMPPO c–OOR spin adducts exhibit more straightforward patterns and are more easily assignable.

ESR Investigation of Chemical Exchange in Geminally Diphosphorylated Linear Nitroxide Radicals

For geminally diphosphorylated linear nitroxide radicals, a marked line width alternation (LWA) can be observed. The LWA appears irrespectively whether the investigated radicals R‘ ‘N(O•)C(P(O)(OEt)2)2R‘ contain any chiral group or not. If R‘ ‘ = tert-butyl and R‘ = H, the small β-hydrogen coupling indicates a completely blocked rotation around the N−C σ-bond and the LWA can be assigned to a chemical exchange between conformations in which the phosphoryl groups have a symmetric and a nonsymmetric geometry. The conformational change is accompanied by the deformation of the CP2 bonding angle and the rate of exchange is slowed in pentane for which the solvent molecules can be trapped by the chelating phosphoryl groups. If R‘ ‘ = benzyl, both the proton hyperfine lines of the CH2 group and the phosphorus lines of the CP2 group produce a LWA; two coalescences can be observed. In the case of a chiral R‘ ‘ group (R‘ ‘ = secondary butyl, R‘ = methyl) a tentative four-site model can explain the highly complex LWA.

Structure and Dynamics of Bis(triphenylphosphine)-3,6-di-tert-butyl-o-semiquinone Radical Anion Complexes of Ag+ and Cu+

Complexes of Ag+ and Cu+ with 3,6-di-tert-butyl-o-semiquinone radical anion (SQ•-) and two triphenylphosphine (PPh3) ligands have been investigated by electron paramagnetic resonance (EPR) in organic solvents. High-temperature EPR spectra of the Ag+ and Cu+ complexes contain lines of hyperfine splittings due to two equivalent 1H nuclei in the 4,5-positions of the aromatic ring of the SQ•- ligand, two equivalent 31P nuclei from the two PPh3 ligands, and 107,109Ag and 63,65Cu nuclear isotopes, respectively. On the basis of the temperature dependence of the 31P hyperfine interaction constant a(31P), the structure of the Ag+ complex at low temperature was determined to be planar, while at higher temperatures the increase in the values of a(31P) shows that phosphorus atoms of the two PPh3 ligands move out of the π-plane of the SQ•- ligand. EPR spectra of the Ag+ complex with variable temperature exhibit alternating line width effect in hyperfine splitting due to the two aromatic protons in the SQ•- ligand. At low temperatures these two protons become nonequivalent. This effect reveals a dynamic charge redistribution within Ag+ and two oxygen atoms of the SQ•- ligand. In the Cu+ complex, we have found a temperature-independent symmetric charge distribution within two oxygen atoms of the radical anion and Cu+, but different positions of the two PPh3 ligands at low temperatures. EPR spectra of the Cu+ complex at variable temperature reveal the dynamic exchange in the positions of PPh3 ligands. The rates of this process are on the order of solvent reorientation frequencies, while intramolecular electron exchange rates in the Ag+ complex are about 2 orders of magnitude lower. The rate of the electron exchange reaction dramatically decreases in viscous mineral oil, which shows that the intramolecular charge redistribution reaction is controlled by nonpolar solvent dynamics.

Dynamic property of acidic protons in Ag 3PW 12O 40 reduced with hydrogen as studied by variable temperature 1H MAS NMR

Ag 3PW 12O 40 generated two kinds of protons by the reduction of Ag + cations with hydrogen. Dynamic properties of these protons were investigated by variable temperature 1H MAS NMR. A dynamic exchange between two kinds of protons was observed as a line-width alternation effect in the NMR spectra above 298 K. The temperature dependence of the spectra was analyzed by using the modified Bloch equations assuming a two-jump model as a function of the correlation time for the dynamic exchange of protons. The activation energy for the exchange was estimated to be 20 kJ mol −1. The dynamic property of the two kinds of protons in Ag 3PW 12O 40 is considered to play an important role in the acid-catalyzed reactions.

Observation of Both Jahn−Teller Distorted Forms (b1g and b2g) of the Cyclooctatetraene Anion Radical in a 1,2-Disubstituted System

The anion radical of cyclooctatetraene (COT) must undergo a Jahn−Teller distortion from its ideal D8h symmetry. Both B3LYP/6-31+G* calculations and EPR observation have shown that the COT moiety in the anion radical of 1,2-dicarbo-tert-butoxycyclooctatetraene exists in a pair of degenerate D4h alternating bond angle conformations (a b1g distortion). The rapid (k = 3 × 107 s-1) interconversion between the two identical conformations leads to specifically predicted line width alternation effects in the EPR spectrum. The COT ring system in the 1,2-dicarbomethoxycyclooctatetraene anion radical [(CH3OOC)2COT-•], on the other hand, has a ground state with alternating bond lengths. Since this b2g distorted (COT-•) system has a plane of symmetry that can be drawn through the molecule and between the two substituents, no line width alternation effects can take place. However, in this system, the B3LYP/6-31+G* calculations predict an energy difference between the b1g and b2g distorted COT-• ring systems of less tha...

Synthesis, Crystal Structure, Magnetic, and Electron Paramagnetic Resonance Properties of a Spiroconjugated Biradical. Evidence for Spiroconjugation Exchange Pathway

A spiroconjugated nitronyl nitroxide biradical, 6,6‘-(4,4,5,5-tetramethylimidazolidine-3-oxide-1-oxyl)-3,3,3‘,3‘-tetramethyl-1,1‘-spirobisindane (1), has been prepared by functionalization of a 3,3,3‘,3‘-tetramethyl-1,1‘-spirobisindane framework followed by Ullman condensation and subsequent oxidation. The biradical crystallizes in the monoclinic space group C2/c with four molecules in the unit cell of dimensions a = 24.861(10) A, b = 12.129(3) A, and c = 12.258(6) A. X-ray analysis of a blue-plate single crystal has revealed dihedral angles of 28° between the nitronyl nitroxide moiety and aromatic ring with intramolecular through space radical−radical distances of 8.25 and 10.11 A. In the solid state, the temperature dependence of the molar magnetic susceptibility reveals antiferromagnetic interactions. These interactions are best fit using a pair model, affording the value J = −4.0 cm^(-1) where J is the interaction parameter appearing in the spin Hamiltonian H = −JS_1·S_2. The field dependence of the magnetization measured at 2 K is consistent with a pair model. Frozen matrix EPR spectra of biradical 1 in CH_2Cl_2 at 100 K shows a half field transition at 1700 G. Temperature dependence of the half field transition intensity has been found to be consistent with a ground singlet state and thermally accessible triplet state. The magnetic interaction observed in the solid state is also observed in solution. Thus, room-temperature solution spectra display a nine-line pattern, with hyperfine coupling to four “equivalent” nitrogen atoms and a hyperfine coupling constant a_N = 3.8 G. Temperature dependence of the solution EPR spectra of biradical 1 displays alternating line width effects caused by conformational dynamics in solution. This behavior has been attributed to modulation of exchange and hyperfine interactions most likely caused by rotational motion about the nitronyl nitroxide−phenyl bond. Biradical 1 therefore exists as a ground-state singlet with a thermally accessible triplet at ca. 4 cm^(-1) higher in energy with a conformational dependence of intramolecular exchange in solution. This coupling may present evidence for spiroconjugation as an exchange pathway. Density functional calculations (B3/6-311G(D)) have been performed to investigate this possibility.

An ESR Study of a Tetrasilaspirobiindane Anion Radical. Stereoelectronic Electron-Accepting Effects of Trialkylsilyl Groups

Abstract ESR spectra of a tetrasilaspirobiindane anion radical (1−) showed a doublet of doublet hfs pattern at low temperatures and significant line-width alternation, indicating that there is remarkable difference in the electron-accepting effects between two silyl substituents at the benzene ring with the unpaired electron due to the ring conformation.

Combined Ring Inversion and Side Group Rotation in Geminal Diphosphoryl Substituted Pyrrolidinoxyl Radicals: ESR Analysis of Chemical Exchange between Four Nonequivalent Sites

The ESR spectra of three diphosphorylated pyrrolidinoxyl radicals have been studied over a large temperature range. While for the trans 2,5-disubstituted compound 1 no line width alternation was found, for the 2,2-disubstituted compounds 2 and 3, dramatic changes in the spectra were observed as a function of the temperature. These changes were explained by a four-site chemical exchange model including both ring inversion and hindered rotation around the carbon−phosphorus bonds. For radical 3, the presence of additional 5,5-dimethyl substitution can completely block rotations around carbon−phosphorus bonds for certain ring geometries, while for other ring conformations chemical exchange still occurs through combined inversion−rotation processes. An effective two-site model composed of nonequivalent sites and a superposition model composed of a pair of exchanging conformers and a pair of nonexchanging conformers were used to simulate spectrum variations. A satisfactory fit was obtained over the entire temperature range investigated. From the temperature dependence of the exchange frequencies the potential barriers for ring inversion (27 kJ/mol), for combined inversion−rotation (11 kJ/mol), and for rotation around the carbon−phosphorus bonds (14 kJ/mol) were estimated.

Conformational Change of Benzoquinone Crown Ether Radical Anions Induced by Complexation with Alkali Metal Cation: An ESR Study

Abstract The crowned p-benzoquinone anion radicals, O5−–O8−, were generated either electrochemically or by using a reducing metal, and their ESR spectra were examined in both the absence and presence of the Na+ or K+ cation. The hyperfine coupling (hfc) constants of the methylene protons bonded to the quinone ring depend largely on the size of the crown ring; the orientation of the benzoquinone skeleton with respect to the crown ring was analyzed. The metal-free O5− has a fixed conformation in which the benzoquinone ring is nearly perpendicular to the crown ring, while O8− interconverts freely between two equivalent conformations. The critical case is O6−; the line-width alternation observed in its ESR spectra was well-analyzed using the two-jump model. Temperature-dependent ESR spectra and MNDO calculations have suggested that O6− undergoes a restricted interconversion between two conformations in which the benzoquinone ring has an orientation similar to that of O5−. An activation energy of 10.0 kcal mol−1 was determined for this motion. The ESR spectra of the ion-paired O5−–O8− anion radicals have suggested that the conformational motion of the anion radicals is fixed by complexation with the Na+ or K+cation. The hfc constants of the methyl protons in the benzoquinone ring increased upon complexation, indirectly confirming the interaction between the carbonyl oxygen of benzoquinone at the crown-ring side and the metal cation.

Re-examination of linewidth alternation in the electron paramagnetic resonance spectrum of the 1,4-dinitrodurene radical anion

The EPR spectrum of the 1,4-dinitrodurene radical anion, prepared by electrochemical reduction in dimethylformamide, has been studied between 223 and 343 K. The EPR spectrum presents a classic example of linewidth alternation (first observed in 1962) and can be computer-simulated on the basis of a two- or a four-jump model. The two-jump model, in which the nitrogen atoms interchange their environment, tends to give poor reproductions of the experimental spectra. However, the four-jump model, based on the change of environment of only one nitro group during a particular jump, gives a much better fit to the experimental spectra over the complete temperature range.

ChemInform Abstract: The Influence of Crown Ethers on Cation Migration Processes. Part 6. The 1,4‐Naphthoquinone Radical Anion

The radical anion of 1,4-naphthoquinone has been prepared by alkali-metal reduction, employing Li, Na, K and Cs, in 1,2-dimethoxyethane in the presence of 18-crown-6, 15-crown-5 and 12-crown-4. Linewidth alternation is observed in the e.s.r. spectra of some of these systems owing to intramolecular cation migration between equivalent sites adjacent to the two carbonyl groups. In some systems a species is observed with a much narrower spectrum than that observed in the absence of a crown ether. Remarkably, it is apparent that the crown ethers do not appear to show great specificity towards the counterions which match the size of their cavities.

ESR studies of free radicals derived from hydroxyquinones: Naphthazarin revised - twenty-five years of the alternating linewidth effect

The neutral semiquinone radical obtained by photolysis of naphthazarin in THF exhibits linewidth alternation. The temperature dependence of the ESR spectrum has been interpreted in terms of a restricted rotation of the hydroxyl groups with an activation energy of 22.7 kJ mol −1.

ChemInform Abstract: ESR Characterization of Ring‐Closed Oxirane Radical Cations via a Novel Alternating Line Width Effect

ChemInformVolume 19, Issue 11 Preparative Organic Chemistry ChemInform Abstract: ESR Characterization of Ring-Closed Oxirane Radical Cations via a Novel Alternating Line Width Effect F. WILLIAMS, F. WILLIAMS Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorS. DAI, S. DAI Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorL. D. SNOW, L. D. SNOW Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorX.-Z. QIN, X.-Z. QIN Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorT. BALLY, T. BALLY Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorS. NITSCHE, S. NITSCHE Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorE. HASELBACH, E. HASELBACH Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorS. F. NELSEN, S. F. NELSEN Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorM. F. TEASLEY, M. F. TEASLEY Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this author F. WILLIAMS, F. WILLIAMS Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorS. DAI, S. DAI Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorL. D. SNOW, L. D. SNOW Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorX.-Z. QIN, X.-Z. QIN Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorT. BALLY, T. BALLY Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorS. NITSCHE, S. NITSCHE Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorE. HASELBACH, E. HASELBACH Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorS. F. NELSEN, S. F. NELSEN Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this authorM. F. TEASLEY, M. F. TEASLEY Dep. Chem., Univ. Tenn., Knoxville, TN 37996, USASearch for more papers by this author First published: March 15, 1988 https://doi.org/10.1002/chin.198811133Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume19, Issue11March 15, 1988 RelatedInformation