Line-width Alternation Effect Research Articles

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.

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.

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...

An electron spin resonance investigation of β-substituted 1,1-diphenylethyl radicals

A large number of β-substituted 1,1-diphenylethyl derivatives Ph2ĊCH2MRn have been generated by addition of ·MRn radicals to 1,1-diphenylethylene. The magnitude of the β-proton hyperfine splitting constants together with the observation of linewidth alternation effects on the CH2 triplet, indicate that the preferred conformation of these radicals is close even though not coincident with that placing the MRn group in an eclipsed position with respect to the 2pz orbital on Cα. The slight departure of these adducts from an eclipsed geometry has been interpreted as due to the presence on Cα of the two phenyl rings which are somewhat twisted with respect to each other thus producing an asymmetric potential around the radical centre. Although all the 1,1-diphenylethyl derivatives which have been investigated adopt the same conformation, the β-proton splitting cover the range from 6 to 11 G. The two common interpretations invoked to explain the effect of β-substituents on this splitting are discussed.

Electron spin resonance of di-t-butyl-ketyl alkali metal ion pairs

Alkali metal radical ion pairs were prepared by reacting di-t-butyl ketone with the whole series of the alkali metals. Two different types of spectra were recorded, one showing hyperfine splitting by one alkali nucleus and one with splitting produced by two counter ions. The latter, though attributed in the literature to a biradical species, is shown to be due to ionic aggregates formed by a ketyl radical, two alkali cations and possibly a diamagnetic anion. Linewidth alternation effects in the alkali multiplets indicate that the alkali nuclei do not occupy equivalent positions and that they are subjected to an intramolecular motion which interchange the two positions. The carbonyl 13C hyperfine splitting constants are shown to be strongly dependent on the counter ion.

ESR study of the steric and spin density effects on the radicals derived from 4-alkoxy-2,3,5,6-tetramethylphenols

The phenoxy and cation radicals derived from 4-methoxy- ( 2) and 4-ethoxy-2,3,5,6-tetramethylphenol ( 3) have been investigated. The phenoxy radicals (PA- 2, PA- 3) show β-alkoxy proton splittings of 0.0 and 0.28 G, whereas the cation radicals A- 2 and A- 3 have β-alkoxy splittings of 2.97 and 2.65 G. These results are interpreted in terms of orthogonal alkoxy groups for the phenoxy radicals and almost coplanar alkoxy groups for the cation radicals. The conformation of the ethoxy group in the cation radical A- 3 is also different from all other ethoxy groups studied to date, due to the steric effects of the two ortho methyl groups. The cation radicals A- 2 and A- 3 also exhibit line width alternation effects which are attributed to the restricted rotation of the hydroxyl group. Simulations of the spectra have been carried out for A- 2 and A- 3 in CH 3NO 2 and CH 2Cl 2CH 3NO 2 mixtures. An interesting solvent effect was also observed for A- 2.

The Intramolecular Dynamic Behavior of Tetraaryl Allyl Radicals

Abstract The electron spin resonance (ESR) and the electron nuclear double resonance (ENDOR) spectra of tetraaryl allyl radicals have been observed. A comparison of the ENDOR spectra of α,α,γ,γ-tetraphenyl allyl, α,α-diphenyl-γ,γ-dianisyl allyl, and α,α,γ,γ-tetraanisyl allyl revealed that the four aryl groups are not equivalent, but are apparently to be classified into two types with different conformations in their molecular frameworks. The molecular orbital calculations on the basis of this model have shown a fairly good agreement with the observed spectra. The temperature dependence of the ENDOR spectra at −60∼−110 °C were analyzed, assuming mutually different twisting angles for the allyl skeleton. The activation energy of the torsional vibration of the allyl skeleton is estimated to be about 2.5 kcal/mol. The ESR spectra of α,α,γ,γ-tetraphenyl allyl and α,α-diphenyl-γ,γ-diphenylene allyl change drastically with the temperature at around +115°C and show a marked line-width alternation effect. It is concluded from the results that the torsional vibration becomes vigorous with an increase in the temperature and that the allyl skeleton begins to rotate internally at high temperatures. The activation energy of the internal rotation of the allyl skeleton has been estimated to be about 15∼20 kcal/mol. The temperature-dependent hyperfine splitting of the methine proton has also been explained by assuming a model of the internal rotation of the allyl skeleton.

ESR Studies of the Conformation and Restricted Rotation of 4-Phenyl-1,3-dioxane Anion

Abstract The ESR of the 4-phenyl-1,3-dioxane anion shows a line-width alternation effect which can be attributed to the restricted rotation between the 1,3-dioxane and benzene rings. From the temperature dependence of the line-width alternation effect, the activation energy was determined to be 180±110 cal·mol−1. The preferential conformation between the two rings was estimated from the hyperfine splitting constant of the proton at the 4 position in the 1,3-dioxane ring, to be that where the two rings have some angle close to 90° rather than the structure where they are parallel. It was also found that an odd electron, which mainly occupies a vacant π orbital in the benzene ring, delocalizes appreciably into a part of the 1,3-dioxane ring.

ESR Studies of the Conformational Interconversion of the 9,10-Dihydrophenanthrene Radical Anion and Its Related Compounds

Abstract The ESR spectra of the 9,10-dihydrophenanthrene radical anion change drastically with the temperature, and at temperatures around −30°C the spectra show a marked line-width alternation effect. These temperature-dependent ESR spectra and the line-width alternation effect have been interpreted in terms of the conformational interconversion of the 9,10-dihydrophenanthrene radical anion molecule. From an analysis of the spectra, the activation energy of the inversion has been estimated to be 6.3±0.6 kcal/mol. It has also been estimated, from the ratio of the axial and equatorial proton splittings, that the dihedral angle of the axial protons is about 11°. These data are compared with those of the 4,5,9,10-tetrahydropyrene radical anion. The ESR of the 4,5-dihydropyrene radical anion is also presented.

Studies of the temperature dependence of the EPR spectra of some peri-alkylnaphthalene anions

Linewidth alternation effects are reported in the EPR spectra of the radical anions of some nonplanar peri-alkylnaphthalene compounds over a range of temperatures. These are analysed using a modified-Bloch-equation approach in terms of the movement of the aliphatic substituents. Estimates of the activation energies for this movement are made and these compare favorably with the value observed for chair-boat interconversion in the cyclohexane molecule.

E.S.R. investigation on durosemiquinone-alkali metal ion pairs

The e.s.r. spectra of the durosemiquinone ion pairs with alkali metal ions have been measured at different temperatures. A linewidth alternation effect is related to a two-sites jump of the cation. The energy barrier opposing the motion is measured for the K, Rb and Cs ion pairs. The hyperfine coupling constants of the alkali metals, their sign and their variation with the temperature, are discussed on the basis of theoretical calculations.

Electron spin resonance spectrum of copper-63 phthalocyanin. A reassessment of the bonding parameters

The e.s.r. spectrum of isotopically pure powdered copper-63 phthalocyanin magnetically dilute in metal-free phthalocyanin has been measured. Much improved resolution of the hyperfine spectrum has allowed more accurate measurement of the nitrogen hyperfine tensor. The bonding parameters β12 and β22 are found to be 0·75 and 1 respectively, whilst ε2 is estimated as 9·51 or 0·60. Much of the earlier work is found to be in error, particularly in the analysis of the spectrum. A line-width alternation effect is detected and accounted for in terms of the anisotropy of the nitrogen hyperfine tensor.

ESR Studies of the Alkali Metal-Acenaphthene Ion Pairs

Abstract Ion pairs formed by alkali metals with acenaphthene in ether-type solvents have been investigated by means of electron spin resonance (ESR). The ESR spectra of the ion pairs were found to show phenomena quite analogous to those found by de Boer et al. in the pyracene-alkali ion system; similar two-line-width alternation effects were also observed in the present work. These observations were contrary to the expectations formed by a consideration of the pyracenealkali metal ion pair, where the phenomena have been explained on the basis of the assumption that a potential well exists near the two ethylene bridges. This paper shows that, in the acenaphthene-alkali metal ion pairs, the alkali ion may be situated above the naphthalene skeleton rather than near the ethylene bridge. One of the line-width alternation effects is explained in terms of the exchange reaction between two ion-pair conformations in which the alkali ion is situated above and under the molecular plane respectively, while the other is ascribed to the oscillation of the cation in the potential well above either side of the molecular plane.

Line width alternation in the electron spin resonance spectrum of the durosemiquinone cation

The e.s.r. spectrum of the durosemiquinone cation exhibits a marked line width alternation effect which is dependent on the temperature and the solvent. The effect is ascribed to cis-trans isomerism, the lifetime of each isomer being comparable with the inverse frequency separation between the proton hyperfine components.