Proton Hyperfine Coupling Constants Research Articles

An electron spin resonance study of the radicals formed on ultraviolet irradiation of the photoallergens fentichlor and bithionol

Radicals formed on u.v. irradiation of aqueous solutions (pH 8) of fentichlor and bithionol have been detected by e.s.r. spectroscopy. The radicals have been identified as substituted benzosemiquinone radicals and the proton hyperfine coupling constants have been assigned on the basis of methyl substitutions.

The conformation of o‐benzosemiquinone radical and its assignment of the proton hyperfine coupling constants by using the indo and the molecular geometry adjusting methods

AbstractThe minimal energy conformations of o‐benzosemiquinone anion radical were calculated for several cases of assignments by using the INDO method and the molecular geometry adjusting method. In order to know the effect of lithium ion in the solvent, the minimal energy conformations of the system of LiOH and o‐benzosemiquinone anion radical were calculated. The calculations of the minimal energy conformations of this radical in t‐butyl alcohol, alkaline aqueous ethanol, alkaline water, neutral methanol, and acetonitrile were carried out. The total energies of the minimal energy conformations in the assignment |A3| > |A4| were lower than those in the assignment |A3| < |A4|.

Radical cations of ethylene oxide and other oxiranes prepared by exposure to ionizing radiation. An electron spin resonance study

Exposure of dilute solutions of tetramethyl oxirane in trichlorofluoromethane to 60C γ-rays at 77 K gave a species identified as the ring-opened radical cation, [graphic omitted]. Its e.s.r. spectrum comprised 13 lines with A(1H)= 15.1 G. This species had an intense violet colour, and there was no evidence for the cyclic cation. Similarly, the cations of cyclopentene oxide and cyclohexene oxide gave red–violet colours and complex e.s.r. spectra which were interpreted in terms of the oxirane ring opening to give 6- and 7-membered cyclic cations only.Both the 2,3-cis-dimethyl and the monomethyl derivatives on exposure at 4 K gave poorly defined e.s.r. spectra and no coloration. On warming to ca. 30 K the spectra became better defined and at 77 K intense red colours appeared together with new e.s.r. features which could be analysed in terms of the open-ring structures.Oxirane itself gave a spectrum with splitting due to four equivalent protons, together with small extra splittings probably due to coupling to a chlorine nucleus. The proton coupling of ca. 16 G is close to that for the two α-protons of the open-chain radical (Ċtext-decoration:overlineHtext-decoration:overlineMtext-decoration:overlineetext-decoration:overlineOĊHMe)+, which strongly supports an open structure. However, there was no visible absorption band under all conditions in CFCl3. In marked contrast, a very similar e.s.r. spectrum of five lines was obtained from solutions in SF6 at 77 K, but these solutions had an intense orange–red coloration. These results are tentatively interpreted in terms of a cyclic cation for oxirange in CFCl3, but an ‘open’ cation in SF6, both having very similar proton hyperfine coupling constants. Similarly, the acquisition of colour and change in the e.s.r. spectrum of the monomethyl and dimethyl derivatives is explained in terms of ring-opening in the 40–70 K region for solutions in CFCl3.

Electron–nuclear double resonance (ENDOR) studies of neutral odd–alternate aromatic hydrocarbon free radicals formed by thermal dissociation

AbstractElectron–nuclear double resonance (ENDOR) spectra are presented for the odd‐alternate aromatic hydrocarbon neutral radicals phenalenyl, [2H5]phenylphenalenyl and for the benzo[c,d]pyrenyl and benz[d,e]anthracenyl radicals formed by thermal dissociative processes in solution. Proton hyperfine coupling constants were determined from the ENDOR spectra and were shown to give an excellent linear correlation with McLachlan spin densities with Q = −26.5 G.

ESR and ENDOR Studies of Cation Radicals Derived from Benzodipyran Compounds. Structure and Dynamic Behavior of Heterocyclic Ring in Tocopherol (Vitamin E)

Abstract ESR and ENDOR spectra of cation radicals of benzodipyran compounds were measured in CH2Cl2, and the proton hyperfine coupling constants were correctly determined for each radical. From detailed analysis of the temperature dependence of hyperfine coupling of β- and γ-methylene protons in the heterocyclic ring, the equilibrium conformation of the β-methylene group and activation energy for the conformational interconversion have been determined. The structure of the heterocyclic ring was compared with those of α-tocopherol (vitamin E) model previously obtained by X-ray analysis. The result suggests that the solution and crystal conformations of the heterocyclic ring are distinctly different.

Theoretical studies on radical formation in the interaction of cysteine with dicarbonyl compounds of biological relevance

A possible pathway for the interaction between cysteine and dicarbonyl compounds of glyoxal and methylglyoxal type was studied by quantum chemical methods (INDO,ab initio). The condensation reaction is likely to occur through intermediate products, formation of which competes with the dimerization reaction. Results fromab initio UHF calculation of Fermi contact interaction for the radical cation of the final product suggest an assignment of the hyperfine coupling constants for protons that differs from the one deduced from the electron spin resonance (ESR) spectrum.

1H-ENDR OF DI-t-BUTYL NITROXIDE EFFECT OF THE 14N HYPERFINE INTERACTION

Abstract ENDOR spectra of di-t-butyl nitroxide have been observed at −90 °C in heptane. Two ENDOR lines near the free proton frequency had different absorption intensities and their line-shape showed a clear dependence on the nitrogen spin quantum number. This indicates an important spin cross relaxation by the nitrogen nucleus in the proton spin system and suggests a negative sign of the proton hyperfine coupling constant.

Radiation effects on 5-bromo-5,6-dihydrothymine and related compounds: an electron spin resonance study

Exposure of 5-bromo-5,6-dihydrothymine, 5-bromo-6-hydroxy-5,6-dihydrothymine, and the two corresponding nucleoside derivatives to 60Co γ-rays at 77 K gave paramagnetic centres identified as the 5-yl radicals formed by dissociative electron capture with concurrent formation of bromide ions. In some cases, electron loss centres formed by loss of a C-6 proton from the parent cations were also detected. The 5-yl radicals are of interest since they are related to radicals which may be formed in the radiolysis of DNA. For the hydroxy derivatives, the evidence suggests that they are initially formed in a conformation such that the C-6 proton is constrained to lie close to the radical plane, resulting in unusually small proton hyperfine coupling constants. This result has significant implications for the use of standard e.s.r. parameters in ‘fingerprinting’ the presence of such radicals.

Preparation and ESR Studies of New Stable Tocopheroxyl Model Radicals

Abstract New tocopherol (vitamin E) model compounds (tocopherol 1; 7-t-butyl-2,2-dimethyl-6-chromanol, tocopherol 2; 7-t-butyl-2,2,5-trimethyl-6-chromanol, and tocopherol 3; 5,7-diisopropyl-2,2-dimethyl-6-chromanol) have been synthesized by condensation of 2-methyl-3-buten-2-ol to the corresponding alkylhydroquinone. Electron spin resonance measurements were performed for the tocopheroxyl radicals obtained from the above tocopherol model compounds by oxidizing the phenol precursors with PbO2 in toluene. The proton hyperfine coupling constants and giso-values were determined. The tocopheroxyl radicals 2 and 3 having large alkyl substituents at both ortho positions (C-5 and C-7) are fairly stable, and the radical stability, as measured by the intensities of the ESR signals, increases in the order tocopheroxyl 1<α-tocopheroxyl model<tocopheroxyl 2<tocopheroxyl 3. The result suggests that the in vitro antioxidant activities of tocopherols 2 and 3 are higher than that of α-tocopherol model.

Electron spin resonance and electron nuclear double resonance studies of cation radicals derived from tocopherol model compounds

Electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) measurements were performed for the cation radicals obtained from the model compounds of α-, β-, γ- and δ-tocopherol (vitamin E) by oxidizing the tocopherol precursors in an AlCl 3-CH 2Cl 2 solution. The proton hyperfine coupling constants g-values were precisely determined. The ENDOR spectra of the cation radicals of α-, β-, γ- and δ-tocopherol models in CH 2Cl 2 at −100°C clearly show 10, 6, 6 and 12 different proton hyperfine couplings, respectively. By varying the temperature, the ESR spectra of the α- and δ-tocopherol model cations exhibit line-width alternation phenomena characteristic of the hindered rotation of the OH group. However, neither the β- nor the γ-tocopherol model cation radical ESR spectra show any sign of an alternative line-width effect. These results are interpreted by assuming that the β- and γ-tocopherol model cations are stabilized in the trans and cis conformations, respectively. On tocopherol model cations are stabilized in the trans and cis conformations, respectively. On the other hand, both the α- and δ-tocopherol model cations exist as cis and trans isomers.

EPR investigation of the intermediate spin exchange regime

The electron paramagnetic resonance spectra of di-tert butylnitroxide (DTBN) are studied as a function of the nitroxide concentration in three hydrocarbon solvents at constant temperature. In the concentration range studied, DTBN undergoes electron spin exchange which leads to broadening, shifting, and eventual collapse of the lines forming the 19-line proton hyperfine pattern. Particular emphasis is given to the spin exchange frequencies ωHE in the region in which they are comparable with the proton hyperfine coupling constants—the intermediate spin exchange regime. Both linewidths and line shapes are found to be in full accord with theory in the intermediate exchange regime which completes the demonstration that theory and experiment are in agreement, the slow and fast exchange regimes having been studied many times in the past. Modeling the spectral lines as Lorentzian–Gaussian convolutions allows a detailed exposition of the collapse of the hyperfine structure as ωHE increases and this collapse, too, is well described by theory. Bimolecular spin exchange rate constants are compared with the prediction of simple hydrodynamic theory.

Anomalous electrochemical and electron spin resonance properties of some biologically relevant methoxyl-substituted quinones

The proton hyperfine coupling constants for the family of methoxyl-substituted semiquinones in aqueous solution have been determined by electron spin resonance (ESR) spectroscopy. These data and the electrochemical midpoint potentials for the corresponding series of substituted quinone/hydroquinone couples are discussed. Neither the additivity principle describing quinoidal hyperfine coupling constants nor the Hammett substituent relationship predicting electrochemical behavior are obeyed by these methoxyl-substituted quinones. It is shown, however, that the experimental data can be rationalized in terms of neighboring group interactions that occur between adjacent methoxyl substituents. Such perturbations apparently lead to a breakdown of hyperconjugation between the electronic bonding structure of the quinone ring and the methyl group of the methoxyl moiety. It is pointed out that ubiquinone is a dimethoxyl compound whose properties are determined by such neighboring group interactions. This observation may have important consquences for the fabrication of ubiquinone analogs and quinone-based drugs.

ChemInform Abstract: QUANTUM CHEMICAL CALCULATIONS ON THE STRUCTURE AND CONFORMATION OF SUBSTITUTED ETHYLENE RADICAL CATIONS AND ON THEIR ESR SPECTRA

Abstract The geometries and conformations of a series of radical cations, derived from ethylene by mono- and distribution with OH or OCH3 groups and CH3 groups, have been determined by ab initio methods and by the INDO method. The calculated barriers to rotation around the CC bonds vary from 35 kJmol−1 for the 1,1-dimethoxyethylene radical cation to over 75–100 kJ mol−1 in monosubstituted derivatives and up to 230 kJ mol−1 for the 1,2-dimethoxyethylene radical cation; an explanation for this variation is provided by analysis of the charge and spin density. The calculated proton hyperfine coupling constants agree well with experimental values. In some cases, it was necessary to take the influence of the barriers to rotation into account explicitly by conformational averaging.

A theoretical interpretation of the electron spin resonance spectrum for BH4 radicals and comparison with results for CH + 4 radical cations

Exposure of sodium borohydride to 60Co γ-rays at 77 K gave a novel radical identified as BH4 from its e.s.r. spectrum. This is dominated by a large hyperfine coupling (ca. 105 G) to two protons, but smaller couplings to two other protons and a boron atom were also detected. The proton hyperfine coupling constants are surprisingly similar to those for the recently identified CH+4 radical, which also has C2v symmetry. These results are strongly supported by ab initio calculations which confirm that such a distortion is most favourable.

Magnetic resonance studies of cation radicals from chromans. Part 2.—Nuclear magnetic resonance, electron spin resonance, ENDOR and TRIPLE spectroscopy of some tricyclic chromans

N.m.r. spectroscopy has been used to study the electron-exchange reaction between the substrate–radical pairs of 2,3,4,7,8,9-hexahydro-2,2,5,7,7,10-hexamethylbenzo[1,2-b : 4,5-b′]-dipyran, 2,3,4,5,6,7-hexahydro-2,2,7,7,9,10-hexamethylbenzo[1,2-b : 4,3-b′]dipyran and 2,3,6,7-tetrahydro-2,2,4,6,6,8-hexamethylbenzo[1,2,-b : 4,5-b′]difuran and their cation radicals. The rate constants and energetics of activation of the reactions have been determined, along with absolute signs of the averaged proton hyperfine coupling constants. ENDOR and TRIPLE measurements provided accurate values of the individual coupling constants and their relative signs: the n.m.r. and ENDOR data taken together gave a complete assignment.

Spin density distribution of the photoexcited triplet state of anthracene via electron spin echo envelope modulation spectroscopy

The values of spin density of the anthracene triplet have been determined from the analyses of electron spin echo envelope modulations and a program of non-linear least-squares fitting as follows: p 1 (α) = 0.116, p 2 (β) = 0.05, p 9(meso) = 0.285 and p 11 = −0.058. The values of anisotropic and isotropic proton hyperfine coupling constants per unit spin density on the adjacent carbon have also been determined. Comparisons with previously reported values are presented.

Quantum chemical calculations on the structure and conformation of substituted ethylene radical cations and on their ESR spectra

The geometries and conformations of a series of radical cations, derived from ethylene by mono- and distribution with OH or OCH 3 groups and CH 3 groups, have been determined by ab initio methods and by the INDO method. The calculated barriers to rotation around the CC bonds vary from 35 kJmol −1 for the 1,1-dimethoxyethylene radical cation to over 75–100 kJ mol −1 in monosubstituted derivatives and up to 230 kJ mol −1 for the 1,2-dimethoxyethylene radical cation; an explanation for this variation is provided by analysis of the charge and spin density. The calculated proton hyperfine coupling constants agree well with experimental values. In some cases, it was necessary to take the influence of the barriers to rotation into account explicitly by conformational averaging.

Laser excitation spectrum and microwave optical double resonance spectrum of the 31 band in the ã 3A2–X̃ 1A1 system of H2CS: The hyperfine structure of the ã 3A2 state

The dye laser excitation spectrum of the 310 band in the ã 3A2 ← X̃ 1A1 transition of H2CS was observed in the region 15 280 to 15 400 cm−1 with Doppler-limited resolution. The observed spectrum was analyzed to yield precise values for the rotational constants, the centrifugal distortion constants, and the spin–spin and the spin-rotation interaction constants in the upper triplet state and for the band origin. Transitions with Ka′=4 and 5 revealed the effect of the c-type Coriolis interaction between the v3′=1 and v6′=1 states, which, when analyzed, allowed determination of the ν3−ν6 difference to be 99.341 cm−1. 15 K-type doubling transitions and two a-type R branch transitions in the ã state were observed by a microwave or radio-frequency optical double resonance technique, of which two K-doubling transitions of Ka=1 and two R branch transitions (all F3 components) were resolved into three hyperfine components. From the observed splittings the proton hyperfine coupling constants were determined to be aF=27.7(25), Tbb+Tcc=12.8(44), and Tbb−Tcc=7.89(19) MHz with one standard errors in parentheses.

Molecular geometry of benzosemiquinone radicals: Radicals from resorcinol and its derivatives

AbstractThe observed proton hyperfine coupling constants of radicals from resorcinol and 2‐carboxylic, 5‐carboxylic, and 5‐methyl resorcinols in acidic and alkaline solution were reproduced by means of the INDO (intermediate neglect of differential overlap) with molecular geometry adjusting method. The bond lengths of CO were longer in acidic solution than in alkaline solution and the total energies were lower in acidic solution than in alkaline solution. The assignments of the coupling constants of the radicals from resorcinols were |A2| > |A5| in alkaline oxidation and |A2| < |A5| in acidic oxidation.

The assignment of the hyperfine coupling constants for the pyridyl protons in aryl and alkyl n-substituted bipyridylium radical cations

The coupling constants for the pyridyl protons of the radical cations of N,N'-dimethyl-4,4'-bipyridylium dichloride PQ + and N,N'-diphenyl-4,4'-bipyridylium dichloride DPQ + have been unambiguously assigned from an ENDOR study of selectively deuterated compounds. The experimental results confirm the early theoretical assignments for PQ + made by Johnson and Gutowsky; however subsequent assignments for DPQ + based on this work have now been revised. The revision of the assignments and the difference between the two compounds are discussed using INDO molecular orbital calculations.