Hyperfine Coupling Constants Of Radicals Research Articles

Electron paramagnetic resonance study of gamma (γ)-irradiated methyl 4-methyl benzoate (C9H10O2)

ABSTRACTIn this study, gamma (γ)-irradiated methyl 4-methyl benzoate (C9H10O2) single crystals were investigated and the structure of the free radical occurring by γ-irradiation was determined by using Electron paramagnetic resonance (EPR) technique. C9H10O2 single crystals were produced by slow evaporation with distilled water solution at room temperature. These single crystals were irradiated by 60Co-gamma-ray source with a dose speed of 0.981 kGy/h at room temperature for 72 h. The EPR spectra of irradiated C9H10O2 single crystals were recorded between 125 and 400 K temperatures in steps of 10° intervals at different orientations in magnetic field with an X-band EPR spectrometer. The spectra were found to be slightly anisotropic and temperature independent; taking into consideration the chemical structure with experimental data we found only one free radical structure . The radical structure was simulated using the Bruker software WinEPR-Simfonia and the experimental g-value with anisotropic hyperfine coupling constants of radical were calculated as follows: g = 2.0124, = 26.51G, = 5.45G.

Communication: spin densities within a unitary group based spin-adapted open-shell coupled-cluster theory: analytic evaluation of isotropic hyperfine-coupling constants for the combinatoric open-shell coupled-cluster scheme.

We report analytical calculations of isotropic hyperfine-coupling constants in radicals using a spin-adapted open-shell coupled-cluster theory, namely, the unitary group based combinatoric open-shell coupled-cluster (COSCC) approach within the singles and doubles approximation. A scheme for the evaluation of the one-particle spin-density matrix required in these calculations is outlined within the spin-free formulation of the COSCC approach. In this scheme, the one-particle spin-density matrix for an open-shell state with spin S and MS = + S is expressed in terms of the one- and two-particle spin-free (charge) density matrices obtained from the Lagrangian formulation that is used for calculating the analytic first derivatives of the energy. Benchmark calculations are presented for NO, NCO, CH2CN, and two conjugated π-radicals, viz., allyl and 1-pyrrolyl in order to demonstrate the performance of the proposed scheme.

Substituent effects on OH bond strength and hyperfine properties of phenol, as model for modified tyrosyl radicals in proteins

Density functional theory is used to investigate the effects of a variety of substituents (CH3, OH, OCH3, SH, SCH3, NH2, NMe2, NO2, F, Cl, CN, and imidazole) on the phenol OH bond dissociation energy (BDE) and phenoxyl radical hyperfine properties. Substitutions are made at the ortho position to model modified tyrosine residues found in enzymes. The calculations show that besides the electronic effects of the substituents, intramolecular hydrogen bonds between OH and the substituents will contribute considerably to stabilize the parent species. Substituent effects on anisole OMe bond strengths can thus not correctly describe the effects on ortho-substituted phenol OH bond strengths, as previously proposed. This fact is supported by a series of calculations on o-substituted anisoles. The odd-alternant spin pattern of the phenoxyl radical is conserved for most of the substitutions. In particular, it is predicted that the cysteine crosslink to tyrosine, present in the radical enzyme galactose oxidase, and the histidine crosslink, present in cytochrome-c oxidase, will only have minor effects on the BDE and the radical hyperfine coupling constants and spin distribution of the tyrosyl radical. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 76: 714–723, 2000

EPR and ENDOR Studies of Single Crystals of α-Glycine X-ray Irradiated at 295 K

Single crystals of the amino acid α-glycine N−CH2−COO- were X-ray irradiated at 280 K and studied at 295 K and at 100 K using EPR, ENDOR, and EIE techniques. Five radicals were detected and characterized. Two of these are well known from previous studies, the oxidation product N−ĊH−COO- (radical I) and the reduction product ĊH2−COO- (radical II). It is shown that at 295 K the ENDOR and EIE spectra from radical I are characteristic of efficient W1x cross relaxation induced by rapid rotation of the amino group. This allows for the determination of the absolute signs of the hyperfine coupling constants of radical I. The other three radicals are two geometrical conformations of the product H2N−ĊH−COOH (radicals III and IV) and a species suggested to be the dimer product N−CH2−CO−ĊH−COO- (radical V). Radical III exhibits spectral parameters related to those previously reported by M. Brustolon et al. (J. Phys. Chem. 1997, 101, 4887). However, on the basis of our analysis of ENDOR and EIE spectra from normal as ...

Density-functional-theory calculations of isotropic hyperfine coupling constants of radicals

Isotropic hyperfine coupling constants for the radicals BH 2, CH 2 −, NH 2, OH 2 +, BH 3 −, CH 3, NH 3 +, H 2CO + and CH 2CH are calculated using density-functional theory with a gradient-corrected local-spin-density approximation and a Slater-type-orbital basis set. The agreement between the calculated and observed results is fairly good. The effect of rare-gas atoms surrounding radicals on the isotropic hyperfine coupling constants is also discussed.

Calculation of hyperfine coupling constants of radicals by density-functional theory.

Hyperfine coupling constants for small radicals have been calculated using the density-functional theory with a gradient-corrected (GC) local-spin-density approximation (LSDA). The agreement between the calculated and observed results is fairly good. The GC-LSDA is essential to calculate the isotropic hyperfine coupling constants for \ensuremath{\pi} radicals. For \ensuremath{\sigma} radicals, the simple LSDA also leads to relatively good results.

Structure and hyperfine coupling constants of radicals in muon-irradiated single-crystal naphthalene

We have observed muonated cyclohexadienyl-type radicals in a naphthalene single crystal, using the standard transverse-fieldμSR technique. Two types of radicals were found, corresponding to muonium addition at theα andβ positions. Owing to the crystal field, the isotropic hyperfine coupling constants show large shifts from the solution values, and there is significant anisotropy in the hyperfine tensors. The results for theα radicals are similar to those observed for protonatedα-hydronaphthyl radicals, but isotope effects are evident. Theβ radicals, whose protonated analogues have not been fully studied, show a more pronounced localized character. Based on the hyperfine tensor directions and geometrical considerations, we have assigned each radical to a specific muon site within the crystal.

Hyperfine and Structural Isotope Effects in Muonated Cyclohexadienyl and Cyclopentyl Radicals

AbstractIncorporation of muonium, the ultra‐light isotope of hydrogen with a mass of only one‐ninth the mass of H, leads to pronounced isotope effects in hyperfine coupling constants of radicals. They reflect the mass dependence of the internal dynamics. A detailed analysis is presented for the cyclohexadienyl and the cyclopentyl radicals. It is concluded that the isotope dependent bond length in an anharmonic potential accounts quantitatively for the observed effects in rigid species.

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.

Proton Hyperfine Coupling Constants in Radicals and the Effect of Orbital Overlap

RECENTLY Symons1 has considered the influence of the excess charge effect on proton hyperfine coupling constants. He observed that cations always seem to have larger proton hyperfine coupling constants than the analogous anions. This was rationalized as the systematic changing of the effective nuclear charge on the proton with the excess charge on the radical. The series of iso-electronic radicals BH3−, CH3 and NH3+ was used to demonstrate the principle very convincingly.

Proton Hyperfine Coupling Constants in Radicals and the Excess Charge-effect

CONSIDERABLE controversy1–3 has centred around the effect of excess charge on the magnitude of proton coupling constants in such radicals as the cations and anions of aromatic hydrocarbons, benzene, naphthalene and anthracene. Simple theories predict that the coupling constants for cations and corresponding anions should be equal, whereas electron spin resonance results show that the cations always have larger values (Table 1).

Aryloxy-radicals. Part VI. Measurement of the electron spin resonance spectra of short-lived substituted phenoxy-radicals in benzene solution

The e.s.r. spectra of short-lived alkyl- and alkoxy-substituted phenoxy-radicals can be obtained by flowing benzene solutions of the corresponding phenols through a bed of either silver oxide or lead dioxide placed within the cavity of an e.s.r. spectrometer. The hyperfine coupling constants of radicals obtained from 4-, 2,4-, and 2,4,6-substituted phenols have distinctly different values when measured in benzene and not in water, but this is not the case with 2,6-substituted phenoxy-radicals. By stopping the flow, e.s.r. spectra of secondary radicals can often be obtained; their structures receive comment.