Anisotropic Hyperfine Coupling Constants Research Articles

2D Ligand ENDOR and TRIPLE resonance studies of Cu(II)-doped DTGS single crystals

The deuteron hyperfine and quadrupole coupling tensors in the bis(glycinato)-Cu(II) ((ND2CH2COO)2Cu) complex in ferroelectric triglycine sulfate (DTGS) single crystals were measured by ENDOR spectroscopy. These data indicate that the molecular symmetry of the Cu complex is C1. From TRIPLE resonance experiments the signs of the isotropic coupling constants were determined. Structural information was obtained from the principal values of the quadrupole coupling tensors of the deuterium atoms and their direction cosines. Using the relationship between the deuterium quadrupole constant and ND bond lengths, the positions of the deuterons in bis(glycinato)-Cu(II) were estimated. The negative signs of the isotropic hyperfine coupling constants were explained by spin polarization of the deuteron 1s orbitals. The values of the anisotropic hyperfine coupling constants are discussed in the framework of conventional molecular-orbital theory and can be understood quite satisfactorily.

Electron spin resonance spectroscopy, stability and spin-probe properties of dithiazolyl, dithiadiazolyl, benzodithiazolyl and disulphenimidyl free radicals

A comparative investigation has been made of the e.s.r. spectra of 1,3,2-dithiazol-2-yl, 1,3,2,4-dithiadiazol-2-yl, 1,3,2-benzodithiazol-2-yl and dibenzenesulphenimidyl free radicals, both in solution and in rigid matrices. The powder spectra were analysed to give values for the anisotropic g-factors and hyperfine coupling constants, and, in turn, these results were used by means of a simple e.s.r. tumbling theory to calculate radical radii from low-temperature fluid e.s.r. spectra. A simple correction was incorporated into the theory to include a contribution from ‘slip’ rotation. The values of the nitrogen isotropic coupling constants provide a useful guide to the electronic and spatial structures of the radicals. It is shown that the 1,3,2-dithiazol-2-yl radicals have the greatest potential as spin probes.

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.

ChemInform Abstract: CALCULATION OF ZERO FIELD SPLITTING PARAMETERS FOR TRIMETHYLENEMETHANE

The spin dipole–dipole contribution to the zero field splitting of the 3A′2 ground state of trimethylenemethane has been computed with ab initio CI wave functions which included selected single and double excitations from the Hartree–Fock configuration. The resulting value of D is 0.020 cm−1. Isotropic and anisotropic proton hyperfine coupling constants were also computed. The difficulty of quantitatively reproducing experimental values for these properties is discussed.

Calculation of zero field splitting parameters for trimethylenemethane

The spin dipole–dipole contribution to the zero field splitting of the 3A′2 ground state of trimethylenemethane has been computed with ab initio CI wave functions which included selected single and double excitations from the Hartree–Fock configuration. The resulting value of D is 0.020 cm−1. Isotropic and anisotropic proton hyperfine coupling constants were also computed. The difficulty of quantitatively reproducing experimental values for these properties is discussed.

Semiempirical molecular orbital calculations of anisotropic1H,13C and19F hyperfine coupling constants in hydrocarbon and fluorocarbon radicals

The anisotropic hyperfine coupling constants (AHCC) from the electron spin resonance (E.S.R.) spectra of a variety of atoms in organic radicals have been calculated by means of semiempirical molecular orbital wavefunctions in the INDO approximation. Hyperfine tensors involving 1H, 13C and 19F nuclei are obtained for the ĊH, ĊH3, CH3ĊH2, (CH3)3Ċ hydrocarbon radicals, malonic acid radical, ĊH2F, ĊF2H, ĊF3 and CF3ĊH2 radicals. The calculated values are compared with available experimental, non-empirical and semiempirical values for these radicals. All integrals of the operator entering the electronic contributions have been evaluated over Slater type orbitals. The introduction of deorthogonalized wavefunctions gives generally better calculated results. In particular, the tensor components of the 19F AHCC are in good agreement with the experimental results without the necessity of readjusting the effective nuclear charges.

CNDO and INDO MO calculations of a few properties of rubredoxins

The CNDO and INDO MO calculations have been performed on the oxidised and reduced species of rubredoxins using Clack, Hush and Yandle’s parameterisation scheme for the transition metal complexes in conjunction with Pople and Nesbet’s open-shell UHF method in an attempt to elicit information on the geometry of the conformation of the iron environment. The properties like isotropic and anisotropic hyperfine coupling constants and quadrupole splitting have been calculated and compared with the corresponding experimental values. The results suggest that both the oxidised and reduced species have the iron atoms attached to four sulphur atoms situated as pairs in two perpendicular planes as a result of a highly distorted tetrahedral arrangement

ChemInform Abstract: ORGANOMETALLIC PEROXY RADICALS. 4. ELECTRON SPIN RESONANCE SPECTRA OF THE RADICALS PRODUCED BY REACTION OF ORGANOPHOSPHORUS(IV) AND ORGANOARSENIC(IV) RADICALS WITH 17O‐ENRICHED OXYGEN

It bas been shown that some tetra-coordinate phosphorus and arsenic radicals combine with oxygen to give peroxy radicals. The magnitude of the oxygen isotropic and anisotropic hyperfine coupling constants indicates that nearly all the unpaired spin is associated with the two oxygen nuclei with ca. 60% on the terminal oxygen. The large isotropic g-factors associated with Group VB peroxy radicals relative to alkylperoxy radicals is attributed to a small change in geometry of the peroxy function.The radicals with ap ∼ 9 G obtained during the reaction of photochemically generated tert-butoxy radicals with trialkylphosphites in the presence of oxygen are secondary species which appear to contain only one oxygen atom.

Organometallic peroxy radicals. 4. Electron spin resonance spectra of the radicals produced by reaction of organophosphorus(IV) and organoarsenic(IV) radicals with 17O-enriched oxygen

It bas been shown that some tetra-coordinate phosphorus and arsenic radicals combine with oxygen to give peroxy radicals. The magnitude of the oxygen isotropic and anisotropic hyperfine coupling constants indicates that nearly all the unpaired spin is associated with the two oxygen nuclei with ca. 60% on the terminal oxygen. The large isotropic g-factors associated with Group VB peroxy radicals relative to alkylperoxy radicals is attributed to a small change in geometry of the peroxy function.The radicals with ap ∼ 9 G obtained during the reaction of photochemically generated tert-butoxy radicals with trialkylphosphites in the presence of oxygen are secondary species which appear to contain only one oxygen atom.

S3− radicals stabilized on the MgO surface

The components of g tensors as well as anisotropic and isotropic hyperfine coupling constants have been calculated for the S 3 − and S 3 + free radicals within the scope of an approximate SCF MO method. The results of calculations are compared with the available experimental data on magnetic resonance parameters of paramagnetic species formed on magnesium oxide and identified as the S 3 − ions.

Defects in hydrogenated SrO

The paramagnetic ${V}_{\mathrm{OH}}$ and ${V}_{\mathrm{F}}$ defects generated by $\ensuremath{\gamma}$ irradiation at 77 K have been observed in hydrogenated SrO crystals. These defects are not stable at room temperature. EPR and electron-nuclear double-resonance measurements have been performed at low temperature in order to identify the impurity nucleus. In both cases, the defect is axially symmetric about a crystalline 100> axis. An analysis of the anisotropic hyperfine coupling constants of these centers in MgO, CaO, and SrO shows that there is an approximate linear dependence between the distance from the trapped hole (oxygen-fluorine for the ${V}_{\mathrm{F}}$ and oxygen-oxygen for the ${V}_{\mathrm{OH}}$ and the crystal lattice parameter. A complex structure in the vicinity of $g\ensuremath{\approx}2$, probably due to aggregate defects, is shown to be associated with hydrogen.

Unstable intermediates. Part 165. Radicals in irradiated organolead compounds: an electron spin resonance study

Exposure of PbPh3Cl and PbBrPh3 to 60Co γ-rays at 77 K gives centres having 207Pb and 35Cl or 81Br hyperfine coupling constants consistent with expectation for the parent radical anions. The electron-loss centres are thought to be the corresponding cations, with the hole largely confined to the halogen atoms. Irradiated PbMe3Cl gives a similar anionic centre in methanolic solution, but the pure material gives largely Me˙ and PbMe2(ĊH2)Cl radicals. Irradiated PbMe4 and PbEt4 pure or in toluene at 77 K. give centres thought to be PbR3˙ radicals having g⊥≈ 2.1 and g∥≈ 1.9, and abnormally large anisotropic 207Pb hyperfine coupling constants. In addition, PbMe4 gives a centre thought to be [PbMe4]–˙ and both give R˙ and PbMe3(ĊH2) or PbEt3(ĊHMe). Irradiated Pb2Ph6 gives only one clearly defined paramagnetic centre, tentatively identified as the parent anion.

On the INDO calculations of N 32− and N 4− free radicals

ĝ-tensors as well as anisotropic and isotropic hyperfine coupling constants have been calculated for the N 3 2− and N 4 − free radicals. The adequacy of the INDO method is discussed in relation with the results of a previous ab initio calculation.

Electron nuclear double resonance studies on heme proteins: interaction of the electron spin with the 57Fe 3+ nucleus in metmyoglobin and methemoglobin

The electron nuclear double resonance (ENDOR) spectrum from the 57Fe nucleus in high-spin ferric heme was obtained in frozen solutions of metmyoglobin and methemoglobin. Values for the isotropic and anisotropic hyperfine coupling constants were obtained from studies on 57Fe-enriched heme proteins. Small but definite differences between the values of the hyperfine constants for the two proteins were observed.

ENDOR of VF and VOH centers in CaO

ENDOR (electron-nuclear double resonance) studies on CaO single crystals positively identify the VF center and resolve a discrepancy in the reported g values of the VOH center. Data for the same two centers in MgO were also obtained and compared with previous work. An analysis of the anisotropic hyperfine coupling constants shows that the relative lattice relaxation for both centers is roughly the same in these hosts. As expected, no motional averaging of the anisotropic resonance spectra was observed for either center.

Calculations of the anisotropic hyperfine coupling constants in free radicals

For a number of free radicals the results of nonempirical ( ab initio) and semi-empirical (CNDO/SP, INDO) calculations of the anisotropic hyperfine coupling constants are compared.

An SCF-MO-INDO Study of Isotropic and Anisotropic Hyperfine Coupling Constants for Sigma Radicals

An SCF-MO-INDO Study of Isotropic and Anisotropic Hyperfine Coupling Constants for Sigma Radicals

The Interpretation of the EPR Spectra of the Amorphous State of Group VIB Hydride Radicals by Means of the Gas-phase Data

Abstract A new procedure for the interpretation of the amorphous EPR spectra of the group VIB hydride radicals, i.e., OH, SH, SeH, and TeH, is proposed. The point of this method is to combine the observed hyperfme structure of these radicals in the gas phase and the calculation of the anisotropic hyperfine coupling constant. For this purpose, the Slater orbital and analytical Hartree-Fock orbitals were examined as unpaired electron orbitals by the comparative calculation of the hfs constants. The accuracy of this method was evaluated successfully by applying it to the OH radical. Moreover, in the case of the SH radical the newly-observed spectra in the amorphous state could be assigned to the SH radical.

On the calculations of the anisotropic hyperfine coupling constants of CH 3 and NH 2 free radicals

Inconsistency in the results of calculations of the isotropic and anisotropic hyperfine coupling constants of π-electronic free radicals is investigated using CH 3 and NH 2 as models. A possible reason of this inconsistency is discussed.

Electron spin resonance study of γ-irradiated ethyleneimine in the solid state

The electron spin resonance (e.s.r.) study of the ethyleneimine in polycrystalline and glassy state shows that the radical [Formula: see text] is formed by γ-irradiation at 77 °K. The features of the e.s.r. spectrum of this radical are discussed in terms of the isotropic and anisotropic hyperfine coupling constants of the hydrogen and nitrogen nuclei. The effects of bleaching the γ-irradiated sample with visible and ultraviolet light are also reported.