Determination of signs of hyperfine coupling constants for an S = ½ system through E.P.R., ENDOR and double ENDOR

A systematic method of obtaining relative signs of hyperfine coupling constants is described. It applies to systems consisting of (a) a set of one or more nuclei coupled fairly strongly to the electron spin, and possessing a two-fold (or higher) axis of symmetry, together with (b) a set of weakly coupled nuclei defining superhyperfine transitions. ENDOR measurements for several E.P.R. hyperfine transitions, with the field oriented along the symmetry axis, give relative signs of hyperfine components for this direction. Signs for the other directions can then be obtained through ENDOR measurements on a single hyperfine transition at various field orientations. Additional double ENDOR measurements may be necessary for very weakly coupled nuclei. This method can complement double ENDOR studies in favourable cases. It is illustrated by the determination of signs of coupling constants of protons and of ⁷⁵As in the AsO₄ ^4- radical in KH₂AsO₄.     

Elektronen-Kern-Doppelresonanz-Untersuchung vonU 2-Zentren in Kaliumchlorid

U ₂-centers in alkali halides are neutral hydrogen atoms in interstitial lattice sites, as has been shown by EPR measurements. The hyperfine interactions with the proton and with the four nearest halogen nuclei are resolved in the EPR spectrum. In order to resolve hyperfine interactions with further nuclei of the surrounding lattice ENDOR measurements have been performed onU ₂-centers in KCl at 77 °K. The analysis of the ENDOR spectra gave precise values for the hyperfine and quadrupole interaction constants of the nearest neighbour chlorine and potassium nuclei. The isotropic hyperfine constant of the chlorine neighbours is 24 times larger than that of the potassium neighbours although both nuclei are on equivalent first shell lattice positions. The hyperfine interactions of second shell potassium nuclei [(1/2, 1/2, 3/2)-position] show an unexpectedly large isotropic hyperfine constant. One expects a pure magnetic dipole-dipole interaction for the outer shell nuclei because of the concentrated hydrogen wave function. Two further chlorine shells could be approximately analysed. A theoretical estimate of the hyperfine and quadrupole interaction constants was made by orthogonalizing the 1s hydrogen wave function to the cores of the surrounding ions. If one takes into account the mutual overlap of neighbouring potassium and chlorine ions, one gets the right order of magnitude of the measured constants and a value of 10.4∶1 for the ratio of the isotropic hyperfine constants of the first shell chlorine and potassium nuclei. The relatively large isotropic constant of the second shell potassium nuclei can also be explained on this basis.     

Theoretical predictions about ENDOR of biradicals in liquid solution

Starting from the equation of motion for the density matrix, ENDOR spectra are computed for a biradical for the first time. The model biradical carries two electronic spins S = 1/2 and two nuclear spins I = 1/2. The relaxation operator is calculated ab initio for three distinct relaxation mechanisms. These mechanisms are the modulation of the isotropic exchange interaction, the hyperfine tensor and the zero-field splitting tensor. The dependence of the ENDOR effect on the amplitudes of the radiofrequency and microwave fields, on the strength of the zero-field splitting tensor, and on the correlation times of the correlation functions is investigated.     

Absolute signs of hyperfine coupling constants as determined by pulse ENDOR of polarized radical pairs

A novel method that allows the determination of absolute signs of hyperfine coupling constants in polarized radical pair (RP) pulse electron-nuclear double resonance (ENDOR) spectra is presented, The variable mixing time (VMT) ENDOR method used here leads to a separation of ENDOR transitions originating from different electron spin manifolds by employing their dependence on the time-dependent parameters of the pulse sequence. The simple kinetic model of the RP VMT ENDOR experiment shows very good agreement with the experiments performed on the P ₇₀₀ ^.+ A ₁ ^.- RP in photosystem I. This method relies on the selective excitation of absorptive or emissive lines of one radical in the RP EPR spectrum and therefore requires high spectral resolution. This condition was fulfilled for the system studied at the low-field edge of the RP EPR spectrum obtained at Q-band. The method presented here has a very high sensitivity and does not require any equipment additional to the one used for RP pulse ENDOR. The VMT ENDOR method offers the possibility for selective suppression of signals from different electron spin manifolds.     

Pauli-forbidden intermediate states and operator identities

The E.S.R. spectra of the β-fluoroisopropyl radical and the β-fluoro, β,β-difluoro and β,β,β-trifluoroethyl radicals have been recorded in fluid solutions. The conformational preferences and barriers to internal rotation in these species have been determined from an investigation of the temperature dependence of the β-fluorine and β-proton hyperfine coupling constants. The experimental hyperfine splittings have been compared with theoretical values calculated by the INDO method.     

Effects of pressure on magnetic properties of F-centers in rubidium halides

Room temperature magnetic resonance measurements have been made on additively colored RbCl and RbBr and on X-irradiated RbH at pressures up to 8 kbar. In RbCl and RbH pressure shifts of the F-center isotropic coupling constant a₁ were obtained from changes in the resolved EPR hyperfine structure. Pressure shifts of first and second shell hyperfine coupling constants of F-centers in RbBr were found from high pressure ENDOR spectra. Effects of the pressure induced polymorphic phase transformations in RbCl and RbBr are discussed. No evidence was found for a structure transformation in RbH. Room pressure ENDOR measurements on additively colored RbI were made at 77°K. The first and second shell hyperfine coupling constants are reported. All results accord well with the behavior of F-centers in other alkali halides.     

On the centrifugal distortion in the microwave spectra of O2 and SO

Stable radicals formed by room temperature X-irradiation of single crystals of acridine and phenazine have been identified and their structures determined by detailed ENDOR studies at 77 K. The E.S.R. spectra are of little assistance in the identification of these radicals. However, the high resolution of ENDOR has made it possible to study all the proton hyperfine interactions in detail. The radicals investigated in this study are formed as a result of the addition of atomic hydrogen to the nitrogen atom of the acridine and phenazine molecules. ENDOR lines of all the protons have been identified and analysed. Spin densities have been deduced from the proton hyperfine tensors. Although the unpaired electron is extensively delocalized throughout the molecule, there are large spin densities (0·475 and 0·584, respectively) on C₉ in acridine and the second nitrogen in phenazine. Excellent agreement has been obtained between the observed spin densities and those computed by INDO molecular orbital calculations. The design and construction of a simple rectangular ENDOR cavity operating in the TE₁₀₁ mode is described.     

1H and 2D N.M.R. studies of the radical anions of biphenyl,fluorenone and phenanthrene

The proton N.M.R. spectra of biphenyl^-, fluorenone^- and phenanthrene^- and the deuterium N.M.R. spectra of biphenyl-d10^- and phenanthrene-d10^- have been measured in ethereal solutions at room temperature. Sign and magnitude of the hyperfine splitting constants derived from the measured contact shifts are reported and compared with E.S.R. data and predictions from theory. From the measured proton and deuterium relaxation times values for the electron spin and the rotational correlation times have been obtained.     

The spin density distribution in some phenyl substituted imidazyl and pyrryl radicals

ENDOR spectroscopy has been used to interpret the E.S.R. spectra of the 2,4,5-triphenylimidazyl, tetraphenylpyrryl, tetrakis (p-tolyl) pyrryl, tetrakis (p-anisyl) pyrryl radicals. Using the hyperfine coupling constants thus obtained in conjunction with a McLachlan-type HMO calculation the spin density distribution in these radicals is calculated. The configuration of the phenyl rings and the deviation of their bond lengths from hexagonal symmetry is inferred from the MO parameters required to fit the experimental couplings.     

Deuteron magnetic resonance of monodeuteroethene: Isotropic and anisotropic phase spectra

Deuteron magnetic resonance spectrum of monodeuteroethene in isotropic phase solution is shown to give the relative signs of the indirect spin-spin coupling constants, while the proton spectrum cannot provide this information. In agreement with previous experimental and theoretical evidence, J _gem is found to have the same sign as the two vicinal coupling constants. High resolution deuteron resonance spectra of monodeuteroethene in the nematic phase solution are analysed in terms of the order matrix and quadrupole splitting. The quadrupole coupling constant and the asymmetry parameter are determined by two partly independent methods.     

Ligand exchange reactions between copper(II)- and nickel(II)-chelates of different sulphur- and selenium-containing ligands

A single-crystal E.S.R. and multi-nuclear ENDOR study of the mixed ligand complex tetra-n-butylammonium(maleonitriledithiolato)(monoethyldithiophosphato)cuprate(II), diamagnetically diluted by the corresponding Ni(II) complex, and the crystal structure of the host complex are reported. n-Bu₄N[Ni(mnt)(HEtdtp)].0·7 acetone is triclinic, space group P1, Z = 2 with a = 9·029(2), b = 13·432(4), c = 17·246(5) Å, α = 108·9(1), β = 90·2(1) and γ = 109·4(1)°. The spin-hamiltonian parameters are calculated from the results of an extended Hückel molecular orbital calculation. The experimental g-tensor and the copper and the four sulphur hyperfine coupling tensors are reporduced well. The almost isotropic and unexpectedly large coupling of the phosphorous atom is negative, which can be understood qualitatively from a spin polarization mechanism.

The hyperfine coupling tensors of eight protons, three of the acetone molecule, four of the tetra-n-butylammonium cation and one intramolecular one, could be determined. The E.S.R. linewidths are unusually small for Cu complexes. Unresolved proton hyperfine interactions are found to give the largest contribution.     

Hyperfine anomaly arising from the nuclear spin-forbidden transition,and absolute sign determination of the zero-field splitting constant

It has been found in the triplet E.S.R. spectra of radical pairs in irradiated potassium deuterium fumarate that the hyperfine structure of the two transitions, M _s = 1?0 and M _s = 0?+1, are entirely different. This anomaly has been interpreted in terms of the forbidden transition arising from the mixing of the nuclear spin states by the anisotropic hyperfine interaction. The theory has been developed for multiplet electron spin systems and includes the nuclear Zeeman interaction which is often neglected. The theoretical predictions are in good agreement with the observed separations and intensities of the anomalous hyperfine lines. In addition, it has been found that since the forbidden lines of the electron spin multiplet system with S ≥ 1 appear strongly only in transitions which include some specific electronic spin states, the anomalous features of the spectra make it possible to determine the absolute sign of the zero-field or hyperfine splitting constant, if the sign for one of them is known. Using this principle, attempts have been made to determine the absolute sign of the zero-field splitting constant for a number of triplet E.S.R. spectra which exhibit a hyperfine anomaly arising from the proposed mechanism.     

Proton hyperfine interactions in the gas-phase E.P.R. spectra of SH (2Π3/2) in two lowest rotational levels

Gas-phase E.P.R. spectra of the SH radical (²Π_3/2) in the J = 5/2 and J = 3/2 rotational levels are described. Analysis of the hyperfine splittings gives the values of axial and non-axial components of the proton hyperfine interaction and the value of the constant d. The frequencies of the two strong lines in the zero-field spectrum are estimated by the use of the observed value of the hyperfine interaction constant d.     

On the influence of second-order splitting and sign of the coupling constant in the E.S.R. spectra of polyatomic free radicals

Second-order splitting has been observed in the E.S.R. spectrum of the monopositive ion of pyracene. Excellent agreement is found upon comparing the experimental transition frequencies and intensities with those predicted by theory. The observed line-width variations of the various hyperfine components indicate that the two isotropic coupling constants have opposite sign.     

ENDOR spectroscopic and molecular orbital study of the dynamical properties of the side chain in radical anions of ubiquinones Q-1, Q-2, Q-6, and Q-10

The dynamics of the side chain of the radical anions of ubiquinones Q-1 (2,3-dimethoxy-5-methyl-6-[3-methyl-2-butenyl]-1,4-benzoquinone), Q-2, Q-6, and Q-10 have been investigated using electron nuclear double-resonance (ENDOR) spectroscopy. When radicals are produced in the liquid phase, secondary radicals are also formed. The EPR spectra of these additional radicals overlap with the radical of interest. ENDOR spectroscopy was found to be capable for studying the dynamical properties of such conditions. The temperature dependence of the isotropic hyperfine coupling constants of the beta- and gamma-protons of the side chain was measured. The activation energy of the rotation and other dynamical properties of the side chain were calculated assuming that rotation can be modeled by the classical two-jump model. The rotation energy barrier for Q-1 was also determined by the hybrid Hartree-Fock/density functional method UB3LYP with the 6-31G(d) basis set. Calculated results were in good agreement with the experimental results. Despite the numerous parameters affecting the ENDOR linewidth ENDOR spectroscopy was shown to be a potential method for studying the dynamical properties of the mixtures of the radicals. Prominent forbidden transitions appear in the ENDOR spectra when alkali ions are present in the sample. From these transitions measured ENDOR-induced EPR spectra showed an additional doublet and phase transition in electron Zeeman frequency.     

ESR, ENDOR, and ESEEM investigations on UV-irradiated 2,4,6-tri-tert-butyl phenol crystals

Electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) measurements were carried out for UV-irradiated 2,4,6-tri-tert-butyl phenol in the polycrystalline state. The radical produced in the crystal was detected by ESR and identified to be the corresponding phenoxyl radical, which is well characterized in the chemical oxidations in solutions. ENDOR and ESEEM spectra were unambiguously analyzed in terms of the hyperfine coupling constants determined from well-resolved ESR in solutions. Radical pairs in the crystals were also ascertained, and together with the single-crystal study the analysis disclosed zero-field splitting parameters in the triplet states. ESEEM time decays gave relaxation timesT ₁ = 5.94 andT ₂ = 1.12 μs at room temperature. These appropriate values permit an easy detection of the spin echoes, and therefore this radical matrix can be used as a useful standard for pulsed ESR investigations.     

E.S.R. and ENDOR of an α-chloro radical in X-irradiated 5-chlorodeoxyuridine single crystals

The most prominent radical formed from X-irradiation of the nucleic acid constituent analogue 5-chlorodeoxyuridine at room temperature is shown to be an α-chloro radical formed by hydrogen addition to C₆. The E.S.R. analysis of the ³⁵Cl hyperfine interaction combined with theoretical simulation of the spin hamiltonian yields tensor components a_xx =46·98 MHz, a_yy =-10·98 MHz and a_zz =-17·01 MHz with a quadrupole coupling constant of eqQ=72 MHz. The principal g-tensor values are g_xx =2·0012, g_yy =2·00862 and g_zz =2·00687. Three additional hyperfine interactions in the radical are observed combining E.S.R. and ENDOR spectroscopy. Besides the two nearly equivalent β-protons on C₆ with principal values of 103·39 MHz and 110·12 MHz, there is hyperfine interaction with the ¹⁴N nucleus of nitrogen N₃ (a_xx =9·81 MHz, a_yy =a_zz ? 0 MHz) and with the proton of the hydrogen bonded to N₃. The latter interaction has tensor components of 2·65 MHz, -10·80 MHz and -8·09 MHz as obtained from ENDOR data. The chlorine hyperfine coupling parameters are related to those observed in other α-chloro radicals. The mechanism of the formation of the radical in 5-chlorodeoxyuridine is discussed briefly.     

E.P.R. of free radicals in an adamantane matrix

Benzyl radicals and the isoelectronic azabenzyl radicals were prepared in an adamantane matrix and their isotropic E.P.R. spectra observed and analysed. The proton hyperfine splittings were assigned by specific deuteration and by analogy with the benzyl radical. The spin density distribution is strongly affected by non-symmetrical nitrogen substitution, but is only weakly affected by symmetrical substitution. INDO calculations on the radicals predict only small deviations from the benzyl radical spin density distribution for all of the azabenzyl radicals. However, the trends in the experimental values can be rationalized in terms of the electronegativity difference between carbon and nitrogen and its effect on the molecular orbitals of benzyl radical. Furthermore the observed trend in the g values not only lends support to this view of the structure but also allows estimation of the approximate energy of the first nπ* excited states.     

Electron paramagnetic resonance study of free radicals inγ-irradiated single crystal of tris(glycine) calcium(II) dibromide

Tris(glycine) calcium(II) dibromide single crystal has been irradiated with aγ-source to produce free radicals and the irradiated sample has been subjected toepr studies. The observed spectra reveal that an NH₄ radical is formed by rupturing glycine molecule due to irradiation. The unpaired electron is localized on the C-N bond. The proton hyperfine interaction on the unpaired spin shows orthorhombic symmetry and the spectroscopic splitting factor remains isotropic.