The E.S.R. spectrum of irradiated ammonium hypophosphite

Electron spin resonance spectra of irradiated crystals of ammonium hypophosphite (NH₄)⁺(H₂PO₂)^- indicate that the ionic radical H[Pdot]O₂ ^- is initially formed. The hyperfine interaction tensor of the phosphorus nucleus has the principal values 1698, 1228 and 1228 Mc/s. Interaction with the proton gives rise to an almost isotropic hyperfine coupling of 230 Mc/s. These results indicate that the species H[Pdot]O₂ ^- is irregular tetrahedral in shape, the unpaired electron occupying a hybrid σ orbital centred on the phosphorus nucleus.

There is evidence that the radical H[Pdot]O₂ ^- reacts with an adjacent H₂PO₂ ^- ion forming the species O₂[Pdot]-PHO₂ ⁼. The principal values of the hyperfine interaction tensors of the two phosphorus nuclei are 1365, 944, 921 Mc/s and 527, 362, 359 Mc/s respectively. The proton has an isotropic hyperfine coupling of 90 Mc/s. The spectra of irradiated deuterated crystals confirmed this analysis.     

An E.S.R. study of electron irradiated K3IrCl6 in a NaCl host lattice

Electron irradiation of the diamagnetic [Ir^IIICl₆]^3- ion in a NaCl host lattice produces two paramagnetic species. ESR studies indicate that these slightly different species can be assigned to a [Ir^IICl₆]^4- complex, with 5d ⁷ low spin configuration, arising from the capture of an electron in a d_z ² orbital. The almost zero value for the quadrupolar interaction and the negative value for the core polarization field per unpaired spin are discussed in terms of a ligand field theory and the group overlap integral for the A _1g molecular orbital is calculated from the experimental data.     

E.S.R. study of the electron trapping centre in a single crystal of maleic acid irradiated at 77 K

Evidence for an electron trapping centre produced by ionizing radiations in a single crystal of maleic acid has been obtained by electron spin resonance. When the crystal is irradiated at 77 K several paramagnetic species are produced. One of them is the species in which the unpaired electron occupies the delocalized π orbital and interacts with the two vinylene protons. The hyperfine tensors of the two vinylene protons, which are typical of α proton coupling, are (-13·0, -8·5, -3·2) and (-6·4, -5·2, -1·9)G, respectively. The g tensor is (2·0043, 2·0040, 2·0024). The principal directions of the hyperfine and g tensors give information about the electron trapping centre in which the framework of the molecule is preserved. The probable structure of this centre is the delocalized carboxyl anion although the possibility of its protonated form cannot be completely excluded.     

E.S.R. study of the conformational kinetics of a biradical

Intramolecular motion in the biradical N,N'-bis(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)ethylenediamine which contains two ionizable amine groups has been studied by E.S.R. spectroscopy. The line-shapes of the spectra as a function of pH and temperature were analysed by density matrix methods to give values for rates and free-energies of activation.     

E.S.R. study of copper and silver N,N-dialkyldiselenocarbamates

Single-crystal E.S.R. studies of Cu(II) diethyldiselenocarbamate and Cu(II) diethyldithiocarbamate, diluted in the corresponding Zn(II) complexes, are reported. The pure copper and zinc crystals consist of dimeric units. When copper is built into the zinc lattice, dimers are formed with one copper and one zinc atom. The g-tensor, the metal hyperfine and quadrupole coupling tensors and also the hyperfine coupling tensors of the ligand selenium atoms have been obtained. Especially, from the measured selenium hyperfine couplings, it could be concluded that the guest molecules do not accept the structure of the host crystal, but have a structure similar to that of the pure copper compound. This is in contrast to the situation in systems where monomeric host crystals (e.g. nickel (II) complexes) are used and where it is found that the guest molecules do accept the structure of the host. As in the monomeric diselenocarbamate systems, in these dimers the principal axes of the g and metal hyperfine coupling tensor do not coincide. In the dimers a rotation of the hyperfine principal axes has taken place in the plane of the molecule, while a rotation of the g-tensor axes in a plane perpendicular to the molecule is typical for the monomeric systems.     

An E.S.R. study of the structure of phosphorus centred radicals in solution

Clementi and Veillard's analysis of the MC SCF theory of closed-shell systems is developed on a different basis. An equivalent Hartree-Fock operator is defined and its physical significance is discussed. A generalized Brillouin theorem is derived, different from that given by Levy and Berthier.     

An E.S.R. study of the triplet dianions of 1,3,5-triphenylbenzene and 2,4,6-triphenyl-sym-triazine

Upon prolonged alkali reduction in ethereal solvents 1,3,5-triphenylbenzene (Tpb), 2,4,6-triphenyl-sym-triazine (Tpt) and 2,4,6-tritolyl-sym-triazine (Ttt) can be reduced to their dinegative ions. Dissolved in rigid solvent matrices these ions produce E.S.R. spectra which are characteristic for randomly oriented triplet systems. Hückel and SCF MO calculations indicate that these dianions have a twofold degenerate lowest anti-bonding level.

Although the ions Tpb^2- and Tpt^2- are iso-electronic their magnetic properties prove to be strikingly different. While Tpb^2- has a triplet ground state irrespective of solvent, counter ion and temperature, Tpt^2- (and also Ttt^2-) has a singlet ground state with a thermally accessible triplet state in solvents with poorly solvating properties and has a triplet ground state in better solvating media. Furthermore, the zero field splitting parameters D and E of the lowest triplet states of Tpt^2- and Ttt^2- are markedly different from those obtained from Tpb^2-. The former show a considerably larger D value than Tpb^2- and, in contrast to Tpb^2-, the ions Tpt^2- and Ttt^2- have a non-zero E value in poorly solvating solvents, pointing to a non-trigonal spin distribution of the triplet systems.

An explanation of the observed differences is given in terms of current MO theories. The results suggest that in the case of Tpt^2- and Ttt^2- the association with the counter ions even in strongly solvating solvents is very strong.     

Electrical and optical properties of neutron irradiated the GeS8 glass

The GeS₈ chalcogenide glass was irradiated by four various doses of fast neutrons. An absorption band has appeared at 450 nm and developed with the neutron doses. It has been concluded that neutral defects are present in the material. From the measurements of the temperature dependence of the d.c. conductivity follows that the activation energy does not depend on the neutron dose within the measured range while the magnitude does with the peak value at neutron dose of 10¹⁷ ncm^?2.     

The E.S.R. spectrum of the tropolonyl radical

The many-body diagrammatic Rayleigh-Schrödinger perturbation theory (RSPT) is used for the calculation of ionization potentials of open-shell systems with one unpaired electron. This theoretical approach is tested on the simple examples of NO₂ and NF₂ molecules described by the INDO semi-empirical hamiltonian. The first- and second-order results are presented.     

E.S.R. spectroscopy of non-cyclic enones

The π-bond model is applied to the calculation of the proton chemical shifts of a range of benzenoid hydrocarbons. The simple calculations give results which compare well with experiment and with calculations based on the ring current model. Least squares fitting of the predicted chemical shifts to the observed values can give an indication of the occurrence of local aromatic groups in larger molecules. This procedure also yields an empirically adjusted estimate of the anisotropy in the molar magnetic susceptibility from the experimental proton chemical shifts. A simple criterion for aromaticity is also given. The overall results suggest that there is no need to associate a special ring current attribute to π conjugation in aromatic molecules.     

Photolysis and E. S. R. Studies on Triphenylphosphinecarbethoxymethylene

Although several different groups have studied the photolysis of phosphorus ylides, none have utilized e. s. r. to support the postulated decomposition mechanisms. The fundamental question which awaits a definitive answer is which bond breaks first upon photolysis. Tschesche¹ and de Silva² utilizing the ylides (1) and (2) suggested that the carbon-phosphorus double bond was initially broken while Nagao³ using (3) proposed phenyl-phosphorus bond cleavage.     

E.S.R. spectra of diboranyl radicals in the solid states

E.S.R. Spectra of the radicals produced from the γ-irradiated diborane (6) in polycrystal or single crystal have been investigated. From the analysis of the results of the single crystal, the radical was identified as neutral diboranyl, ·B₂H₅, produced by extracting one of the two bridging hydrogen atoms of diborane. The major portion of the spin density on the radical is equally shared by the two boron atoms, a considerable portion exists on the bridging hydrogen atom, and a small portion with negative sign on the four terminal hydrogens. The orbitals of the unpaired electron on the two borons are in the same plane at an angle of 73°. These results are in good agreement with those from INDO calculation for the radicals. The structure of the radical is discussed in comparison with some other radicals having a similar structure.     

Studies of annealing of point defects and their influence on the electrical degradation and recovery behaviors of heavily neutron irradiated silicon

We have investigated the annealing behaviors of point defects and their influence on the electrical degradation and recovery of heavily neutron irradiated silicon. It is found that high concentrations of divacancy (V₂) and vacancy-oxygen (VO) complexes are introduced in heavily irradiated silicon, which is responsible for the enhanced carrier recombination and therefore the observed drastic decrease of carrier lifetime. While the dopants deactivation from vacancy-phosphorus (VP) complexes and carrier compensation from VO and V₂ complexes result in the remarkable increase of resistivity. After post-irradiation isochronal annealing at 200–400°C the carrier lifetime and resistivity exhibit insignificant changes, which is attributed to the transformation of V₂ and VO complexes into [VO?+?O_i], V₂O, V₃O complexes at 200–300°C and the possible formation of VP-O complexes at 400°C. While the heat treatments at elevated temperatures (>400°C) result in the elimination of the majority of electrically active V_mO_n and the possible VP-O complexes, and therefore the carrier lifetime and resistivity of silicon begin to recover at 500°C and 650°C, respectively. However, the recovery of carrier lifetime is incomplete, which is due to the enhanced carrier-recombination from the survival defects with deep levels at E_C ? 0.24?eV and E_C ? 0.44?eV during annealing.     

Rapid computer simulation of E.S.R. spectra

A fast computer algorithm is presented which permits simulation of the effects of rotational diffusion, electron and nuclear relaxation, microwave power, and modulation frequency upon saturation transfer (passage) E.S.R. spectra. Comparison of theoretical and experimental spectra for nitroxide spin-labelled biomolecules suggests that while the dependence of electron spin-lattice relaxation time upon rotational correlation time is weak, the variation of the ratio of the electron to nuclear spin-lattice relaxation times is significant and consideration of strong nuclear relaxation is necessary for the simulation of spectra characterized by correlation times near the reciprocal of the nitrogen nuclear resonance frequency.     

E.S.R. of Cl17O2

The E.S.R. spectrum of ¹⁷O-labelled ClO₂ was studied in an X-irradiated single crystal of KClO₃ and in toluene solution. The isotropic hyperfine constant measured in solution is 12·0 gauss and allows the correct choice of signs to be made in analysing the dipolar tensor in the solid. The values of the anisotropic tensor indicate a spin density of 0·27 on each oxygen.     

E.S.R. evidence for the planarity of the t-butyl radical

Monte Carlo calculations are reported for the radial distribution function g ₂(r; λ) of a fluid in which the intermolecular pair potential is [u ^ref(r) + λu ^p(r)], u ^ref(r) being the Weeks-Chandler-Andersen (WCA) reference fluid, and [u ^ref(r) + u ^p(r)] being the Lennard-Jones (6, 12) fluid. The calculations are performed for λ values in the range 0 to 1, at the state condition ρσ³ = 0·80, kT/ε = 0·719. It is shown that at high densities the perturbation expansion of g ₂(r; λ = 1) about g ₂(r; λ = 0) is rapidly convergent, but that the corresponding expansion for y ₂(r; λ) = exp [βu(r; λ)] × g ₂(r; λ) is not. In addition Monte Carlo estimates of the individual terms that contribute to the first-order perturbation term, (?g ₂/?λ)_λ=0, are presented. It is shown that these terms are individually large, but that (?g ₂/?λ)_λ=0 is small because there is strong cancellation between the various terms. Consequently, the calculation of (?g ₂/?λ)_λ=0 is highly sensitive to the approximation used to evaluate the individual terms.     

A study by E.S.R. of Mn2+ in potassium chromate

Divalent manganese ions are found in potassium chromate single crystals after heating at 600°C in the presence of iodine. These ions occupy three differently oriented sites with the same spin Hamiltonian parameters: g_z = 2·000, g_x = 2·008, g_y = 2·015; A_z = ?88·4 G, A_x = A_y = ?91 G; D = 190 and E = ?30 G.The authors suggest that Mn²⁺ occupies a substitutional (K⁺) position rather than an interstitial one. The influence of iodine is discussed.     

Photo-EPR study of gamma and neutron irradiated SI GaAs:Cr

The photo-EPR study of semi-insulating GaAs:Cr is reported. For non-irradiated samples the temperature dependence of Cr EPR signal amplitude was measured and the abnormal increasing of the Cr⁴⁺ EPR signal at the time the illumination was stopped was discovered. The neutron irradiation suppressed the Cr⁴⁺ EPR signal, but had almost no influence on the Cr²⁺ signal. The gamma irradiation induced a new spectrum.     

E.S.R. spectra of X-irradiated methylene diphosphonic acid

X-irradiation of single crystals of methylene diphosphonic acid gives rise to E.S.R. spectra which have been assigned to (OH)₂(O)P-?H₂ and

The former species is the more abundant of the two and the methylene group undergoes rapid re-orientation about a two-fold axis at room temperature but is stationary at 77 K. The ¹H and ³¹P coupling constants in this radical are very similar to those in Ph₃P⁺CH₂, and support the previous conclusions [1] that there is little delocalization of the unpaired electron to the phosphorus atom.