T. Cole

Paramagnetic Defects in Irradiated NH4ClO4

Analysis of the electron magnetic resonance of x‐ray damaged crystals of NH4ClO4 shows that the long‐lived defects are NH3+ ions. Isotropic hyperfine splittings are found to be 54.6 Mc for nitrogen and 72.5 Mc for hydrogen. These radicals appear to be planar and execute restricted rotation at room temperature. A second transient defect is identified as ClO3.

13C chemical shift tensor in K2Pt(CN)4Br0.3 · 3H2O

Single crystal rotation spectra have been obtained for the 13C resonance in K2Pt(CN)4Br0.3 · 3H2O and the 13C chemical shift tensor determined. The principal values are (relative to the carboxyl carbon in CH3COOH): σ11=261±10 ppm (parallel to C–N bond), σ22 = −48±10 ppm (perpendicular to C–N bond and Pt–Pt chain), and σ33=−10±5 ppm (parallel to Pt–Pt direction). Since the 14N quadrupole interaction is of similar size to the 14N Zeeman interaction in this material, large effects of the 14N quadrupole interaction are observed on the 13C spectra.

Electron Magnetic Resonance of an Oriented Allyl‐Type Free Radical

It has been found that x‐irradiation of a single crystal of glutaconic acid (HOOC)CH=CH–CH2(COOH) produces the oriented allyl‐type free radical (HOOC)C(1)H=C(2)H–C(3)H(COOH). A detailed analysis of the electron magnetic resonance spectra of this radical confirms that the spin density on carbon 2 is opposite in sign to the spin densities on carbons 1 and 3. Additional structural features determined are: (a) The unpaired electron occupies a delocalized π molecular orbital extending over carbon atoms 1, 2, and 3; (b) spin densities ρii for carbon p orbitals on carbon atoms 1, 2, and 3 are ρ11=ρ33=+0.57, ρ22=−0.19; (c) principal values of the hyperfine tensors for protons 1 and 3 are −53, −36, and −18 Mc, and for proton 2 are +17, +12, +7 Mc; (d) all C–H bonds in the radical are nearly parallel. The parallelism of all C–H bonds indicates that the undamaged parent molecule is the trans isomer of the acid.

ESR and ENDOR Spectra of Gd3+ in CdF2

ENDOR and ESR have been used to measure the spin‐Hamiltonian parameters for Gd3+ ions in single crystals of CdF2. Measurements were made in insulating and semiconducting crystals. In both cases the predominant ESR spectrum arose from substitutional Gd3+ in cubic sites. Hyperfine splittings for the 155Gd and 157Gd were 12.8414 and 16.856 MHz, respectively. Parallel and perpendicular components of the hyperfine tensors of surrounding 19F nuclei were; A| = 8.693 MHz and A⊥ = − 6.952 MHz for the nearest‐neighbor fluorines. More distant fluorine hyperfine splittings out to the fourth‐nearest‐neighbor shell have been measured and are found to fit closely to a point‐dipole model. No differences in ESR and ENDOR Hamiltonian parameters, outside experimental errors have been found for Gd3+ between insulating and semiconducting CdF2 at low temperatures. Specifically, no charge‐compensating F− ion was found within 7 A of the Gd3+. The ENDOR measurements give values for the magnetic moments of 155Gd and 157Gd of − 0.2...

ZERO-FIELD ELECTRON MAGNETIC RESONANCE IN SOME INORGANIC AND ORGANIC RADICALS

Zero‐field electron magnetic resonance (EMR) spectra have been observed for several dilute paramagnetic systems. Among these are peroxylamine disulfonate ions in aqueous solutions, Cr3+ in single crystals of MgO, free radicals in x‐irradiated powders of malonic acid, potassium hydrogen malonate, dipotassium malonate, succinic acid, and glutaconic acid. Zero‐field spectra were observed by means of a marginal oscillator—detector similar to that of Benedek and Kushida. The zero‐field splittings in the systems studied were due exclusively to the hyperfine couplings between electronic and nuclear spins. Results obtained were in good agreement with high‐field EMR measurements on these systems. In several cases the hyperfine couplings were obtained with greater precision than at high field. In addition, the radical —OOC–ĊH–COO— was identified, for the first time, in irradiated dipotassium malonate by means of its zero‐field spectrum. No zero‐field lines could be detected in electron‐irradiated polyethylene. Adva...

Production of Free Radicals in Organic Solids by Hydrogen‐Atom Bombardment

A beam of hydrogen (or deuterium) atoms, produced by a microwave electrodeless discharge, has been found capable of producing free radicals upon striking polycrystalline samples of malonic acid, malonamide, methyl malonic acid, and methyl malonamide. The radicals produced in these compounds, identified by means of their electron magnetic resonance signals, arise from the removal of a methylene hydrogen of the parent molecule. Only one radical species is produced in each compound, in contrast to x irradiation which often produces several types of radicals. The present study strongly suggests that the impinging hydrogen atoms (a) abstract methylene hydrogens from the organic molecules to form radicals and H2 and (b) penetrate several molecular layers of the crystallites before they react.Radicals were also formed by impinging hydrogen atoms in naphthalene, hexamethylene tetramine, calcium malonate, calcium formate, and aluminum acetyl acetonate. However, no positive identification of these radicals could be...

Electron Nuclear Double Resonance in Irradiated Organic Crystals

The electron nuclear double resonance (ENDOR) spectrum in irradiated single crystals of succinic acid is shown with the magnetic field along the b axis. At 14.2 Mc the intense line obscuring the CH proton is due to depolarization.

Noncubic EPR Spectra of Gd3+ in CdF2

Spectra of Ge3+ with orthorhombic symmetry have been observed in CdF2 crystals containing both Gd3+ and Na+ as impurity ions, where the charge‐compensating Na+ ions occupy nearest‐neighbor Cd2+ sites. The orthorhombic field parameters are D = 116 ± 1 MHz, E = 42 ± 1 MHz, and the cubic field parameter is B4 = − 2.444 ± 0.005 MHz. The variation of the intensity of the orthorhombic spectrum with heat treatment has been observed. Also, a complicated spectrum has been observed in CdF2 containing both Gd3+ and Y3+; it is suggested that this spectrum can be interpreted in terms of the exchange coupling between Gd3+ ions and trapped conduction electrons.