ESR Line Shape Research Articles

Spin‐probe studies. II. Applications to polymer characterization

AbstractThe spin‐probe technique has been employed to study interactions between several small organic nitroxides and host polymers in which they are dissolved. By this method one is able to study the mobility of the dissolved molecule in its microscopic environment using electron spin resonance spectroscopy. The behavior of one nitroxide has been examined in twelve different polymers and copolymers. An ESR line shape parameter has been correlated with Tm and Tg. In addition, energy barriers for the rotation of the dissolved nitroxides in the polymers have been calculated. They range from 7.8 to 18 kcal/mole, depending on the structure of the nitroxide and the polymer. Similar experiments in a hydrocarbon solvent afford a rotational energy barrier of 3.8 kcal/mole.

ESR line shape in the very vicinity of a critical point

It is suggested that a drastic change of the ESR line shape occurs due to the short range order in the very vicinity of the critical point.

On the ESR line shapes of systems undergoing intermolecular exchange

Attention is drawn to the simplicity of Alexander's equations for the density matrix of a spin system of radical-ions undergoing electron exchange with neutral molecules. The potential usefulness of the equations for describing the line shapes in the ESR spectra of radicals undergoing simultaneuous intra- and intermolecular exchange processes is also pointed out.

ESR Line Shape Due to Anisotropic α-Fluorine Coupling in the Polycrystalline State

The ESR line shape due to an anisotropic hyperfine coupling was calculated including the nuclear Zeeman term in the spin Hamiltonian. Since it was desirable to see the effect of the nuclear Zeeman term explicitly, the line shape was calculated for the ideal case where the component linewidth is infinitesimally small. In such a case, it was possible to derive the shape function analytically for an axially symmetric hyperfine tensor, and even for an orthorhombic symmetry the numerical integration in the line-shape function was not so difficult. The numerical calculation was executed for the α-fluorine coupling with the tensor elements of 189, −4, and −16 G, and the result was compared with the simpler cases in which the nuclear Zeeman term is neglected.

Study on the Quinquevalent Chromium Complex

Accurate values of the parallel and perpendicular components of g-factor and hyperfine coupling tensor were determined for [CrOCln]—(n−3) where n=4 and 5, by the ESR line shape analysis. It was shown by a molecular orbital treatment that these magnetic parameters can be explained consistently by taking into account of an excited electronic configuration in which an electron in a filled orbital is excited to the half-filled level. An assignment of a hitherto unexplained visible absorption band was proposed on the same ground.

Physical Properties and Structure of Polyacetonitrile

Abstract The fact that a polymer containing C=N double bonds is formed from saturated nitrile complexes with metal chlorides is supported by the infrared absorption at about 1600 cm−1 and the characteristic addition of silver nitrate. The polyacetonitrile has an electric conductivity from 10−10 to 10−5Ω−1 cm−1 and an energy gap from 0.4 to 1.2 eV., indicating that the polymer is an organic semiconductor. The electric conductivity increases and the energy gap decreases with increases in the heating time and temperature, suggesting a more extensive formation of a conjugated system, including also C=C double bonds among polymer chains at a higher temperature. This theory is supported by the evolution of ammonia, resulting in simultaneous decreases of the H and N contents of the polymer, the appearance of the absorption at 840 cm−1, by oxidation by potassium permanganate, by analyses of ESR line shapes, and by viscometric measurements. This structual change may bring about a noncrystalline pattern in X-ray diffraction. Therefore, it is indicated that polyacetonitrile does not have a complete C=N conjugated sequence because of some pyrolysis, accompanied by the extension of a conjugated sustem such as cross-linking at a higher temperature and a longer heating.

Ionic Paramagnetic Species Associated with γ-Irradiated Amyl Disulfide

ESR and optical bleaching studies of n-amyl disulfide, γ-irradiated at 77°K in the amorphous state indicate the presence of ionic paramagnetic species in the irradiated material involving unpaired spins localized mainly at the sulfur. A comparison of ESR derivative peak positions from such samples with those arising from an apparent negative ion species produced from amyl disulfide in a tetrahydrofuran matrix indicates that this species may be common to both types of samples. This finding is supported by the fact that the threshold energy for the destruction of the ionic centers by optical bleaching is the same for both types of samples, being 2.1±0.2 eV. If the ESR spectrum of the negative-ion species is substracted from the initial ionic species spectrum of amorphous amyl disulfide an ESR line shape is obtained which is postulated to result from a positive-ion species. The principal g values obtained for this species are 2.003±0.001, 2.035±0.001, and approximately 2.018. Calculated g values based on a model in which the electron hole is localized on a nonbonding 3p sulfur orbital are in qualitative agreement with the observed g values. The The negative-ion species appearing to be common to both types of samples exhibits an axially symmetric g tensor (g‖ = 2.003±0.001, g⊥ = 2.022±0.001). This g tensor symmetry is obtained theoretically using a model in which the unpaired spin is in 3dσ bonding orbitals associated with both sulfurs.

Modulation Broadening of NMR and ESR Line Shapes

An analysis is made of the phase sensitive detection of magnetic resonances at various harmonics of the modulation frequency. The results are applicable to any experiment in which phase sensitive detection is employed. A mathematical treatment of modulation broadening is developed. Unlike the Taylor expansion method this is applicable in all practical cases. Signal amplitude and line shape of Lorentz and Gaussian resonances are calculated for the first three harmonics as functions of the modulation amplitude. Expressions which relate the Fourier coefficients of a modulation-broadened resonance line shape to those of the true integrated line shape are derived. The use of these expressions in correcting observed line shapes for modulation broadening is compared with that of another method involving the summation of a convergent series.

ESR Line Shapes in Glasses of Copper Complexes

The study by Sands of ESR line shapes of polycrystalline copper complexes has been extended and various ``anomalies'' discovered in the line shapes. The spectrum of copper phthalocyanine dissolved in concentrated H2SO4 has been studied at 77°K. The M=−32 hyperfine line has a behavior very different from that discussed by Sands. Isotropic extra hyperfine structure, arising from the presence of the ligand nitrogens, is easily discernible even in the glass.