Robert D. Allendoerfer

A phenomenological description of ENDOR in solution; example: The tri-t-butyl phenoxyl radical

Abstract The ENDOR spectrum of the tri- t -butyl phenoxyl radical is interpreted and precise values of the coupling constants reported. The relative intensities of the lines are reported under various conditions of microwave power, rf power, temperature, viscosity, and the Zeeman modulation, and a phenomenological theory beginning with the Bloch equations is developed to explain each effect. The optimization of each parameter for solution ENDOR is discussed.

Structure and properties of a new κ-phase organic metal: (BEDT-TTF)2Cu2(CN)3

Abstract The electro-synthesis, structure and conductivity of a new κ-phase organic metal (BEDT-TTF)2Cu2(CN)3 are described [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene C10H8S8]. Partially oxidized BEDT-TTF cations form a structure similar to that found in the κ-phase superconductors, while Cu2(CN)3−1 anions form infinite two-dimensional sheets which alternate with layers of BEDT-TTF cations along the crystallographic a axis. The morphology and conductivity of the crystals are dependent on the electrocrystallization conditions. Samples prepared under certain conditions show a semiconductor-to-metal transition in the 10–30K range, while others have an immeasurably large resistance in this temperature range, even though the room-temperature crystal structures are identical, and EDX shows all samples to have the same composition. However, application of a thin layer of epoxy induces the transition in all samples, and enhances the low-temperature conductivity of some samples by a factor of about 100. Low temperature crystallographic measurements of the unit cell change on cooling indicate the epoxy-coating to reduce the thermal contraction of the samples, in particular in the direction of the crystallographic a-axis.

A coaxial microwave cavity for improved electron paramagnetic resonance sensitivity with lossy solvents

Abstract A design for a coaxial TE 011 -mode microwave cavity to optimize EPR sensitivity for measurements involving lossy samples is presented. A prototype cavity has been constructed and its sensitivity compared to the sensitivities of commercial rectangular TE 102 - and cylindrical TM 110 -mode cavities under a variety of experimental conditions. For saturable samples, the TE 011 and TM 110 cavities are approximately equivalent in sensitivity and twice as sensitive as the TE 102 cavity. For nonsaturable samples the sensitivity ratio is approximately 3 : 2 : 1 for TM 110 : TE 011 : TE 102 . The magnetic field modulation available from the TE 102 cavity is roughly double that available from the other two, so when broad lines are studied, all three cavities are equivalent. If the experiment requires temperature control deviating from ambient temperature by more than 25°C, the TE 011 coaxial cavity is two to three times more sensitive than the other two. It is concluded that the strength of the coaxial design is not only its sensitivity, but the ease with which its novel sample geometry permits the experimenter to modify the sample during an EPR experiment without adversely affecting the EPR sensitivity.

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.

Endor of a randomly oriented carbazyl radical

Abstract The ESR and ENDOR spectra of the 1, 3, 6, 8 tetra- t -butyl carbazyl radical have been determined in glassy perdeuterotoluene at 77°K. By performing the ENDOR experiment at the various turning points of the ESR spectrum,the complete hyperfine tensor of the protons at the 2 and 7 positions has been determined without any single crystal measurements.

Synthesis of an organogallium(I) compound [Ga(CH2CMe2Ph)]n with EPR spectral evidence for gallium clusters

Abstract The organogallium(I) compound [Ga(CH 2 CMe 2 Ph)] n has been prepared by the reduction of Ga(CH 2 CMe 2 Ph) 2 Cl by using either sodium or lithium with naphthalene in THF. The yellow dihydronaphthalene gallium(III) intermediate M 2 {C 10 H 8 [Ga(CH 2 CMe 2 Ph) 2 Cl] 2 } initially formed at −78°C but then decomposed at higher temperatures to form [Ga(CH 2 CMe 2 Ph)] n , Ga(CH 2 CMe 2 Ph) 3 and MCl. EPR spectra, which were recorded as the two yellow intermediates Na 2 {C 10 H 8 [Ga(CH 2 CMe 2 R) 2 Cl] 2 } (R=Ph, Me) decomposed, indicated the presence of radicals. The first and second derivatives of the EPR signals, line-widths, g -values and hyperfine coupling constants are consistent with the radicals being clusters of organogallium species. The experimental spectra were simulated by the superposition of two spectra, a single Gaussian shaped line with a peak-to-peak width of 14 mT (92%) with the spectrum of five equivalent gallium nuclei, A =2.1 mT (8%). The even number of lines observed in the experimental spectra indicate an odd number of gallium nuclei with at least five being required to give the number of observed lines.

An Electronic Curriculum for Introductory Chemistry

We are creating a World-Wide Web (WWW) introductory chemistry site that will incorporate many key features of an electronic chemistry curriculum, including flexibility, interactivity, extensibility, diversity, and self-assessment. The site will be accessible from browsers such as Mosaic and Netscape. Advantages of serving introductory chemistry from a WWW site include expanding access to chemistry instruction, platform independence, easing transitions between university units, and creating a curriculum responsive to diverse needs. Contributions and suggestions from the chemistry community can be incorporated easily and rapidly into the electronic curriculum. Since the WWW site could be used by any campus in the world having Internet access, we expect that our electronic curriculum will have a far-ranging impact on the way introductory chemistry is taught.

Conformation of the m-dibenzoylbenzene radical anion

Abstract The hyperfine coupling constants for the m-dibenzoylbenzene radical anion previously determined by Chaudhuri et al. have been confirmed by ENDOR. It is shown that two reasonable assignments of these constants simulate the ESR spectrum equally well and that an HMO molecular orbital calculation can predict either assignment depending on whether the two carbonyl groups are chosen syn or anti. Thus, based on magnetic resonance data, no conclusion can be drawn about the conformation of the radical anion.

Solution ENDQR from dispersion mode ESR

Abstract The Bloch equations are used to obtain an expression for the magnitude of the ENDOR enhancement of a dispersion mode ESR signal. The maximum predicted dispersion mode enhancement is 1.79 times greater than that predicted for adsorption mode ESR signals. Experimentally, a ratio of 2.0 ± 0.4 is found when γ 2 H 1 2 T 1 T 2 is set equal to 1.