Ira B. Goldberg

Electron spin resonance of polyacetylene and AsF5‐doped polyacetylene

The results of an electron spin resonance (ESR) study of trans‐polyacetylene, before and after doping with AsF5, are reported. The undoped polymer exhibits a Lorentzian line (g=2.00263) indicative of motional narrowing from a mobile unpaired electron species. Measurement of the intensity indicates a Curie law susceptibility with approximately one unpaired electron per 3×103 carbon atoms in agreement with earlier studies. From these results and comparison with earlier data on the cis polymer, it is suggested that the ESR signal arises from a mobile defect in the π system. Utilizing an apparatus which allows in situ AsF5 doping of a polyacetylene sample in the microwave cavity, we have monitored the ESR signal during the doping process. The spin resonance line narrows and increases in intensity, with no detectable g shift, as the doping proceeds. At high doping levels the line becomes asymmetric with a characteristic Dysonian line shape consistent with metallic behavior.

Hydrogen incorporation during coal liquefaction

Coal hydrogenation reactions have been investigated using a deuterium tracer method which makes it possible to determine which structural positions in the coal react with hydrogen gas or donor solvent during liquefaction. 2H2 and/or tetralin-d12 were reacted with a Pittsburgh Seam coal at 13.8 to 22.1 MPa and 360 to 425 °C for 0.25 to 1.0 h. Hydrogenation and exchange indices were formulated to indicate the relative contribution of each type of reaction to the total H incorporation. In the coal-deuterium gas system, deuterium incorporation in the solvent-separated products increases in the order oil < asphaltene < preasphaltene < residue. However, in the coal-tetralin-d12-deuterium gas system, deuterium incorporation is similar in each of these four fractions. In both systems, 2H incorporation varies with structural position, with the α-aliphatic positions exhibiting the greatest extent of incorporation. The α-tetralyl radical appears to be an important intermediate in hydrogen transfer to and exchange with the coal. The results indicate that in the donor system the abstraction of hydrogen from the solvent by coal-derived radicals is involved in the rate-determining step of the formation of the soluble products. Evidence indicates that considerable direct interaction of the gas-phase hydrogen with the coal also occurs in the donor solvent system.

Improving the analytical accuracy of electron paramagnetic resonance spectroscopy

Abstract Precisions and accuracies of approximately ±1% can be obtained for concentration measurements by using proper methods for double integration of the electron paramagnetic resonance signal. To obtain this level of performance, precautions also need to be taken concerning the actual values of the modulation amplitude, signal amplification, and power level. In addition, the manner in which the microwave cavity is tuned can significantly alter results. This report discusses methods for double integration as well as the significance of these other precautions which need to be taken.

Kinetic study of the reaction of F with H2 and CF3H by ESR methods

The reactions between F and H2, H and F2, and F and CF3H were studied by using a fast flow reactor with electron spin resonance detection. The reaction rate constant for F+H2→HF+H was determined to be 1.00±0.08×1013 cm3u2009mole−1⋅sec−1 at 298u2009°K. The rate of the reaction H+F2→HF+F was determined in a system where known amounts of F and F2 were injected into a stream of H2 in He. Using finite difference techniques to solve the rate equations, the rate constant was found to be 0.26±0.06×1013 cm3u2009mole−1⋅sec−1. These reaction rates are in good agreement with other published values. The rate constant determined for the reaction F+CF3H→HF+CF3 was 0.90±0.08×1011 cm3u2009mole−1⋅sec−1. Techniques are presented which permit ESR to be used in studies of complex reaction mechanisms and in nonuniform flow velocities.

Radical formation during vacuum drying of Wyoming subbituminous and Morwell brown coals

The weight loss on drying a Wyoming subbituminous coal containing 50% moisture and a Morwell (Australia) brown coal containing 59% moisture (based on drying at 110°C) was correlated with changes in radical concentration, g-factor and linewidth of the electron paramagnetic resonance spectra, infrared spectra and elemental analyses. Both coals exhibited a substantial increase in readical concentration on drying at temperatures from 20 to 150°C. Elemental analyses showed that the Wyoming coal lost H2O and CO2 on extended drying. The loss of CO2 was substantiated by infrared spectroscopy. These observations are consistent with the high carboxyl content of this coal revealed by structural studies.

Computer control of electron spin resonance spectroscopy

Abstract A laboratory computer interface with an electron spin resonance (ESR) spectrometer is reported, which permits operator control of the magnetic field through the computer. Two programs are described. In the first, the field is stepped and checked for stability at preset increments before data from a signal channel and a reference channel are collected. Any number of increments can be accumulated, and the signal channel can also be recorded as a function of time. The second program uses a continuous sweep to collect data. Sweep rates up to 10 kG/min have been used with no auxiliary coils. The data acquisition program is started by a signal from the reference channel, and the reference absorptions are counted on the return sweep to insure reproducible starting fields. Signals from the reference or from the sample channels can be recorded during a sweep. Both programs are sufficiently open so that other peripheral functions can be executed.

Relation of the chemical structure of coal to its hydropyrolysis

The hydropyrolysis of Illinois No. 6 coal has been studied in a batch reactor, in which the reactions were initiated by explosion of H2O2 mixtures. The ratio of H2 to O2 was kept at 8, while the total pressure of the gas mixture was changed to vary the reaction temperature. The heating rate was ≈ 50 000 °C s−1, and the reaction time was < 50 ms. The conversion of the feed coal increased from 19% at 620 °C to 81%at ⪢ 1500 °C. At conversions < 50%, the gaseous product consisted of mainly CH4 and CO in almost equal proportions, and at conversions ⪢ 60% the concentration of CO increased. Comparison with results from a large flow reactor revealed that comparable conversions were obtained in the present batch reactor, although product distributions were markedly different from each other. The dissimilar product distribution is attributed to different reacting media: preburning of H2 and O2 in the flow reactor versus in situ burning of the mixture in the batch reactor. The H/C ratios of solid residues after the hydropyrolysis decreased linearly as the conversion increased, revealing that the portions of coal having high H/C ratios were preferentially gasified. This observation was substantiated by a high H/C ratio, 1.74 of the first portion of coal gasified, and by a sharp decrease in H/C ratio in subsequent gasified portions. These data indicated that aliphatic side chains (or linkages) and single-ring aromatic clusters in the feed coal were gasified first, followed by larger aromatic clusters. Semi-quantitative determination of the distribution of different aromatic clusters showed good agreement with current structural information on coal. Thus, the effects of reaction variables were explained in terms of the structural features of coal, and the ratelimiting steps in the hydropyrolysis process were identified.

Magnetic Susceptibility of Inconel Alloys 718, 625, and 600 at Cryogenic Temperatures

In June 1988, the Discovery Space Shuttle mission was delayed because of a malfunctioning hydrogen fuel bleed valve system. The problem was traced to the linear variable differential transformer (LVDT) which produced erroneous readings for the valve position. Near liquid hydrogen temperatures, Inconel 718 used in the armature of the LVDT became strongly magnetic. The AC magnetic susceptibility of three samples of Inconel 718 of slightly different compositions, one sample of Inconel 625, and one sample of Inconel 600 were measured as a function of temperature. Inconel 718 behaves as a spin glass. Its susceptibility reaches a maximum between 15 and 19 K, near the liquid hydrogen boiling point, 20 K. The susceptibility increases by an order of magnitude as the iron content increases by 1.2% and the nickel content decreases by 1.5%. The nominal composition is 12–20% iron and 50–55% nickel. Inconel 625, which contains about 4% iron, was paramagnetic. Inconel 600 exhibited spin glass properties below 6 K, short-range ferromagnetism between 6 and 92 K, and paramagnetism above 92 K.

Kinetics of the reaction of atomic fluorine with phosphorous trifluoride: ESR determination

Abstract The reaction rate constant of the addition of atomic fluorine to excess PF 3 was determined by fast flow ESR measurements to be (8.6 ± 0.6) × 10 12 cm 3 mole −1 s −1 at 300 K. The stoichiometry of the overall reaction of F with PF 3 was 2:1. Finite difference calculations simulating several reaction mechanisms suggest that the reaction occurs in two consecutive addition steps, F + PF 3 = PF 4 and F + PF 4 = PF 5 . Assuming this mechanism is correct, the rate constant for the second reaction would be approximately (1.2 ± 0.2) × 10 13 cm 3 mole −1 s −1 .

High modulation amplitude modification for Varian ESR spectrometers

A water cooling accessory was constructed to enable the use of high amplitudes on field modulation electron spin resonance spectrometers. Field modulations greater than 50 G (100 G peak‐to‐peak) can be used for long periods without damage to the coils by heating.