H. A. Resing

Apparent Phase—Transition Effect in the NMR Spin—Spin Relaxation Time Caused by a Distribution of Correlation Times

It is shown that a broad distribution of correlation times produces an apparent‐phase‐transition effect in the NMR spin—spin relaxation time just before the onset of rigid‐lattice behavior. NMR relaxation‐time data for liquid water filling the pores of a charcoal adsorbent provide a sensitive test of the theory. The occurrence of the apparent phase transition for molecules adsorbed on the surfaces of solids provides a method of estimating the spread of activation energies for surface diffusion and thereby provides a new method for studying the heterogeneity of surfaces.

Determination of aromatic hydrocarbon fraction in oil shale by 13C n.m.r. with magic-angle spinning

Abstract A method for estimation of aromatic content in oil shales is demonstrated. Magic-angle spinning at 2 kHz is shown to remove chemical shift anisotropy to a sufficient degree to resolve aromatic and aliphatic 13 C n.m.r. spectral regions for a lithic oil shale specimen. The proton and carbon n.m.r. relaxation parameters are such as to allow room-temperature use of this proton-enhanced 13 C n.m.r. technique as a quantitative analytical tool. Cross polarization times of a millisecond or less and experiment repetition periods of 0.5 s or less are optimum. The specimen examined is represented by an aromatic carbon fraction 0.264 ± 0.007; this determination is quite insensitive to the proton-carbon cross polarization time. Spectra for kerogen, shale oil, and dawsonite are also presented. Dawsonite may interfere in the determination of the aromatic fraction.

NMR Relaxation Times in Solid White Phosphorus: Diffusion and Rotation

Nuclear magnetic resonance relaxation times have been measured for solid white phosphorus by spin‐echo techniques. The diffusion coefficient calculated from T2 in the α phase can be represented as D=D0exp(−ΔH/RT), where D0=0.077±0.014 cm2/sec, and ΔH=12.1±0.1 kcal/mole. The correlation time for rotation calculated from T1 in the β phase can be represented by tc=t0exp(ΔH/RT), where t0=6.04×10—14 sec, and ΔH=4.02±0.02 kcal/mole. The distance between phosphorus atoms in the P4 molecule is found to be 2.24±0.03 A from the value of T1 at the T1 minimum in the β phase.

NMR Relaxation in Adsorbed Molecules. V. SF6 on Faujasite: Dipolar Coupling of Fluorine Nuclei to Ferric‐Ion Impurities

The spin—lattice and spin—spin relaxation times for fluorine nuclei of SF6 adsorbed to maximum capacity on dehydrated synthetic faujasite have been measured as functions of temperature from 77° to 300°K. These have been interpreted according to the theory of Torrey as arising from diffusion of SF6 molecules in the magnetic field of paramagnetic iron impurities in the faujasite. The mean time between diffusional jumps is well represented as τ=1.14×10−12 exp(4000/RT) sec.

NMR Relaxation Times of Benzene Adsorbed on Charcoal: Molecular Rotation and Diffusion

NMR relaxation times have been determined as a function of temperature and composition for the protons of benzene adsorbed on charcoal. These relaxation times have been interpreted to give the following conclusions. The benzene molecule rotates more rapidly in a plane perpendicular to its sixfold axis than it does about other axes. The average enthalpy of activation for surface diffusion lies in the range 5–7 kcal/mole. At any composition, surface diffusion is characterized by a distribution of activation enthalpies with a standard deviation of about 1 kcal/mole. There are close to 1020 unpaired electronic spin/cm3. Some evidence for high‐energy adsorption sites is presented.

Identification of intercalated species in AsF5NO2SbF6 intercalated graphite using 19F N.m.r.; fluorine exchange between the various species

Abstract An intercalated graphite was prepared by reaction of polycrystalline graphite with AsF 5 followed by reaction with NO 2 SbF 6 . At −73%, four narrow lines are found in the 19 F n.m.r. spectrum at +124.1, +112.7, +66.7, and +46.8 ppm (relative to CFCl 3 ). These lines are attributed to SbF 6 − , SbF 5 , AsF 6 − in rapid chemical exchange with AsF 5 , and AsF 3 , respectively. This demonstrates the use of n.m.r. for the qualitative analysis of the chemical species present in the interlamellar space. As the temperature is increased, the lines broaden (except for that of AsF 3 ) and subsequently narrow, indicating, further, that chemical exchange of fluorine is occurring among SbF 6 − , SbF 5 , AsF 6 − and AsF 5 . At 22 °C the exchange rate is ∼ 10 3 s −1 for SbF 6 − and >3 × 10 3 s −1 for the other species. The data are interpreted in terms of a model in which fluorine exchange between SbF 6 − and the fluoro-arsenic species occurs via SbF 5 .

Graphite-AsF5 intercalation kinetics and diffusion by NMR imaging

Abstract One-dimensional NMR images of the 19F concentrations have been recorded as functions of time during the intercalation of AsF5 into HOPG at room temperature. The image maintained a constant shape at all times during intercalation, indicating the absence of AsF5 concentration gradient in the radial direction. The NMR gradient diffusion technique was used to determine the macroscopic 19F diffusion coefficient for stage I HOPG/AsF5 at room temperature; the result is D=4×10−6 cm2/sec.

Potential energy of KC24(NH3)4 on an ionic model: Order parameter of the ammonia molecule

Abstract The only significant ordering potential energy of an NH 3 molecule in KC 24 (NH 3 ) 4 is that due to the interaction of its dipole (taken as distributed charges) with the K + ion: V = 15750(1 − cos χ ) cal/mole of NH 3 . This is sufficient to maintain the C 3 axis parallel to the layer planes of the compound and thereby to support those planes at the observed d-spacing. The K + ion finds itself in a flat minimum with “particle in a box” energy level spacing of ≅5.6 mev at room temperature. The magnitude and temperature dependence calculated for the NMR order parameter S for the C 3 axis of the co-intercalated NH 3 agree with experiment and fix the sign of S as negative.

NMR determination of the ordering of CH3NO2 co-intercalated with PF6−, AsF6−, or SbF6− in graphite

Abstract Comparison of the intensities of the 19 F and 1 H NMR spectra of the graphite intercalation compound prepared by reaction of highly-oriented pyrolytic graphite with NO 2 PF 6 dissolved in CH 3 NO 2 showed that the number of CH 3 NO 2 molecules present per PF 6 − ion varied from 1.64 to 1.14 depending upon sample preparation. The 1 H NMR spectra of CH 3 NO 2 in single pieces of HOPG intercalated with PF 6 − , AsF 6 − , or SbF 6 − show 1:2:1 triplets due to dipolar splittings resulting from partial ordering of the CH 3 NO 2 molecules with the molecular symmetry axis parallel to the graphite planes.

AsF5 intercalated graphite: Conductivity and the anisotropy of the skin depth via NMR spectroscopy

Intercalation of AsF5 into highly oriented pyrolytic graphite (HOPG) produces a compound which is a highly anisotropic, two‐dimensional electrical conductor. Despite the high conductivity in the graphite planes, an 19F NMR experiment can detect all of the fluorine in the sample when the c axis of the HOPG is oriented perpendicular to the rf magnetic field B1. It is not necessary to pulverize the sample. The signal intensity is reduced when the c axis is oriented parallel to B1, thereby enabling estimates to be made of the skin depth (∼12 μm) and the in‐plane conductivity (∼2.7×105 Ω−1 cm−1).