Robert A. Wind

Comparison of the 1H NMR analysis of solids by the CRAMPS and MAS-only techniques

1H NMR spectra are reported on eight representative solid samples, including pure powdered crystalline samples, synthetic organic polymers, a silica gel, HY zeolite, and a lignite. Spectra were obtained by the following three approaches: (1) single pulse on a static sample, (2) CRAMPS, and (3) single pulse with magic-angle spinning (MAS-only). The MAS-only results were obtained as a function of MAS speed. Although the MAS-only technique is capable of achieving a significant degree of line narrowing, even with modest MAS speeds, MAS-only spectra of the general quality of the apparently undistorted high-resolution 1H spectra obtained by the CRAMPS technique are not obtained at the highest MAS speeds examined (21 kHz for a polymethylmethacrylate sample), unless the 1H-1H dipolar interactions in the sample are rather weak, as with silica gel or a zeolite. Thus, caution should be exercised in interpreting 1H MAS-only spectra, especially if CRAMPS results are not available as a calibration.

The stop-and-go spinning technique in MAS experiments

Abstract High-resolution solid-state NMR experiments are described in which detection occurs under magic-angle spinning conditions, but other portions of the experimental sequence are carried out with the sample static. This is achieved by starting and stopping the spinner during the sequence. In a 2D FT experiment designed to provide a chemical-shift tensor pattern for each isotropic chemical-shift tensor average, the sample is static during the evolution period and the magnetization evolves according to the complete chemical-shift tensor during that period. In another experiment, I to S cross polarization is carried out under static-sample conditions, avoiding the well-known modulation patterns in the Hartmann-Hahn match condition for systems with weakly coupled I spins. Examples of the application of these techniques are presented and prospects for such techniques are discussed.

An investigation of coal by means of e.s.r., 1H n.m.r., 13C n.m.r. and dynamic nuclear polarization☆

Abstract Sixty coal samples of different rank and origin have been investigated by means of e.s.r., 1 H n.m.r. and 13 C n.m.r., the last two in combination with dynamic nuclear polarization (DNP). The following parameters have been determined: the number of free radicals, the e.s.r. linewidth, the 1 H Zeeman relaxation rate, the 1 H relaxation rate in the rotating frame, the 1 H DNP enhancement, the 13 C DNP enhancement, the 13 C Zeeman relaxation rate and the 13 C aromaticity, observed via 1 H 13 C cross-polarization (CP), both with and without magic-angle spinning (MAS). The relations between these parameters and coal rank have been investigated. Moreover, with DNP special experiments have been performed which provide information about the localization and the mobility of the unpaired electrons present in these coals. Finally, DNP has been used to investigate various features of the quantitative analysis of coal via 13 C n.m.r. MAS was found to reduce the measured 13 C aromaticity, and for three coals it was shown that even without MAS only ≈ 50% of the aromatic 13 C nuclei are detected by the CP technique.

A study of 13C‐enriched chemical vapor deposited diamond film by means of 13C nuclear magnetic resonance, electron paramagnetic resonance, and dynamic nuclear polarization

13C nuclear magnetic resonance (NMR), with and without dynamic nuclear polarization (DNP), and electron paramagnetic resonance (EPR) measurements are presented on two 13C‐enriched chemical vapor deposited (CVD) diamond films. The samples were prepared by the decomposition in a microwave discharge of a gas mixture of CH4 and H2, with and without O2. 13C NMR spectra, obtained with or without magic‐angle spinning (MAS) both at a field of 14 T using direct polarization (DP) and at 3.5 T via cross polarization (CP), are presented. Because the samples contain both nuclear and (unpaired) electron spins, irradiation of the sample with microwaves with a frequency at or near the electron Larmor frequency leads to an enhancement of the nuclear spin polarization via the effect referred to as dynamic nuclear polarization (DNP). This enhancement (at a field of 1.4 T), combined with the effects of isotopic enrichment (14% 13C), makes it possible to measure 13C DNP–CP–MAS spectra within a couple of hours, compared to alm...

Characterization of electron spin exchange interactions in cellulose chars by means of ESR,1H NMR, and Dynamic Nuclear Polarization

Cellulose chars heat treated under nitrogen atmosphere for six hours over a range of heating temperatures from 250 to 1000°C were studied by ESR, broadline1H NMR, and Dynamic Nuclear Polarization (DNP). Chars heated below 450°C exhibited DNP enhancements predominantly due to the solid state effect resulting from static electron-nuclear spinspin interactions, while chars heated at higher temperatures exhibited Overhauser enhancements, which result from time dependent interactions. It was found that, while the maximum number of unpaired electrons was obtained at a heating temperature of 700°C, the maximum Zeeman and rotating-frame1H relaxation rates were achieved at much lower temperatures. Moreover, small Overhauser enhancements were observed even at the lower heating temperatures, where the time dependent electron-nuclear interactions are expected to be minimal, and the solid state enhancements decrease more rapidly than expected for samples heated above 350°C. These effects are explained in terms of a distribution of rates of electron-electron spin-exchange interactions. The charred and carbonized cellulose samples provided a set of solids in which the number of unpaired electrons varied over a large range and exhibited a broad distribution of spin-exchange rates. It was shown that DNP-NMR is a powerful method for probing this distribution, and for detecting small fractions of rapidly exchanging and static electrons.

Recent advances in coal characterization by 13C and 1H n.m.r.

Abstract Ultralarge magic-angle spinning (MAS) systems (2.4–6.5 cm3) provide a sufficiently large increase in signal-to-noise ratio that 13C MAS experiments without cross-polarization (CP), as well as time-domain 13C CP/MAS studies, can be carried out expeditiously. The non-CP approach allows one to circumvent the uncertainties in quantitation by 13C CP/MAS techniques; however, a more direct determination of TH1ϱ values than by variable contact-time experiments yields promising results on 13C spin counting in coal. The 1H combined rotation/multiple-pulse spectroscopy technique, based on multiple-pulse dipolar line narrowing and MAS, provides a useful complement to 13C MAS techniques, typically yielding poorly resolved 1HCsp2 and 1HCsp3 peaks from which deconvolution nevertheless provides at least a semiquantitative assessment of amounts. A dipolar-dephasing approach and imbibing with perdeutero-pyridine, employed either separately or together, dramatically improve resolution and provide strategies for examining the mobilities of individual coal components.

The use of magnetic resonance parameters in the characterization of premium coals and other coals of various rank

Abstract Eight premium coals and twelve Polish coals were investigated by means of e.s.r., 1H n.m.r., 13C CP-n.m.r. and dynamic nuclear polarization (DNP). The following parameters were studied: the concentration of free radicals, Ne; the 1H Zeeman relaxation rate, WZH; the 1H rotating-frame relaxation rate, WρH; the 1H DNP enhancement factor, PH; and the CP-determined carbon aromaticity, (fa)CP. The results generally agree with the patterns established in an earlier study of the same parameters of 60 other coals. The following relationships have been found between the above parameters and parameters obtained via proximate and ultimate analyses: Ne versus percentage volatile matter, WZH versus oxygen content, WρH versus Ne, PH versus carbon content and (fa)CP versus percentage volatile matter. For the premium coals it was observed that exposure to air prior to degassing resulted in irreversible changes in Ne, WZH and PH.

13C knight shift saturation and 1H dynamic nuclear polarization in a polycrystalline sample of the organic conductor (fluoranthenyl)2PF6

Abstract It is demonstrated that the 013 C Knight shift in the organic conductor (fluoranthenyl) 2 PF 6 can be reduced by saturating the conduction electron ESR line with microwave irradiation. It is also found that microwave irradiation leads to a nuclear polarization enhancement due to the Overhauser effect. The measurements have been used to separate the 13 C Knight shift and the ordinary chemical shift.

1H n.m.r. Zeeman relaxation in premium coals and other coals of various rank

Abstract Proton n.m.r. spin-lattice relaxation measurements are reported of eight available Argonne premium coals and five other coals. Possible relaxation mechanisms are considered, including proton-proton interactions, interactions between the protons and the unpaired electrons present in organic radicals, and interactions between the protons and the unpaired electrons present in paramagnetic oxygen. The effect of possible limitations in proton spin diffusion on relaxation behaviour has also been taken into account. For the two highest rank premium coals an unusually long relaxation time was observed. This was found to be due to the fact that these coals have never been exposed to air. The relaxation rates of the other coals were found to increase strongly with decreasing coal rank. It was concluded that this is due to an increasing amount of paramagnetic oxygen, trapped in the coals even after evacuation. It was also observed that for most coals the relaxation is non-exponential, with a non-exponentiality which increases for decreasing coal rank. This has been attributed to the presence of protons in both a molecular and a macromolecular phase in the coal, with an increasing amount of protons in the molecular phase for decreasing coal rank. The results were found to be consistent with those obtained in other coals of similar rank.

29Si dynamic nuclear polarization of dehydrogenated amorphous silicon

Abstract 29Si NMR spectra were obtained on a dehydrogenated amorphous silicon sample by means of dynamic nuclear polarization (DNP). The dependence of the DNP enhancement factor, which peaked at about 40, upon the microwave frequency offset (ω - ωe) from the electron spin Larmor frequency is essentially antisymmetric about ω - ωe = 0, which corresponds to a g value of 2.0059. This observation shows that the paramagnetic centers are fixed, immobile on the time scale of electron spin larmor precession, requiring a reexamination of recently advanced models of the paramagnetic defects. Prospects for the application of DNP in the study of amorphous silicon materials are discussed.