In solids containing both unpaired electrons and nuclear spins, the dynamic nuclear polarization (DNP) technique can be applied to enhance the NMR signal. DNP requires irradiation at or near the electron Larmor frequency, and in this paper two probes are described to achieve this goal. Two new DNP probes were designed for electromagnets operating at a field of 1.4 T. In one probe, DNP can be combined with CPMAS. Compared to a similar probe reported previously, the main advantage of the new probe is the capability to spin vacuum-sealed samples with an improved filling factor. A second probe is based on an oversize microwave cavity. In this probe, large DNP enhancement factors can be obtained in nonlossy samples, e.g., a 1H enhancement factor of 260 for doped polystyrene.

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 almost 20 h for the CP–MAS spectra taken at high field without DNP. Data based on DNP–CP–MAS experiments with variable CP-match time or dipolar-dephasing time are shown and discussed in terms of structural models. The cross-polarization signals are rationalized in terms of estimates of the chemical shifts of hydrogenated diamond surfaces; this approach indicates that most of the hydrogens in the polycrystalline sample are at the intergrain boundaries.

In solids containing both magnetic nuclei and unpaired electrons, the nuclear NMR signal can be enhanced via irradiation at or near the electron Larmor frequency to yield the dynamic nuclear polarization (DNP) effect DNP combined with modem solid-state NMR spectroscopy can be used to study properties of materials that could not be investigated by NMR spectroscopy alone, such as the dynamics of the unpaired electrons, molecular structures in the vicinity of the unpaired electrons, and the presence of small amounts of nuclei. In this chapter a review of the various mechanisms that can determine the DNP enhancement is given, and applications of the DNP NMR technique are shown in coal, undoped trans-polyacetylene, (fluoranthenyl)2PF6, a ceramic fiber, and a vapor-deposited diamond.

This chapter provides an overview of the fundamental issues concerning quantitation in C-13 NMR spectroscopy of carbonaceous solids. General factors governing quantitation in solid-state C-13 NMR spectroscopy (such as sample heterogeneity, the presence of unpaired electron spins, interference of proton decoupling by molecular motion, magic-angle spinning (MAS) effects, and implementation of the proper recycle-delay time) are discussed together with those factors that play a major role in cross-polarization (CP) experiments (Hartmann-Hahn match, proton spin-locking, cross-polarization spin dynamics, and interference of cross-polarization from MAS). Technical aspects and requirements of the solid-state C-13 NMR experiment are outlined, and effective strategies to obtain the most reliable results are presented.

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.

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.

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.

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.

Recent evidence concerning the molecular phase present in coals is discussed. It is proposed that pyrolysis-field ionization mass spectrometry (py-f.i.m.s.) provides information on the chemical nature of such material, but caution is also suggested in these interpretations. N.m.r. experiments can give information concerning the spatial location of molecular species, and domains with different mobilities can be recognised. The importance of considering coal origins and petrology in discussions of coal structure is emphasized. An empirical perspective of the molecular phase is afforded by solvent-free catalytic hydrogenation. Coal depolymerization by solvated electrons yields products consistent with polymethylene as a major part of the macromolecular network. The relation of these observations to the molecular mass of extracted material is suggested as crucial.

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.

> This paper studies ten Chinese coals at different ranks by means of cross polarization and magic angle spinning, combined rotation and multiple-pulse spectroscopy, and dynamic nuclear polarization. NMR is first reported. It is found that, compared with those obtained previously coat somples from other parts of the world, some Chinese coals at lower ranks display unusual characteristics, which imply peculiar chemical structures in the coals.

Application de la technique a l'etude d'un polyamide-imide (le Torlon) en variant les conditions operatoires. Mise en evidence de distorsions dans les intensites relatives

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.

Sixty coal samples of different rank and origin have been investigated by means of e.s.r., 1H n.m.r. and 13C 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 1H Zeeman relaxation rate, the 1H relaxation rate in the rotating frame, the 1H DNP enhancement, the 13C DNP enhancement, the 13C Zeeman relaxation rate and the 13C aromaticity, observed via 1H13C 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 13C n.m.r. MAS was found to reduce the measured 13C aromaticity, and for three coals it was shown that even without MAS only ≈ 50% of the aromatic 13C nuclei are detected by the CP technique.

1H13C polarization transfer via the nuclear solid effect (NSE) is investigated. Polarization transfer is obtained via irradiation at a frequency equal to the difference or sum of the proton and carbon Larmor frequencies, the “forbidden” transitions. 13C spectra of polyethylene, polypropylene, and hexamethylbenzene obtained by NSE, conventional cross-polarization, and thermal polarization are shown. It is found that for direct CH bonds the intensities of the 13C NSE spectra are often considerably larger than those of the spectra obtained by the cross-polarization method, and also provide more quantitative information. The 13C signals due to indirect CH bonds are suppressed in the NSE case, mainly due to the technological problem of the heating due to the finite high-power irradiation time at the forbidden transition. Experiments to alleviate these problems are proposed.

It is demonstrated that the 013C Knight shift in the organic conductor (fluoranthenyl)2PF6 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 13C Knight shift and the ordinary chemical shift.

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.

A field-cycling NMR investigation of 1H–13C polarisation transfer using cross-relaxation and the nuclear solid effect (NSE) is described. Dynamic nuclear polarisation (DNP) of the 13C spins is observed when forbidden transitions are driven by r.f. irradiation at the sum and difference Larmor frequencies of the two nuclei. When the 1H spins are pre-polarised, a significant transfer of polarisation to the 13C nuclei is achieved in a time short compared with the spin-lattice relaxation time of 13C. The cross-polarisation arising from the NSE is studied as a function of B-field and time. These results are compared with the solutions of the differential equations that govern the coupled system of 1H–13C spins. The effects of cross-relaxation are incorporated into the model for the first time and good agreement between theory and experiment is obtained. The experiments have been conducted at 20 K on a 13C-enriched sample of benzoic acid.