3 Solid Materials Research with NMR and Dynamic Nuclear Polarization Spectroscopy Downloaded by FUDAN UNIV on February 15, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch003
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Robert A. Wind , Russ Lewis , Herman Lock , and Gary E . Maciel
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Chemagnetics, Inc., 2555 Midpoint Drive, Fort Collins, CO 80525 Department of Chemistry, Colorado State University, Fort Collins, CO 80523
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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 modern 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)PF ,a ceramic fiber, and a vapor-deposited diamond. 2
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R j L A R I Z A T I O N TRANSFER between the electron and nuclear spin systems can be obtained in solids containing both magnetic nuclei and unpaired electrons via irradiation at or near the electron Larmor frequency. The result is an enhanced nuclear polarization and, henceforth, an enhanced N M R signal. This effect is called dynamic nuclear polarization (DNP) (1-4). DNP is an old technique, and the first DNP experi0065-2393/93/0229-0045S06.00/0 © 1993 American Chemical Society
Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.
Downloaded by FUDAN UNIV on February 15, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch003
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MAGNETIC RESONANCE OF CARBONACEOUS SOLIDS
ment dates as early as 1953 (5). In the earlier days of magnetic resonance, DNP was applied mainly at low temperatures in high-energy physics, where it was used for the production of polarized targets or to study mag netic ordering at extremely low spin temperatures. During the past decade, however, other applications have been developed in which DNP was combined with modern solid-state N M R techniques such as crosspolarization-magic-angle spinning (CP-MAS) and two-dimensional (2D) N M R spectroscopy (6—8). DNP N M R spectroscopy can be applied successfully in a large variety of solid materials in which unpaired electrons occur naturally, in materials that contain paramagnetic impurities, or in materials in which unpaired electrons have been embedded by doping the solid with a suitable agent. Examples of materials in which unpaired electrons occur naturally are fos sil fuels and carbonaceous chars containing ττ-type organic radicals (9,10); amorphous materials in which unpaired electrons occur in the form of fixed dangling bonds (11); and undoped tfms-polyacetylene containing mobile dangling bonds, the so-called "solitons" (12, 13). Examples of materials containing paramagnetic impurities are natural and artificial dia monds (14), and examples of doped materials are doped polymers (15) and organic conductors (16). In this chapter a review of applications of DNP N M R spectroscopy in solid materials research will be given. First, a brief description of the DNP phenomenon will be given. The conditions under which a noticeable DNP enhancement can be expected will be discussed, and the loss in N M R sig nal due to the presence of the unpaired electrons will be addressed. Appli cations of DNP N M R spectroscopy will be given, and representative illus trations of these applications will be shown. Finally, future developments of DNP N M R spectroscopy will be discussed.
Theory Overview of Dynamic Nuclear Polarization.
Extensive re views of the different mechanisms that can dominate in a DNP experiment are given in references 1-4, 7, and 15. Here we confine ourselves to a treatment of the main features of these mechanisms. Moreover, we re strict ourselves to DNP of nuclear spin-% systems, and we assume that the spin diffusion among the nuclei is fast enough to average out possible differences in enhancement factors. The kind of DNP experiment that can be applied, the magnitude of the nuclear polarization enhancement, and the circumstances under which the maximal enhancement occurs depend on the nature and time dependence of the electron-nuclear interaction term H . If H contains time-independent terms, the nuclear polariza tion enhancement can arise from three DNP mechanisms: the solid-state en
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Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.
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WIND ET AL.
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NMR & DNP Spectroscopy
effect, the direct thermal mixing effect, and the indirect thermal mixing effect. For these effects the enhancement of the nuclear polarization minus unity, Ε - 1, as a function of the frequency of the microwave field, ω, is antisymmetrical about the electron Larmorfrequencyw . We assume that the nuclear Larmor frequency, ω , is larger than the electron spin resonance (ESR) line width, w . Then the solid-state effect gives a maximum when ω « w ± ω , the direct thermal mixing effect becomes maximal when ω « ω ± w , and the nuclear polarization enhancement due to the indirect thermal mixing effect becomes optimal when ω = ω ± ωV with u) „%< ω,1 < ωη. Q
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If H contains time-dependent terms with frequencies comparable to ω , an Overhauser effect is observed (17). Then the enhancement curve, (Ε - 1) as a function of the microwave offset frequency, ω - w , which reflects the (saturated) ESR line shape, is often symmetrical about o>, and (Ε - 1) becomes maximal when ω = w In Figure 1 characteristic DNP enhancement curves are given for the four DNP mechanisms mentioned. In theory the DNP enhancement can be very large, of the order of (/ η > ^ i practice the observed enhancement is often consid erably less. To investigate this issue, in the following sections theoretical expressions of the DNP enhancement factors will be given for static and time-dependent electron-nuclear interactions.
Downloaded by FUDAN UNIV on February 15, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch003
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Figure I. Frequency dependence of the various DNP effects: curve 1, solid-state effect; curve 2, indirect thermal mixing effect; curve 3, direct thermalf^ge
