Magnetic Resonance of Carbonaceous Solids - American Chemical

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Downloaded by CORNELL UNIV on August 19, 2016 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch027

Electron Magnetic Resonance of Standard Coal Samples at Multiple Microwave Frequencies R. B. Clarkson, Wei Wang, D. R. Brown, H . C. Crookham, and R. L . Belford Department of Chemistry and Illinois EPR Research Center, University of Illinois, Urbana, IL 61801 The naturally occurring unpaired electrons in coal provide a unique route for the nondestructive study of coal structure. Electron paramagnetic resonance (EPR), electron—nuclear double resonance (ENDOR), dynamic nuclear polarization (DNP), and electron spin-echo (ESE) techniques use the paramagnetic spins as probes of their environment, and the spins provide information about their immediate chemical surroundings as well as about the nature of more distant regions in the coal. In the context of an ongoing program in our laboratory to better determine the structure and bonding in the organic (maceral) components of whole coals (including Argonne Premium coal samples), coal extracts, and model coal systems by electron magnetic resonance methods (EPR, ENDOR, ESE), we have begun a multifrequency EMR study of coal, with the frequency range spanning nearly 2 orders of magnitude (2—250 GHz), to improve our understanding of the complex magnetic interactions present in the coal material and the origin of these effects in molecular structure and bonding. With this approach, we are developing methods for the elucidation of atomic and molecular structure in coal and other complex, disordered materials. 0065-2393/93/0229-0507$06.50/0 © 1993 American Chemical Society

Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.


508

M A G N E T O RESONANCE OF CARBONACEOUS SOLIDS

IVIumFREQUENCY E L E C T R O N M A G N E T I C R E S O N A N C E (EMR) spectroscopy is the method of making E M R measurements [e.g., electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and electron spin-echo (ESE)] on the same material at several substantially different microwave frequencies. Previous applications of this approach to EPR spectroscopy of coal have demonstrated its effec tiveness, even over a modest two-frequency range (9.5 and 35 GHz) (J-2), and the entire subject was reviewed in 1987 by Belford et al. (5). Multifrequency studies of complex systems often can facilitate reliable interpreta tions of structure, bonding, and magnetic interactions that would other wise remain uncertain or impossible to analyze in single-frequency experi ments. A n examination of the principal terms of the spin Hamiltonian provides a useful starting point for understanding the utility of multifrequency experiments. The spin Hamiltonian, H , can be written as the sum of electronic (H ) and nuclear (if ) spin operators, s

e

n

H

= H

s

e

+ H

(1)

n

in which

H

e

= +I

μ

Β

IB

0

·g ·S

(2)

e

where the spin angular momentum 5 = 1/2, μ is the Bohr magneton, B is the external magnetic field vector, g is the matrix of g factors, S is the electronic spin angular momentum vector (matrix), and Β

H

n

Q

= "gnUnl Β ·Ι + S ' A I 0

(3)

where the nuclear spin quantum number J = 1/2, μ is the nuclear magne ton, I is the nuclear spin angular momentum vector (matrix), and A represents the hyperfine interaction matrix. The first term in each part of H describes the Zeeman interaction (electronic or nuclear) and is charac terized by a dependence on B and expressions containing g-factors. The second term in H describes the hyperfine interactions between electrons and nuclei. Information on structure and bonding is contained in both the Zeeman and hyperfine interactions, which together form the basis for an exη

s

Q

n


27.

CLARKSON ET A L

509

EMR at Multiple Microwave Frequencies

perimentally observed E M R spectrum. Because of the complexity of the spectra of disordered, heterogeneous systems such as coal, analyzing or unambiguously interpreting the data can be very difficult. For example, the E P R line widths of coal spectra can include contributions from ganisotropy and hyperfine interactions, and determining the relative impor tance of each contribution is usually difficult. In this case, one would like to "switch off' the hyperfine interaction for one observation and then compare the resulting spectrum with a con ventional one containing both Zeeman and hyperfine terms in order to separate the two contributions. Magic-angle spinning (MAS) attempts to do just this, for example, by averaging away the dipolar portion of Ι · I interactions in N M R spectroscopy, and other N M R techniques address different portions of H to simplify spectra. Unfortunately, the much more rapid electronic relaxation rates exhib ited by paramagnetic systems have thus far made it impossible to apply techniques such as MAS to E M R spectroscopy. What is needed for E M R spectroscopy is a method for varying the importance of the terms in the spin Hamiltonian. This method must not depend critically on relaxation rates—a goal that the multifrequency approach accomplishes. By applying EPR, E N D O R , or E S E techniques at different micro wave frequencies (and hence in different B ranges), Zeeman interactions can be emphasized or de-emphasized relative to hyperfine terms, allowing a more critical evaluation of many spectral effects, including g-dispersion (EPR) (4); nuclear Larmor frequencies and orientation selection (ENDOR) (5); echo-envelope modulation depth (6); orientation selection (7-8); exact cancellation (ESE) (9); and relaxation rates (seen in all methods). For example, Figure 1 shows EPR spectra taken at three dif ferent frequencies of an Illinois No. 6 coal (4). As g-dispersion increases with 2? , a low-field shoulder appears. In this example, important chemi cal information contained in the electronic Zeeman term could be revealed only at higher frequencies. Figure 2 also demonstrates the poten tial utility of the multifrequency approach. Here, the electron spin-echo envelope modulation (ESEEM) from the same Illinois coal is shown at two frequencies. The depth of the E S E E M pattern is greater at lower fre quencies because of a reduction in the energy separation between elec tronic spin states and a corresponding increase in the transition probabil ity of the nearly forbidden (electron—nuclear spin-flip) transitions. In this chapter, we review our very high-frequency (VHF) (96 and 250 GHz) EPR work (4, 10) and report new low-frequency (3 GHz) pulsed EPR results on Argonne Premium coal samples. Related ongoing work in our laboratory includes multifrequency E N D O R spectroscopy of coal.


χ

s

Q

0


2


510

MAGNETIC RESONANCE OF CARBONACEOUS SOLIDS

Figure 1. Dependence of EPR spectra of a powdered Illinois coal sample upon microwavefrequencyof the spectrometer. The magnetic field scale is centered at a value corresponding to g = 2.003 for eachfrequency.Thefrequenciesare designated as microwave bands: X = 9.5 GHz, Q = 35 GHz, and W = 94 GHz.

Figure 2. ESEEM time-domain patternsfroma powdered Illinois coal sample taken at 3-GHz (---), and 9.4-GHz (—) microwave frequencies. (Reproducedfromreference 6. Copyright 1989 American Chemical Society.)


27.

CLARKSON ET AL. EMR at Multiple Microwave Frequencies

511


Experimental Details Samples of coal from the Argonne Premium Coal Sample Program were received in sealed glass ampules under nitrogen. The compositions of these eight samples have been reported (11). For EPR spectroscopy at 96 GHz (VHF EPR), the ampules were opened in air just prior to taking the spectra. Samples were loaded into 0.5-mm i.d. capillary tubes and placed in the cavity of the spectrometer. The W-band (96 GHz) instrument has been described elsewhere (4). Field strengths were measured with an NMR gaussmeter (Metrolab model 2025), and frequencies were monitored with a digitalfrequencycounter (EIP model 578). A variety of sharp, solution-phase samples were used as g-markers. A sample of Herrin No. 6 (essentially the same as Illinois No. 6) coal from the Illinois Coal Basin Sample Program (ICBSP No. 1, Southern Illinois Univer sity Registry No. 1822) was separated by density-gradient centrifiigation prior to study, and vitrinite, sporinite, and fusinite maceralfractionswere analyzed before taking EPR spectra. The analysis data for these samples are given in Table I. The maceral composition of the Herrin No. 6 coal was determined to be 87% vitrinite, 4.4% liptinite, and 8.6% inertinite. The separated vitrinite from this coal was also studied on a 250-GHz spectrometer built by Lynch et al. at Cornell University (12). Model compounds (perylene, dibenzothiophene, and dibenzofuran) were prepared as cation radicals either by adsorbing the materials as gases onto an activated silica-alumina catalyst (Houdry M-46) (15) or by UV-irradiating boric acid glasses containing the compounds in 10-50-mM concentrations. Lower con centrations would, in some cases, be required to obtain the highest spectral reso lution. Samples for the ESE study were evacuated for 24 h at room temperature and at pressures of

Magnetic Resonance of Carbonaceous Solids - American Chemical

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