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Anal. Chem. 1982, 5 4 , 1602-1606
Binuclear Shift Reagents for Nuclear Magnetic Resonance Spectrometry of Aromatic and Polycyclic Aromatic Compounds Thomas J. Wenrel' and Robert E. Severs* Department of Chemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), Campus Box 449, University of Colorado, Boulder, Colorado 80309
The effects of addlng blnuclear lanthanide( I I I)-sllver( I ) shlft reagents to aromatic compounds and phosphines are dlscussed. Wlth aromatic compounds, the sllver bonds at sltes away from alkyl substltuents so that selectlve shlfts, leading to slgnlflcant spectral clarlflcatlon, are observed. Because of dlfferent shlfts, the analysls of complex mlxtures such as the methylbenzenes In gasoline can be performed In the presence of these shlft reagents. A study of the relatlve shlftlng ablllty of the binuclear complexes formed wlth the ligand 6,6,7,7,8,8,8-heptafluoro-2,2dlmethyl-3,5~ctanedlone(fod), vs. 2,2,0,0-tetramethyl-3,5-heptanedlone (thd), revealed that the fluorlnated llgands are much more effecthre shlft reagents.
We have previously reported the use of binuclear complexes formed from a lanthanide(II1) P-diketonate and silver(1) 0diketonate as shift reagents for aromatics, olefins, and certain halogenated compounds (1-3). In the presence of these binuclear complexes, the NMR spectra obtained for toluene and p-diphenylbenzene become first order and much easier to interpret (1). The shifts we observe by using these shift reagents are substantially larger than those reported in previous studies by Evans et al. (4) and Dambska and Janowski (5). Studies by other workers were with either silver heptafluorobutyrate or silver trifluoroacetate with lanthanide 0diketonates. In this paper, the use of these shift reagents is demonstrated for more complex polycyclic aromatics. Effects of adding shift reagents on both the IH and 13C NMR spectra of substrates will be considered. In addition, mixtures of methylbenzenes can be analyzed and quantitated by using these shift reagents. Phosphines constitute another class of compounds that are considered soft Lewis bases and which bond only weakly, if at all, to conventional mononuclear lanthanide shift reagents. Silver readily bonds to phosphine donors and the binuclear complexes can be used to induce shifts in the NMR spectra of phosphines that are not observed in the absence of the silver.
EXPERIMENTAL SECTION Apparatus. Proton NMR spectra were recorded with a Varian EM-390 spectrometer. Phosphorus decoupled proton NMR spectra and 13Cand 31PNMR spectra were recorded with a JEOL PFT-100 spectrometer equipped with a Nicolet 1080 data system. Procedures. The synthesis of the silver P-diketonatesas well as the procedure for using these shift reagents was previously described (2). Reagents. All chemicals were used as received without further purification. The L n ( f ~ dchelates )~ were either purchased from Aldrich Chemical Co., Milwaukee, WI, or synthesized according to the procedure of Richardson and Sievers (6). The silver(1) compounds, as well as the lanthanide(II1)chelates used to form the binuclear shift reagents in situ are now available commercially from Aldrich Chemical Co., Milwaukee, WI. Chloroform-d was Present address: Bates College, Department of Chemistry, Lewiston, Maine 04240. 0003-2700/82/0354-160260 1.2510
obtained from Stohler Isotope Chemicals, Waltham, MA. The substrates used in these studies were obtained from a variety of sources and were of the best purity available.
RESULTS AND DISCUSSION Studies of NMR Spectra of Aromatic Compounds Bonded to Binuclear Shift Reagents. The three isomers of xylene have very similar properties. The NMR spectra of the individual xylenes consist of what appears to be a single unresolved peak for the four aromatic protons and another single peak from the methyl groups. o-Xylene has two nonequivalent aromatic protons, and the NMR spectrum obtained with 0.2 M Pr(fod),, 0.2 M Ag(fod), and 0.1 M o-xylene exhibited two resonances in the aromatic region. The stabilities of silver complexes with olefins are known to be very dependent on steric factors and the silver preferentially bonds at sites away from steric encumbrances (7-9). The resonance that shifts the farthest was assigned to the two aromatic protons farther away from the methyl groups. This assignment is made on the basis that the silver prefers to bond on the side of the ring opposite to the methyl groups, and protons closer to the site of the silver bonding should shift farther. This site of bonding has also been substantiated by I3CNMR data (vide infra). The compound m-xylene has three different aromatic protons and addition of these binuclear shift reagents leads to unique reasonances for all three (2). Once again, the relative order of the shifts agrees with the premise that the silver is preferentially bonded at sites remote from the methyl groups. For p-xylene, all four aromatic proton resonances are equivalent. In the presence of the shift reagents they shift and remain as a singlet. Mixtures of the xylenes and ethylbenzene are easily quantitated with these shift reagents. In the NMR spectrum of a mixture 0.1 M in ethylbenzene and each xylene isomer shown in Figure la, the aromatic, as well as the methyl, resonances overlap, and quantitative data cannot be obtained for all four compounds. Addition of enough Pr(fod)a and Ag(fod) to produce a solution 0.3 M in the binuclear shift reagent leads to a complete resolution of the methyl resonances as shown in Figure lb. All four components can now be easily quantitated. Assignment of the resonances was confirmed by selective spiking with the pure components. One further test of the ability of these shift reagents to distinguish mixtures of substituted benzenes was performed on unleaded gasoline, which contains a number of methylbenzene compounds. The proton NMR spectrum of a sample of unleaded gasoline is shown in Figure 2a. Due to the appearance of hydrocarbon resonances in the 1-2 ppm area, a downfield shift reagent was used. The spectrum obtained by using Yb(fod)3 with Ag(fod) is shown in Figure 2b. Through a set of selective spiking experiments, the following components were identified: toluene; 0-,m-, and p-xylene; 1,2,3-trimethylbenzene; 1,2,4-trimethylbenzene; 1,3,5-trimethylbenzene (mesitylene); and 1,2,4,5-tetramethylbenzene (durene). These constitute most of the major resonances attributable to substituted aromatics that are observed in the 0 1982 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
1603
Figure 3. Proton NMR spectra of 0.1 M 2-methyianthracene in CDCI, with (a) no shift reagent, (b) 0.1 M Yb(fod), and 0.1 M Ag(fod), and (c) 0.2 M Yb (fod):,and 0.2 M Ag(fod). Flgure 1. Proton NMR spectra of a mixture 0.1 M in each xylene isomer and 0.1 M in ethylbenzene in CDCi, with (a) no shift reagent and (b) 0.3 M Pr(fod), and 0.3 M Ag(fod).
cHcg
I,Zb-trimthylbenra
Figure 2. Proton NMR spectra of 60 pL of unleaded gasoline in 1 mL of CDCI, with (a) no shift roagent and (b) 0.3 M Yb(fod), and 0.3 M
Ag(fod). spectrum. The spectrum in Figure 2b demonstrates clearly that shift reagents can be used to alter selectively the spectra of particular compound classes of interest to extract information from very complex matrices. We have also studied a number of polycyclic, fused ring aromatic compounds. The spectra of these compounds generally consist of a complex set of resonances which cannot be assigned with any certainty. The compounds pyrene, 1,2:5,6-dibenzanthracene,anthracene, phenanthrene, and benzo[a]pyrene are all unsubstituted polycyclic compounds. The spectra of these shift in the presence of Y b ( f ~ dand )~ Ag(fod) or Pr(fod), and &$fa); however, no additional clarity is observed. In general, for unsubstituted ring compounds, the entire set of resonances shifts as a unit. This indicates that there is no strongly preferred site of silver complexation with these compounds. The structures of silver complexes with unsubstituted polyqy3c fused ring compounds have been previously studied ( 7 , 1 0 , I I ) .In these compounds preferred sites of silver complexation were observed; however, these studies were performed with the solid complexes and generally involved a silver to aromlatic ratio of one or less. In order to observe useful shifts with these shift reagents on these types of compounds, we usually employed silver to aromatic ratios greater than one. At these ratios in solution, the specificity for silver bonding at a particular site is reduced and selective shifts leading to improved spectral clarity are not observed. For polycyclic aromatics with methyl substituents, the silver appears to bond at selective sites and more dramatic improvements are observeld in the shifted spectra.
Figure 4. Proton NMR spectra of 0.1 M 9-methylanthracene in CDCI, with (a) no shift reagent, (b) 0.1 M Yb(fod), and 0.1 M Ag(fod),(c) 0.2 M Yb(fod), and 0.2 M Ag(fod), (d) 0.3 M Yb(fodI3and 0.3 M Ag(fod), (e) 0.4 M Yb(fod),and 0.4 M Ag(fod),and (f) 0.5 M Yb(fod), and 0.5 M Ag(fod). In 2-methylanthracene, the aromatic protons are all nonequivalent. The series of spectra obtained with increasing amounts of Yb(fod)3 and Ag(fod) is shown in Figure 3. There are a number of sites where the silver may bond, but the more favorable locations would be those removed from the methyl group. This results in more selectivity in the shifts than observed with anthracene. In Figure 3b, the resonances of three of the protons are resolved. The two doublets are coupled to each other and are assigned to H3 and H4. The singlet, due to its small shift, is assumed to be close to the methyl group and is msigned to H1.In Figure 3c, the spectrum is much more descriptive. Two more singlets are now apparent. The protons on the center ring, Hg and Hlo, have no neighboring protons on adjacent carbons and should be singlets. Since Hg is closer to the methyl group than Hlo, it is assigned to the resonance which shifts the least. The complex pattern downfield of these singlets corresponds to four protons and is similar to the pattern observed in the shifted spectrum of o-xylene. The end ring in this compound is ortho substituted but the silver bonding is apparently not sufficiently localized to allow the four nonequivalent protons to exhibit unique resonances in the spectrum. However, the resulting shifted spectra are quite descriptive and give more information about the identity of the compound than the original spectrum. Substitution of a methyl group at the 9-position of anthracene creates a symmetrical derivative with five sets of equivalent protons. The series of spectra obtained with in-
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
9
IO
7ppm
8
Figure 6. Proton NMR spectra of 0.1 M acenaphthene in CDCI, with (a) no shift reagent and (b) 0.1 M Yb(fod), and 0.1 M Ag(fod).
12 II IO 9 B 7ppm Flgure 5. Proton NMR spectra of 0.1 M 1-methylphenanthrenein CDCI, wlth (a) no shift reagent, (b) 0.2 M Yb(fod), and 0.2 M Ag(fod),(c) 0.4 M Yb(fod), and 0.4 M Ag(fod), and (d) 0.5 M Yb(fod), and 0.5 M Ag(fod).
creasing concentrations of Yb(fodI3 and Ag(fod) is shown in Figure 4. Once again, the unshifted spectrum is not too useful and quite a bit more is learned using the shift reagent. In Figure 4c, two triplets have separated, each corresponding to two protons. These protons must have two adjacent protons on the ring and are assigned to H2,7and H3,& The triplets result because the neighboring protons exhibit essentially the same coupling constant. In this instance, the doublet of doublets that should be observed appears as a triplet. As the concentration of the binuclear shift reagent increases, the two triplets exhibit different shifts and the one that shifts the farthest is assigned to H3,& In Figure 4d, the situation is further improved. The two triplets are still present, as well as an additional doublet. Based on the relative shifts observed as the shift reagent was increased, this doublet, since it shifts the least, is believed to correspond to Hl@ The f i a l resonance of relative area three appears to result from the overlap of a doublet and a singlet, which correspond to H4,5 and Hlo, respectively. Continued addition of shift reagent leads to the spectrum in Figure 4e. The doublet and singlet of previously overlapping multiplets are now more discernible, and these are assigned to H4,5and Hlo. In 1-methylphenanthrene,a three-ring compound with a staggered configuration, all of the aromatic protons are nonequivalent. The spectra obtained with increasing concentrations of Y b ( f ~ dand ) ~ Ag(fod) with 1-methylphenanthrene are shown in Figure 5. In Figure 5d, individual resonances are observed for seven of the nine protons, and the remaining two protons appear as one singlet. In this spectrum, four doublets centered at 10.4, 11.2, 12.3, and 12.8 ppm are observed. In addition, three doublets of doublets, which appear as triplets centered at 10.7, 11.0, and 11.5 ppm, are apparent. In a first-order spectrum of this compound, H3, H6, and H7 would be expected to appear as doublets of doublets. All of the other aromatic protons should appear as doublets. Assignments were based on both spin decoupling experiments and relative shifts. The distinction of H6 and H8 was based on the initial unshifted spectrum in which steric pressure causes H6 and H, to appear furthest downfield (12). These proton resonances remain furthest downfield in the spectra obtained after addition of the shift reagent. The NMR spectrum of naphthalene in the presence of these shift reagents exhibited very little bonding specificity and selective shifting. The unshifted spectrum of acenaphthene is shown in Figure 6a. This compound has three types of
,H6*?*(
I2
II
,
,
IO
9
8
7 w
Flgure 7. Proton NMR spectra of 0.1 M 1-methylnaphthalenein CDCI, with (a)no shift reagent, (b) 0.1 M Yb(fod), and 0.1 M Ag(fod), (c) 0.2 M Yb(fod), and 0.2 M Ag(fod), and (d) 0.3 M Yb(fodb and 0.3 M Ag(fod). aromatic protons. Addition of Yb(fod), and Ag(fod) leads to complete resolution of the resonances for each proton, as seen in Figure 6b. The assignments are made on the bases of relative shifts and coupling. The silver is expected to bond away from the substituted positions and A is assigned to the doublet that shifts the furthest. The spectrum of 1-methylnaphthalene in CDC1, is shown in Figure 7a. The spectrum is quite complex and the addition of Y b ( f ~ dand ) ~ Ag(fod) clarify it considerably. There are seven distinct aromatic protons, and unique resonances are observed for six of them at various points in the series of shifted spectra shown in Figure 7b-d. In spectrum c, three doublets and two triplets are resolved. The assignments are made on the basis of decoupling data and relative shifts. Unfortunately, H6 and H7 cannot yet be distinguished and assigned Comparison of fod and thd Chelates as Binuclear Shift Reagents. In all of the studies just discussed, as well as in our previous reports, primarily the lanthanide complexes of fod were used to form the binuclear shifts reagents. The lanthanide complexes of 2,2,6,6-tetramethyl-3,5-heptanedione, H(thd), have been widely used as NMR shift reagents and were tested for their ability to form the binuclear complexes with silver P-diketonates and shift the NMR spectra of olefins and aromatics. The shifts in the NMR spectrum of toluene using Pr(thd), and Yb(thd)3with various silver P-diketonates were recorded. In solutions 0.2 M Pr(thd)3(or Yb(thd),) in CDCl, with 0.2 M Ag(fod), Ag(tfa), or Ag(thd) and 0.1 M toluene, very small, if any, shifts were observed in the NMR spectrum of toluene. A similar result was observed for 1-hexenewhen Yb(thd), with either Ag(fod) or Ag(tfa) was added. The poor shifts with the Ln(thd)3-silver diketonate mixtures may result from the inability of the tris chelates to form the tetrakis chelate ion pair, the species believed to be the
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
active shift reagent complex. An alternative explanation may be the expected poorer ]Lewis acidity of the thd binuclear complex relative to those containing fluorinated ligands. Shifts of the spectra oQ' toluene were also recorded after adding Pr(fodI3 or Yb(focI), with Ag(thd). In both cases, the shifts observed with Ag(thd) were smaller than those observed using Ag(fod) or Ag(tfa), The solubility of Ag(thd) in the presence of the lanthanide chelate and the toluene is comparable to that of Ag(fod) and Ag(tfa), and the smaller shifts may also result in part from subtle changes in the magnetic environment of the toluene in the resulting complex. Carbon-13 NMR Spectra of Aromatic Compounds. In previous 13CNMR studies of diamagnetic silver(1) complexes with aromatic compouncb, upfield shifts were observed for the resonances of those carbons where the silver bonded to the ring (13,141.In a 13C NMR study of the AgBF4 complex with toluene, upfield shifts for carbons 2, 3, and 4 were reported (14). Carbon 1exhibited downfield shifts on com-
Toluene
o-Xylene
plexation. With paramagnetic binuclear complexes (0.2 M Eu(fod), with 0.2 M Ag(fod) and 0.1 M toluene) we have observed upfield shifts for carbons 3 and 4. Carbons 1 and 2 and the methyl carbon resonance shift downfield. Carbon 2, in the presence of the paramagnetic binuclear shift reagent, undoubtedly experiences two effects. Any silver bonding occurring at the 2-3 position should shift the resonance upfield; however, bonding at the 3-4 position results in a downfield shift induced l ~ the y paramagnetic europium(II1). The overall result for carbon 2 is a downfield shift. Carbons 3 and 4 shift upfield further than reported by Crist, but this may be due to the increased concentration of silver that we have added. A 13CNMR study of the AgBF, complex with o-xylene has been reported and upfieid shifts were observed for carbons 3 and 4 while carbon 2 shifted downfield (13). In our results using 0.2 M E u ( f ~ dand ) ~ 0.2 M Ag(fod) with 0.1 M o-xylene, carbons 1,2, and 3 all shifted downfield while the resonance for carbon 4 shifted upfield. These 13CNMR spectra for both toluene and o-xylene agree with the results of the proton NMR spectra of these compounds. The silver preferentially bonds away from the steric encumbrance of the methyl groups and those protons closer to the site of silver complexation exhibit larger shifts. Carbon-13 NMR data, when used in conjunction with proton NMR data, should greatly aid in the assignment of configurations of aromatic compounds studied with these shift reagents. The 13CNMR spectra of more complex aromatics can also be improved. The compound 1-methylnaphthalene has ten nonequivalent aromatic carbons. In the unshifted spectrum only nine carbon resonainces are observed. With a mixture of 0.2 M Yb(fod)3, 0.2 M Ag(fod), and 0.1 M l-methylnaphthalene, all ten of the aromatic carbon resonances appear in the spectrum. The carbon in the CF3 group on the fod ligands does appear in this region of the spectrum as a large quartet due to the fluorine coupling and must be taken into account. In most cases, only the two center lines of the quartet are actually observed, and they are much smaller than the substrate resonances at these concentrations. We have also examined the lSC NMR spectrum of 1-methylnaphthalene with a 360-MHz instrumlent. Again, only nine carbon resonances were observed in the aromatic region. All ten were apparent after the addition of Yb(fod), and Ag(fod). Studies of NMR Spectra of Phosphines with Silver(I)-Lanthanide Shift Reagents. Very few studies have appeared using lanthanide shift reagents to study phos-
1605
j .-RH] 3
Figure 8. Proton NMR spectra of 0.1 M triphenylphosphine in CDCI, with (a: no shifl reagent and (b) 0.2 M Pr(fod), and 0.2 M Ag(tfa).
phine-containing compounds. Gerken and Ritchey (15)have reported induced shifts in the proton spectrum of triethylphosphine using Pr(II1) and Eu(II1) chelates of thd and fod. The shifts were quite small and were attributed to the formation of a relatively weak lanthanide-phosphine complex. Mandel et al. (16) observed induced shifts in the proton spectrum of ethyldiphenylphosphine using Pr(fod), and Yb(fad),, but these were also rather small. Since lanthanide shift reagents are hard acids, and the phosphorus in phosphines constitutes a soft base, only weak lanthanide-phosphine complexes are expected, and the resulting lanthanide-induced shifts are quite small. The ability of silver to bind to phosphines is well-known. Partenheimer has aptly demonstrated this for silver p-diketonates by exchanging phosphine donors for olefins previously bound to the silver (17). The silver p-diketonate-phosphine complex apparently forms a binuclear complex when a lanthanide chelate is added, and selective lanthanide-induced shifts are observed in the NMR spectrum of the phosphine. Triphenylphosphine in the presence of Pr(fod), (without Ag(1)) does not exhibit any observable shifts. Addition of Ag(tfa) to this mixture results in substantial shifts. In Figure 8, the spectrum of a 0.1 M solution of triphenylphosphine before and after the addition of 0.2 M Pr(fod), and 0.2 M Ag(tfa) is shown. The resonance that shifts upfield the furthest is a doublet of doublets and is assigned to Ho Coupling to HB and phosphorus accounts for the shape of this resonance. The resonances for HA and HB exhibit a small splitting from long range coupling to the phosphorus. The resonance for HA shifts the least and appears as a triplet of relative area one. In a proton spectrum obtained with the phosphorus decoupled, the resonance for Hc collapsed to a doublet. The small splitting in the peaks for HA and HBwas also absent. The relative shifts for the protons in triphenylphosphine indicate that the silver preferentially bonds to the phosphorus rather than the aromatic rings. This conclusion is substantiated by the 31PNMR spectrum. In the presence of the shift reagent, the 31Pspectrum consists of two doublets that result from the 107Ag-31Pand lOeAg-,lP spin-spin coupling. At 22 OC, with 0.2 M Pr(fod),, 0.2 M Ag(fod), and 0.15 M PPh,, a 107Ag-31Pcoupling constant of 734 Hz is obtained for the Pr-Ag-PPh, complex. Silverphosphorus coupling has been observed in previous studies and the coupling constant can be related to the amount of s-orbital character in the silver-phosphorus bond (1419). In these earlier reports, complexes with two or more phosphine ligands were studied and the coupling constants (224-503 Hz) are substantially smaller than the value we observe. This suggests that the bond formed between the silver, which we believe forms an ion pair with the lanthanide tetrakis chelate anion, and the phosphorus contains a considerable amount of s-orbital character. As the concentration of triphenylphosphine is increased, the intensity of the doublets gradually
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Anal. Chem. 1982. 5 4 . 1606-1615
is reduced and a single resonance upfield of the center of the doublets starts to appear in the spectrum. Presumably, a rapid exchange of the triphenylphosphine ligands takes place and the singlet is for the [Pr(f~d)~]Ag(PPh,), species. When the concentration of PPh3 is twice that of silver, only a singlet is observed in the 31Pspectrum. A similar coupling is observed for the compound triphenylphosphite. In the earlier studies, silver-phosphorus coupling was observed for complexes of silver with triethylphosphite (18).Once again, the 107Ag-31Pcoupling constant of 1081 Hz that we observed is substantially larger than the values reported in these previous studies (324-756 Hz).
ACKNOWLEDGMENT The encouragement of William Harris is gratefully acknowledged. The assistance of Martin Ashley in obtaining 13C and 31PNMR spectra is greatly appreciated. LITERATURE CITED (1) Wenzel, T. J.; Bettes, T. C.; Sadlowskl, J. E.; Slevers, R. E. J . Am. Chem. SOC. 1980, 102, 5003. (2) Wenzel, T . J.; Slevers, R. E. Anal. Chem. 1981, 53, 303. (3) Wenzel, T. J.; Slevers, R. E. J . Am. Chem. SOC. 1982, 104, 382. (4) Evans, D. F.; Tucker, J. N.; deVlllardl, G. C. J . Chem. Soc., Chem. Commun. 1975. 205.
Dambska, A.; Janowski, A. Org. Magn. Reson. 1960, 13, 122. Richardson, M. F.; Slevers, R. E. Inorg. Chen?. 1971, 70, 408. BeverwlJk.C. D. M.; van der Kerk, 0. J. M.; Leusink, A. J.; Nones. J. G. Organomet. Chem. Rev., Sect. A 19’10, 5 , 215. Muhs, M. A.; Welss, F. T. J . Am. Chem. SOC. 1962, 8 4 , 4607. GII-Av, E.; Herllng, J. J . Fhys. Chem. 1962, 66, 1208. Peyronel, G.; Vezzosl, I. M.; Buffagnl, S. Qarz.Chlm. Ita/. 1959, 89, 1863, 1869. Vezzosl, I . M.; Buffagnl, S.; Peyronel, G. &zz. Chim. Ita/. 1967, 97, 859. Jonathan, N.; Gordon, S.; Dailey, B. P. J . Chem. Fhys. 1962, 36, 2443. van Dongen, J. P. C. M.; Beverwljk, C. D. M. J . Organomet. Chem. 1973, 51, C36. Crlst, D. R.; Hsleh, 2. H.; Jordan, G. J.; Schlnco, F. P.;Maclorowskl, C. A. J . Am. Chem. SOC. 1974, 96, 4932. Gerken, T. A.; Rltchey, W. M. J . Magn. Reson. 1976, 24, 155. Mandel, F. S.; Cox, R. H.; Taylor, R. C. J . Magn. Reson. 1974, 14, 235. Partenhelmer, W.; Johnson, E. H. Inorg Chem. 1973, 12, 1274. Muettertles, E. L.; Alegranti, C. W, J . Am. Chem. SOC. 1972, 94, 6386. Hollander, F. J.; Ip, Y. L.; Coucouvanls, D. Inorg. Chem. 1978, 15, 2230.
RECEIVED for review August 10, 1981. Resubmitted and accepted April 16,1982. The support of the National Science Foundation through Grant CHE 79-13022 is gratefully acknowledged.
Structural Resolution in the Carbon- 13 Nuclear Magnetic Resonance Spectrometric Analysis of Coal by Cross Polarization and Magic-Angle Spinning Mark J. Sullivan and Gary E. Maclel* Depadment of Chemistry, Colorado Stafe Un/vers& Fod Colllns, Colorado 80523
The origin of the line widths and the prospects for improved structural resolution in “C CP/MAS spectra of coal are addressed. Hole-burning and two-dimensional FT experiments indicate that inhomogeneous broadening influences are the largest contributors to the “C line wldths In the spectrum of Powhatan No. 5 coal. This conclusion Is conflrmed by T 2 measurements, which indicate homogeneous line widlhs In the range 2.5-8.5 ppm. The widths of the “aromatic” and “aliphatic” bands are therefore attributed mainly to distributions of similar structures and chemical shifts. Double exponential multiplication and convolution difference methods have been found useful In bringing out additional fine structure in the spectra, features that can reasonably be assigned to specific structural moieties. Differential relaxation behavior shows considerable promise in distinguishing structural differences in the time domain that would not be apparent in the normal ‘‘C CP/MAS spectrum. However, higher spectrometer sensitivities will be required before this potential can be realized fully.
The advent of pulse Fourier transform (FT) NMR techniques in the late 1960s set the stage for the use of 13CNMR for qualitative and quantitative analysis in liquids, including a variety of sample types encountered in fossil fuel technology. However, for solid samples the effects of lH-13C magnetic dipole-dipole interactions and I3Cchemical shift anisotropies and the time bottleneck of long 13C spin-lattice relaxation 0003-2700/82/0354-1606$01.25/0
times render the direct application of the liquid-state 13C NMR technique essentially useless-yielding, broad, featureless spectra of low intensity. Pines, Gibby, and Waugh introduced the technique of high-power lH decoupling for eliminating the broadening effect of lH-13C dipolar interactions, with 13C-lH cross polarization (CP) to circumvent the 13C Tl bottleneck ( I ) . Schaefer and Stejskal(2) then introduced the use of MAS to average out the 13C chemical shift anisotropy (3, 4 ) and demonstrated that the CP/MAS combination provides an approach that is capable of yielding high-resolution 13CNMFt spectra of solid samples. Line widths on the order of 1ppm or less can be achieved by this method on crystalline samples, often providing a higher order of structural discrimination than one can achieve in a corresponding liquid (e.g., because of motional averaging of different conformations in the liquid state) (5). Unfortunately, the 13CNMR spectra typically obtained on solid fossil fuels (coals, oil shales) do not exhibit a high level of spectral resolution (6-16).Most fossil-fuel 13CCP/MAS spectra essentially consist of two broad bands-one in the aromatic/olefinic region from about 170 ppm to 95 ppm and one in the aliphatic region from about 90 ppm to -5 ppm. These “aromatic” and “aliphatic” bands are usually relatively featureless, as seen for Powhatan No. 5 coal in several of the figures that follow, but may sometimes show reproducible shoulders or multiple maxima, especially for lower-rank coals (10).Indeed, some lignites show a considerable amount of fine structure, as seen in the several spectra of Byrd Stadium lignite that follow. 0 1982 American Chemical Society
