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Observation of natural-abundance, 13C13C dipolar satellites in ultraoriented polyethylene
JOURNAL
OF MAGNETIC
RESONANCE
ti,467-470
(1976)
Observation of Natural-Abundance,13C-13C Dipolar Satellitesin Ultraoriented Polyethylene The recently developed 13C-‘IH cross-polarization techniques in solids (I) have provided a means of increasing botlh sensitivity and resolution in the 13C NMR of solids. Further variations of these experiments in both oriented (Z-4) and polycrystalline (5) solids have demonstrated that structural information may also be obtained by following the development of the 13C magnetization under the influence of the nearby ‘H dipolar fields. This structural information is often useful in assigning principal axes of the chemical shift tensors in a solid where the carbon and hydrogen atomic positions are known; or, if the shift tensor assignment is known, one may determine C-H structural parameters. This communication shows that weak 13C-13C dipolar satellites in spectra of oriented systems can be observed and, therefore, can be used to assign principal axes to 13C chemical shift tensors. Figure 1 shows 13C spectra of oriented linear polyethylene (M,, = 33,000, AI, = 161,000) at three different temperatures. The spectra were obtained using the spinlocking 13C-lH cross-polarization method (I) in which both rf fields were matched at 70 kHz. The oriented polyethylene sample was obtained from Professor R. S. Porter (6) and has been extruded at 0.243 GPa (2400 atm) and 130°C through a conical die of such dimensions as to produce a draw ratio of 11.8 : 1. The sample is oriented so that the draw direction is parallel (+I “) to H,, the magnetic-field direction. The alignment of the polyethylene chains in the ordered, crystalline regions of this semicrystalline substance is estimated by X ray (17) to be within 5” of the draw direction. Therefore, the system has a high degree of order even though it is not as perfect as a single crystal. Figure lc shows the low-temperature spectrum using three amplifications. On the x 1 scale, one can see that the bulk of the intensity is centred at 181 ppm relative to CS,. The methylene chemical shift tensor of polyethylene was examined previously (8, 9) and this resonance position is predicted when the chain axes are parallel to Ho. Under x 16 amplification, the spectra of Fig. 1 display significant intensity for about 40 ppm downfield from the main resonance. The chemical shift range for polycrystalline polyethylene spans the range from 142 to 181 ppm; therefore, this intensity arises from chains which have not been fully aligned by the drawing process. As one raises the temperature, one can see qualitatively that these misaligned chains also show greater motional averaging via the decrease in width of this shoulder. This topic will be treated in another paper (10). The dipolar 13C--13Csatellites do not show up until one gets to a x 128 amplification. (For this amplification an exponential multiplication corresponding to a 0.8 ppm line broadening was applied to the FID, whereas the FID’s were used directly at the lower amplifications.) The 13C-13C satellites arise from the statistical chance that two 13C nuclei happen to be close enough to produce an observable dipolar interaction. Because of the sample’s high orientation, these peaks are at discrete positions such that Copyright 0 1976 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
467
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COMMUNICATIONS
the satellite splitting, A (in energy units), arising from the ith pair is given by A = 1.5y;,G2(1 - 3cos28,)/r2~, [II where Ri is the distance between the 13C nuclei and 0, is the angle between R, and Ho. Equation [l] is based on the assumption that during the acquisition of data, the protondecoupling field is strong enough to decouple protons and carbons sufficiently to preserve the secular&y of the (31ZiIZj - I,.Ij) 13C spin operator. This condition is satisfied for polyethylene in the presence of resonant 70 kHz rf fields. These samples contain 13C at natural abundance (1.1%). Therefore, the satellite spectra ought to be as
Jllllll~~ll~l~l~llllr(l(l(lllllll~~~~~~~~~~~~l~~~l~l~~~l~~~l~l~~~l~ 50 100 150
200
PPM(cs2)
250
300
FIG. 1. Carbon-13 cross-polarization spectra of ultraoriented linear polyethylene with an 11.8: 1 draw ratio. The draw direction is parallel to the magnetic field. The three pairs of 13C-13C dipolar satellite positions based on RI = 0.153 nm and w = 112” are given by the dashed lines. Temperatures and spectral amplification factors are also given. sharp as the main line (50 Hz full width at half-height), although the satellites will be broadened by imperfect alignment according to Eq. [I]. The room-temperature structure of polyethylene has been determined (II, 12). Furthermore, the temperature dependence of the unit cell parameters has been measured (13, 24). The labeled dashed lines in Fig. 1 correspond to the satellite positions which are to be expected on the basis of the usually accepted (12) picture of all-tvans polyethylene chains having a C-C bond distance of 0. I53 nm and a C-C-C bond angle, !P, of 112”. A calculation of 13C-13C satellite splittings in polyethylene for this orientation indicated that the three strongest interactions come from pairs of carbons on the same chain which are one, two, and three bonds separated.
COMMUNICATIONS
469
Only the R, pair of satellites corresponding to two directly bonded r3C nuclei can be resolved on both sides of the main resonance. These satellite positions agree within experimental error with those predicted from X-ray structural parameters. These resonances are quite broad (z 30 ppm), partly because of the &5” uncertainty in chain alignment as predicted by Eq. [I]. The sharpening of these satellites at 102°C is most easily interpreted as arising from a rotational averaging of Bi about the chain axis direction for these chains which are slightly misaligned relative to H,,, although no distinction between jump and continuous models of averaging can be made from these data alone. The decrease in the magnitude of the splitting might be due to an increase in conformational disorder such as gauche bonds because the observed splittings are really time-averaged resonance positions, averaged over a time comparable to the inverse of the splitting. For the R, and R3satellites corresponding to 13C pairs two and three bonds away, only the high-field components are clearly visible. The low-field satellites overlap, with intensity coming from unoriented chains, although the break in the downfield resonance of Fig. lc (x 16) is probably associated with the R, satellite. The position of the R3 peak is about what one expects on the basis of X-ray parameters, but the R, satellite peak is consistently about 39 ppm from the main resonance instead of the predicted 46 ppm. The carbon-carbon distance giving rise to this satellite is exactly the cdimension of the orthorhombic unit cell, which has a commonly accepted value (21-15) of c = 0.2543 + 0.0005 nm over a wide temperature range. Because of the discrepancy between this distance and the distance implied by a 39 ppm satellite (0.269 nm), an X-ray determination of the c axis was made for this drawn polyethylene material. The room-temperature result was c = 0.2546 + 0.0005 nm. This value was calibrated against an internal silicon standard, and convergence was good under the assumption of an orthorhombic unit cell. It is hard to account for the magnitude of this discrepancy, particularly in view of the agreement of the R, and R3 satellite positions with the normally assumed structure. Furthermore, this satellite position persists after cycling the sample temperature up to 102°C and down to -94°C. The same position is also observed for a similar sample having an average draw ratio of 25 : 1. The following explanations have been considered for reconciling the difference between the NMR and X-ray results for R2: (a) imperfect orientation of polymer chains, (b) motional averaging, (c) isotopic 13C downfield shift due to the substitution of a 13C for a lzC two bonds away, (d) a 13C isotope effect on R,, (e) pseudo-dipolar interaction, (f) scalar spin-spin interaction, and (g) NMR artifacts. Geometric considerations dismiss (a) and (b) at room temperature and below; the R, and R3satellites do not show effects (c) and (d); furthermore, an isotopic 7 ppm downfield shift should have beeln observable in Fig. lb; the required pseudo-dipolar interaction would have to be an order of magnitude greater than typical two-bond 13C-13C J couplings (16), and the scalar spin-spin interaction should not contribute to the dipolar splitting under these conditions. On the subject of artifacts, there are two characteristics of this satellite which are unexpected: namely, the shape is somewhat asymmetric, and the intensity is a.bout 60 % of the expected I. 1% of the total intensity. The following precautions have been taken to minimize artifacts in the R2 satellite region: external lock field modu18ation frequencies are much greater than the splitting, spectra have been taken at different RF reference offsets, and data have been accumulated so as to avoid baseline artifacts (17).
470
COMMUNICATIONS
It would appear that the discrepancy in R, probably arises from a pseudo-dipolar interaction or an NMR artifact which is not understood at this time. Alternatively, but less likely, is the possibility that isotopic substitution produces substantial changes in the atomic positions. In summary, the observation of 13C-13C dipolar satellites at natural abundance in highly oriented systems is clearly useful for assigning chemical shift tensor orientations, and obtaining geometric information about the solid, although some questions remain regarding the ultimate precision of this method. This method also shows promise for the study of anisotropic molecular mobility, or the study of pseudodipolar interactions in systems where the structure is well known. Signal-to-noise will probably be the limiting consideration, but for samples as rich in carbon as polyethylene, this is not a problem. Isotopic 13C enrichment at sites of interest in more complicated systems would extend the usefulness of this technique so long as adjacent enriched sites are separated by more than, say, 0.8 nm. ACKNOWLEDGMENTS
The author gratefully acknowledges the generosity of Professor R. S. Porter in supplying the polyethylene samples, and in addition, thanks Dr. J. Mazur for calculating the carbon-carbon distances in crystalline polyethylene, and Dr. G. T. Davis for the X-ray determination of the c axis in this sample. REFERENCES
1. A. PINES, M. G. GIBBY, AND J. S. WAUGH, Bull. Amer. Phys. Sac. 16,1403 (1971); J. Chem. Phys. 56, 1776 (1972); J. Chem. Phys. 59, 569 (1973). 2. L. MULLER, A. KUMAR, T. BAUMANN, AND R. R. ERNST, Phys. Rev. Lett. 32,1402 (1974). 3. R. K. HESTER, J. L. ACKERMAN, V. R. CROSS, AIYD J. S. WAUGH, Phys. Rev. Lett. 34,993 (1975); R. K. HESTER, J. L. ACKERMAN, B. L. NEFF, AND J. S. WAUGH, Phys. Rev. Lett. 36,lOSl (1976); R. K. HESTER, V. R. CROSS, J. L. ACKERMAN, AND J. S. WAUGH, J. Chem. Phys. 63,3606 (1975). 4. J. S. WAUGH, Proc. Nat. Acad. Sci. U.S.A. 73, 1394 (1976). 5. M. E. STOLL, A. J. VEGA, AND R. L. VAUGHAN, J. Chem. Phys. in press. 6. A description of this sample and its preparation may be found in the text and references of N. E. WEEKS, S. MORI, AND R. S. PORTER, J. Poly. Sci. B 13,203 l(l975); N. E. WEEKS AND R. S. PORTER, J. Poly. Sci. B 13,2049 (1975). 7. W. T. MEAD AND R. S. PORTER, private communication. 8. J. URBINO AND J. S. WAUGH, Proc. Nat. Acad. Sci. U.S.A. 71, 5062 (1974). 9. D. L. VANDERHART, J. Chem. Phys. 64,830 (1976). 10. To be published. 11. C. W. BIJNN, Trans. Faraday Sot. 35,482 (1939). 12. S. KAVESH AND J. M. SCHULTZ, J. Poly. Sci. A-2 8,243 (1970), and references cited therein. 13. P. R. SWAN, J. Poly. Sci. 56, 403 (1962). 14. G. T. DAVIS, R. K. EBY, AND J. P. COLSON, J. Appl. Phys. 41,4316 (1970). 15. G. AVITABILE, R. NAPOLITANO, B. PIROZZI, K. D. ROUSE, M. W. THOMAS, AND B. T. M. WILLIS, J. PoZy. Sci. B 13, 351 (1975). 16. J. B. STOTHERS, “Carbon-13 NMR Spectroscopy,” Academic Press, New York, 1972. 17. E. 0. STEJSKAL AND J. SCHAEFER, J. Magn. Resonance 18,560 (1975).
United States Department of Commerce National Bureau of Standards Washington, D.C. 20234 Received September 2, 1976
D.
L.
VANDERHART
OF MAGNETIC
RESONANCE
ti,467-470
(1976)
Observation of Natural-Abundance,13C-13C Dipolar Satellitesin Ultraoriented Polyethylene The recently developed 13C-‘IH cross-polarization techniques in solids (I) have provided a means of increasing botlh sensitivity and resolution in the 13C NMR of solids. Further variations of these experiments in both oriented (Z-4) and polycrystalline (5) solids have demonstrated that structural information may also be obtained by following the development of the 13C magnetization under the influence of the nearby ‘H dipolar fields. This structural information is often useful in assigning principal axes of the chemical shift tensors in a solid where the carbon and hydrogen atomic positions are known; or, if the shift tensor assignment is known, one may determine C-H structural parameters. This communication shows that weak 13C-13C dipolar satellites in spectra of oriented systems can be observed and, therefore, can be used to assign principal axes to 13C chemical shift tensors. Figure 1 shows 13C spectra of oriented linear polyethylene (M,, = 33,000, AI, = 161,000) at three different temperatures. The spectra were obtained using the spinlocking 13C-lH cross-polarization method (I) in which both rf fields were matched at 70 kHz. The oriented polyethylene sample was obtained from Professor R. S. Porter (6) and has been extruded at 0.243 GPa (2400 atm) and 130°C through a conical die of such dimensions as to produce a draw ratio of 11.8 : 1. The sample is oriented so that the draw direction is parallel (+I “) to H,, the magnetic-field direction. The alignment of the polyethylene chains in the ordered, crystalline regions of this semicrystalline substance is estimated by X ray (17) to be within 5” of the draw direction. Therefore, the system has a high degree of order even though it is not as perfect as a single crystal. Figure lc shows the low-temperature spectrum using three amplifications. On the x 1 scale, one can see that the bulk of the intensity is centred at 181 ppm relative to CS,. The methylene chemical shift tensor of polyethylene was examined previously (8, 9) and this resonance position is predicted when the chain axes are parallel to Ho. Under x 16 amplification, the spectra of Fig. 1 display significant intensity for about 40 ppm downfield from the main resonance. The chemical shift range for polycrystalline polyethylene spans the range from 142 to 181 ppm; therefore, this intensity arises from chains which have not been fully aligned by the drawing process. As one raises the temperature, one can see qualitatively that these misaligned chains also show greater motional averaging via the decrease in width of this shoulder. This topic will be treated in another paper (10). The dipolar 13C--13Csatellites do not show up until one gets to a x 128 amplification. (For this amplification an exponential multiplication corresponding to a 0.8 ppm line broadening was applied to the FID, whereas the FID’s were used directly at the lower amplifications.) The 13C-13C satellites arise from the statistical chance that two 13C nuclei happen to be close enough to produce an observable dipolar interaction. Because of the sample’s high orientation, these peaks are at discrete positions such that Copyright 0 1976 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
467
468
COMMUNICATIONS
the satellite splitting, A (in energy units), arising from the ith pair is given by A = 1.5y;,G2(1 - 3cos28,)/r2~, [II where Ri is the distance between the 13C nuclei and 0, is the angle between R, and Ho. Equation [l] is based on the assumption that during the acquisition of data, the protondecoupling field is strong enough to decouple protons and carbons sufficiently to preserve the secular&y of the (31ZiIZj - I,.Ij) 13C spin operator. This condition is satisfied for polyethylene in the presence of resonant 70 kHz rf fields. These samples contain 13C at natural abundance (1.1%). Therefore, the satellite spectra ought to be as
Jllllll~~ll~l~l~llllr(l(l(lllllll~~~~~~~~~~~~l~~~l~l~~~l~~~l~l~~~l~ 50 100 150
200
PPM(cs2)
250
300
FIG. 1. Carbon-13 cross-polarization spectra of ultraoriented linear polyethylene with an 11.8: 1 draw ratio. The draw direction is parallel to the magnetic field. The three pairs of 13C-13C dipolar satellite positions based on RI = 0.153 nm and w = 112” are given by the dashed lines. Temperatures and spectral amplification factors are also given. sharp as the main line (50 Hz full width at half-height), although the satellites will be broadened by imperfect alignment according to Eq. [I]. The room-temperature structure of polyethylene has been determined (II, 12). Furthermore, the temperature dependence of the unit cell parameters has been measured (13, 24). The labeled dashed lines in Fig. 1 correspond to the satellite positions which are to be expected on the basis of the usually accepted (12) picture of all-tvans polyethylene chains having a C-C bond distance of 0. I53 nm and a C-C-C bond angle, !P, of 112”. A calculation of 13C-13C satellite splittings in polyethylene for this orientation indicated that the three strongest interactions come from pairs of carbons on the same chain which are one, two, and three bonds separated.
COMMUNICATIONS
469
Only the R, pair of satellites corresponding to two directly bonded r3C nuclei can be resolved on both sides of the main resonance. These satellite positions agree within experimental error with those predicted from X-ray structural parameters. These resonances are quite broad (z 30 ppm), partly because of the &5” uncertainty in chain alignment as predicted by Eq. [I]. The sharpening of these satellites at 102°C is most easily interpreted as arising from a rotational averaging of Bi about the chain axis direction for these chains which are slightly misaligned relative to H,,, although no distinction between jump and continuous models of averaging can be made from these data alone. The decrease in the magnitude of the splitting might be due to an increase in conformational disorder such as gauche bonds because the observed splittings are really time-averaged resonance positions, averaged over a time comparable to the inverse of the splitting. For the R, and R3satellites corresponding to 13C pairs two and three bonds away, only the high-field components are clearly visible. The low-field satellites overlap, with intensity coming from unoriented chains, although the break in the downfield resonance of Fig. lc (x 16) is probably associated with the R, satellite. The position of the R3 peak is about what one expects on the basis of X-ray parameters, but the R, satellite peak is consistently about 39 ppm from the main resonance instead of the predicted 46 ppm. The carbon-carbon distance giving rise to this satellite is exactly the cdimension of the orthorhombic unit cell, which has a commonly accepted value (21-15) of c = 0.2543 + 0.0005 nm over a wide temperature range. Because of the discrepancy between this distance and the distance implied by a 39 ppm satellite (0.269 nm), an X-ray determination of the c axis was made for this drawn polyethylene material. The room-temperature result was c = 0.2546 + 0.0005 nm. This value was calibrated against an internal silicon standard, and convergence was good under the assumption of an orthorhombic unit cell. It is hard to account for the magnitude of this discrepancy, particularly in view of the agreement of the R, and R3 satellite positions with the normally assumed structure. Furthermore, this satellite position persists after cycling the sample temperature up to 102°C and down to -94°C. The same position is also observed for a similar sample having an average draw ratio of 25 : 1. The following explanations have been considered for reconciling the difference between the NMR and X-ray results for R2: (a) imperfect orientation of polymer chains, (b) motional averaging, (c) isotopic 13C downfield shift due to the substitution of a 13C for a lzC two bonds away, (d) a 13C isotope effect on R,, (e) pseudo-dipolar interaction, (f) scalar spin-spin interaction, and (g) NMR artifacts. Geometric considerations dismiss (a) and (b) at room temperature and below; the R, and R3satellites do not show effects (c) and (d); furthermore, an isotopic 7 ppm downfield shift should have beeln observable in Fig. lb; the required pseudo-dipolar interaction would have to be an order of magnitude greater than typical two-bond 13C-13C J couplings (16), and the scalar spin-spin interaction should not contribute to the dipolar splitting under these conditions. On the subject of artifacts, there are two characteristics of this satellite which are unexpected: namely, the shape is somewhat asymmetric, and the intensity is a.bout 60 % of the expected I. 1% of the total intensity. The following precautions have been taken to minimize artifacts in the R2 satellite region: external lock field modu18ation frequencies are much greater than the splitting, spectra have been taken at different RF reference offsets, and data have been accumulated so as to avoid baseline artifacts (17).
470
COMMUNICATIONS
It would appear that the discrepancy in R, probably arises from a pseudo-dipolar interaction or an NMR artifact which is not understood at this time. Alternatively, but less likely, is the possibility that isotopic substitution produces substantial changes in the atomic positions. In summary, the observation of 13C-13C dipolar satellites at natural abundance in highly oriented systems is clearly useful for assigning chemical shift tensor orientations, and obtaining geometric information about the solid, although some questions remain regarding the ultimate precision of this method. This method also shows promise for the study of anisotropic molecular mobility, or the study of pseudodipolar interactions in systems where the structure is well known. Signal-to-noise will probably be the limiting consideration, but for samples as rich in carbon as polyethylene, this is not a problem. Isotopic 13C enrichment at sites of interest in more complicated systems would extend the usefulness of this technique so long as adjacent enriched sites are separated by more than, say, 0.8 nm. ACKNOWLEDGMENTS
The author gratefully acknowledges the generosity of Professor R. S. Porter in supplying the polyethylene samples, and in addition, thanks Dr. J. Mazur for calculating the carbon-carbon distances in crystalline polyethylene, and Dr. G. T. Davis for the X-ray determination of the c axis in this sample. REFERENCES
1. A. PINES, M. G. GIBBY, AND J. S. WAUGH, Bull. Amer. Phys. Sac. 16,1403 (1971); J. Chem. Phys. 56, 1776 (1972); J. Chem. Phys. 59, 569 (1973). 2. L. MULLER, A. KUMAR, T. BAUMANN, AND R. R. ERNST, Phys. Rev. Lett. 32,1402 (1974). 3. R. K. HESTER, J. L. ACKERMAN, V. R. CROSS, AIYD J. S. WAUGH, Phys. Rev. Lett. 34,993 (1975); R. K. HESTER, J. L. ACKERMAN, B. L. NEFF, AND J. S. WAUGH, Phys. Rev. Lett. 36,lOSl (1976); R. K. HESTER, V. R. CROSS, J. L. ACKERMAN, AND J. S. WAUGH, J. Chem. Phys. 63,3606 (1975). 4. J. S. WAUGH, Proc. Nat. Acad. Sci. U.S.A. 73, 1394 (1976). 5. M. E. STOLL, A. J. VEGA, AND R. L. VAUGHAN, J. Chem. Phys. in press. 6. A description of this sample and its preparation may be found in the text and references of N. E. WEEKS, S. MORI, AND R. S. PORTER, J. Poly. Sci. B 13,203 l(l975); N. E. WEEKS AND R. S. PORTER, J. Poly. Sci. B 13,2049 (1975). 7. W. T. MEAD AND R. S. PORTER, private communication. 8. J. URBINO AND J. S. WAUGH, Proc. Nat. Acad. Sci. U.S.A. 71, 5062 (1974). 9. D. L. VANDERHART, J. Chem. Phys. 64,830 (1976). 10. To be published. 11. C. W. BIJNN, Trans. Faraday Sot. 35,482 (1939). 12. S. KAVESH AND J. M. SCHULTZ, J. Poly. Sci. A-2 8,243 (1970), and references cited therein. 13. P. R. SWAN, J. Poly. Sci. 56, 403 (1962). 14. G. T. DAVIS, R. K. EBY, AND J. P. COLSON, J. Appl. Phys. 41,4316 (1970). 15. G. AVITABILE, R. NAPOLITANO, B. PIROZZI, K. D. ROUSE, M. W. THOMAS, AND B. T. M. WILLIS, J. PoZy. Sci. B 13, 351 (1975). 16. J. B. STOTHERS, “Carbon-13 NMR Spectroscopy,” Academic Press, New York, 1972. 17. E. 0. STEJSKAL AND J. SCHAEFER, J. Magn. Resonance 18,560 (1975).
United States Department of Commerce National Bureau of Standards Washington, D.C. 20234 Received September 2, 1976
D.
L.
VANDERHART