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High resolution13C NMR studies of one- and two-dimensional polymerized C60 under high pressure
Pergamon
PIh S0022-3697(97)00044-9
J. Phys. Chem Solids Vo158, No. 1I, pp. 1645-1647, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/97 $17.00 + 0.00
HIGH RESOLUTION 13C NMR STUDIES OF ONE- AND TWO-DIMENSIONAL POLYMERIZED C60 UNDER HIGH PRESSURE F. RACHDI a'*, C. GOZE a, L. HAJJI a, M. N01Z~EZ-REGUEIROb, L. MARQUES c, J.-L. HODEAU c and M. MEHRING d ~Groupede Dynamiquedes Phases Condensers,Universit~MontpellierII, PI. Eugrne Bataillon,34095 Montpellier,France bEPM-Matformag,CNRS, BP 166, 38042 Grenoble,France CLaboratoirede Cristallographie,CNRS, BP 166, 38042 Grenoble,France dPhysikalischesInstitut,UniversifiitStuttgart, Pfaffenwaldring57, D70550 Stuttgart, Germany Abstract--We report on ~3CNMR measurementsof orthorhombic one-dimensionaland rhombohedral twodimensional polymers of C60 obtained under high pressure. The obtained ~3C MAS NMR spectrum of the orthorhombicpolymershowstwo resonancesat 146 ppm and 73.5 ppm, and the one of the rhombohedralpolymer presents six isotropic lines at 149.1, 147.9, 145.2, 139.6, 134.8 and 73.5 ppm. The static distortionof the C6o moleculesinducedby the transformationunderpressuremustbe at the originof the observedinequivalentcarbons in both systems. The t3C NMR lineshapesimulationof the obtainedspectra are compatiblewith the suggested polymeric structureswhere the C60moleculesare connectedby [2 + 2] cycloadditions.© 1997 ElsevierScience Ltd.
Keywords: magnetism,structure 1. INTRODUCTION Bulk polymerized C60 phases obtained at high pressure and high temperature have been reported [1, 2]. At least three new phases have been synthesized and characterized [3]. The pressure, temperature and duration synthesis conditions are essential to obtain one of the rhombohedral, orthorhombic or tetragonal phases. As with other non-saturated organic molecules similar reactions as in photopolymerization [4] were expected to appear in solid C60 under pressure and temperature. The cycloaddition [2 + 2] mechanism between two adjacent C60 molecules was proposed to be at the origin of the observed lattice contraction and increase of disorder in these systems. In this paper we report on 13C MAS NMR measurements on orthorhombic onedimensional o-lD and rhombohedral two-dimensional rh-2D polymerized phases. The observed inequivalent carbons in both studied polymers are discussed in terms of the bonding nature between adjacent C60 molecules and distortion from the spherical shape of the molecules. Also, we discuss the molecular dynamics in these systems.
2. EXPERIMENT We performed 13C NMR, measurements at ambient temperature on o-lD and rh-2D polymerized fullerene C60. About 30 mg of each sample have been used. The pristine C60 starting materials were 10% 13C enriched in order to increase the NMR signal and the samples were *Correspondingauthor.
kept in air. NMR measurements were performed using 200 and 400 MHz Bruker NMR spectrometers with ~3C NMR Larmor frequencies of 50.2 and 100.4 MHz, respectively. The spectra were obtained by a one-pulse sequence 7r/2-acquisition with repetition time of 200 and 900 s. The samples were obtained by compressing the enriched C6o (encapsulated in a Pt container) in a belt apparatus for periods not exceeding 2 h. The o- 1D sample was obtained at 4 GPa and 300°C, the rh-2D sample was made at 5 Gpa and 700°C. The X-ray diffraction patterns of the polymerized fullerene are similar to those published earlier [3]. 3. RESULTSAND DISCUSSION Fig. 1 shows the ~3C MAS NMR spectra of the polymerized C60 o-lD phase obtained under pressure with different repetition times of 200 and 900 s. Both spectra present two isotropic lines at positions 146 and 73.5 ppm, and two sidebands corresponding to the line at 146 ppm, which appear at 266 and 26 ppm. The relative intensities of the isotropic lines including the sidebands are reported in Table 1. The observed broadening of the line around 146 ppm could be related to a continuous distribution of C-C distances due to a partial disorder in the chain arrangement that can be partly curved or staggered. Fig. 2 shows the ~3C MAS NMR spectra of the rh-2D polymerized phase with repetition times of 200 and 900 s. Again, we observe two isotropic lines at the same positions as the o-lD phase. The line at position 146 ppm for the rh-2D phase exhibits a structure. A
1645
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.i,~;t~5~.
~f:~'%
oo,
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9
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I 300
200
'
I - 250
'
I 200
'
I 150
'
I 100
'
l 50
~
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ppm Fig. 1. 13CMAS NMR spectraof the orthorhombic(ID) onedimensional phases with a spinning rate of 12 kHz and repetition times of 200 and 900 s. The stars indicate the sidebands of the line at 146 ppm. Inset shows the structural model of the orthorhombic 1D phase. deconvolution of this group of lines gives us six isotropic lines at the following positions: 134.8, 139.6, 142.8, 145.2, 147.9 and 149.1 ppm. We suspect the line at 142.8 - 1 ppm is from unreacted pristine C6o and evaluate its contribution to be about 5% of the total intensity. The relative intensities of the others five isotropic lines are reported in Table 2. Using a lower spinning rate (spectra not shown here), all the sidebands which mimic the envelope of the static spectrum show an extent of anisotropy of about 300 ppm in both phases. As expected in such frozen systems of polymerized C6o, we found long spin-lattice relaxation time T~ and very broad static powder spectra. As it has been shown for the A6Cro phase, with A = K, Rb and Cs [5], the large extent of anisotropy of about 300 ppm indicates the freezing in of the C60 molecules. The latter can be blocked by electrostatic forces as in m6Cr0 or by bonding between the C60 molecule and its neighbour C60 molecules in polymer configurations. The 13C MAS NMR spectra reveal inequivalent carbons sites which can be interpreted as sp 2 carbons around the 146-ppm line position and sp 3 bonding carbons between C6o molecules at 73.5 ppm. The disagreement in the relative intensities of the sp 2 and sp 3 carbons, calculated from structural studies and obtained from experiment, with a repetition time of 200 s comes from the very long TI of the sp 3 carbons (see Tables 1 and 2). The intensity of the sp 2 carbon is always overestimated in comparison with the sp 3 carbon intensity due to the Table 1. Relative intensities of the isotropic lines including the sidebands in the o-lD phase ppm
146 73.5
Spectrum at:
calculated from Ref. [6]
900 s
200 s
54.5 5.5
56.9 3.4
56 4
I 300
s
"sp3 -~-fi~,.,~';~.~'/
.
I 250
I 200
i 150 ppm
i 1O0
I 50
I 0
Fig. 2. '3C MAS NMR spectra of rhombohedral (2D) twodimensional phase with a spinning rate of 12 kHz and repetition times of 200 and 900 s. Inset shows the structural model of this phase. chemical shift anisotropy relaxation mechanism which is usually more efficient for sp 2 carbons. Such behavior is visible in the ~3C MAS NMR spectrum of the rh-2D polymer phase obtained with a repetition time of 900 s which shows an increase of the sp 3 carbon resonance intensity compared with the one obtained with a 200-s repetition time (see Fig. 2). But here, again, the intensity of all sp 3 carbons is not totally covered because of the long Tl; nevertheless, the obtained line intensities are very close to what is expected from the cycloaddition model. If we suppose that the NMR signal is affected in the same manner in the o - l D and rh-2D phases, by the short repetition time of 200 s, we can compare the relative intensity ratios in the o- 1D and rh-2D phases. By doing so we see that the sp 3 ratio is smaller in the o-lD, as expected, due to the decrease of the number of intermolecular [2 -4- 2] bonds. As can be seen in Table I, a disagreement between the experimental and calculated relative intensities of sp 2 and sp 3 carbons in the o-lD polymer is obtained even with a 900-s repetition time. This could be related to a small mixed amount of other phases present in the o - l D sample. In the rhombohedric phase, it has been shown [3] that the fullerene molecule are no longer spheres, but are deformed by the new bonding and are being oblated perpendicular to the plane of polymerization. It is tempting to interpret the splitting of the line at 146 ppm in this phase as coming Table 2. Relative intensities of the isotropic lines including the sidebands in the rh-2D phase ppm
149.1 147.9 145.2 139.6 134.8 73.5
Spectrum at 990s
49 11
Spectrum at 200s 13.9 16.1 13.3 5.4 5.2 6.1
53.9
calculated from Ref. [6] 12 12 12 6 6 12
48
High resolution ~3CNMR studies of one- and two-dimensionalpolymerizedC6o under high pressure from this distortion from sphericity of the C60 molecule. On the same C60 molecule we can expect inequivalent carbons due to different neighbouring. The carbons involved here are all sp 2 hybridized and they have three neighbours which can be at slightly different positions due to the non-sphericity. The small changes in the angles and the distances could be at the origin of the five inequivalent carbons observed. From X-ray studies we know the space group R3m and the six inequivalent coordinates are needed to generate all the other carbon positions. We found here evidence for the distortion of the molecule from sphericity. The number of inequivalent sp 2 carbons predicted by the theoretical [6] studies are in good agreement with our NMR results (see also Refs, [7, 8]).
4. CONCLUSION We have shown here that the [2 + 2] cycloaddition reactions predicted in polymerized C60 is clearly verified by our 13C MAS NMR measurements on the rhombohedric (2D) phase. For the orthorhombic (1D) phase, as mentioned above, a slight disagreement in the relative intensities of the sp 2 and sp 3 carbons between experimental and calculated values does not allow to check correctly the [2 + 2] cycloaddition structure. The static spectra indicate the freezing in of the C60
1647
molecules. MAS NMR measurements reveal a very long spin-lattice relaxation time of carbons, and the observed resonance positions and relative intensities are in good agreement with the expected behavior of sp 2 and spa carbons in both systems. Finally, in the rhombohedric phase, the splitting of the line at 146 ppm in five components around the position of pristine C6o indicates the deformation of the spherical shape of the C60 molecule which leads to the observed inequivalent carbon sites.
REFERENCES
1. Iwasa, Y. et al., Science, 1994, 264, 1570-1572. 2. B&houx, O., Ntifiez-Regueiro, M., Marques, L., Hodeau, J.-L., P&roux, M., In: Proceedings of the Materials Research Society, Boston, 1993, Abstract of Contributed Papers (Materials Research Society, Pittsburg, 1993) Abstract No. G2.9, p. 202. 3. N6fiez-Regueiro,M. et al., Phys. Rev. Lett., 1995, 74, 278. 4. Rao, M. et al., Science, 1993, 259, 955. 5. Rachdi, F., Reichenbach,J., Firlej, L., Bernier, P., Ribet, M., Aznar, R., Zimmer, G., Helme, M. and Mehring, M., Solid State Commun., 1993, 87, 547. 6. Xu, C.H. and Scuseria,G.E., Phys. Rev. Lett., 1995,74, 274. 7. Goze, C., Contribution h l'6tude du fuller~ne C6o: Etudes RMN et RPE, PhD Thesis, University of Montpellier, France, 1996. 8. Goze, C., Rachdi, F., Hajji, L., Nfifiez-Regueiro, M., Marques, L., Hodeau, J.L. and Mehring, M., Phys. Rev., 1996, B 54, 1.
PIh S0022-3697(97)00044-9
J. Phys. Chem Solids Vo158, No. 1I, pp. 1645-1647, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/97 $17.00 + 0.00
HIGH RESOLUTION 13C NMR STUDIES OF ONE- AND TWO-DIMENSIONAL POLYMERIZED C60 UNDER HIGH PRESSURE F. RACHDI a'*, C. GOZE a, L. HAJJI a, M. N01Z~EZ-REGUEIROb, L. MARQUES c, J.-L. HODEAU c and M. MEHRING d ~Groupede Dynamiquedes Phases Condensers,Universit~MontpellierII, PI. Eugrne Bataillon,34095 Montpellier,France bEPM-Matformag,CNRS, BP 166, 38042 Grenoble,France CLaboratoirede Cristallographie,CNRS, BP 166, 38042 Grenoble,France dPhysikalischesInstitut,UniversifiitStuttgart, Pfaffenwaldring57, D70550 Stuttgart, Germany Abstract--We report on ~3CNMR measurementsof orthorhombic one-dimensionaland rhombohedral twodimensional polymers of C60 obtained under high pressure. The obtained ~3C MAS NMR spectrum of the orthorhombicpolymershowstwo resonancesat 146 ppm and 73.5 ppm, and the one of the rhombohedralpolymer presents six isotropic lines at 149.1, 147.9, 145.2, 139.6, 134.8 and 73.5 ppm. The static distortionof the C6o moleculesinducedby the transformationunderpressuremustbe at the originof the observedinequivalentcarbons in both systems. The t3C NMR lineshapesimulationof the obtainedspectra are compatiblewith the suggested polymeric structureswhere the C60moleculesare connectedby [2 + 2] cycloadditions.© 1997 ElsevierScience Ltd.
Keywords: magnetism,structure 1. INTRODUCTION Bulk polymerized C60 phases obtained at high pressure and high temperature have been reported [1, 2]. At least three new phases have been synthesized and characterized [3]. The pressure, temperature and duration synthesis conditions are essential to obtain one of the rhombohedral, orthorhombic or tetragonal phases. As with other non-saturated organic molecules similar reactions as in photopolymerization [4] were expected to appear in solid C60 under pressure and temperature. The cycloaddition [2 + 2] mechanism between two adjacent C60 molecules was proposed to be at the origin of the observed lattice contraction and increase of disorder in these systems. In this paper we report on 13C MAS NMR measurements on orthorhombic onedimensional o-lD and rhombohedral two-dimensional rh-2D polymerized phases. The observed inequivalent carbons in both studied polymers are discussed in terms of the bonding nature between adjacent C60 molecules and distortion from the spherical shape of the molecules. Also, we discuss the molecular dynamics in these systems.
2. EXPERIMENT We performed 13C NMR, measurements at ambient temperature on o-lD and rh-2D polymerized fullerene C60. About 30 mg of each sample have been used. The pristine C60 starting materials were 10% 13C enriched in order to increase the NMR signal and the samples were *Correspondingauthor.
kept in air. NMR measurements were performed using 200 and 400 MHz Bruker NMR spectrometers with ~3C NMR Larmor frequencies of 50.2 and 100.4 MHz, respectively. The spectra were obtained by a one-pulse sequence 7r/2-acquisition with repetition time of 200 and 900 s. The samples were obtained by compressing the enriched C6o (encapsulated in a Pt container) in a belt apparatus for periods not exceeding 2 h. The o- 1D sample was obtained at 4 GPa and 300°C, the rh-2D sample was made at 5 Gpa and 700°C. The X-ray diffraction patterns of the polymerized fullerene are similar to those published earlier [3]. 3. RESULTSAND DISCUSSION Fig. 1 shows the ~3C MAS NMR spectra of the polymerized C60 o-lD phase obtained under pressure with different repetition times of 200 and 900 s. Both spectra present two isotropic lines at positions 146 and 73.5 ppm, and two sidebands corresponding to the line at 146 ppm, which appear at 266 and 26 ppm. The relative intensities of the isotropic lines including the sidebands are reported in Table 1. The observed broadening of the line around 146 ppm could be related to a continuous distribution of C-C distances due to a partial disorder in the chain arrangement that can be partly curved or staggered. Fig. 2 shows the ~3C MAS NMR spectra of the rh-2D polymerized phase with repetition times of 200 and 900 s. Again, we observe two isotropic lines at the same positions as the o-lD phase. The line at position 146 ppm for the rh-2D phase exhibits a structure. A
1645
1646
F. RACHDI et al. sP 2
. ~
.i,~;t~5~.
~f:~'%
oo,
.A.__
.J'-,_
9
/~ ooos
I 300
200
'
I - 250
'
I 200
'
I 150
'
I 100
'
l 50
~
I 0
ppm Fig. 1. 13CMAS NMR spectraof the orthorhombic(ID) onedimensional phases with a spinning rate of 12 kHz and repetition times of 200 and 900 s. The stars indicate the sidebands of the line at 146 ppm. Inset shows the structural model of the orthorhombic 1D phase. deconvolution of this group of lines gives us six isotropic lines at the following positions: 134.8, 139.6, 142.8, 145.2, 147.9 and 149.1 ppm. We suspect the line at 142.8 - 1 ppm is from unreacted pristine C6o and evaluate its contribution to be about 5% of the total intensity. The relative intensities of the others five isotropic lines are reported in Table 2. Using a lower spinning rate (spectra not shown here), all the sidebands which mimic the envelope of the static spectrum show an extent of anisotropy of about 300 ppm in both phases. As expected in such frozen systems of polymerized C6o, we found long spin-lattice relaxation time T~ and very broad static powder spectra. As it has been shown for the A6Cro phase, with A = K, Rb and Cs [5], the large extent of anisotropy of about 300 ppm indicates the freezing in of the C60 molecules. The latter can be blocked by electrostatic forces as in m6Cr0 or by bonding between the C60 molecule and its neighbour C60 molecules in polymer configurations. The 13C MAS NMR spectra reveal inequivalent carbons sites which can be interpreted as sp 2 carbons around the 146-ppm line position and sp 3 bonding carbons between C6o molecules at 73.5 ppm. The disagreement in the relative intensities of the sp 2 and sp 3 carbons, calculated from structural studies and obtained from experiment, with a repetition time of 200 s comes from the very long TI of the sp 3 carbons (see Tables 1 and 2). The intensity of the sp 2 carbon is always overestimated in comparison with the sp 3 carbon intensity due to the Table 1. Relative intensities of the isotropic lines including the sidebands in the o-lD phase ppm
146 73.5
Spectrum at:
calculated from Ref. [6]
900 s
200 s
54.5 5.5
56.9 3.4
56 4
I 300
s
"sp3 -~-fi~,.,~';~.~'/
.
I 250
I 200
i 150 ppm
i 1O0
I 50
I 0
Fig. 2. '3C MAS NMR spectra of rhombohedral (2D) twodimensional phase with a spinning rate of 12 kHz and repetition times of 200 and 900 s. Inset shows the structural model of this phase. chemical shift anisotropy relaxation mechanism which is usually more efficient for sp 2 carbons. Such behavior is visible in the ~3C MAS NMR spectrum of the rh-2D polymer phase obtained with a repetition time of 900 s which shows an increase of the sp 3 carbon resonance intensity compared with the one obtained with a 200-s repetition time (see Fig. 2). But here, again, the intensity of all sp 3 carbons is not totally covered because of the long Tl; nevertheless, the obtained line intensities are very close to what is expected from the cycloaddition model. If we suppose that the NMR signal is affected in the same manner in the o - l D and rh-2D phases, by the short repetition time of 200 s, we can compare the relative intensity ratios in the o- 1D and rh-2D phases. By doing so we see that the sp 3 ratio is smaller in the o-lD, as expected, due to the decrease of the number of intermolecular [2 -4- 2] bonds. As can be seen in Table I, a disagreement between the experimental and calculated relative intensities of sp 2 and sp 3 carbons in the o-lD polymer is obtained even with a 900-s repetition time. This could be related to a small mixed amount of other phases present in the o - l D sample. In the rhombohedric phase, it has been shown [3] that the fullerene molecule are no longer spheres, but are deformed by the new bonding and are being oblated perpendicular to the plane of polymerization. It is tempting to interpret the splitting of the line at 146 ppm in this phase as coming Table 2. Relative intensities of the isotropic lines including the sidebands in the rh-2D phase ppm
149.1 147.9 145.2 139.6 134.8 73.5
Spectrum at 990s
49 11
Spectrum at 200s 13.9 16.1 13.3 5.4 5.2 6.1
53.9
calculated from Ref. [6] 12 12 12 6 6 12
48
High resolution ~3CNMR studies of one- and two-dimensionalpolymerizedC6o under high pressure from this distortion from sphericity of the C60 molecule. On the same C60 molecule we can expect inequivalent carbons due to different neighbouring. The carbons involved here are all sp 2 hybridized and they have three neighbours which can be at slightly different positions due to the non-sphericity. The small changes in the angles and the distances could be at the origin of the five inequivalent carbons observed. From X-ray studies we know the space group R3m and the six inequivalent coordinates are needed to generate all the other carbon positions. We found here evidence for the distortion of the molecule from sphericity. The number of inequivalent sp 2 carbons predicted by the theoretical [6] studies are in good agreement with our NMR results (see also Refs, [7, 8]).
4. CONCLUSION We have shown here that the [2 + 2] cycloaddition reactions predicted in polymerized C60 is clearly verified by our 13C MAS NMR measurements on the rhombohedric (2D) phase. For the orthorhombic (1D) phase, as mentioned above, a slight disagreement in the relative intensities of the sp 2 and sp 3 carbons between experimental and calculated values does not allow to check correctly the [2 + 2] cycloaddition structure. The static spectra indicate the freezing in of the C60
1647
molecules. MAS NMR measurements reveal a very long spin-lattice relaxation time of carbons, and the observed resonance positions and relative intensities are in good agreement with the expected behavior of sp 2 and spa carbons in both systems. Finally, in the rhombohedric phase, the splitting of the line at 146 ppm in five components around the position of pristine C6o indicates the deformation of the spherical shape of the C60 molecule which leads to the observed inequivalent carbon sites.
REFERENCES
1. Iwasa, Y. et al., Science, 1994, 264, 1570-1572. 2. B&houx, O., Ntifiez-Regueiro, M., Marques, L., Hodeau, J.-L., P&roux, M., In: Proceedings of the Materials Research Society, Boston, 1993, Abstract of Contributed Papers (Materials Research Society, Pittsburg, 1993) Abstract No. G2.9, p. 202. 3. N6fiez-Regueiro,M. et al., Phys. Rev. Lett., 1995, 74, 278. 4. Rao, M. et al., Science, 1993, 259, 955. 5. Rachdi, F., Reichenbach,J., Firlej, L., Bernier, P., Ribet, M., Aznar, R., Zimmer, G., Helme, M. and Mehring, M., Solid State Commun., 1993, 87, 547. 6. Xu, C.H. and Scuseria,G.E., Phys. Rev. Lett., 1995,74, 274. 7. Goze, C., Contribution h l'6tude du fuller~ne C6o: Etudes RMN et RPE, PhD Thesis, University of Montpellier, France, 1996. 8. Goze, C., Rachdi, F., Hajji, L., Nfifiez-Regueiro, M., Marques, L., Hodeau, J.L. and Mehring, M., Phys. Rev., 1996, B 54, 1.