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Deformation of guest molecules in the C60 based mixed crystals
Vibrational Spectroscopy 16 Ž1998. 31–34
Deformation of guest molecules in the C 60 based mixed crystals K.T. Antonova a
a,)
, M.K. Marchewka b, E. Kowalska c , P. Byszewski
c,d
Institute of Solid State Physics, Bulgarian Academy of Sciences, blÕd. Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria Institute of Low Temperature and Structural Research, Polish Academy of Sciences, ul. Okolna 2, Wroclaw, Poland c Institute of Vacuum Technology, ul. Dluga 44 r 5O, 00-241 Warsaw, Poland d Institute of Physics PAS, al. Lotnikow 32 r 46, 02-668 Warsaw, Poland
b
Received 13 June 1997; accepted 23 October 1997
Abstract Methylnaphthalenes form mixed crystals with C 60 . The molecules in these crystals are so densely packed, that the flat methylnaphthalene molecules undergo a deformation. The deformation is observed by the shift of the characteristic infrared and Raman active modes. However the modes of the rigid C 60 cage remain at the same positions, indicating that the C 60 molecules are not significantly deformed. q 1998 Elsevier Science B.V. Keywords: Infrared modes; Raman modes; Methylnaphthalenes
1. Introduction Weak van der Waals intermolecular interaction between fullerenes allows investigations of the host–guest type systems. The deviation from the uniform distribution of electrons in fullerenes is responsible for the orientational ordering, phase transition in the crystal and the fullerenes’ hindered rotation below 260 K w1x. The strongest interaction occurs between electron rich double bonds of one molecule with the pentagon of the other. The interaction with the guest molecules can be of a different nature and can be responsible for various phenomena. Rigid skeletons of some molecules having strong C–C or C–H bonds prevent strong interaction with C 60 . The resulting material consists of molecules maintaining their individual properties. However, if a ) Corresponding author. Fax: q359-2-757032; e-mail: [email protected].
strong coupling of guest and host pi-electrons occurs, it can cause a deformation of molecules and can lead to a shift of the electronic levels. Aromatic hydrocarbons like benzene, toluene and methylnaphthalene ŽMNAP. are good solvents of fullerenes and the interaction between solvent–solute molecules leads to modification of both species w2x. The relatively strong interactions between dimethylnaphthalene ŽDMNAP. molecules and C 60 have been observed by the modifications of the UV-VIS absorption spectra in the solution w3x. In this work we continue the investigation of the intermolecular interactions between C 60 and different mono- and di-substituted methylnaphthalene molecules by means of IR and Raman spectroscopy. The planar molecules of methylnaphthalene, almost equal in size to the diameter of C 60 molecules, easily enter the C 60 based lattice and readily form mixed crystals with fullerenes. However, the presence of two kinds of molecules in the lattice results in the deformation of
0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 4 - 2 0 3 1 Ž 9 7 . 0 0 0 4 5 - 3
32
K.T. AntonoÕa et al.r Vibrational Spectroscopy 16 (1998) 31–34
their charge distribution w4x. This interaction can cause lowering of the symmetry of the molecules and relaxation of the selection rules of the optical activity.
2. Experimental It has been previously reported that the solubility of C 60 in ŽD.MNAP is very high and reaches 33–36 mgrml w2x. When the solvent is evaporated at room temperature, the dried product still contains the solvent molecules and does not exhibit any crystallisation, in contrast to fullerene crystalline powders obtained from other solvates e.g. from toluene. Therefore, we looked further into the system trying to prepare crystalline powders reversing the role of the solvent and solute. In those experiments we tried to dissolve limited amounts of ŽD.MNAPs in C 60 crystalline powder. The crystalline powders were prepared by grinding chromatographically purified C 60 Ž100–150 mg. with steadily increasing amounts of ŽD.MNAPs in agate mortar ŽMerck, for synthesis grade. and checking the product by the power X-ray diffraction method w5x. It was proven, in the course of experiments, that a small admixture of ŽD.MNAP distorted the X-ray pattern, where on the familiar diffraction lines, characteristic for the C 60 in the fcc structure, new broad diffraction lines could be distinguished. With the increased amount of ŽD.MNAP, the fcc reflections disappeared altogether but the new ones became narrower. It was not possible to determine the composition of the powder directly from these experiments because of the partial evaporation of ŽD.MNAP during preparation. The composition of the samples exhibiting the best diffraction pattern, i.e. the narrowest set of the new lines, was determined from thermogravimetry experiments Žusing the DuPont 1090 Thermoanalyser. w5x. It was found by the mass loss during annealing that the samples prepared by this method exhibited well determined composition of C 60 :1MNAPs 1:1 molar ratio and C 60 :ŽD.MNAPs 1:2 molar ratio. We were never able to prepare the C 60 :ŽD.MNAP samples of the composition 1:1 using this method. However, samples of this composition were prepared by partial decomposition of the C 60 :ŽD.MNAPs 1:2 samples. The 1:2 samples decomposed in two steps while
annealed. In the first step at low temperature, approximately 80–1008C, one ŽD.MNAP molecule was released, at a high temperature of approximately 120–1608C the other molecule was freed and the C 60 powder adopted the cubic lattice. The X-ray diffraction showed that the powders exhibited a relatively good crystalline structure with a unit cell of a triclinic symmetry Že.g. in the case of C 60 :1,4DMNAP s 1:2 the lattice parameters were found to be: a s 1.059 nm, b s 1.391 nm, c s 1.541 nm, a s 97.98, b s 81.98 and g s 98.98 w5x.. FT IR and Raman spectra were measured using a BRUKER IFS 88 instrument. The polycrystalline powders were put between KBr wafers. The resolution was 0.5 cmy1 and the signalrnoise ratio was established by 100 scans.
3. Results and discussion In Fig. 1 the spectra of pristine C 60 , of pure 1,4DMNAP and of the crystal C 60 :1,4DMNAPs 1:2 are represented, respectively, by curves 1, 2 and 3. In Fig. 2, the FT Raman spectra of the same samples are given. Looking at the IR spectrum of the C 60 :1,4DMNAPs 1:2 ŽFig. 1, curve 3. one can see a very rich set of bands which are assigned to the molecular vibrational modes of C 60 and 1,4DMNAP. The main absorption bands of the pristine C 60 vibrations of T1u symmetry are at their usual places: 1427, 1183, 577 and 526 cmy1 but the pentagon pinch mode at 1427 cmy1 is strongly affected. It is seen as a weak band among three other weak bands at 1456, 1438 and 1419 cmy1 . We are unable to say whether a threefold degeneracy of the pinch mode occurred because the band overlaps with a set of strong bands of the methyl group asymmetric deformation vibrations Žsee curve 2.. They are also Raman active ŽFig. 2, curve 2. but when situated in the interstitial sites between the C 60 molecules the methyl group C–H vibrations do not show these Raman bands ŽFig. 2, curve 3.. We ascribe the modification of the bands in this spectral region from strong to weak intensity to the strong intermolecular interaction. Further, a doublet of the methyl group symmetric deformation vibrations is seen at 1390 and 1374 cmy1 in Fig. 1, curve 3. In comparison with curve 2 both bands are
K.T. AntonoÕa et al.r Vibrational Spectroscopy 16 (1998) 31–34
33
with the C 60 molecules almost destroys the Raman band ŽFig. 2, curve 3. and downshifts IR band to 942 cmy1 ŽFig. 1, curve 3.. The modifications Žlowering the intensity and shift of the frequency. of the in-plane vibrational modes of 1,4DMNAP when surrounded by C 60 molecules demonstrate a significant deformation of the DMNAP planar structure. Such deformations have been obtained as well by the computational optimization of the crystal structure of C 60 :1,4DMNAPs 1:2 w5x. The IR bands of the C–H out-of-plane vibrations remain strong but are downshifted with respect to pure 1,4DMNAP ŽFig. 1, curve 2 and 3.: from 823 to 815 cmy1 and from 753 to 746 cmy1 . The band at 695 cmy1 which is both IR and Raman active loses in its intensity. The IR breathing modes of the relatively rigid C 60 molecules are not disturbed by the organic molecules
Fig. 1. FT IR spectra of pristine C 60 Žcurve 1., of pure 1,4 DMNAP Žcurve 2. and of C 60 :1,4DMNAPs1:2 Žcurve 3..
deformed. The broadening can be attributed to the asymmetric surrounding of the methyl group by the neighboring C 60 molecules in the crystalline lattice. These vibrations are Raman active also Ž1364 cmy1 . and there is a significant difference in the corresponding band intensity in curve 3 and curve 2 probably because of the strong deformation of the symmetry of the methyl group. The bands corresponding to the C–H in-plane bending vibrations are seen at 1155 and 1142 cmy1 with a lowered intensity with respect to these bands for the 1,4DMNAP ŽFig. 1, curves 2 and 3.. The ring breathing mode of the C–C bond stretching in phase vibration in benzene is only Raman active Ž1023 cmy1 . w6x. The addition of the second ring in naphthalene makes this mode IR active too Ž959 cmy1 . w6x. The two extra methyl groups make this band for 1,4DMNAP complicated and some structure is seen ŽFig. 1, curve 3.. The interaction
Fig. 2. FT Raman spectra of pristine C 60 Žcurve 1., of pure 1,4DMNAP Žcurve 2. and of C 60 :1,4DMNAPs1:2 Žcurve 3..
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K.T. AntonoÕa et al.r Vibrational Spectroscopy 16 (1998) 31–34
but the C–CH 3 umbrella’s symmetric breathing vibrations of the 1,4DMNAP lose their symmetry in the surrounding of C 60 molecules. As a result, the strong 1,4DMNAP Raman mode at 538 cmy1 in Fig. 2, curve 2, became a weak band at 537 cmy1 Žcurve 3. and the relatively strong IR band at 563 cmy1 in Fig. 1, curve 2 is seen as a shoulder of the 577 cmy1 C 60 breathing band Žcurve 3.. The IR bands of ring C–C bonds out-of-plane deformations at 408, 450 and 484 cmy1 in Fig. 1, curve 2 lose in their intensity but are clearly seen in curve 3. In Fig. 2 the Raman active mode is at 356 cmy1 . We do not represent the IR and Raman spectra of the samples with other ŽD.MNAPs because the modifications of the bands are similar to those already discussed.
4. Conclusion The materials investigated here consist of strongly interacting molecules forming mixed crystals. The spectra show that relatively strong intermolecular interaction causes a deformation of the charge distribution in both components of these materials w5x. However, because of the stiffness of the fullerene’s cage, only the skeletons of DMNAP molecules expe-
rience deformation which could be observed as shifted IR and Raman active modes. We would like to emphasize the role of methyl groups with their vibration frequencies in the middle IR spectral region facilitating investigation of the intermolecular interactions with fullerenes.
Acknowledgements This work was partly financed by the Bulgarian National Foundation for Scientific Investigations, under No. F-529.
References w1x T. Atake, T. Tanaka, H. Kawaji, K. Kikuchi, K. Saito, S. Suzuki, I. Ikemoto, Y. Achiba, Phys. C 185–189 Ž1991. 427. w2x R.S. Ruoff, J. Phys. Chem. 97 Ž1993. 3379. w3x P. Byszewski, E. Kowalska, J. Radomska, R. Diduszko, in: K. Sangwal, E. Jartych, J.M. Olchowik ŽEds.., Proceedings of the Intermolecular Interactions in Matter, Politechnica Lubelska, Lublin, 1995, p. 98. w4x K. Harigaya, S. Abe, Phys. Rev. B 49 Ž1994. 16746. w5x P. Byszewski, E. Kowalska, R. Diduszko, R. Swietlik, H. Lange, J. Mieczkowski, J. Chem. Phys., in press. w6x B. Schrader, W. Meier ŽEds.., DMS RamanrIR Atlas of Organic Compounds, Verlag Chemie, Weinheim, 1977.
Deformation of guest molecules in the C 60 based mixed crystals K.T. Antonova a
a,)
, M.K. Marchewka b, E. Kowalska c , P. Byszewski
c,d
Institute of Solid State Physics, Bulgarian Academy of Sciences, blÕd. Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria Institute of Low Temperature and Structural Research, Polish Academy of Sciences, ul. Okolna 2, Wroclaw, Poland c Institute of Vacuum Technology, ul. Dluga 44 r 5O, 00-241 Warsaw, Poland d Institute of Physics PAS, al. Lotnikow 32 r 46, 02-668 Warsaw, Poland
b
Received 13 June 1997; accepted 23 October 1997
Abstract Methylnaphthalenes form mixed crystals with C 60 . The molecules in these crystals are so densely packed, that the flat methylnaphthalene molecules undergo a deformation. The deformation is observed by the shift of the characteristic infrared and Raman active modes. However the modes of the rigid C 60 cage remain at the same positions, indicating that the C 60 molecules are not significantly deformed. q 1998 Elsevier Science B.V. Keywords: Infrared modes; Raman modes; Methylnaphthalenes
1. Introduction Weak van der Waals intermolecular interaction between fullerenes allows investigations of the host–guest type systems. The deviation from the uniform distribution of electrons in fullerenes is responsible for the orientational ordering, phase transition in the crystal and the fullerenes’ hindered rotation below 260 K w1x. The strongest interaction occurs between electron rich double bonds of one molecule with the pentagon of the other. The interaction with the guest molecules can be of a different nature and can be responsible for various phenomena. Rigid skeletons of some molecules having strong C–C or C–H bonds prevent strong interaction with C 60 . The resulting material consists of molecules maintaining their individual properties. However, if a ) Corresponding author. Fax: q359-2-757032; e-mail: [email protected].
strong coupling of guest and host pi-electrons occurs, it can cause a deformation of molecules and can lead to a shift of the electronic levels. Aromatic hydrocarbons like benzene, toluene and methylnaphthalene ŽMNAP. are good solvents of fullerenes and the interaction between solvent–solute molecules leads to modification of both species w2x. The relatively strong interactions between dimethylnaphthalene ŽDMNAP. molecules and C 60 have been observed by the modifications of the UV-VIS absorption spectra in the solution w3x. In this work we continue the investigation of the intermolecular interactions between C 60 and different mono- and di-substituted methylnaphthalene molecules by means of IR and Raman spectroscopy. The planar molecules of methylnaphthalene, almost equal in size to the diameter of C 60 molecules, easily enter the C 60 based lattice and readily form mixed crystals with fullerenes. However, the presence of two kinds of molecules in the lattice results in the deformation of
0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 4 - 2 0 3 1 Ž 9 7 . 0 0 0 4 5 - 3
32
K.T. AntonoÕa et al.r Vibrational Spectroscopy 16 (1998) 31–34
their charge distribution w4x. This interaction can cause lowering of the symmetry of the molecules and relaxation of the selection rules of the optical activity.
2. Experimental It has been previously reported that the solubility of C 60 in ŽD.MNAP is very high and reaches 33–36 mgrml w2x. When the solvent is evaporated at room temperature, the dried product still contains the solvent molecules and does not exhibit any crystallisation, in contrast to fullerene crystalline powders obtained from other solvates e.g. from toluene. Therefore, we looked further into the system trying to prepare crystalline powders reversing the role of the solvent and solute. In those experiments we tried to dissolve limited amounts of ŽD.MNAPs in C 60 crystalline powder. The crystalline powders were prepared by grinding chromatographically purified C 60 Ž100–150 mg. with steadily increasing amounts of ŽD.MNAPs in agate mortar ŽMerck, for synthesis grade. and checking the product by the power X-ray diffraction method w5x. It was proven, in the course of experiments, that a small admixture of ŽD.MNAP distorted the X-ray pattern, where on the familiar diffraction lines, characteristic for the C 60 in the fcc structure, new broad diffraction lines could be distinguished. With the increased amount of ŽD.MNAP, the fcc reflections disappeared altogether but the new ones became narrower. It was not possible to determine the composition of the powder directly from these experiments because of the partial evaporation of ŽD.MNAP during preparation. The composition of the samples exhibiting the best diffraction pattern, i.e. the narrowest set of the new lines, was determined from thermogravimetry experiments Žusing the DuPont 1090 Thermoanalyser. w5x. It was found by the mass loss during annealing that the samples prepared by this method exhibited well determined composition of C 60 :1MNAPs 1:1 molar ratio and C 60 :ŽD.MNAPs 1:2 molar ratio. We were never able to prepare the C 60 :ŽD.MNAP samples of the composition 1:1 using this method. However, samples of this composition were prepared by partial decomposition of the C 60 :ŽD.MNAPs 1:2 samples. The 1:2 samples decomposed in two steps while
annealed. In the first step at low temperature, approximately 80–1008C, one ŽD.MNAP molecule was released, at a high temperature of approximately 120–1608C the other molecule was freed and the C 60 powder adopted the cubic lattice. The X-ray diffraction showed that the powders exhibited a relatively good crystalline structure with a unit cell of a triclinic symmetry Že.g. in the case of C 60 :1,4DMNAP s 1:2 the lattice parameters were found to be: a s 1.059 nm, b s 1.391 nm, c s 1.541 nm, a s 97.98, b s 81.98 and g s 98.98 w5x.. FT IR and Raman spectra were measured using a BRUKER IFS 88 instrument. The polycrystalline powders were put between KBr wafers. The resolution was 0.5 cmy1 and the signalrnoise ratio was established by 100 scans.
3. Results and discussion In Fig. 1 the spectra of pristine C 60 , of pure 1,4DMNAP and of the crystal C 60 :1,4DMNAPs 1:2 are represented, respectively, by curves 1, 2 and 3. In Fig. 2, the FT Raman spectra of the same samples are given. Looking at the IR spectrum of the C 60 :1,4DMNAPs 1:2 ŽFig. 1, curve 3. one can see a very rich set of bands which are assigned to the molecular vibrational modes of C 60 and 1,4DMNAP. The main absorption bands of the pristine C 60 vibrations of T1u symmetry are at their usual places: 1427, 1183, 577 and 526 cmy1 but the pentagon pinch mode at 1427 cmy1 is strongly affected. It is seen as a weak band among three other weak bands at 1456, 1438 and 1419 cmy1 . We are unable to say whether a threefold degeneracy of the pinch mode occurred because the band overlaps with a set of strong bands of the methyl group asymmetric deformation vibrations Žsee curve 2.. They are also Raman active ŽFig. 2, curve 2. but when situated in the interstitial sites between the C 60 molecules the methyl group C–H vibrations do not show these Raman bands ŽFig. 2, curve 3.. We ascribe the modification of the bands in this spectral region from strong to weak intensity to the strong intermolecular interaction. Further, a doublet of the methyl group symmetric deformation vibrations is seen at 1390 and 1374 cmy1 in Fig. 1, curve 3. In comparison with curve 2 both bands are
K.T. AntonoÕa et al.r Vibrational Spectroscopy 16 (1998) 31–34
33
with the C 60 molecules almost destroys the Raman band ŽFig. 2, curve 3. and downshifts IR band to 942 cmy1 ŽFig. 1, curve 3.. The modifications Žlowering the intensity and shift of the frequency. of the in-plane vibrational modes of 1,4DMNAP when surrounded by C 60 molecules demonstrate a significant deformation of the DMNAP planar structure. Such deformations have been obtained as well by the computational optimization of the crystal structure of C 60 :1,4DMNAPs 1:2 w5x. The IR bands of the C–H out-of-plane vibrations remain strong but are downshifted with respect to pure 1,4DMNAP ŽFig. 1, curve 2 and 3.: from 823 to 815 cmy1 and from 753 to 746 cmy1 . The band at 695 cmy1 which is both IR and Raman active loses in its intensity. The IR breathing modes of the relatively rigid C 60 molecules are not disturbed by the organic molecules
Fig. 1. FT IR spectra of pristine C 60 Žcurve 1., of pure 1,4 DMNAP Žcurve 2. and of C 60 :1,4DMNAPs1:2 Žcurve 3..
deformed. The broadening can be attributed to the asymmetric surrounding of the methyl group by the neighboring C 60 molecules in the crystalline lattice. These vibrations are Raman active also Ž1364 cmy1 . and there is a significant difference in the corresponding band intensity in curve 3 and curve 2 probably because of the strong deformation of the symmetry of the methyl group. The bands corresponding to the C–H in-plane bending vibrations are seen at 1155 and 1142 cmy1 with a lowered intensity with respect to these bands for the 1,4DMNAP ŽFig. 1, curves 2 and 3.. The ring breathing mode of the C–C bond stretching in phase vibration in benzene is only Raman active Ž1023 cmy1 . w6x. The addition of the second ring in naphthalene makes this mode IR active too Ž959 cmy1 . w6x. The two extra methyl groups make this band for 1,4DMNAP complicated and some structure is seen ŽFig. 1, curve 3.. The interaction
Fig. 2. FT Raman spectra of pristine C 60 Žcurve 1., of pure 1,4DMNAP Žcurve 2. and of C 60 :1,4DMNAPs1:2 Žcurve 3..
34
K.T. AntonoÕa et al.r Vibrational Spectroscopy 16 (1998) 31–34
but the C–CH 3 umbrella’s symmetric breathing vibrations of the 1,4DMNAP lose their symmetry in the surrounding of C 60 molecules. As a result, the strong 1,4DMNAP Raman mode at 538 cmy1 in Fig. 2, curve 2, became a weak band at 537 cmy1 Žcurve 3. and the relatively strong IR band at 563 cmy1 in Fig. 1, curve 2 is seen as a shoulder of the 577 cmy1 C 60 breathing band Žcurve 3.. The IR bands of ring C–C bonds out-of-plane deformations at 408, 450 and 484 cmy1 in Fig. 1, curve 2 lose in their intensity but are clearly seen in curve 3. In Fig. 2 the Raman active mode is at 356 cmy1 . We do not represent the IR and Raman spectra of the samples with other ŽD.MNAPs because the modifications of the bands are similar to those already discussed.
4. Conclusion The materials investigated here consist of strongly interacting molecules forming mixed crystals. The spectra show that relatively strong intermolecular interaction causes a deformation of the charge distribution in both components of these materials w5x. However, because of the stiffness of the fullerene’s cage, only the skeletons of DMNAP molecules expe-
rience deformation which could be observed as shifted IR and Raman active modes. We would like to emphasize the role of methyl groups with their vibration frequencies in the middle IR spectral region facilitating investigation of the intermolecular interactions with fullerenes.
Acknowledgements This work was partly financed by the Bulgarian National Foundation for Scientific Investigations, under No. F-529.
References w1x T. Atake, T. Tanaka, H. Kawaji, K. Kikuchi, K. Saito, S. Suzuki, I. Ikemoto, Y. Achiba, Phys. C 185–189 Ž1991. 427. w2x R.S. Ruoff, J. Phys. Chem. 97 Ž1993. 3379. w3x P. Byszewski, E. Kowalska, J. Radomska, R. Diduszko, in: K. Sangwal, E. Jartych, J.M. Olchowik ŽEds.., Proceedings of the Intermolecular Interactions in Matter, Politechnica Lubelska, Lublin, 1995, p. 98. w4x K. Harigaya, S. Abe, Phys. Rev. B 49 Ž1994. 16746. w5x P. Byszewski, E. Kowalska, R. Diduszko, R. Swietlik, H. Lange, J. Mieczkowski, J. Chem. Phys., in press. w6x B. Schrader, W. Meier ŽEds.., DMS RamanrIR Atlas of Organic Compounds, Verlag Chemie, Weinheim, 1977.