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Comment on “Genetic relationship between intrinsic Raman and infrared fundamental vibrations of the C60 and C70 fullerenes”
Journal of Molecular Structure 407 (1997) 81–83
Comment on ‘‘Genetic relationship between intrinsic Raman and infrared fundamental vibrations of the C 60 and C 70 fullerenes’’ [J. Mol. Struct. 378 (1996) 147] Sean H. Gallagher a, Robert S. Armstrong a,*, Robert D. Bolskar b, Peter A. Lay a, Christopher A. Reed b b
a School of Chemistry, University of Sydney, Sydney, New South Wales, 2006, Australia Department of Chemistry, University of Southern California, Los Angeles, CA 90089-0744, USA
Received 8 August 1996; revised 18 November 1996; accepted 19 November 1996
Abstract Brockner and Menzel [J. Mol. Struct. 378 (1996) 147] have recently presented the view that the effects of resonance in the Raman spectrum of C 70 are unclear and that the anomalies in polarization data have not been considered in the literature. In fact, three papers have addressed these issues, removing inconsistencies in the assignments of the vibrational modes of C 70. In the work on C 60, Brockner and Menzel [ibid] assign the H g(2) mode to a Raman band at 485 cm −1. This contradicts earlier assignments to a band at 430 cm −1 and a recent reaffirmation at 427 cm −1 via resonance Raman spectroscopy. Additional features in the Raman spectrum of C 60, proposed by Brockner and Menzel to arise from isomers of C 60, are better explained by 13 C splitting and the symmetry-lowering effects of solute–solvent interactions. q 1997 Elsevier Science B.V. Keywords: C 60 fullerene; C 70 fullerene; Vibration; Resonance Raman spectroscopy
1. Introduction
2. Experimental
In a recent paper in this journal [1], Brockner and Menzel report the correlation of vibrational modes of C 60 with those of C 70 and offer a complete band assignment for these modes based on their earlier FT–Raman spectroscopic results [2]. While we agree with most of the assignments, the purpose of this comment is to emphasize that the authors did not consider all relevant work and as a result, some of their claims are not novel and others have more likely alternative explanations.
FT–Raman spectra were recorded on a Bruker RFS 100 spectrometer with 1064-nm Nd:YAG laser excitation (200 mW) using a 1 cm pathlength cuvette. The spectra were recorded (1024 scans) at 1.5 cm −1 resolution for C 60 (5 mM) in CS 2 solvent.
* Corresponding author. Tel.: +61-2-93513104; fax: +61-293513329; e-mail: [email protected].
3. Results and discussion Brockner and Menzel [1] are quite correct in highlighting the inconsistencies amongst the literature vibrational spectra and assignments published for
0022-2860/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S 0 02 2- 2 86 0 (9 6 )0 9 72 5 -6
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S.H. Gallagher et al./Journal of Molecular Structure 407 (1997) 81–83
C 70 prior to 1995. They cite reasons such as an ‘‘unclear influence by visible laser light excitation’’, presumably for the Raman spectra, and a ‘‘disregard for spectroscopy principles and thoughtless treatment of experimental data’’ with respect to polarization data. In early 1995, we reported the resonance Raman spectrum of C 70 in benzene solution using a range of visible laser excitations [3]. These results clearly revealed the manner in which the intensity of the 25 most intense Raman bands of C 70 depended upon the resonance conditions and an explanation of the enhancement mechanisms was given. However, this was not cited in their paper. In addition, we [3] and others [4,5] have reported and discussed the problems associated with the published polarization data on C 70, but these papers were also not cited. The lack of recognition of this literature led to outdated assertions on the Raman spectra of C 70 in the paper of Brockner and Menzel [1]. In their analysis of the Raman spectrum of C 60, Brockner and Menzel [1] propose the assignment of the H g(2) mode results in the band at 485 cm −1. This is based on their earlier analysis of the FT–Raman spectrum and on PM3 calculations [2] but contradicts the accepted 430/435 cm −1 assignment as discussed below. Part of the justification for their assignment lies in the absence of a band at ca. 430 cm −1 in the FT–Raman spectrum of C 60 in CS 2 solution and yet, the authors did not report the presence of a band at 485 cm −1 either [2]. We have performed experiments under the same conditions and the results are consistent with those in Ref. [2]. We believe that a combination of the weak intensity of the band at ca. 430 cm −1 and the coincidence of a CS 2 solvent band at ca. 400 cm −1 precludes its observation. Brockner and Menzel have not considered the significance of the reduction of symmetry that arises from solvent effects on the resonance Raman spectra of C 60, communicated by us in 1994 [6]. Solute–solvent interactions were shown to remove the degeneracy of the band due to the H g(7) mode at 1421 cm −1 in toluene solution. Subsequent to this, we have established that the symmetry-lowering effects of solvents (including CS 2) allow many bands due to IR–Raman-silent modes of C 60 to become Raman-activated and resonance enhanced [7]. In addition, these results clearly show an enhancement (resonance Raman) of intensity for a band at 427 cm −1, but no spectroscopic evidence for a
band at 485 cm −1. The intensity behaviour of this band is explained in terms of a B-term scattering mechanism, consistent with group theoretical requirements for a mode of H g symmetry [7]. Thus, we find no support for the assignment of the H g(2) mode to a band at 485 cm −1 and we reaffirm the 430/435 cm −1 assignment. Finally, we make a comment on the proposed existence of isomeric forms of C 60 [1,2]. The authors invoke the presence of isomers to explain the ‘‘C 60 surplus Raman frequencies’’. In their earlier paper, they cited that in addition to the I h form of C 60, there are 70 isomers of lower symmetry that are theoretically possible [8] and some (C2v , D 2d, D 2h, etc.) are calculated to be only a few electron volts less stable than the icosahedral form of C 60 [9]. Brockner and Menzel suggest that one or more of these isomers exists in their C 60 sample and that these give rise to the extra Raman features [1,2]. Assuming the presence of C 60 isomers, the observed intensity of some of the unexplained features in the FT–Raman spectrum of C 60 would imply that only a few isomers exist and that their abundance is significant. If this were really the case, other evidence for the existence of these isomers should exist in the IR- or 13C-NMR spectra of C 60 samples. This is not the case. While other C 60 isomers may putatively exist, we believe that the ‘‘surplus Raman frequencies’’ of C 60 are better explained by the symmetry-lowering effects of solvents [7] and 13C isotope effects [10]. In summary, the anomalies in the literature for the polarization data in the Raman spectrum of C 70 have been considered in depth [3–5] and the resonance problems are not unclear but are well defined [3]. For C 60, the assignment of the 427 cm −1 band to the H g(2) mode is reaffirmed [7] and additional features in the Raman spectrum can be explained by 13C splitting [10], and solute–solvent interactions [6,7]. There is no need to invoke isomers of C 60 and indeed, such an assertion is inconsistent with other experimental evidence.1
Acknowledgements The authors are grateful for the support from the 1
Full papers, in follow up to our C 60 and C 70 communications, have been submitted for publication [11,12].
S.H. Gallagher et al./Journal of Molecular Structure 407 (1997) 81–83
Australian Postgraduate Award Scheme (SHG), the Australian Research Council (RSA and PAL) and the National Institutes of Health (GM 23851) (CAR).
References [1] W. Brockner and F. Menzel, J. Mol. Struct., 378 (1996) 147. [2] K. Lynch, C. Tanke, F. Menzel, W. Brockner, P. Scharff and E. Stumpp, J. Phys. Chem., 99 (1995) 7985. [3] S.H. Gallagher, R.S. Armstrong, P.A. Lay and C.A. Reed, Chem. Phys. Lett., 234 (1995) 245. [4] G. Onida, W. Andreoni, J. Kohanoff and M. Parrinello, Chem. Phys. Lett., 219 (1994) 1.
83
[5] N. Chandrabhas, K. Jayaram, D.V.S. Muthu, A.K. Sood, R. Seshadri and C.N.R. Rao, Phys. Rev. B, 47 (1993) 10 963. [6] S.H. Gallagher, R.S. Armstrong, P.A. Lay and C.A. Reed, J. Am. Chem. Soc., 116 (1994) 12 091. [7] S.H. Gallagher, R.S. Armstrong, P.A. Lay and C.A. Reed, Chem. Phys. Lett., 248 (1996) 353. [8] C. Coulombeau and A. Rassat, J. Chim. Phys., 88 (1991) 173. [9] K. Raghavachari and C.M. Rohlfing, J. Phys. Chem., 96 (1992) 2463. [10] S.P. Love, D. McBranch, M.I. Salkola, N.V. Coppa, J.M. Robinson, B.I. Swanson and A.R. Bishop, Chem. Phys. Lett., 225 (1994) 170. [11] S.H. Gallagher, R.S. Armstrong, W.A. Clucas, P.A. Lay and C.A. Reed, J. Phys. Chem., 101 (1997) in press. [12] S.H. Gallagher, R.S. Armstrong, R.D. Bolskar, P.A. Lay and C.A. Reed, J. Am. Chem. Soc., 119 (1997) in press.
Comment on ‘‘Genetic relationship between intrinsic Raman and infrared fundamental vibrations of the C 60 and C 70 fullerenes’’ [J. Mol. Struct. 378 (1996) 147] Sean H. Gallagher a, Robert S. Armstrong a,*, Robert D. Bolskar b, Peter A. Lay a, Christopher A. Reed b b
a School of Chemistry, University of Sydney, Sydney, New South Wales, 2006, Australia Department of Chemistry, University of Southern California, Los Angeles, CA 90089-0744, USA
Received 8 August 1996; revised 18 November 1996; accepted 19 November 1996
Abstract Brockner and Menzel [J. Mol. Struct. 378 (1996) 147] have recently presented the view that the effects of resonance in the Raman spectrum of C 70 are unclear and that the anomalies in polarization data have not been considered in the literature. In fact, three papers have addressed these issues, removing inconsistencies in the assignments of the vibrational modes of C 70. In the work on C 60, Brockner and Menzel [ibid] assign the H g(2) mode to a Raman band at 485 cm −1. This contradicts earlier assignments to a band at 430 cm −1 and a recent reaffirmation at 427 cm −1 via resonance Raman spectroscopy. Additional features in the Raman spectrum of C 60, proposed by Brockner and Menzel to arise from isomers of C 60, are better explained by 13 C splitting and the symmetry-lowering effects of solute–solvent interactions. q 1997 Elsevier Science B.V. Keywords: C 60 fullerene; C 70 fullerene; Vibration; Resonance Raman spectroscopy
1. Introduction
2. Experimental
In a recent paper in this journal [1], Brockner and Menzel report the correlation of vibrational modes of C 60 with those of C 70 and offer a complete band assignment for these modes based on their earlier FT–Raman spectroscopic results [2]. While we agree with most of the assignments, the purpose of this comment is to emphasize that the authors did not consider all relevant work and as a result, some of their claims are not novel and others have more likely alternative explanations.
FT–Raman spectra were recorded on a Bruker RFS 100 spectrometer with 1064-nm Nd:YAG laser excitation (200 mW) using a 1 cm pathlength cuvette. The spectra were recorded (1024 scans) at 1.5 cm −1 resolution for C 60 (5 mM) in CS 2 solvent.
* Corresponding author. Tel.: +61-2-93513104; fax: +61-293513329; e-mail: [email protected].
3. Results and discussion Brockner and Menzel [1] are quite correct in highlighting the inconsistencies amongst the literature vibrational spectra and assignments published for
0022-2860/97/$17.00 q 1997 Elsevier Science B.V. All rights reserved PII S 0 02 2- 2 86 0 (9 6 )0 9 72 5 -6
82
S.H. Gallagher et al./Journal of Molecular Structure 407 (1997) 81–83
C 70 prior to 1995. They cite reasons such as an ‘‘unclear influence by visible laser light excitation’’, presumably for the Raman spectra, and a ‘‘disregard for spectroscopy principles and thoughtless treatment of experimental data’’ with respect to polarization data. In early 1995, we reported the resonance Raman spectrum of C 70 in benzene solution using a range of visible laser excitations [3]. These results clearly revealed the manner in which the intensity of the 25 most intense Raman bands of C 70 depended upon the resonance conditions and an explanation of the enhancement mechanisms was given. However, this was not cited in their paper. In addition, we [3] and others [4,5] have reported and discussed the problems associated with the published polarization data on C 70, but these papers were also not cited. The lack of recognition of this literature led to outdated assertions on the Raman spectra of C 70 in the paper of Brockner and Menzel [1]. In their analysis of the Raman spectrum of C 60, Brockner and Menzel [1] propose the assignment of the H g(2) mode results in the band at 485 cm −1. This is based on their earlier analysis of the FT–Raman spectrum and on PM3 calculations [2] but contradicts the accepted 430/435 cm −1 assignment as discussed below. Part of the justification for their assignment lies in the absence of a band at ca. 430 cm −1 in the FT–Raman spectrum of C 60 in CS 2 solution and yet, the authors did not report the presence of a band at 485 cm −1 either [2]. We have performed experiments under the same conditions and the results are consistent with those in Ref. [2]. We believe that a combination of the weak intensity of the band at ca. 430 cm −1 and the coincidence of a CS 2 solvent band at ca. 400 cm −1 precludes its observation. Brockner and Menzel have not considered the significance of the reduction of symmetry that arises from solvent effects on the resonance Raman spectra of C 60, communicated by us in 1994 [6]. Solute–solvent interactions were shown to remove the degeneracy of the band due to the H g(7) mode at 1421 cm −1 in toluene solution. Subsequent to this, we have established that the symmetry-lowering effects of solvents (including CS 2) allow many bands due to IR–Raman-silent modes of C 60 to become Raman-activated and resonance enhanced [7]. In addition, these results clearly show an enhancement (resonance Raman) of intensity for a band at 427 cm −1, but no spectroscopic evidence for a
band at 485 cm −1. The intensity behaviour of this band is explained in terms of a B-term scattering mechanism, consistent with group theoretical requirements for a mode of H g symmetry [7]. Thus, we find no support for the assignment of the H g(2) mode to a band at 485 cm −1 and we reaffirm the 430/435 cm −1 assignment. Finally, we make a comment on the proposed existence of isomeric forms of C 60 [1,2]. The authors invoke the presence of isomers to explain the ‘‘C 60 surplus Raman frequencies’’. In their earlier paper, they cited that in addition to the I h form of C 60, there are 70 isomers of lower symmetry that are theoretically possible [8] and some (C2v , D 2d, D 2h, etc.) are calculated to be only a few electron volts less stable than the icosahedral form of C 60 [9]. Brockner and Menzel suggest that one or more of these isomers exists in their C 60 sample and that these give rise to the extra Raman features [1,2]. Assuming the presence of C 60 isomers, the observed intensity of some of the unexplained features in the FT–Raman spectrum of C 60 would imply that only a few isomers exist and that their abundance is significant. If this were really the case, other evidence for the existence of these isomers should exist in the IR- or 13C-NMR spectra of C 60 samples. This is not the case. While other C 60 isomers may putatively exist, we believe that the ‘‘surplus Raman frequencies’’ of C 60 are better explained by the symmetry-lowering effects of solvents [7] and 13C isotope effects [10]. In summary, the anomalies in the literature for the polarization data in the Raman spectrum of C 70 have been considered in depth [3–5] and the resonance problems are not unclear but are well defined [3]. For C 60, the assignment of the 427 cm −1 band to the H g(2) mode is reaffirmed [7] and additional features in the Raman spectrum can be explained by 13C splitting [10], and solute–solvent interactions [6,7]. There is no need to invoke isomers of C 60 and indeed, such an assertion is inconsistent with other experimental evidence.1
Acknowledgements The authors are grateful for the support from the 1
Full papers, in follow up to our C 60 and C 70 communications, have been submitted for publication [11,12].
S.H. Gallagher et al./Journal of Molecular Structure 407 (1997) 81–83
Australian Postgraduate Award Scheme (SHG), the Australian Research Council (RSA and PAL) and the National Institutes of Health (GM 23851) (CAR).
References [1] W. Brockner and F. Menzel, J. Mol. Struct., 378 (1996) 147. [2] K. Lynch, C. Tanke, F. Menzel, W. Brockner, P. Scharff and E. Stumpp, J. Phys. Chem., 99 (1995) 7985. [3] S.H. Gallagher, R.S. Armstrong, P.A. Lay and C.A. Reed, Chem. Phys. Lett., 234 (1995) 245. [4] G. Onida, W. Andreoni, J. Kohanoff and M. Parrinello, Chem. Phys. Lett., 219 (1994) 1.
83
[5] N. Chandrabhas, K. Jayaram, D.V.S. Muthu, A.K. Sood, R. Seshadri and C.N.R. Rao, Phys. Rev. B, 47 (1993) 10 963. [6] S.H. Gallagher, R.S. Armstrong, P.A. Lay and C.A. Reed, J. Am. Chem. Soc., 116 (1994) 12 091. [7] S.H. Gallagher, R.S. Armstrong, P.A. Lay and C.A. Reed, Chem. Phys. Lett., 248 (1996) 353. [8] C. Coulombeau and A. Rassat, J. Chim. Phys., 88 (1991) 173. [9] K. Raghavachari and C.M. Rohlfing, J. Phys. Chem., 96 (1992) 2463. [10] S.P. Love, D. McBranch, M.I. Salkola, N.V. Coppa, J.M. Robinson, B.I. Swanson and A.R. Bishop, Chem. Phys. Lett., 225 (1994) 170. [11] S.H. Gallagher, R.S. Armstrong, W.A. Clucas, P.A. Lay and C.A. Reed, J. Phys. Chem., 101 (1997) in press. [12] S.H. Gallagher, R.S. Armstrong, R.D. Bolskar, P.A. Lay and C.A. Reed, J. Am. Chem. Soc., 119 (1997) in press.