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Assignment of the Internal Vibrational Modes of C70 by Inelastic Neutron Scattering Spectroscopy and Periodic-DFT.

Refson K, Parker SF - ChemistryOpen (2015)

Bottom Line: Unfortunately, many of the modes are either forbidden or have very low infrared or Raman intensity, even if allowed.We have obtained a new INS spectrum from a large sample recorded at the highest resolution available.We demonstrate that all previous assignments are incorrect in at least some respects and propose a new assignment based on periodic density functional theory (DFT) that successfully reproduces the INS, infrared, and Raman spectra.

View Article: PubMed Central - PubMed

Affiliation: ISIS Facility, Science and Technology Facilities Council (STFC), Rutherford Appleton Laboratory Chilton, Didcot, OX11 0QX, UK ; Department of Physics, Royal Holloway, University of London Egham, TW20 0EX, UK.

ABSTRACT
The fullerene C70 may be considered as the shortest possible nanotube capped by a hemisphere of C60 at each end. Vibrational spectroscopy is a key tool in characterising fullerenes, and C70 has been studied several times and spectral assignments proposed. Unfortunately, many of the modes are either forbidden or have very low infrared or Raman intensity, even if allowed. Inelastic neutron scattering (INS) spectroscopy is not subject to selection rules, and all the modes are allowed. We have obtained a new INS spectrum from a large sample recorded at the highest resolution available. An advantage of INS spectroscopy is that it is straightforward to calculate the spectral intensity from a model. We demonstrate that all previous assignments are incorrect in at least some respects and propose a new assignment based on periodic density functional theory (DFT) that successfully reproduces the INS, infrared, and Raman spectra.

No MeSH data available.


Calculated INS spectra of C70 modes involving motion of: a) the ‘belt’ atoms and b) the ‘cap’ atoms. The spectra are normalised to the intensity per carbon atom.
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fig09: Calculated INS spectra of C70 modes involving motion of: a) the ‘belt’ atoms and b) the ‘cap’ atoms. The spectra are normalised to the intensity per carbon atom.

Mentions: There have been attempts19,26 to identify modes that are characteristic of the ‘belt’ of the additional ten atoms that distinguish C60 from C70. By generating INS ‘spectra’ where the cross section of all the atoms except those of interest are set to zero, it is possible to see whether there are any modes localised in either the belt or the caps. Figure 9 shows the result, and it is clear that for almost all the modes, both the belt and cap atoms are involved. There are a few modes where there is no motion of the belt atoms (mode at 307 cm−1, mode at 336 cm−1, mode at 705 cm−1, mode at 705 cm−1, and the mode at 707 cm−1), and these are shown in Figure S1 in the Supporting Information. We find no modes that only involve motion of the belt atoms. We also disagree with the conclusion19 that the amplitude of motion of the belt atoms is larger than that of the cap: the intensities in Figure 9 are per carbon atom, and both spectra are plotted on the same ordinate scale. Thus the amplitudes of the atoms are similar in both cases.


Assignment of the Internal Vibrational Modes of C70 by Inelastic Neutron Scattering Spectroscopy and Periodic-DFT.

Refson K, Parker SF - ChemistryOpen (2015)

Calculated INS spectra of C70 modes involving motion of: a) the ‘belt’ atoms and b) the ‘cap’ atoms. The spectra are normalised to the intensity per carbon atom.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4608530&req=5

fig09: Calculated INS spectra of C70 modes involving motion of: a) the ‘belt’ atoms and b) the ‘cap’ atoms. The spectra are normalised to the intensity per carbon atom.
Mentions: There have been attempts19,26 to identify modes that are characteristic of the ‘belt’ of the additional ten atoms that distinguish C60 from C70. By generating INS ‘spectra’ where the cross section of all the atoms except those of interest are set to zero, it is possible to see whether there are any modes localised in either the belt or the caps. Figure 9 shows the result, and it is clear that for almost all the modes, both the belt and cap atoms are involved. There are a few modes where there is no motion of the belt atoms (mode at 307 cm−1, mode at 336 cm−1, mode at 705 cm−1, mode at 705 cm−1, and the mode at 707 cm−1), and these are shown in Figure S1 in the Supporting Information. We find no modes that only involve motion of the belt atoms. We also disagree with the conclusion19 that the amplitude of motion of the belt atoms is larger than that of the cap: the intensities in Figure 9 are per carbon atom, and both spectra are plotted on the same ordinate scale. Thus the amplitudes of the atoms are similar in both cases.

Bottom Line: Unfortunately, many of the modes are either forbidden or have very low infrared or Raman intensity, even if allowed.We have obtained a new INS spectrum from a large sample recorded at the highest resolution available.We demonstrate that all previous assignments are incorrect in at least some respects and propose a new assignment based on periodic density functional theory (DFT) that successfully reproduces the INS, infrared, and Raman spectra.

View Article: PubMed Central - PubMed

Affiliation: ISIS Facility, Science and Technology Facilities Council (STFC), Rutherford Appleton Laboratory Chilton, Didcot, OX11 0QX, UK ; Department of Physics, Royal Holloway, University of London Egham, TW20 0EX, UK.

ABSTRACT
The fullerene C70 may be considered as the shortest possible nanotube capped by a hemisphere of C60 at each end. Vibrational spectroscopy is a key tool in characterising fullerenes, and C70 has been studied several times and spectral assignments proposed. Unfortunately, many of the modes are either forbidden or have very low infrared or Raman intensity, even if allowed. Inelastic neutron scattering (INS) spectroscopy is not subject to selection rules, and all the modes are allowed. We have obtained a new INS spectrum from a large sample recorded at the highest resolution available. An advantage of INS spectroscopy is that it is straightforward to calculate the spectral intensity from a model. We demonstrate that all previous assignments are incorrect in at least some respects and propose a new assignment based on periodic density functional theory (DFT) that successfully reproduces the INS, infrared, and Raman spectra.

No MeSH data available.