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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.


Comparison of the TOSCA INS spectrum of C70 at 7 K (blue) with that calculated for the solid state by CASTEP (red, site symmetry Cs) after individual scaling of the modes and inclusion of the phonon wings.
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fig08: Comparison of the TOSCA INS spectrum of C70 at 7 K (blue) with that calculated for the solid state by CASTEP (red, site symmetry Cs) after individual scaling of the modes and inclusion of the phonon wings.

Mentions: Close inspection of Figure 5 a, b shows that the calculated and observed INS spectra differ in minor regards, in that some of the calculated transition energies are slightly (a few wavenumbers) inaccurate. We have previously shown that the calculated intensities (eigenvectors) are not very sensitive to the transition energy,35 so it is possible to make small changes in the transition energies and use the same eigenvectors. The procedure is to shift a calculated band to the nearest experimental feature of the correct intensity. In essence, this amounts to individual scaling factors for each of the modes and was successfully used for C60.27 Figure 8 shows the result, and it can be seen that the agreement is excellent; based on this, Table S2 in the Supporting Information lists the internal modes of C70 in D5h symmetry. The transition energies reported are a combination of experimental and our ab initio results and represent the best available description of the vibrational modes of C70.


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

Refson K, Parker SF - ChemistryOpen (2015)

Comparison of the TOSCA INS spectrum of C70 at 7 K (blue) with that calculated for the solid state by CASTEP (red, site symmetry Cs) after individual scaling of the modes and inclusion of the phonon wings.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig08: Comparison of the TOSCA INS spectrum of C70 at 7 K (blue) with that calculated for the solid state by CASTEP (red, site symmetry Cs) after individual scaling of the modes and inclusion of the phonon wings.
Mentions: Close inspection of Figure 5 a, b shows that the calculated and observed INS spectra differ in minor regards, in that some of the calculated transition energies are slightly (a few wavenumbers) inaccurate. We have previously shown that the calculated intensities (eigenvectors) are not very sensitive to the transition energy,35 so it is possible to make small changes in the transition energies and use the same eigenvectors. The procedure is to shift a calculated band to the nearest experimental feature of the correct intensity. In essence, this amounts to individual scaling factors for each of the modes and was successfully used for C60.27 Figure 8 shows the result, and it can be seen that the agreement is excellent; based on this, Table S2 in the Supporting Information lists the internal modes of C70 in D5h symmetry. The transition energies reported are a combination of experimental and our ab initio results and represent the best available description of the vibrational modes of C70.

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.