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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 a) TOSCA INS spectrum of C70 at 7 K with that calculated for the b) solid state by CASTEP (site symmetry Cs) and c) for the gas phase by GAUSSIAN 03 (D5h symmetry).
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fig05: Comparison of the a) TOSCA INS spectrum of C70 at 7 K with that calculated for the b) solid state by CASTEP (site symmetry Cs) and c) for the gas phase by GAUSSIAN 03 (D5h symmetry).

Mentions: A comparison of the TOSCA INS spectrum with that calculated for the gas phase and the solid state is shown in Figure 5. It can be seen from the comparisons of the observed and calculated spectra that the present results provide a better description of the vibrational modes of C70 than all previous computational studies.19,23–25 In most cases, these have imposed D5h point-group symmetry on the calculation, that is, a gas-phase calculation, hence mode assignment is straightforward. In our case, we have a Cs symmetry molecule, and relating the modes back to the parent D5h symmetry is more complex. To proceed, we note that the highest subgroup of D5h that is compatible with translational symmetry is C2v. Imposing C2v symmetry on the Pm C70 structure raises the crystal symmetry to Pmm2. This structure was then geometry optimised, and the vibrational modes were calculated. This results in an INS spectrum that has the same profile as that from the Pm structure, but slightly shifted to lower wavenumber. Thus, the modes have been calculated in the same order in both systems. Using the correlation given in Table S1 in the Supporting Information, and assuming that modes that are degenerate under D5h symmetry will be very close (a few wavenumbers at most), it can be seen that the modes can be readily assigned as A′, E′, A“, and E”.


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 a) TOSCA INS spectrum of C70 at 7 K with that calculated for the b) solid state by CASTEP (site symmetry Cs) and c) for the gas phase by GAUSSIAN 03 (D5h symmetry).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig05: Comparison of the a) TOSCA INS spectrum of C70 at 7 K with that calculated for the b) solid state by CASTEP (site symmetry Cs) and c) for the gas phase by GAUSSIAN 03 (D5h symmetry).
Mentions: A comparison of the TOSCA INS spectrum with that calculated for the gas phase and the solid state is shown in Figure 5. It can be seen from the comparisons of the observed and calculated spectra that the present results provide a better description of the vibrational modes of C70 than all previous computational studies.19,23–25 In most cases, these have imposed D5h point-group symmetry on the calculation, that is, a gas-phase calculation, hence mode assignment is straightforward. In our case, we have a Cs symmetry molecule, and relating the modes back to the parent D5h symmetry is more complex. To proceed, we note that the highest subgroup of D5h that is compatible with translational symmetry is C2v. Imposing C2v symmetry on the Pm C70 structure raises the crystal symmetry to Pmm2. This structure was then geometry optimised, and the vibrational modes were calculated. This results in an INS spectrum that has the same profile as that from the Pm structure, but slightly shifted to lower wavenumber. Thus, the modes have been calculated in the same order in both systems. Using the correlation given in Table S1 in the Supporting Information, and assuming that modes that are degenerate under D5h symmetry will be very close (a few wavenumbers at most), it can be seen that the modes can be readily assigned as A′, E′, A“, and E”.

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.