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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.


Related in: MedlinePlus

Vibrational spectra of C70 at 7 K. a) INS spectrum recorded on TOSCA, b) INS spectrum recorded on MAPS with an incident energy of 1452 cm−1, and c) INS spectrum recorded on MAPS with an incident energy of 2017 cm−1.
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fig02: Vibrational spectra of C70 at 7 K. a) INS spectrum recorded on TOSCA, b) INS spectrum recorded on MAPS with an incident energy of 1452 cm−1, and c) INS spectrum recorded on MAPS with an incident energy of 2017 cm−1.

Mentions: As described in the Introduction, there is no agreement as to the exact form of the low-temperature structure of C70, with both orthorhombic and monoclinic structures proposed.9–11 Since fivefold symmetry is incompatible with long-range order,28 it follows that the site symmetry must be simultaneously a subgroup of D5h and also a permitted site symmetry of Pbnm or a monoclinic space group. Only Cs, C2, and C1 meet these constraints. Spectroscopically, all three are equivalent: all degeneracies are lifted and all modes are allowed in both the infrared and Raman spectra. (The three possibilities are only distinguishable by single crystal studies with polarised radiation.) However, the intensities will vary considerably, and modes that are forbidden under D5h symmetry are likely to be weak in the solid-state spectra and may be confused with combination modes, as found with C6029,30 All the modes are allowed in INS spectroscopy and all will have similar intensities, since all the atoms in the mode have the same scattering cross section and will have similar amplitudes of vibration. Figure 2 shows the INS spectra recorded with the TOSCA and MAPS instruments (ISIS Facility, Rutherford Appleton Laboratory, Chilton, UK). Figure 3 a shows the Raman spectrum at 20 K recorded simultaneously with the TOSCA spectrum, while Figure 3 b shows the infrared spectrum at 113 K recorded by attenuated total reflectance (ATR). In comparison to previous work,22 the INS spectra exhibit a signal-to-noise ratio more than an order of magnitude better and with significantly improved resolution, particularly in the 800–1600 cm−1 region. While the infrared and Raman spectra show many modes across the range 200–3000 cm−1, there is a sharp cut-off at 1600 cm−1 in the INS spectra, demonstrating that all modes to higher energy must be overtones or combinations.


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

Refson K, Parker SF - ChemistryOpen (2015)

Vibrational spectra of C70 at 7 K. a) INS spectrum recorded on TOSCA, b) INS spectrum recorded on MAPS with an incident energy of 1452 cm−1, and c) INS spectrum recorded on MAPS with an incident energy of 2017 cm−1.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig02: Vibrational spectra of C70 at 7 K. a) INS spectrum recorded on TOSCA, b) INS spectrum recorded on MAPS with an incident energy of 1452 cm−1, and c) INS spectrum recorded on MAPS with an incident energy of 2017 cm−1.
Mentions: As described in the Introduction, there is no agreement as to the exact form of the low-temperature structure of C70, with both orthorhombic and monoclinic structures proposed.9–11 Since fivefold symmetry is incompatible with long-range order,28 it follows that the site symmetry must be simultaneously a subgroup of D5h and also a permitted site symmetry of Pbnm or a monoclinic space group. Only Cs, C2, and C1 meet these constraints. Spectroscopically, all three are equivalent: all degeneracies are lifted and all modes are allowed in both the infrared and Raman spectra. (The three possibilities are only distinguishable by single crystal studies with polarised radiation.) However, the intensities will vary considerably, and modes that are forbidden under D5h symmetry are likely to be weak in the solid-state spectra and may be confused with combination modes, as found with C6029,30 All the modes are allowed in INS spectroscopy and all will have similar intensities, since all the atoms in the mode have the same scattering cross section and will have similar amplitudes of vibration. Figure 2 shows the INS spectra recorded with the TOSCA and MAPS instruments (ISIS Facility, Rutherford Appleton Laboratory, Chilton, UK). Figure 3 a shows the Raman spectrum at 20 K recorded simultaneously with the TOSCA spectrum, while Figure 3 b shows the infrared spectrum at 113 K recorded by attenuated total reflectance (ATR). In comparison to previous work,22 the INS spectra exhibit a signal-to-noise ratio more than an order of magnitude better and with significantly improved resolution, particularly in the 800–1600 cm−1 region. While the infrared and Raman spectra show many modes across the range 200–3000 cm−1, there is a sharp cut-off at 1600 cm−1 in the INS spectra, demonstrating that all modes to higher energy must be overtones or combinations.

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.


Related in: MedlinePlus