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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 a) the observed infrared spectrum of C70 at 113 K with b) the calculated VDOS and c) calculated infrared spectrum; (b) and (c) are calculated for the solid state by CASTEP with Cs site symmetry.
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fig07: Comparison of a) the observed infrared spectrum of C70 at 113 K with b) the calculated VDOS and c) calculated infrared spectrum; (b) and (c) are calculated for the solid state by CASTEP with Cs site symmetry.

Mentions: Further support for the validity of our model is provided by comparison of the observed and calculated Raman and infrared spectra (Figures 6 and 7). Our Raman spectrum was recorded using 785 nm excitation, which is well removed from the lowest-energy electronic absorption bands at 467 and 545 nm,33 so the spectrum should not be affected by resonance or preresonance effects,18 and the agreement is very good. The infrared spectrum (Figure 7 a), was recorded by ATR and is markedly different from those recorded by transmission infrared spectroscopy.13,15,16,19,20 Comparison with the vibrational density of states (VDOS) (Figure 7 b) and the calculated infrared spectrum (Figure 7 c) shows much better agreement with the former. Many more modes are observed than expected, most of which are fundamentals. It is not at all clear why this occurs. The ATR device uses a clamp to ensure good contact between the sample and the diamond ATR element; however, pressure alone cannot be the explanation, as transmission infrared measurements20 up to 10 GPa do not result in a similar spectrum. We (and others)34 have observed a similar effect in C60.


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

Refson K, Parker SF - ChemistryOpen (2015)

Comparison of a) the observed infrared spectrum of C70 at 113 K with b) the calculated VDOS and c) calculated infrared spectrum; (b) and (c) are calculated for the solid state by CASTEP with Cs site symmetry.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig07: Comparison of a) the observed infrared spectrum of C70 at 113 K with b) the calculated VDOS and c) calculated infrared spectrum; (b) and (c) are calculated for the solid state by CASTEP with Cs site symmetry.
Mentions: Further support for the validity of our model is provided by comparison of the observed and calculated Raman and infrared spectra (Figures 6 and 7). Our Raman spectrum was recorded using 785 nm excitation, which is well removed from the lowest-energy electronic absorption bands at 467 and 545 nm,33 so the spectrum should not be affected by resonance or preresonance effects,18 and the agreement is very good. The infrared spectrum (Figure 7 a), was recorded by ATR and is markedly different from those recorded by transmission infrared spectroscopy.13,15,16,19,20 Comparison with the vibrational density of states (VDOS) (Figure 7 b) and the calculated infrared spectrum (Figure 7 c) shows much better agreement with the former. Many more modes are observed than expected, most of which are fundamentals. It is not at all clear why this occurs. The ATR device uses a clamp to ensure good contact between the sample and the diamond ATR element; however, pressure alone cannot be the explanation, as transmission infrared measurements20 up to 10 GPa do not result in a similar spectrum. We (and others)34 have observed a similar effect in C60.

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.