Orientational ordering and low-temperature libration in the rotor-stator cocrystals of fullerenes and cubane

Norbert M. Nemes, Mar García-Hernández, G. Bortel, Éva Kováts, Bence J. Nagy, István Jalsovszky, S. Pekker

Research output: Contribution to journalArticle

13 Citations (Scopus)

Abstract

Cocrystals of cubane and fullerenes, C60-cubane and C 70-cubane, show distinct rotational ordering transitions. We studied the corresponding structural changes with temperature-dependent X-ray diffraction and the thermodynamics of the phase transitions with adiabatic microcalorimetry and differential scanning calorimetry. C60-cubane has one phase transition around 130 K from a high-temperature fcc phase with freely rotating C60 to a low-temperature orthorombic phase in which the fullerene rotation is frozen. The corresponding enthalpy change is approximately 1170 J/mol, and the entropy change is 9.6 J/(mol K). C70-cubane has two phase transitions. Around 380 K, the high-temperature fcc phase with freely rotating C70 transforms into a bct phase in which the C70 rotates uniaxially around an axis that precesses around the c direction with a full opening angle of 40*. Around 170 K, the uniaxial rotation also freezes out, with an accompanying structural transition to monoclinic and enthalpy and entropy changes of 620 J/mol and 8.7 J/(mol K), respectively. The low-temperature specific heat was analyzed in terms of the Debye-Einstein model to estimate the librational energies of the fullerenes and Debye temperatures. We found very similar values for the two cocrystals, approximately Elib ) 2.2 meV and TDebye ) 23 K. For reference, we also measured the specific heats of pure C60 and C70 and found Elib ) 2.96 meV and TDebye ) 32 K for C60 and Elib ) 1.9 meV and TDebye ) 20 K for C70.

Original languageEnglish
Pages (from-to)2042-2049
Number of pages8
JournalJournal of Physical Chemistry B
Volume113
Issue number7
DOIs
Publication statusPublished - Feb 19 2009

Fingerprint

Fullerenes
cubane
libration
stators
Stators
rotors
fullerenes
Rotors
Phase transitions
specific heat
Specific heat
Enthalpy
Entropy
Temperature
enthalpy
entropy
Debye temperature
Differential scanning calorimetry
heat measurement
Thermodynamics

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Materials Chemistry
  • Surfaces, Coatings and Films

Cite this

Orientational ordering and low-temperature libration in the rotor-stator cocrystals of fullerenes and cubane. / Nemes, Norbert M.; García-Hernández, Mar; Bortel, G.; Kováts, Éva; Nagy, Bence J.; Jalsovszky, István; Pekker, S.

In: Journal of Physical Chemistry B, Vol. 113, No. 7, 19.02.2009, p. 2042-2049.

Research output: Contribution to journalArticle

Nemes, Norbert M. ; García-Hernández, Mar ; Bortel, G. ; Kováts, Éva ; Nagy, Bence J. ; Jalsovszky, István ; Pekker, S. / Orientational ordering and low-temperature libration in the rotor-stator cocrystals of fullerenes and cubane. In: Journal of Physical Chemistry B. 2009 ; Vol. 113, No. 7. pp. 2042-2049.
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AU - Nemes, Norbert M.

AU - García-Hernández, Mar

AU - Bortel, G.

AU - Kováts, Éva

AU - Nagy, Bence J.

AU - Jalsovszky, István

AU - Pekker, S.

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N2 - Cocrystals of cubane and fullerenes, C60-cubane and C 70-cubane, show distinct rotational ordering transitions. We studied the corresponding structural changes with temperature-dependent X-ray diffraction and the thermodynamics of the phase transitions with adiabatic microcalorimetry and differential scanning calorimetry. C60-cubane has one phase transition around 130 K from a high-temperature fcc phase with freely rotating C60 to a low-temperature orthorombic phase in which the fullerene rotation is frozen. The corresponding enthalpy change is approximately 1170 J/mol, and the entropy change is 9.6 J/(mol K). C70-cubane has two phase transitions. Around 380 K, the high-temperature fcc phase with freely rotating C70 transforms into a bct phase in which the C70 rotates uniaxially around an axis that precesses around the c direction with a full opening angle of 40*. Around 170 K, the uniaxial rotation also freezes out, with an accompanying structural transition to monoclinic and enthalpy and entropy changes of 620 J/mol and 8.7 J/(mol K), respectively. The low-temperature specific heat was analyzed in terms of the Debye-Einstein model to estimate the librational energies of the fullerenes and Debye temperatures. We found very similar values for the two cocrystals, approximately Elib ) 2.2 meV and TDebye ) 23 K. For reference, we also measured the specific heats of pure C60 and C70 and found Elib ) 2.96 meV and TDebye ) 32 K for C60 and Elib ) 1.9 meV and TDebye ) 20 K for C70.

AB - Cocrystals of cubane and fullerenes, C60-cubane and C 70-cubane, show distinct rotational ordering transitions. We studied the corresponding structural changes with temperature-dependent X-ray diffraction and the thermodynamics of the phase transitions with adiabatic microcalorimetry and differential scanning calorimetry. C60-cubane has one phase transition around 130 K from a high-temperature fcc phase with freely rotating C60 to a low-temperature orthorombic phase in which the fullerene rotation is frozen. The corresponding enthalpy change is approximately 1170 J/mol, and the entropy change is 9.6 J/(mol K). C70-cubane has two phase transitions. Around 380 K, the high-temperature fcc phase with freely rotating C70 transforms into a bct phase in which the C70 rotates uniaxially around an axis that precesses around the c direction with a full opening angle of 40*. Around 170 K, the uniaxial rotation also freezes out, with an accompanying structural transition to monoclinic and enthalpy and entropy changes of 620 J/mol and 8.7 J/(mol K), respectively. The low-temperature specific heat was analyzed in terms of the Debye-Einstein model to estimate the librational energies of the fullerenes and Debye temperatures. We found very similar values for the two cocrystals, approximately Elib ) 2.2 meV and TDebye ) 23 K. For reference, we also measured the specific heats of pure C60 and C70 and found Elib ) 2.96 meV and TDebye ) 32 K for C60 and Elib ) 1.9 meV and TDebye ) 20 K for C70.

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