There is currently great interest in replacing the harmful volatile hydrofluorocarbon fluids used in refrigeration and air-conditioning with solid materials that display magnetocaloric, ...electrocaloric or mechanocaloric effects. However, the field-driven thermal changes in all of these caloric materials fall short with respect to their fluid counterparts. Here we show that plastic crystals of neopentylglycol (CH
)
C(CH
OH)
display extremely large pressure-driven thermal changes near room temperature due to molecular reconfiguration, that these changes outperform those observed in any type of caloric material, and that these changes are comparable with those exploited commercially in hydrofluorocarbons. Our discovery of colossal barocaloric effects in a plastic crystal should bring barocaloric materials to the forefront of research and development in order to achieve safe environmentally friendly cooling without compromising performance.
In this paper, we present the experimental and theoretical investigation of 1-ethyl-3-methylquinoxaline-2-thione, molecule exhibiting an important biological activity. We used X-ray diffraction, ...UV-visible absorption spectroscopy and DFT calculations with 6-31G(
d,p
) basis sets. According to the results, the compound crystallizes in the monoclinic system,
P
2
1
/
c
with eight molecules per unit cell (
Z
= 8,
Z
' = 2) and with the lattice parameters (
a
= 7.7882(8) Å,
b
= 15.964(2) Å,
c
= 16.519(2) Å, α = 90°, β = 94.413(11)°, γ = 90°), in which the two fused-rings adopt a planar geometry, and assumes a dihedral angle with the ethyl group, presenting the most stable conformation. UV–Vis absorption spectroscopic properties of 1-ethyl-3-methylquinoxaline-2-thione showed a redshift when the solvent polarity increases from ethanol to DMSO. Detailed analysis of intra and intermolecular interactions as such H-bonding, π–π stacking, electronic transitions, HOMO–LUMO patterns, molecular electrostatic potential maps and Mulliken charges distribution was carried out in order to investigate charge transfer within the molecule for the needs of biomedical applications.
Crystals of the solvate C
60
·2CBr
2
H
2
(monoclinic
C
2/
m
), which is stable in air, were grown by slow evaporation of solutions of C
60
in CBr
2
H
2
at room temperature. The high enthalpy change ...for the complete desolvation process, 54.9 kJ mol
−1
of solvent, as well as the relatively large negative excess volume of −49.6 Å
3
indicate the presence of strong intermolecular interactions between C
60
and CBr
2
H
2
. The strong intermolecular interactions are consistent with an overall orientational order for the C
60
and the CBr
2
H
2
molecules in the solvate as found by the Rietveld refinement of its crystal structure.
5-Azidomethyl-8-hydroxyquinoline has been synthesized and characterized using IR,
1
H and
13
C NMR spectroscopic methods. Thermal analysis revealed no solid-solid phase transitions. The crystal ...structure of this compound was refined by Rietveld method from powder X-ray diffraction data at 295 K. The single- crystal structure of the compound at 260 K was solved and refined using SHELX 97 program. According to the data obtained by both methods, the structure of the compound is monoclinic, space group
P
2
1
/
c
, with
Z
= 4 and
Z
' = 1. For the single crystal at 260 K,
a
= 12.2879 (9) Å,
b
= 4.8782 (3) Å,
c
= 15.7423 (12) Å, β=100.807(14)°. Mechanisms of deformation resulting from intra- and intermolecular interactions, such as hydrogen bonding, induced slight torsions in the crystal structure. The optimized molecular geometry of 5-azidomethyl-8-hydroxyquinoline in the ground state is calculated using density functional theory (B3LYP) and Hartree-Fock (HF) methods with the 6-311G(
d,p
) basis set. The calculated results show good agreement with experimental values. Energy gap of the molecule was found using HOMO and LUMO calculation which reveals that charge transfer occurs within the molecule.
The polymorphism and the dynamics of a simple rigid molecule (1-fluoro-adamantane) have been studied by means of X-ray powder diffraction and broadband dielectric spectroscopy. At temperatures below ...the melting point, the molecule forms an orientationally disordered Phase I with a cubic-centered structure (Phase I, Fm3¯m, Z = 4). This phase possesses eight equilibrium positions for the fluorine atom, with equal occupancy factors of 1/8. A solid-solid phase transition to a low-temperature tetragonal phase (Phase II, P4¯2(1)c, Z = 2) reduces the statistical disorder to only four possible equivalent sites for the fluorine atom, with fractional occupancies of 1/4. The dynamics has been rationalized under the constraints imposed by the space group of the crystal structure determined by powder X-ray diffraction. The dielectric spectroscopy study reveals that the statistical disorder in Phase II is dynamic in character and is associated with reorientational jumps along the two- and three-fold axes. In the dielectric loss spectra, the cooperative (α) relaxation exhibits a shoulder on the high-frequency side. This remarkable finding clearly reveals the existence of two intrinsic reorientational processes associated with the exchange of the F atom along the four sites. In addition to such "bimodal" relaxation, a secondary Johari-Goldstein relaxation is detected at lower temperatures.
Phase equilibria involving orientationally disordered (OD) and liquid phases of the two-component system between carbon tetrachloride (CCl4) and 2-methyl-2-bromomethane ((CH3)3CBr) have been ...determined by means of X-ray powder diffraction and thermal analysis techniques from 210 K up to the liquid state. The isomorphism relation between the OD stable face-centered cubic (FCC) phase of (CH3)3CBr and the metastable FCC phase of CCl4 has been demonstrated throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium FCC + L for the whole range of composition, despite the monotropy of the FCC phase for the CCl4 component with respect to its OD rhombohedral (R) stable phase. A continuous series of OD R mixed crystals is found, which confirms the R lattice symmetry of the OD phase II of (CH3)3CBr, for which the crystallographic results have been long-time misinterpreted. X-ray patterns of such a phase were indexed according to the recent single-crystal results obtained by Rudman (Rudman, R. J. Mol. Struct. 2001, 569, 157). In addition, some experimental evidences are given to confirm the number of molecules per unit cell (Z = 21). The thermodynamic assessment reproduces coherently the phase diagram for the stable R + L and R + FCC two-phase equilibria as well as for the partially metastable FCC + L two-phase equilibrium and provides a set of data for the thermodynamic properties of nonexperimentally available phase transitions of pure components. Surprisingly, the phase equilibrium involving R and FCC OD phases appears as one of the very few showing a solid−solid equilibrium with two extremes.
There is currently great interest in replacing the harmful volatile hydrofluorocarbonfluidsused in refrigeration and air-conditioning with solid materials that display magnetocaloric,electrocaloric ...or mechanocaloric effects. However, thefield-driven thermal changes in all ofthese caloric materials fall short with respect to theirfluid counterparts. Here we show thatplastic crystals of neopentylglycol (CH3)2C(CH2OH)2display extremely large pressure-driven thermal changes near room temperature due to molecular reconfiguration, that thesechanges outperform those observed in any type of caloric material, and that these changesare comparable with those exploited commercially in hydrofluorocarbons. Our discovery ofcolossal barocaloric effects in a plastic crystal should bring barocaloric materials to theforefront of research and development in order to achieve safe environmentally friendlycooling without compromising performance.
The polymorphism and the dynamics of a simple rigid molecule (1-fluoro-adamantane) have been studied by means of X-ray powder diffraction and broadband dielectric spectroscopy. At temperatures below ...the melting point, the molecule forms an orientationally disordered Phase I with a cubic-centered structure (Phase I,
Fm
3&cmb.macr;
m
,
Z
= 4). This phase possesses eight equilibrium positions for the fluorine atom, with equal occupancy factors of 1/8. A solid-solid phase transition to a low-temperature tetragonal phase (Phase II,
P
4&cmb.macr;2
1
c
,
Z
= 2) reduces the statistical disorder to only four possible equivalent sites for the fluorine atom, with fractional occupancies of 1/4. The dynamics has been rationalized under the constraints imposed by the space group of the crystal structure determined by powder X-ray diffraction. The dielectric spectroscopy study reveals that the statistical disorder in Phase II is dynamic in character and is associated with reorientational jumps along the two- and three-fold axes. In the dielectric loss spectra, the cooperative (
α
) relaxation exhibits a shoulder on the high-frequency side. This remarkable finding clearly reveals the existence of two intrinsic reorientational processes associated with the exchange of the F atom along the four sites. In addition to such "bimodal" relaxation, a secondary Johari-Goldstein relaxation is detected at lower temperatures.
Bimodal reorientational relaxations along the twofold (
α
) and threefold (
α
′) axes of the disordered Phase II (
P
4&cmb.macr;2
1
c
) of 1-F-adamantane.
Crystals of the solvate C 60 ·2CBr 2 H 2 (monoclinic C2/m), which is stable in air, were grown by slow evaporation of solutions of C 60 in CBr 2 H 2 at room temperature. The high enthalpy change for ...the complete desolvation process, 54.9 kJ mol-1 of solvent, as well as the relatively large negative excess volume of-49.6 Å 3 indicate the presence of strong intermolecular interactions between C 60 and CBr 2 H 2. The strong intermolecular interactions are consistent with an overall orientational order for the C 60 and the CBr 2 H 2 molecules in the solvate as found by the Rietveld refinement of its crystal structure.
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