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  • Author or Editor: Yulia Kovalevskaya x
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Abstract  

Heat capacity C p(T) of the crystalline dl-cysteine was measured on heating the system from 6 to 309 K by adiabatic calorimetry; thermodynamic functions were calculated based on these data smoothed in the temperature range 6–273.15 K. The values of heat capacity, entropy, and enthalpy at 273.15 K were equal to 142.4, 153.3, and 213.80 J K−1 mol−1, respectively. At about 300 K, a heat capacity peak was observed, which was interpreted as an evidence of a first-order phase transition. The enthalpy and the entropy of the transition are equal, respectively, to 2300 ± 50 and 7.6 ± 0.1 J K−1 mol−1.

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Abstract  

Heat capacity C p(T) of the orthorhombic polymorph of L-cysteine was measured in the temperature range 6–300 K by adiabatic calorimetry; thermodynamic functions were calculated based on these measurements. At 298.15 K the values of heat capacity, C p; entropy, S m 0(T)-S m 0(0); difference in the enthalpy, H m 0(T)-H m 0(0), are equal, respectively, to 144.6±0.3 J K−1 mol−1, 169.0±0.4 J K−1 mol−1 and 24960±50 J mol−1. An anomaly of heat capacity near 70 K was registered as a small, 3–5% height, diffuse ‘jump’ accompanied by the substantial increase in the thermal relaxation time. The shape of the anomaly is sensitive to thermal pre-history of the sample.

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Abstract

Heat capacity of crystalline L- and DL-phenylglycines was measured in the temperature range from 6 to 305 K. For L-phenylglycine, no anomalies in the C p (T) dependence were observed. For DL-phenylglycine, however, an anomaly in the temperature range 50–75 K with a maximum at about 60 K was registered. The enthalpy and the entropy changes corresponding to this anomaly were estimated as 20 J mol−1 and 0.33 J K−1 mol−1, respectively. In the temperature range 205–225 K, an unusually large dispersion of the experimental points and a small change in the slope of the C p (T) curve were noticed. Thermodynamic functions for L- and DL-phenylglycines in the temperature range 0–305 K were calculated. At 298.15 K, the values of heat capacity, entropy, and enthalpy are equal to 179.1, 195.3 J K−1 mol−1, and 28590 J mol−1 for L-phenylglycine and 177.7, 196.3 J K−1 mol−1 and 28570 J mol−1 for DL-phenylglycine. For both L- and DL-phenylglycine, the C p (T) at very low temperatures does not follow the Debye law CT 3. The heat capacity C p (T) is slightly higher for L-phenylglycine, than for the racemic DL-crystal, with the exception of the phase transition region. The difference is smaller than was observed previously for the L-/DL-cysteines, and considerably smaller, than that for L-/DL- serines.

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Abstract  

Heat capacity of D- and DL-serine was measured using adiabatic calorimetry in a temperature range of 5.5 to 300 K, and then thermodynamic functions were calculated. The difference in heat capacity (C PD-C PDL) between two species indicates a small anomaly in D-serine near 15 K and a systematic excess over DL for temperatures > 30 K. This is much larger, than a difference in thermodynamic functions measured so far for the polymorphs of organic molecular crystals. The excess is fitted well to Einstein contribution with characteristic temperature of 185 K which is equivalent to vibrational mode at 129 cm−1.

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Abstract  

Thermodynamic properties of β-alanine in the temperature range 6.3–301 K were studied. No phase transitions were observed for the sample specially prepared to contain no solvent inclusions. At 298.15 K the calorimetric entropy and the difference in the enthalpy values are equal, respectively, to 126.6 JK−1 mol−1 and 19.220 Jmol−1. The C p (T) in the temperature range 6–16 K can be well described by Debye equation C p = AT 3. A comparison of the data on the entropies of glycine polymorphs and of β-alanine was used to show, that the empirical Parks–Huffman rule holds in the case of these compounds.

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Abstract  

Synthetic enstatite MgSiO3 was crystallized from a melt, quenched into water, and then annealed at 873 K. The product is the monoclinic polymorph with the unit cell parameters of a=0.9619(7), b=0.8832(3), c=0.5177(4) nm, β=108.27(5)°. Heat capacity was measured from 6 to 305 K using an adiabatic vacuum calorimeter. Thermodynamic functions for clinoenstatite differ by about 5% from those predicted after a thermodynamic model in the literature, but are very close to those measured for orthorhombic enstatite.

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Low-temperature heat capacity of diglycylglycine

Some summaries and forecasts for the heat capacity of amino acids and peptides

Journal of Thermal Analysis and Calorimetry
Authors: V. Drebushchak, Yulia Kovalevskaya, I. Paukov, and Elena Boldyreva

Abstract  

Heat capacity of tripeptide diglycylglycine was measured in a temperature range from 6.5 to 304 K. The results were compared with those for glycine and glycylglycine. Peptide bonding was found not to change C P(T) virtually above 70 K, where heat capacity does not obey the Debye model. Comparison with literature data allows one to expect a significant difference in the heat capacity for enantiomorph and racemic species of valine and leucine, like it was found recently for D-and DL-serine.

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Abstract  

Heat capacity of stoichiometric homogeneous spinel MgFe2O4 was measured from 5 to 305 K and thermodynamic functions were derived for temperatures up to 725 K using our previous high-temperature experimental data for the same sample. Anomaly in C p was found at very low temperatures. Experimental data below 20 K contain large (up to 25% near 5 K) error arising from the difference in the thermal history between the experimental series. Magnetic contribution to the low-temperature heat capacity was tested, and the linear function was found to fit experimental data better than the three-halves power derived from the spin-wave theory.

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Journal of Thermal Analysis and Calorimetry
Authors: I. Paukov, Yulia Kovalevskaya, Irina Kiseleva, and Tatiana Shuriga

Abstract

Low-temperature heat capacity of natural zinnwaldite was measured at temperatures from 6 to 303 K in a vacuum adiabatic calorimeter. An anomalous behavior of heat capacity function C p(T) has been revealed at very low temperatures, where this function does not tend to zero. Thermodynamic functions of zinnwaldite have been calculated from the experimental data. At 298.15 K, heat capacity C p(T) = 339.8 J K−1mol−1, calorimetric entropy S o(&) – S o(6.08) = 329.1 J K−1 mol−1, and enthalpy & o(&) − & o(6.08) = 54,000 J mol−1. Heat capacity and thermodynamic functions at 298.15 K for zinnwaldite having theoretical composition were estimated using additive method of calculation.

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Journal of Thermal Analysis and Calorimetry
Authors: Elena Boldyreva, V. Drebushchak, I. Paukov, Yulia Kovalevskaya, and Tatiana Drebushchak

Abstract  

Monoclinic (I) and orthorhombic (II) polymorphs of paracetamol were studied by DSC and adiabatic calorimetry in the temperature range 5 - 450 K. At all the stages of the study, the samples (single crystals and powders) were characterized using X-ray diffraction. A single crystal → polycrystal II→ I transformation was observed on heating polymorph II, after which polymorph I melted at 442 K. The previously reported fact that the two polymorphs melt at different temperatures could not be confirmed. The temperature of the II→I transformation varied from crystal to crystal. On cooling the crystals of paracetamol II from ambient temperature to 5 K, a II→ I transformation was also observed, if the 'cooling-heating' cycles were repeated several times. Inclusions of solvent (water) into the starting crystals were shown to be important for this transformation. The values of the low-temperature heat-capacity of the I and II polymorphs of paracetamol were compared, and the thermodynamic functions calculated for the two polymorphs.

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