Authors:I. Paukov, Yulia Kovalevskaya and Elena Boldyreva
Heat capacity Cp(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, Cp; entropy, Sm0(T)-Sm0(0); difference in the enthalpy, Hm0(T)-Hm0(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.
Authors:I. Paukov, Yulia Kovalevskaya and Elena Boldyreva
Heat capacity Cp(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.
Authors:I. E. Paukov, Yulia A. Kovalevskaya and Elena V. Boldyreva
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 Cp(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 Cp(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 Cp(T) at very low temperatures does not follow the Debye law C – T3. The heat capacity Cp(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.
Authors:V. Drebushchak, Tatiana Drebushchak, N. Chukanov and Elena Boldyreva
Five polymorphs of chlorpropamide (α, β, δ, γ, and ε) were investigated near the melting point by using DSC. Structure of
samples was tested by X-ray powder diffraction. Four first polymorphs were found to transform into ε-polymorph, which melts
at Tm=128°C, ΔmH=24 kJ mol−1. Enthalpy of the polymorph transitions ranges from +3 kJ mol−1 for α→ε to −0.8 kJ mol−1 for β→ε.
Structure of three first polymorphs was published elsewhere, and the structure of δ-polymorph is published for the first time.
XRPD patterns for all polymorphs are reported, together with the atomic coordinates for the δ-polymorph.
Authors:V. Drebushchak, Yulia Kovalevskaya, I. Paukov and Elena Boldyreva
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 CP(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.
Authors:Igor Paukov, Yulia Kovalevskaya, Elena Boldyreva and Valery Drebushchak
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 Cp(T) in the temperature range 6–16 K can be well described by Debye equation Cp = AT3. 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.
Authors:Elena Boldyreva, V. Drebushchak, I. Paukov, Yulia Kovalevskaya and Tatiana Drebushchak
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.