Reference Materials for Calorimetry and Differential Thermal Analysis -[updated version of two previous documents produced by the IUPAC Commission; many refs.]. -(SABBAH, R.; AN, XU-WU; CHICKOS, J. S.; PLANAS LEITAO, M. L.; ROUX, M. V.; TORRES, L. A.; Thermochim. Acta 331 (1999) 2, 93-204; Cent. Thermodyn. Microcalorim., CNRS, F-13003 Marseille, Fr.; EN)
The low temperature crystalline and glassy phases of methyl a-hydroxy-isobutyrate (MHib) were identiÐed and characterized structurally by di †erential scanning calorimetry, IR and Raman spectroscopy and molecular modeling. Within the temperature range 13È171 K, MHib exists as a glassy state, where individual molecules may assume the two conformational states previously observed for this compound isolated in an argon matrix and in the liquid phase [S. Jarmelo and R. Fausto, J. Mol. Struct., 1999, 509, 193]. At ca. 171 K, devitriÐcation occurs and a crystalline phase may then be formed [T (onset) B 213 K], the enthalpy of crystallization being ca. 5 kJ mol~1. The crystalline phase was found to exhibit conformational selectivityÈin this phase all individual molecules assume a conformation analogous to the most stable conformer found for the isolated molecule and in the liquid (the Syn-syn s-cis conformer, where the HÈOÈCÈC, OÈCÈC2O and O2CÈOÈC dihedrals are ca. 0¡). Molecular modeling and Raman data are consistent with a structural unit within the crystal where two MHib molecules form a centrosymmetrical hydrogen bonded dimer. The observed temperature of fusion for the crystal is 240 K. [T f (peak)]
Density functional theory (Becke3LYP/6-311++G**) conformational analysis was carried out for all positional butanediol isomers. Taking into account the relative populations of the most stable conformers at 298.15 K, the weighted mean enthalpies of each butanediol isomer in the gas state were computed. Combining these results with the experimental values for the enthalpies of vaporization at 298.15 K, an estimate of the enthalpy of each of the butanediol isomers in the liquid state was obtained and discussed. The insight into the structural changes at the molecular level from the isolated molecule to the condensed state was improved by an infrared spectroscopy study in the OH stretching region, which was carried out for a wide range of concentrations of carbon tetrachloride solutions and pure liquids. The spectroscopic studies essentially confirmed the results derived from the combination of the computational and calorimetric studies.
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