The low temperature crystalline and glassy states of methyl α-hydroxy-isobutyrate†
The low temperature crystalline and glassy phases of methyl α-hydroxy-isobutyrate (MHib) were identified and characterized structurally by differential scanning calorimetry, IR and Raman spectroscopy and molecular modeling. Within the temperature range 13–171 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, devitrification occurs and a crystalline phase may then be formed [T(onset)≈213 K], the enthalpy of crystallization being ca. 5 kJ mol−1. The crystalline phase was found to exhibit conformational selectivity—in 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 H–O–C–C, O–C–CO and OC–O–C 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 [Tf(peak)] for the crystal is 240 K.