The low temperature crystalline and glassy states of methyl alpha-hydroxy-isobutyrate

The low temperature crystalline and glassy phases of methyl alpha -hydroxy- isobutyrate (MHib) were identified and characterized structurally by differ ential scanning calorimetry, IR and Raman spectroscopy and molecular modeli ng. Within the temperature range 13-171 K, MHib exists as a glassy state, w here individual molecules may assume the two conformational states previous ly 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) approximate to 213 K], the enthalpy of crystallization being ca. 5 kJ mol(-1). The crystalline phase was found to exhibit conformational selec tivity-in this phase all individual molecules assume a conformation analogo us 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-C=O and O=C-O-C dihedrals are ca. 0 degrees). Molecular modeling and Raman data are consis tent with a structural unit within the crystal where two MHib molecules for m a centrosymmetrical hydrogen bonded dimer. The observed temperature of fu sion [T-f(peak)] for the crystal is 240 K.