Structural and vibrational characterization of methyl glycolate in the lowtemperature crystalline and glassy states

The low temperature phases of methyl glycolate (MGly) were identified and c haracterized structurally by differential scanning calorimetry, infrared an d Raman spectroscopies and molecular modeling. Within the temperature range 13-273 K, MGly may exist in three solid phases. A crystalline phase (I) ca n be formed from the liquid upon slow cooling [T-onset=222-227 K] or from t he low temperature glassy state resulting from fast deposition of the vapou r onto a cold substrate at 13 K and subsequent warming. A mixture of the gl assy state and crystalline phase (I) is obtained by cooling the liquid at h igher cooling rates (v(cooling)greater than or equal to 10 K min(-1)). Upon heating this mixture, devitrification occurs at ca. 175 K, the cold liquid then formed giving rise to a second crystalline variety (II) at T-onset=19 8-207 K. In the glassy state, individual MGly 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. Str uct., 1999, 509, 183]. On the contrary, the crystalline phase I was found t o exhibit conformational selectivity-in this phase, all individual molecule s assume a conformation analogous to the most stable conformer found for th e isolated molecule and in the liquid (the syn-syn s-cis conformer, where t he H-O-C-C, O-C-C=O and O=C-O-C dihedrals are ca. 0 degrees). In agreement with the spectroscopic results, a molecular modeling analysis reveals that, in this phase, two non-equivalent molecules exhibiting an intramolecular O H ... O=hydrogen bond exist, which are connected by a relatively strong int ermolecular OH ... O'2=hydrogen bond. Crystalline state II could not be cha racterized in detail structurally, but the thermodynamic studies seem to in dicate that it corresponds to a metastable crystalline form having a more r elaxed structure and a slightly higher energy than crystalline state I. The observed temperature of fusion for the two observed crystalline forms are: I, 264 K and II, 260 K.