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

Abstract

The low temperature phases of methyl glycolate (MGly) were identified and characterized structurally by differential scanning calorimetry, infrared and Raman spectroscopies and molecular modeling. Within the temperature range 13–273 K, MGly may exist in three solid phases. A crystalline phase (I) can be formed from the liquid upon slow cooling [T_onset=222–227 K] or from the low temperature glassy state resulting from fast deposition of the vapour onto a cold substrate at 13 K and subsequent warming. A mixture of the glassy state and crystalline phase (I) is obtained by cooling the liquid at higher cooling rates (v_cooling10 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=198–207 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. Struct., 1999, 509, 183]. On the contrary, the crystalline phase I 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°). In agreement with the spectroscopic results, a molecular modeling analysis reveals that, in this phase, two non-equivalent molecules exhibiting an intramolecular OH···O hydrogen bond exist, which are connected by a relatively strong intermolecular OH···O′ hydrogen bond. Crystalline state II could not be characterized in detail structurally, but the thermodynamic studies seem to indicate that it corresponds to a metastable crystalline form having a more relaxed 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.