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

Abstract

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)]

Matrix-isolation FT-IR spectra and molecular orbital calculations on neutral N,N-dimethylglycine

“…The frequency of this mode may be correlated with that of the O-H stretching, since it is well known that a stronger hydrogen bond leads to a lower O-H stretching frequency and to a higher tC-O torsion. [31][32][33][34] Indeed, the observed frequency for the hydrogen bonded conformer of DMG was found to fit well a linear plot of tC-O vs. nOH (Fig. 6) derived for a series of other carboxylic acids isolated in inert matrices, including several molecules that also exhibit intramolecular hydrogen bonds (formic, pyruvic, glycolic, oxalic, maleic, malonic 25,31,32,[34][35][36] ).…”

“…[31][32][33][34] Indeed, the observed frequency for the hydrogen bonded conformer of DMG was found to fit well a linear plot of tC-O vs. nOH (Fig. 6) derived for a series of other carboxylic acids isolated in inert matrices, including several molecules that also exhibit intramolecular hydrogen bonds (formic, pyruvic, glycolic, oxalic, maleic, malonic 25,31,32,[34][35][36] ). This fact gives further support to the assignments made here, although this particular correlation is not a universal one, since besides hydrogen bonding other effects (e.g., different vibrational coupling and packing) might affect the frequencies, in particular that of the torsional mode.…”

“…Note that the intramolecular hydrogen bond in DMG is considerably strong. For example, the observed shifts to lower frequencies in the nO-H mode due to intramolecular hydrogen bonding in glycolic (CH 2 OHCOOH), oxalic (HOOC=COOH) and pyruvic (CH 3 COCOOH) acids are much lower (80, 70 and 115 cm À1 , respectively 24,31,32 ). In malonic acid, whose most stable form was previously pointed out as possessing a quite strong intramolecular hydrogen bond, this shift is also smaller than in DMG (295 cm À1 25 ).…”

Chirality-dependent sublimation of -(trifluoromethyl)-lactic acid: Relative vapor pressures of racemic, eutectic, and enantiomerically pure forms, and vibrational spectroscopy of isolated (S,S) and (S,R) dimers

“…Analogously to similar compounds such as lactic acid [35] and ahydroxy isobutyric acid [36] three monomer conformations were taken into account. Each hydroxy group can act as donor or acceptor for an intramolecular hydrogen bond and an additional acceptor is provided by the carbonyl functionality.…”

Matrix isolation FTIR spectroscopic and theoretical study of methyl lactate*1

“…The second conformer, GskC, is characterized by a weaker intramolecular hydrogen bond of the type OH Á Á Á O ester . It can be obtained from the most stable form by internal rotation around the C-C bond and has an energy higher than this latter by 9.0 and 6.6 kJ mol À1 , for MGly and MHIb, respectively (calculated HF/6-31G * [13][14][15]). Methyl lactate (MLac) differs from MGly by substitution of one of the a-hydrogen atoms by a methyl group.…”