Molecular structure and vibrational spectra of methyl glycolate and methyl α-hydroxy isobutyrate

Abstract

Conformational isomerism in isolated and liquid methyl glycolate and methyl a-hydroxy isobutyrate was investigated by a concerted molecular orbital and vibrational spectroscopic approach (infrared and Raman). The molecular structures, relative energies, dipole moments and vibrational spectra of the various possible conformers of the studied compound were calculated, using the extended 6-31G* basis set at the HF-SCF ab initio level of theory. The theoretical results were then used to interpret infrared and Raman data obtained under different experimental conditions. It was found that both in the liquid and gaseous phase the studied molecules exist in two experimentally observed conformational states, the C s point group Syn-syn conformer (Ss), which exhibits an intramolecular OH … Oy hydrogen bond, being the most stable form. As expected, the relative populations of the second more stable conformers increase in the liquid phase, since intermolecular interactions tend to reduce the importance of the intramolecular H-bonding that is the main stabilizing factor of the Ss forms.

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

“…1) which has an energy 6.6 kJ mol~1 higher than the conformational ground state, in the gaseous phase. 8 In the present study, structural, thermodynamic and vibrational data for the solid phases and phase transitions of MHib within the temperature range 13È273 K are reported. Di †eren-tial scanning calorimetry was used to detect and characterize low temperature phase transitions and vibrational spectroscopy (both Raman and IR) was used to shed light on the nature of the di †erent phases observed.…”

“…As a starting point to interpret the experimental data for the condensed phases we used the information previously obtained for monomeric methyl glycolate by matrix-isolation IR spectroscopy and quantum chemical calculations. 8 We have found that, within the temperature range 13È273 K, methyl glycolate may exist in three solid phases : a glassy state, existing at the lowest temperatures (\174 K), and two crystalline phasesÈa stable crystalline phase, which can be formed from the liquid upon slow cooling or by heating the completely vitriÐed sample, and a metastable crystalline variety, which can be obtained by heating a sample previously cooled from the liquid using higher cooling rates K min~1). 7 The stable crystalline phase of (v cooling P 10 methyl glycolate was found to exhibit conformational selectivity, all individual molecules assuming a conformation similar to the most stable conformer found for the isolated ¤ Electronic Supplementary Information available.…”

“…7 To the best of our knowledge MHib has not yet been the subject of any experimental or theoretical spectroscopic or structural study in the condensed phases. On the other hand, this compound was also studied in our previous matrixisolation spectroscopic and quantum chemical study, 8 where the preferred conformations assumed by its monomer could be well established. Indeed, as methyl glycolate, MHib exists in two experimentally detectable conformational states in the isolated molecule situation : the Syn-syn s-cis conformer (Ss ; see Fig.…”

Matrix isolation FTIR spectroscopic and theoretical study of methyl lactate

“…Among the methyl a-hydroxy carboxylates, methyl glycolate (MGly) and methyl a-hydroxy-isobutyrate (MHIb) have been the most studied species, having already been the subject of detailed structural, thermodynamic and vibrational analysis, both for the isolated molecule and in the condensed phases [13][14][15][16][17][18]. Matrix isolation infrared spectroscopy studies previously undertaken on these two compounds in our laboratory allowed the identification and characterization, in both cases, of two different conformers 1 SsC and GskC, which were also shown to be present in the gaseous phase [13][14][15].…”

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.…”