Methyl lactate [CH(CH 3)OHC(¼O)OCH 3 ] conformational space was investigated by extensive DFT/B3LYP/6-311þþG(d,p) and MP2/6-31þþG(d,p) calculations and matrix-isolation FTIR spectroscopy in argon and xenon matrices. From the seven different conformers predicted by the calculations, two forms (the most stable conformers yielded by the calculations: SsC and GskC, where the letters refer to the conformations assumed by the HOCC, OCC¼O and O¼COC dihedrals, respectively) were observed and characterized spectroscopically. Conformer interconversion pathways were also theoretically investigated. The low calculated barrier associated with the G 0 sk 0 C ! GskC conversion ($1 kJ mol À1) explains the absence of the third most stable form (G 0 sk 0 C) of methyl lactate in low-temperature matrices. Deposition of methyl lactate at different temperatures, together with the theoretical data, aided to the full assignment of the observed spectra. #
Lactic acid {2-hydroxypropionic acid [CH(CH 3)OHCOOH]} monomer was studied by matrix isolation FT-IR spectroscopy and molecular orbital calculations undertaken at both DFT(B3LYP)/6-311þþG(d,p) and MP2/ 6-31G(d,p) levels of approximation. The theoretical calculations predicted the conformer (SsC) with the carboxylic group and both the CCOH (alcohol) and O=CCO moieties in a cis configuration as the most stable form. In this conformer, the a-hydroxy hydrogen atom is involved in an intramolecular hydrogen bond with the carbonyl oxygen atom. The second most stable conformer (GskC) also shows a cis carboxylic group and differs from SsC in the CCOH (alcohol) and O=CCO dihedral angles, which are equal to 43.3 and 156.8 respectively. These angles are equal to À51.5 and À149.9 in the third most stable conformer (G 0 sk 0 C). These forms are characterized by showing a relatively weak intramolecular H (alcohol) Á Á ÁO (acid) hydrogen bond. In the AaT conformer, the carboxylic group is trans and the CCOH (alcohol) and O=CCO dihedral angles are 162.2 and 174.1 , respectively. This conformer shows a relatively strong H (acid) Á Á ÁO (alcohol) hydrogen bond. In consonance with the theoretical results, the matrix isolation experiments confirmed the predominance of conformer SsC in argon and xenon matrices, and provide the first experimental evidence of conformers GskC and AaT. Since the barrier for interconversion G 0 sk 0 C $ GskC is only $2 kJ mol À1 , these two conformers are in equilibrium in the matrices and, at low temperature, the population of the less stable G 0 sk 0 C form is too small to enable its observation. Full assignment of the observed spectra was undertaken on the basis of comparison with the theoretical data and temperature variation studies. Water doping of matrices enabled the identification of spectral features due to weakly bound complexes of lactic acid with water.
The structures and vibrational spectra of the preferred conformers of the neutral form of N,N-dimethylglycine (DMG) were studied by a combined approach, using DFT(B3LYP)/6-311++G** and MP2/6-31++G** calculations and low temperature matrix isolation IR spectroscopy. The conformational ground state was found to be the intramolecularly O-HÁ Á ÁN hydrogen-bonded GAT form, where the (lone pair)-N-C-C and N-C-C=O dihedral angles are 30 (gauche; G) and ca. 180 (anti; A), respectively, and the carboxylic group assumes the trans (T ) configuration (O=C-O-H dihedral angle equal to 180). The presence in the matrices of two additional conformers, where the carboxylic moiety assumes the most commonly found cis (C) conformation and the N-C-C=O axis adopts the syn arrangement (the two conformers differ only in the positions of the methyl groups), could also be established. Observation of these conformers is in consonance with the theoretical predictions, which indicate that the observed conformers should differ in energy by less than 7 kJ mol À1. Full assignment of the observed infrared spectra of both DMG and its -OD isotopomer in Ar and Xe matrices was carried out on the basis of comparison with the theoretically predicted spectra and temperature variation experiments.
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