Rotational isomers of lactic acid: first experimental observation of higher energy forms

Abstract

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.

Self-Aggregation in Pyrrole: Matrix Isolation, Solid State Infrared Spectroscopy, and DFT Study

“…The value of the scaling factor has been chosen in consonance with previously reported calculations, performed at the same level of theory on other molecules containing first row elements and where H-bonding was found to play an important role. 69,70,72 Corrections to calculated energies due to basis set superposition error (BSSE) were obtained by the full counterpoise procedure. 73 For a complex between N molecules (N g 2), the BSSE correction is described by the following equation…”

“…66 The level of theory was selected taking into consideration the available information on the good general quality of the B3LYP functional to deal with intermolecular forces, in particular, hydrogen bonding. [67][68][69][70] All structures were optimized using the geometry direct inversion of the invariant subspace (GDIIS) method, 71 with the TIGHT convergence criteria defined in Gaussian. Calculations were undertaken with and without symmetry constrains, all calculations converging to the symmetrical structures discussed in detail in the next sections.…”

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

“…All optimized structures were confirmed to be minimum energy conformations. The calculated frequencies were scaled down by a single factor (0.978) [7,20] to correct them for the effects of basis set limitations, neglected part of electron correlation and anharmonicity effects, and used to assist the analysis of the experimental spectra and to account for the zero-point vibrational energy contribution to the total energy. Normal coordinate analysis was undertaken in the internal coordinates space as described by Schachtschneider [34] using the program BALGA and the optimized geometries and harmonic force constants resulting from the DFT(B3LYP)/ 6-311þþG(d,p) calculations.…”

“…1 Table 1). In lactic acid, the fourth more stable conformer is AaT (with a relative energy within a range of 10 kJ mol À1 [7]), which has no equivalent form in MLac since in this latter molecule the stabilizing OH acid Á Á Á O alcohol hydrogen bond cannot operate due to the substitution of the carboxylic hydrogen atom by the methyl ester group. Indeed, in the ester, the close proximity of the methyl ester group and the hydroxyl oxygen atom leads to a strong repulsion that is reflected in the fact that the AaT conformation is not a minimum energy species.…”

“…On the other hand, since the energy barrier to convert the G 0 sk 0 C conformer into the GskC is only $2 kJ mol À1 , conformational cooling [19,20] precludes the observation of the G 0 sk 0 C form in low-temperature matrices. This last form converts easily to the slightly more stable GskC conformer [7].…”