| Abstract
| - The FT-IR spectra of two diastereomers of 2,3-butanediol, (R,S) and (S,S), isolated in low-temperature argonand xenon matrixes were studied, allowing the identification of two different conformers for each compound.These conformers were characterized by a ±gauche arrangement around the O−C−C−O dihedral angle,thus enabling the establishment of a very weak intramolecular hydrogen bond of the O···H−O type. Noother forms of these compounds were identified in matrixes, despite the fact that these four conformers hadcalculated relative energies from 0 to 5.1 kJ mol-1 and were expected to be thermally populated from 50 to6% in the gaseous phase of each compound. The nonobservation of additional conformers was explained interms of low barriers to intramolecular rotation, resulting in the conformational relaxation of the compoundsduring deposition of the matrixes. The barriers to internal rotation of the OH groups were computed to beless than 4 kJ mol-1 and are easily overcome in matrixes within the family of conformers with the sameheavy atom backbone. The barriers for intramolecular rearrangement of the O−C−C−O dihedral angle inboth diastereomers were calculated to range from 20 to 30 kJ mol-1. Interconversions between the latterconformers were not observed in matrixes, even after annealing up to 65 K. Energy calculations, barriers,and calculated infrared spectra were carried out at the DFT(B3LYP)/6-311++G** theory. Additional MP2/6-311++G** calculations of energies and vibrational frequencies were performed on the most relevantconformers. Finally, independent estimations of the hydrogen-bond enthalpy in the studied molecules werealso obtained based on theoretical structural data and from vibrational frequencies (using well-establishedempirical correlations). The obtained values for −ΔH for both diastereomers of 2,3-butanediol amount toca. 6−8 kJ mol-1.
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