A study of the solvent dependence of the structures and the vibrational, 1H and 13C NMR spectra of l- and dl-mandelic acid and l- and dl-3-phenyllactic acid

• The structure of mandelic and 3-phenyllactic acids was optimized at the DFT B3LYP level.
• The vibrational wavenumbers and NMR chemical shifts were calculated.
• Vibrational and NMR assignments were provided by combining experimental and theoretical data.
• The Gibbs free energy changes of solvation were computed.

The structural stability of mandelic acid and 3-phenyllactic acid was investigated by Density Functional B3LYP calculations with the 6-311G∗∗ basis set. The two acids were predicted to have non-planar forms with cisoid α-hydroxy moiety with respect to the CO group as their lowest energy structures. From the SCRF = SMD calculations the stability of both acids is predicted to be significantly dependent on the dielectric constant of the solvent. The infrared and Raman spectra of l-mandelic and l-3-phenyllactic acids were compared to the corresponding solid state spectra of the dl-racemic mixtures. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of the molecules. The 1H and 13C NMR spectra of both acids were interpreted by experimental and DFT calculated chemical shifts of the two acids. The RMSD between experimental and theoretical 1H and 13C chemical shifts for mandelic acid is 0.961 and 6.119 ppm, whereas for 3-phenyllactic acid, it is 0.611 and 5.214 ppm, respectively.

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