A study of the experimental and theoretical infrared, Raman, ¹H and ¹³C NMR spectra of the biochemicals valeric and valproic acids

The structural stability, vibrational, ¹H and ¹³C NMR spectra of valeric and valproic acids were investigated by the B3LYP calculations with the 6-311G^∗∗basis set. Valeric acid is predicted to exist predominantly in the planar cis form (80% abundance). Valproic acid is predicted to have an equilibrium mixture of 68% gauche-1 and 32% gauche-2 structures at 298.15 K. The spectral feature of the OH stretching mode in the infrared spectra of both acids suggests the presence of strong H-bonding in the condensed phase of valeric acid and weak H-bonding in the case of valproic acid. The harmonic and anharmonic 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. Not all of the calculated anharmonic wavenumbers showed a consistent trend with the observed wavenumbers. 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 valeric acid is 1.8 and 3.8 ppm, whereas for valproic acid, it is 1.4 and 4.5 ppm, respectively.