FT-IR, FT-Raman, UV–Visible, and NMR spectroscopy and vibrational properties of the labdane-type diterpene 13-epi-sclareol

Abstract In this work, FT-IR, FT-Raman, UV–Visible and NMR spectroscopies and density functional theory (DFT) calculations were employed to study the structural and vibrational properties of the labdane-type diterpene 13- epi -sclareol using the hybrid B3LYP method together with the 6-31G ∗ basis set. Three stable structures with minimum energy found on the potential energy curves (PES) were optimized, and the corresponding molecular electrostatic potentials, atomic charges, bond orders, stabilization energies and topological properties were computed at the same approximation level. The complete assignment of the bands observed in the vibrational spectrum of 13- epi -sclareol was performed taking into account the internal symmetry coordinates for the three structures using the scaled quantum mechanical force field (SQMFF) methodology at the same level of theory. In addition, the force constants were calculated and compared with those reported in the literature for similar compounds. The predicted vibrational spectrum and the calculated 1 H NMR and 13 C NMR chemical shifts are in good agreement with the corresponding experimental results. The theoretical UV–Vis spectra for the most stable structure of 13- epi -sclareol demonstrate a better correlation with the corresponding experimental spectrum. The study of the three conformers by means of the theory of atoms in molecules (AIM) revealed different H bond interactions and a strong dependence of the interactions on the distance between the involved atoms. Furthermore, the natural bond orbital (NBO) calculations showed the characteristics of the electronic delocalization for the two six-membered rings with chair conformations.