Molecular structure of 4-hidroxy-3-(3-methyl-2-butenyl) acetophenone, a plant antifungal, by X-ray diffraction, DFT calculation, and NMR and FTIR spectroscopy.

The molecular structure of two mixed and closely related conformers of the title compound, C13H16O2, found in the solid with unequal occupancies has been determined by X-ray diffraction methods. The substance crystallizes in the monoclinic Pca2(1) space group with a=17.279(2), b=5.1716(7), c=12.549(2)Å, and Z=4 molecules per unit cell. The structure was solved from 1314 reflections with I>2σI and refined to an agreement R1-factor of 0.049. The minor conformer (34.7%) is nearly mirror-related to and extensively overlapped with the major one. The skeleton of the 4-hydroxyacetophenone molecular fragment and the prenyl group, (CH2)(CH)C(CH3)2, pendant arm attached to it are both planar and perpendicular to each other. A strong intermolecular O-H⋯O bond links neighboring molecules in the lattice to produce a polymeric structure. The conformational structures of the compound in the gas phase have been calculated by the DFT method and the geometrical results have been compared with the X-ray data. These data allow a complete assignment of vibration modes in the solid state FTIR and Raman spectra. The calculated 1H and 13C chemicals shifts are in good agreement with the corresponding experimental NMR spectra of the compound in solution.

Structural study, coordinated normal analysis and vibrational spectra of 4-hydroxy-3-(3-methyl-2-butenyl)acetophenone.

Structural and vibrational properties of 4-hydroxy-3-(3-methyl-2-butenyl)acetophenone, isolated from Senecio nutans Sch. Bip. (Asteraceae) were studied by infrared and Raman spectroscopies in solid phase. The Density Functional Theory (DFT) method together with Pople's basis set show seven stable conformers for the compound in the gas phase and that only two conformations are probably present in the solid phase. The harmonic vibrational wavenumbers for the optimized geometry were calculated at B3LYP/6-31G and B3LYP/6-311++G levels. For a complete assignment of the vibrational spectra, DFT calculations were combined with Pulay´s Scaled Quantum Mechanics Force Field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental ones. Then, a complete assignment of all the observed bands in the vibrational spectra was performed. The natural bond orbital (NBO) study reveals the characteristics of the electronic delocalization of the two stable structures, while the corresponding topological properties of electronic charge density were analyzed by employing Bader's Atoms in the Molecules theory (AIM).