Molecular vibrations of acetylacetone enol. Infrared polarization spectroscopy and theoretical predictions

The IR polarization spectrum of acetylacetone enol (AAe, (3Z)-4-hydroxy-3-penten-2-one) was recorded in the region 2000 - 450 cm^-1 using stretched polyethylene as an anisotropic solvent. The measured orientation factors were consistent with C_s molecular symmetry of AAe and provided an experimental distinction between in-plane and out-of-plane polarized spectral features. The results suggest the assignment of at least one previously unrecognized fundamental transition. Excellent agreement with presently and previously observed transition wavenumbers in the investigated region was obtained by theoretical predictions in the harmonic approximation using B3LYP-D3 Density Functional Theory (DFT); calculated diagrams are provided for all 39 normal modes of AAe. The prediction of in-plane transition moment directions was found to be very sensitive to calculational details. The wavenumbers computed with the anharmonic models VPT2 (2nd-order Vibrational Perturbation Theory) and DVPT2 (Deperturbed VPT2) were consistent with those of the harmonic analysis, but the predictions obtained using GVPT2 (Generalized VPT2) deviated significantly.