Vibrational properties of levulinic acid and furan derivatives: Raman spectroscopy and theoretical calculations

In this work, the Raman spectra of furan, furfuryl alcohol (FA), furfural, hydroxymethylfurfural (HMF), and levulinic acid were obtained in the 500 to 4000 cm^-1 spectral region at room temperature. Vibrational wavenumbers were calculated for these compounds with the B3LYP method using the 6-31 + G(2df,p) basis set. The experimentally determined C=C and C-C wavenumbers for furan and furan derivatives were in good agreement with the calculated wavenumbers without scaling factor, while the calculated C=O and C-H wavenumbers at ∼1660 and 3000 cm^-1, respectively, showed larger deviations from the measured ones. The Raman spectra for furan and furan derivatives showed intense C=C bands, whereas the levulinic acid spectrum showed intense C-H vibrations with broad doublet C=O bands. We also found that an empirical method based on the chemical structure similarities is able to predict the HMF Raman spectrum from the combined furfural and FA spectra.