Interpretation of vibrational and NMR spectra of allyl acrylate: An evidence for several conformers

The mid-IR, far-IR, and Raman spectra of allyl acrylate were measured and interpreted with support of the B3LYP/aug-cc-pVDZ calculated anharmonic vibrational spectra followed by the potential energy distribution analysis. The experimental 1H and 13C NMR spectra of allyl acrylate dissolved in CDCl3 or C6D6 were interpreted by means of the B3LYP/aug-cc-pVDZ-su2 calculated NMR chemical shifts and J(1H,1H) and J(1H,13C) coupling constants. Exactly ten stable allyl acrylate conformers (five s-cis and five s-trans) were found after careful B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ scan of the conformational space. The experimental IR and Raman spectra are in good agreement with the theoretical spectra of the most stable conformers 1 with a presence of the second stable conformer 2, both exhibiting cis arrangement of the acrylic moiety. There are however two bands in the IR spectra, at ca. 1270 and 1260 cm−1, that definitely indicate the conformers with trans arrangement of the acrylic moiety to be present in liquid allyl acrylate. The bands at ca. 2990 and 1650 cm−1 are suggested to be due to Fermi resonances engaging CH and CC stretching vibrations, respectively. The careful inspection of the room temperature 1H and 13C NMR spectra of allyl acrylate suggest that a dominating form of the allyl acrylate molecule in an inert solvent exhibits the cis conformation of the acrylic moiety and an extended allyl group.