Raman and infrared spectra, conformations and ab initio calculations of 3-methoxymethylene-2,4-pentanedione

The IR (4000-400 cm^-1) and Raman (4000-50 cm^-1) spectra of 3-methoxymethylene-2,4-pentanedione [H₃C-O-CH=C(COCH₃)₂] as a liquid and solutes in various solvents of different polarity have been recorded at ambient temperature. Additional IR and Raman spectra were obtained for amorphous and crystalline solid state at low temperature. The vibrational spectra revealed that the compound exists at least in two dominant conformers of different polarity and that the conformer present in the solid phase is less polar. NMR spectra in various solvents at different temperatures were also obtained.

The compound can exist in several conformers due to the rotation around O-C= and both =C-C bonds with planar or nonplanar arrangements of heavy atoms. Ab initio MP2 and DFT calculations using a wide scale of basis sets were carried out. According to these calculations six conformational structures of the eight theoretically possible conformational structures with the methoxy group oriented as anti or syn and carbonyl groups oriented as Z or E towards the C=C double bond were obtained at potential energy surface. It has been shown that the conformers with the E orientations of both acetyl groups are not the stable ones. The calculated ab initio MP2 and DFT energies of all found conformers in vacuum suggest the most stable ZEa conformer where Z and E regard to the trans and cis acetyl groups, respectively and a denotes the orientation of the methoxy group. The EZa conformer was calculated as the second most stable one with the energy by at least 10 kJ mol^-1 higher. Corrections of the relative energies of single conformers obtained in vacuum on the polar surroundings were done by including the solvent effect into the calculations using IEF Polarizable Continuum Model.

Assignments of the vibrational spectra for the studied compound were made with the aid of normal coordinate calculations employing scaled ab initio force field. The scaled ab initio frequencies as well as calculated energies indicate that ZEa is the conformer present in the solid phase.