Theoretical molecular structure determination infrared frequencies with solvent effect of some α-diketones

Abstract EN

Molecular structure determination for six α-dicarbonyl compounds as well as their vibrational frequencies have been theoretically investigated by molecular orbital by ab initio calculation based on DFT theory level using 3-21G basis set performed by B3PW91 method.

Assignments for vibrational frequencies were made, and solvent effect on the IR frequencies in the carbonyl region was studied using four solvents ; carbon tetrachloride, chloroform, methanol and tetrahydrofuran.

α-dicarbonyl compounds exist in Tran's conformation with planar structure.

Replacement of H atoms in glyoxal by CH3 or phenyl groups has no effect on the planarity of the molecules.

Carbon-carbon bond length of dicarbonyl moiety is shorter in dibenzoylmethane compared with glyoxal, whereas C = O bond shows opposite behavior.

The molecules have two frequencies in the carbonyl region that arise from coupling between the two carbonyl groups in the symmetrical molecules, assigned to two internal normal vibrational modes.