Vibrational spectra of 2-methylpropanal: Assignments, self-associations and solvent effects

Abstract Raman and infrared spectra of 2-methylpropanal (CH 3 ) 2 CHCHO, (CH 3 ) 2 CDCHO and (CH 3 ) 2 CHCDO in different physical states (liquid, solid and solution) have been investigated between 4000 and 40 cm −1 . A complete assignment is carried out on the basis of one predominant conformer (anticlinal) in equilibrium with another less stable one (synperiplanar). Some vibrational modes of the synperiplanar form are identified. The CO stretching region is carefully examined. The complex structure of this band as a function of the physical state of the pure aldehyde and the polarity of the solvent for solutions is discussed. The structure can be explained by Fermi resonances between combinations and the fundamental νCO mode. In the pure liquid, as well as in solution, the broadness of the band can be due to random dipolar interactions. In polar basic solvents (CH 3 CN and DMSO), these interactions implicate both aldehyde and solvent. When the solvent is slightly acidic (HCCl 3 ), weak hydrogen bonds between the aldehydic group and the solvent may occur. Moving from the liquid to the solid state, in addition to the fact that the less stable conformer disappears, molecular association between the CO groups by charge transfer may account for the observed changes.