Self-association and hydrogen bonding of propionaldehyde in binary mixtures with water and methanol investigated by concentration-dependent polarized Raman study and DFT calculations

Abstract

The Raman spectra of neat propionaldehyde [CH3CH2CHO or propanal (Pr)] and its binary mixtures with hydrogen-donor solvents, water (W) and methanol (M), [CH3CH2CHO H2O] and CH3CH2CHO CH3OH] with different mole fractions of the reference system, Pr varying from 0.1 to 0.9 at a regular interval of 0.1, were recorded in the nu(C=O) stretching region, 1600-1800 cm(-1). The isotropic parts of the Raman spectra were analyzed for both the cases. The wavenumber positions and line widths of the component bands were determined by a rigorous line-shape analysis, and the peaks corresponding to self-associated and hydrogen-bonded species were identified. Raman peak at similar to 1721 cm(-1) in neat Pr, which has been attributed to the self-associated species, downshifts slightly (similar to 1 cm(-1)) in going from mole fraction 0.9 to 0.6 in (Pr W) binary mixture, but on further dilution it shows a sudden downshift of similar to 7 cm(-1). This has been attributed to the low solubility of Pr in W (similar to 30%), which does not permit a hydrogen-bonded network to form at higher concentrations of Pr. A significant decrease in the intensity of this peak in the Raman spectra of Pr in a nonpolar solvent, n-heptane, at high dilution (C = 0.05) further confirms that this peak corresponds to the self-associated species. In case of the (Pr M) binary mixture, however, the spectral changes with concentration show a rather regular trend and no special features were observed. Copyright (C) 2010 John Wiley & Sons, Ltd.