Intermolecular hydrogen bonding in the binary mixture [(C 2H 5) 2C [dbnd]O + CH 3OH] probed by polarized Raman measurements and DFT calculations

Abstract

The Raman spectra of neat (C 2H 5) 2C O (pentanone) and its binary mixtures with hydrogen donor solvent (CH 3OH), [(C 2H 5) 2C O + CH 3OH] having different mole fractions of the reference system, (C 2H 5) 2C O in the range 0.1–0.9 at a regular interval of 0.1 were recorded in the C O stretching region. In neat liquid, the Raman peak appears asymmetric. The asymmetric nature of the peak has been attributed to the C O stretching mode of the two conformers of (C 2H 5) 2C O having C 2 and C 2v point groups and the corresponding bands at ∼1711 and ∼1718 cm −1, respectively. A careful analysis of the I iso (isotropic component of the Raman scattered intensity) at different concentrations reveals that upon dilution with methanol, at mole fraction C = 0.6, an additional peak in the C O stretching region is observed at ∼1703 cm −1 which is attributed to the hydrogen bonding with methanol. A peculiar feature in this study is that upon dilution, the peak at ∼1718 cm −1 shows a minimum at C = 0.6, but on further dilution it shows a blue shift. However, the other peak at ∼1711 cm −1 shows a continuous red shift with dilution as well as a maximum at C = 0.7 in the linewidth vs. concentration plot, which is essentially due to competition between motional narrowing and diffusion phenomena. A significant amount of narrowing in the Raman band at ∼1718 cm −1 can be understood in terms of caging effect of the reference molecule by the solvent molecules at high dilution. A density functional theoretic (DFT) calculation on optimized geometries and vibrational frequencies of two conformers of neat (C 2H 5) 2C O in C 2 ad C 2v forms and the complexes with one and two CH 3OH molecules with both the conformers was performed. The experimental results and theoretical calculations together indicate a co-existence of two conformers as well as hydrogen bonded complex with methanol in the binary mixture, [(C 2H 5) 2C O + CH 3OH] at intermediate concentrations.