Intermolecular hydrogen bonding in the binary mixture [(C2H5)2COÂ +Â CH3OH] probed by polarized Raman measurements and DFT calculations

The Raman spectra of neat (C2H5)2CO (pentanone) and its binary mixtures with hydrogen donor solvent (CH3OH), [(C2H5)2CO + CH3OH] having different mole fractions of the reference system, (C2H5)2CO in the range 0.1-0.9 at a regular interval of 0.1 were recorded in the CO stretching region. In neat liquid, the Raman peak appears asymmetric. The asymmetric nature of the peak has been attributed to the CO stretching mode of the two conformers of (C2H5)2CO having C2 and C2v point groups and the corresponding bands at ∼1711 and ∼1718 cm−1, respectively. A careful analysis of the Iiso (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 CO 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 (C2H5)2CO in C2 ad C2v forms and the complexes with one and two CH3OH 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, [(C2H5)2CO + CH3OH] at intermediate concentrations.