2 research outputs found
C–H···O Interaction in Methanol–Water Solution Revealed from Raman Spectroscopy and Theoretical Calculations
A combination
of temperature-dependent Raman spectroscopy and quantum chemistry
calculation was employed to investigate the blue shift of CH<sub>3</sub> stretching vibration in methanol–water mixtures. It shows
that the conventional O–H···O hydrogen bonds
do not fully dominate the origin of the C–H blue shift and
the weak C–H···O interactions also contribute
to it. This is consistent with the temperature-dependent results,
which reveal that the C–H···O interaction is
enhanced upon increasing the temperature, leading to further C–H
blue shift in observed spectra at high temperature. This behavior
is in contrast with the general trend that the conventional O–H···O
hydrogen bond is destroyed by the temperature. The results will shed
new light onto the nature of the C–H···O interaction
and be helpful to understand hydrophilic and hydrophobic interactions
of amphiphilic molecules in different environments
Identification of Alcohol Conformers by Raman Spectra in the C–H Stretching Region
The
spontaneous polarized Raman spectra of normal and deuterated
alcohols (C<sub>2</sub>–C<sub>5</sub>) have been recorded in
the C–H stretching region. In the isotropic Raman spectra,
a doublet of −C<sub>α</sub>H stretching vibration is
found for all alcohols at below 2900 cm<sup>–1</sup> and above
2950 cm<sup>–1</sup>. By comparing the experimental and calculated
spectra of various deuterated alcohols, the doublets are attributed
to the −C<sub>α</sub>H stretching vibration of different
conformers. For ethanol, the band observed at 2970 cm<sup>–1</sup> is assigned as the stretching vibration of −C<sub>α</sub>H in the C<sub>α</sub>–O–H plane of the <i>gauche-</i>conformer, while the band at 2895 cm<sup>–1</sup> is contributed from both the −C<sub>α</sub>H<sub>2</sub> symmetrical stretching vibration of the <i>trans-</i>conformer
and the −C<sub>α</sub>H stretching vibration out of the
C<sub>α</sub>–O–H plane of the <i>gauche-</i>conformer. The population of <i>gauche</i>-conformer is
estimated to be 54% in liquid ethanol. For the larger alcohols, the
same assignments for the doublet are obtained, and the populations
of <i>gauche</i>-conformers with plane carbon skeleton are
found to be slightly larger than that of ethanol, which is consistent
with results from molecular dynamics simulations