252
10 Applications: Organic Interfaces
amplitude to both components. As a result, the calculated Im[χ (2) ] spectrum does
not apparently show the ν 2,9 modes. However, if we tentatively augmented the force
constant of the ν 2,9 modes with the other conditions intact, the MD simulation would
yield the Im[χ (2) ] spectrum with the red dashed line. This is a “thought experiment”,
feasible with MD simulation. Then a positive band of the asymmetric stretching
modes emerges and moves to the blue, and consequently the positive band is
separated from the original high-frequency negative component. 2 We also find that
the original negative component at ∼2950 cm −1 increases its negative amplitude
after the asymetric modes are separated out. This EPSA result of Panel (c) manifests
the spectral overlap of the asymmetric stretching and the Fermi component depicted
in Panel (b).
10.1.4 Ethanol C–H Vibrations
Ethanol contains both methyl and methylene groups, and also has trans and gauche
conformers. Therefore, ethanol is a simplest model to incorporate all the complicating factors (a)–(c) in the alkyl C–H vibrations mentioned in Sect. 10.1.1; (a)
different functional groups, (b) vibrational modes, (c) conformational isomers. An
intensive experimental analysis of the ethanol C–H vibrations has been performed
by Gan et al. [6, 8], who separated the methyl and methylene vibrations using
normal and partially deuterated ethanol species, i.e. CH 3 CH 2 OH, CH 3 CD 2 OH,
CD 3 CH 2 OH. The latter two species allow us to focus on the methyl and methylene
C–H vibrations, respectively, in the ethanol molecule. The comprehensive set
of experimental SFG spectra of ethanol, including the different isotopes and
polarization combinations, were analyzed by the MD simulation with extending the
above modeling of methanol.
MD calculation of liquid ethanol can reproduce and elucidate the C–H vibrational
spectra of infrared, Raman and SFG in a unified manner [34, 35]. One of the
important findings is that the trans and gauche conformers show rather different
spectral components in the SFG spectra. In particular, methylene C–H vibrations
near a gauche defect show a distinct feature in the vibrational SFG spectra. Accurate
modeling of ethanol C–H vibrations is a significant step toward general simulation
of alkyl molecules.
2 The asymmetric C–H stretching mode has a positive amplitude when the molecule is tilted, as
illustrated in Fig. 10.2c.
10 Applications: Organic Interfaces
amplitude to both components. As a result, the calculated Im[χ (2) ] spectrum does
not apparently show the ν 2,9 modes. However, if we tentatively augmented the force
constant of the ν 2,9 modes with the other conditions intact, the MD simulation would
yield the Im[χ (2) ] spectrum with the red dashed line. This is a “thought experiment”,
feasible with MD simulation. Then a positive band of the asymmetric stretching
modes emerges and moves to the blue, and consequently the positive band is
separated from the original high-frequency negative component. 2 We also find that
the original negative component at ∼2950 cm −1 increases its negative amplitude
after the asymetric modes are separated out. This EPSA result of Panel (c) manifests
the spectral overlap of the asymmetric stretching and the Fermi component depicted
in Panel (b).
10.1.4 Ethanol C–H Vibrations
Ethanol contains both methyl and methylene groups, and also has trans and gauche
conformers. Therefore, ethanol is a simplest model to incorporate all the complicating factors (a)–(c) in the alkyl C–H vibrations mentioned in Sect. 10.1.1; (a)
different functional groups, (b) vibrational modes, (c) conformational isomers. An
intensive experimental analysis of the ethanol C–H vibrations has been performed
by Gan et al. [6, 8], who separated the methyl and methylene vibrations using
normal and partially deuterated ethanol species, i.e. CH 3 CH 2 OH, CH 3 CD 2 OH,
CD 3 CH 2 OH. The latter two species allow us to focus on the methyl and methylene
C–H vibrations, respectively, in the ethanol molecule. The comprehensive set
of experimental SFG spectra of ethanol, including the different isotopes and
polarization combinations, were analyzed by the MD simulation with extending the
above modeling of methanol.
MD calculation of liquid ethanol can reproduce and elucidate the C–H vibrational
spectra of infrared, Raman and SFG in a unified manner [34, 35]. One of the
important findings is that the trans and gauche conformers show rather different
spectral components in the SFG spectra. In particular, methylene C–H vibrations
near a gauche defect show a distinct feature in the vibrational SFG spectra. Accurate
modeling of ethanol C–H vibrations is a significant step toward general simulation
of alkyl molecules.
2 The asymmetric C–H stretching mode has a positive amplitude when the molecule is tilted, as
illustrated in Fig. 10.2c.