248
10 Applications: Organic Interfaces
essentially an issue of molecular modeling for the SFG calculations, and the general
theory of polarization in Chap. 6 is particularly valuable for this requirement.
Second, even the C–H bands of organic species are actually quite complicated and
challenging to the analysis of SFG spectra. General formulas of C–H vibrations in
the SFG spectroscopy have been proposed by Hirose and co-workers in early SFG
theory [11, 12], and sophisticated by Wang and co-workers [7, 32]. These formulas
are useful to characterize the SFG signals of the C–H band in various polarizations.
A remaining issue in the theoretical analysis is to advance the MD simulation toward
reproducing and even predicting the observed SFG spectra of organic interfaces.
The MD analysis aims at elucidating observed SFG spectra of various species
using realistic molecular properties of constituent species. Theoretical analysis of
organic SFG spectra has been relatively less developed in comparison to those
of aqueous systems at present. The reliable theoretical support is definitely called
for in practical interpretation of SFG spectra of organic species, particularly when
examples of related infrared or Raman vibrational spectra do not offer a useful
clue to help understanding the spectral features of SFG. This chapter summarizes
the basis of theoretical SFG analysis of organic interfaces with the help of MD
simulation.
10.1 C–H Bands of Alkyl Groups
The C–H vibrational band of alkyl groups appears in 2800–3000 cm −1 range, and
has been one of the most common targets in the SFG measurement to date, as
well as the O-H band of aqueous systems. The C–H band is ubiquitous in organic
molecules, and its band shapes offer detailed information on the interface. Precise
theoretical analysis of the C–H band is particularly beneficial to the application of
SFG spectroscopy.
10.1.1 C–H Modes
Conventional assignment of the C–H band is performed in comparison with the
infrared and Raman spectra of the corresponding C–H vibrations. The components
in the C–H band are usually assigned with the notations in Fig. 10.1. In a methyl
(CH 3 –) group, the symmetric and asymmetric C–H vibrations are labeled with r +
and r − , respectively. The asymmetric vibration consists of two modes, classified
into in-plane (r
−
IP ) and out-of-plane (r
+
OP ) modes. In a methylene (–CH 2 –) group,
the symmetric and asymmetric C–H vibrations are labeled with d + and d − ,
respectively. These C–H stretching modes are generally close to the overtone or
combination of bending modes, and thus often split by the Fermi resonance. The
satellite modes of the Fermi resonace are labeled with FR, such as r
+
FR and d
+
FR .
10 Applications: Organic Interfaces
essentially an issue of molecular modeling for the SFG calculations, and the general
theory of polarization in Chap. 6 is particularly valuable for this requirement.
Second, even the C–H bands of organic species are actually quite complicated and
challenging to the analysis of SFG spectra. General formulas of C–H vibrations in
the SFG spectroscopy have been proposed by Hirose and co-workers in early SFG
theory [11, 12], and sophisticated by Wang and co-workers [7, 32]. These formulas
are useful to characterize the SFG signals of the C–H band in various polarizations.
A remaining issue in the theoretical analysis is to advance the MD simulation toward
reproducing and even predicting the observed SFG spectra of organic interfaces.
The MD analysis aims at elucidating observed SFG spectra of various species
using realistic molecular properties of constituent species. Theoretical analysis of
organic SFG spectra has been relatively less developed in comparison to those
of aqueous systems at present. The reliable theoretical support is definitely called
for in practical interpretation of SFG spectra of organic species, particularly when
examples of related infrared or Raman vibrational spectra do not offer a useful
clue to help understanding the spectral features of SFG. This chapter summarizes
the basis of theoretical SFG analysis of organic interfaces with the help of MD
simulation.
10.1 C–H Bands of Alkyl Groups
The C–H vibrational band of alkyl groups appears in 2800–3000 cm −1 range, and
has been one of the most common targets in the SFG measurement to date, as
well as the O-H band of aqueous systems. The C–H band is ubiquitous in organic
molecules, and its band shapes offer detailed information on the interface. Precise
theoretical analysis of the C–H band is particularly beneficial to the application of
SFG spectroscopy.
10.1.1 C–H Modes
Conventional assignment of the C–H band is performed in comparison with the
infrared and Raman spectra of the corresponding C–H vibrations. The components
in the C–H band are usually assigned with the notations in Fig. 10.1. In a methyl
(CH 3 –) group, the symmetric and asymmetric C–H vibrations are labeled with r +
and r − , respectively. The asymmetric vibration consists of two modes, classified
into in-plane (r
−
IP ) and out-of-plane (r
+
OP ) modes. In a methylene (–CH 2 –) group,
the symmetric and asymmetric C–H vibrations are labeled with d + and d − ,
respectively. These C–H stretching modes are generally close to the overtone or
combination of bending modes, and thus often split by the Fermi resonance. The
satellite modes of the Fermi resonace are labeled with FR, such as r
+
FR and d
+
FR .