angle of incidence should be changed to achieve comparable transmittance of the pand s-polarized light into the aqueous phase. This leads to a large decrease in the
MSEFS values (Fig. 2.5b). The spectral range of the transmission of the electromagnetic radiation by Ge is in the range of 2 μm to 16 μm. Use of the Si prism limits
the available spectral region from 1.2 μm to 8 μm. The optimal angles of incidence
for the use of Ge and Si prisms are in the range of 15
–20
(Table 2.1). These angles
of incidence are difficult to achieve in the experimental IRRAS set-up.
2.3.3 Composition of the Sample: Isotopic Substitution in IRS
The main analytical problem in studies of self-assembled, multicomponent molecular films arises from their compositional complexity. A large number of organic
compounds contains methylene, methyl, carboxylic, amide, amine or hydroxyl
groups. The same functional groups, even if located in different molecules, absorb
the IR light at very similar wavenumbers. Thus, different components of a system
give overlapping IR absorption bands which cannot be deconvoluted. From a
practical point of view the analysis of methyl and methylene groups is very important, because they give IR absorption modes which are separated from the IR
absorption modes of other functional groups and are commonly present in organic
molecules. For these reasons the stretching modes of the methylene and methyl
groups are widely investigated [56–60]. Replacement of H by D, due to twofold
increase in mass of the D atom, leads to a large bathochromic shift of the IR
absorption modes of perdeuterated molecules. Isotopic substitution is a powerful
tool in the vibrational spectroscopy which gives large spectral changes with very
little change in the chemical nature of the system. A mass change of isotopes of
hydrogen causes large changes in the position of the stretching modes involving
carbon–hydrogen atoms (Table 2.2).
For molecules containing a hydrocarbon chain, the substitution of H by D atoms
causes ca. 700 cm
À1 bathochromic shift of the ν(CD) modes compared to
ν(CH) modes (Table 2.2). Despite the fact that the synthesis of specifically deuterated molecules is expensive and time-consuming, the need for this kind of molecules
in research is clear. For example, a large variety of lipids containing either deuterated
or per-deuterated hydrocarbon chains are commercially available [62]. Figure 2.12
Table 2.2 Reduced mass and wavenumber of carbon–hydrogen isotope stretching mode
Functional group
Reduced mass (kg)
Wavenumber of the stretching mode (cm
À1
)
C–H
1.544 Â 10
À27
3050–2800
C–D
2.868 Â 10
À27
2300–2100
C–T
4.010 Â 10
À27 [61]
ca. 1800 [61]
2.3 Experimental Considerations Prior to In Situ Spectroelectrochemical Experiments
25
MSEFS values (Fig. 2.5b). The spectral range of the transmission of the electromagnetic radiation by Ge is in the range of 2 μm to 16 μm. Use of the Si prism limits
the available spectral region from 1.2 μm to 8 μm. The optimal angles of incidence
for the use of Ge and Si prisms are in the range of 15
–20
(Table 2.1). These angles
of incidence are difficult to achieve in the experimental IRRAS set-up.
2.3.3 Composition of the Sample: Isotopic Substitution in IRS
The main analytical problem in studies of self-assembled, multicomponent molecular films arises from their compositional complexity. A large number of organic
compounds contains methylene, methyl, carboxylic, amide, amine or hydroxyl
groups. The same functional groups, even if located in different molecules, absorb
the IR light at very similar wavenumbers. Thus, different components of a system
give overlapping IR absorption bands which cannot be deconvoluted. From a
practical point of view the analysis of methyl and methylene groups is very important, because they give IR absorption modes which are separated from the IR
absorption modes of other functional groups and are commonly present in organic
molecules. For these reasons the stretching modes of the methylene and methyl
groups are widely investigated [56–60]. Replacement of H by D, due to twofold
increase in mass of the D atom, leads to a large bathochromic shift of the IR
absorption modes of perdeuterated molecules. Isotopic substitution is a powerful
tool in the vibrational spectroscopy which gives large spectral changes with very
little change in the chemical nature of the system. A mass change of isotopes of
hydrogen causes large changes in the position of the stretching modes involving
carbon–hydrogen atoms (Table 2.2).
For molecules containing a hydrocarbon chain, the substitution of H by D atoms
causes ca. 700 cm
À1 bathochromic shift of the ν(CD) modes compared to
ν(CH) modes (Table 2.2). Despite the fact that the synthesis of specifically deuterated molecules is expensive and time-consuming, the need for this kind of molecules
in research is clear. For example, a large variety of lipids containing either deuterated
or per-deuterated hydrocarbon chains are commercially available [62]. Figure 2.12
Table 2.2 Reduced mass and wavenumber of carbon–hydrogen isotope stretching mode
Functional group
Reduced mass (kg)
Wavenumber of the stretching mode (cm
À1
)
C–H
1.544 Â 10
À27
3050–2800
C–D
2.868 Â 10
À27
2300–2100
C–T
4.010 Â 10
À27 [61]
ca. 1800 [61]
2.3 Experimental Considerations Prior to In Situ Spectroelectrochemical Experiments
25