13 Overview of Application of NIR Spectroscopy to Physical …
307
Highly accurate calculations of vibrational levels and transition dipole moments is
indispensable in reproducing fine effects observed in NIR spectra that are induced by
the interactions with solvent molecules. The spectral manifestation of the interaction
with solvent was investigated in detail by Futami et al. using similar methods [33].
Again, a pyrrole molecule was examined while the selection of solvents (n–hexane,
CCl 4 , CHCl 3 , and CH 2 Cl 2 ) was dictated by their gradually changing properties,
being dielectric constant, polarity, and acidity. These solvents are suitable for spectral measurements in the NIR and IR regions where ν(NH) fundamental and the first
overtone bands appear. A variation in the shift and intensity of the ν(NH) and 2ν(NH)
bands of pyrrole was observed depending on the solvent used. It was observed that the
shift in the wavenumbers decreases in the following order of solvents: CCl 4 > CHCl 3
> CH 2 Cl 2 . At the same time, the absorption intensity of these two bands increases
in the same order and is more pronounced for the fundamental than the overtone
band. These trends correspond directly to the increasing order of the static solvent
permittivity of the solvents being CCl 4 > CHCl 3 > CH 2 Cl 2 . The study suggested
that the dependency of the solvent shift on the solvent permittivity results from the
anharmonicity of the vibrational potential. However, the intensity variations result
from changes in the slope of the dipole moment function (Fig. 13.6) [33]. Therefore, the study suggested that mechanical and electrical anharmonicity each have a
distinct and non-trivial impact on the observed NIR spectra [33]. Interestingly, the
spectral variability of NH stretching bands of pyrrole caused by solvent effects is
quite different from the trends resulting from hydrogen bonding.
Further insights into the dependency X–H stretching vibrations of a solvated
molecule with respect to the solvent permittivity were reported by Futami et al. in their
examination of the HF molecule [34]. HF is an archetypal polar molecule and a simple
electronic system suitable for application of more advanced theoretical methods.
In addition to density functional theory (DFT) calculations on B3LYP/6–311 +
+G(3df,3pd) level, the significantly more reliable coupled-cluster singles and doubles
method (CCSD) in conjunction with the aug-cc-pVQZ basis set was applied as well.
Both approaches utilize a solvent cavity model by means of SCRF/IPCM [34]. The
study revealed that the vibrational potential and dipole moment function of a solvated
HF molecule vary in accordance with the permittivity of the solvent. Another finding
indicated that the absorption intensities of the fundamental increase proportionally
to the permittivity. However, the intensities of the first, second, and third overtones
do not increase continuously. In addition, the study demonstrated the accuracy of the
applied quantum chemical approaches, with the DFT–B3LYP and CCSD methods
leading to substantially different calculation results in the dependence of absorption
intensities on static solvent permittivity [34].
The impact of solvent effects on the anharmonicity of the potential was further
explored by Gonjo et al. [2]. They examined solvent effects in MIR and NIR spectra
of phenol and its 2,6–dihalogenated derivatives with F, Br, and Cl atoms (Fig. 13.7).
The experimental study revealed a characteristic pattern in the intensity of the consecutive OH stretching band of the fundamental, first, second, and third overtone. Moreover, it was deemed sensitive to the interaction with solvent molecules. Again, the
same solvents as in previous studies were used, namely CH 2 Cl 2 , CHCl 3 , CCl 4 , and
307
Highly accurate calculations of vibrational levels and transition dipole moments is
indispensable in reproducing fine effects observed in NIR spectra that are induced by
the interactions with solvent molecules. The spectral manifestation of the interaction
with solvent was investigated in detail by Futami et al. using similar methods [33].
Again, a pyrrole molecule was examined while the selection of solvents (n–hexane,
CCl 4 , CHCl 3 , and CH 2 Cl 2 ) was dictated by their gradually changing properties,
being dielectric constant, polarity, and acidity. These solvents are suitable for spectral measurements in the NIR and IR regions where ν(NH) fundamental and the first
overtone bands appear. A variation in the shift and intensity of the ν(NH) and 2ν(NH)
bands of pyrrole was observed depending on the solvent used. It was observed that the
shift in the wavenumbers decreases in the following order of solvents: CCl 4 > CHCl 3
> CH 2 Cl 2 . At the same time, the absorption intensity of these two bands increases
in the same order and is more pronounced for the fundamental than the overtone
band. These trends correspond directly to the increasing order of the static solvent
permittivity of the solvents being CCl 4 > CHCl 3 > CH 2 Cl 2 . The study suggested
that the dependency of the solvent shift on the solvent permittivity results from the
anharmonicity of the vibrational potential. However, the intensity variations result
from changes in the slope of the dipole moment function (Fig. 13.6) [33]. Therefore, the study suggested that mechanical and electrical anharmonicity each have a
distinct and non-trivial impact on the observed NIR spectra [33]. Interestingly, the
spectral variability of NH stretching bands of pyrrole caused by solvent effects is
quite different from the trends resulting from hydrogen bonding.
Further insights into the dependency X–H stretching vibrations of a solvated
molecule with respect to the solvent permittivity were reported by Futami et al. in their
examination of the HF molecule [34]. HF is an archetypal polar molecule and a simple
electronic system suitable for application of more advanced theoretical methods.
In addition to density functional theory (DFT) calculations on B3LYP/6–311 +
+G(3df,3pd) level, the significantly more reliable coupled-cluster singles and doubles
method (CCSD) in conjunction with the aug-cc-pVQZ basis set was applied as well.
Both approaches utilize a solvent cavity model by means of SCRF/IPCM [34]. The
study revealed that the vibrational potential and dipole moment function of a solvated
HF molecule vary in accordance with the permittivity of the solvent. Another finding
indicated that the absorption intensities of the fundamental increase proportionally
to the permittivity. However, the intensities of the first, second, and third overtones
do not increase continuously. In addition, the study demonstrated the accuracy of the
applied quantum chemical approaches, with the DFT–B3LYP and CCSD methods
leading to substantially different calculation results in the dependence of absorption
intensities on static solvent permittivity [34].
The impact of solvent effects on the anharmonicity of the potential was further
explored by Gonjo et al. [2]. They examined solvent effects in MIR and NIR spectra
of phenol and its 2,6–dihalogenated derivatives with F, Br, and Cl atoms (Fig. 13.7).
The experimental study revealed a characteristic pattern in the intensity of the consecutive OH stretching band of the fundamental, first, second, and third overtone. Moreover, it was deemed sensitive to the interaction with solvent molecules. Again, the
same solvents as in previous studies were used, namely CH 2 Cl 2 , CHCl 3 , CCl 4 , and