13 Overview of Application of NIR Spectroscopy to Physical …
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bands of melamine are more sensitive to the molecule’s chemical neighborhood as
compared to NIR bands. It was reasoned that neglecting the chemical environment
as done in simple models (e.g. single molecule) largely reduces the accuracy in the
description of those vibrations. In contrast, NIR bands seem to be less dependent on
the chemical environment, in particular to the adjacent crystalline planes in the structure of melamine [53]. Comprehension of the relationships between MIR and NIR
spectra is important from the point of view of spectroscopic studies and applications.
Further investigations are necessary to provide a more complete picture; however,
it is clear that spectral simulations play an important role in the exploration of this
important aspect.
It is to be noted that new developments in anharmonic theories and applications
are predominantly validated on the basis of MIR spectra, which are readily available and simpler for analysis [40]. However, results of these studies have important
implications for NIR spectroscopy as well [3]. For instance, an in–depth understanding of the origin and nature of overtone and combination bands in the MIR
range may be achieved [54]. Interestingly, also in MIR region, these bands are far
more numerous than fundamental counterparts [54]. Typically, overtones and combinations bands are very weak, unless they are in resonance with the fundamental
bands. In practical applications, calculations based on VSCF have a less favorable
efficiency–to–accuracy ratio and a number of improvements have been proposed in
recent years. These improvements increase the efficiency of the VSCF approach, e.g.
by reducing the grid density for potential evaluations or by employing more efficient
ways for determination of the electronic structure underlying of anharmonic vibrational analysis (e.g. resolution of the identity (RI) approximation in connection with
Moller–Plesset second-order perturbation, i.e. RI–MP2). Interestingly, anharmonic
(VSCF and VPT2) calculations have been used in connection with IR power spectra
predicted by velocity autocorrelation of ab initio QMCF–MD and QM/MM–MD
trajectories [40]. Such approaches can substantially increase the potential of interpretation of MIR spectra measured for highly labile systems, e.g. hydrogen–bonded
molecules in aqueous solution [40].
13.5 Solution Chemistry
In the NIR region, molar absorption coefficients of many molecules are small, and
thus, NIR spectroscopy is highly suitable for the investigation of hydrogen bonding
in condensed phase. One can measure bands such as those due to water and solutions
easily with high reproducibility. One can also reduce the effect of interface which
often yields a serious issue in IR and ATR–IR spectroscopy. Using these advantages,
Ikehata et al. [55] explored miscibility of solutions. They investigated the thermal
phase behavior of triethylamine (TEA)–water mixtures which show phase separation
with the lower critical solution temperature (LCST) type by NIR spectroscopy. They
paid attention on a band shift of the first overtone of the C–H stretching modes
of TEA and made a phase diagram of the mixtures. They originally thought that
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bands of melamine are more sensitive to the molecule’s chemical neighborhood as
compared to NIR bands. It was reasoned that neglecting the chemical environment
as done in simple models (e.g. single molecule) largely reduces the accuracy in the
description of those vibrations. In contrast, NIR bands seem to be less dependent on
the chemical environment, in particular to the adjacent crystalline planes in the structure of melamine [53]. Comprehension of the relationships between MIR and NIR
spectra is important from the point of view of spectroscopic studies and applications.
Further investigations are necessary to provide a more complete picture; however,
it is clear that spectral simulations play an important role in the exploration of this
important aspect.
It is to be noted that new developments in anharmonic theories and applications
are predominantly validated on the basis of MIR spectra, which are readily available and simpler for analysis [40]. However, results of these studies have important
implications for NIR spectroscopy as well [3]. For instance, an in–depth understanding of the origin and nature of overtone and combination bands in the MIR
range may be achieved [54]. Interestingly, also in MIR region, these bands are far
more numerous than fundamental counterparts [54]. Typically, overtones and combinations bands are very weak, unless they are in resonance with the fundamental
bands. In practical applications, calculations based on VSCF have a less favorable
efficiency–to–accuracy ratio and a number of improvements have been proposed in
recent years. These improvements increase the efficiency of the VSCF approach, e.g.
by reducing the grid density for potential evaluations or by employing more efficient
ways for determination of the electronic structure underlying of anharmonic vibrational analysis (e.g. resolution of the identity (RI) approximation in connection with
Moller–Plesset second-order perturbation, i.e. RI–MP2). Interestingly, anharmonic
(VSCF and VPT2) calculations have been used in connection with IR power spectra
predicted by velocity autocorrelation of ab initio QMCF–MD and QM/MM–MD
trajectories [40]. Such approaches can substantially increase the potential of interpretation of MIR spectra measured for highly labile systems, e.g. hydrogen–bonded
molecules in aqueous solution [40].
13.5 Solution Chemistry
In the NIR region, molar absorption coefficients of many molecules are small, and
thus, NIR spectroscopy is highly suitable for the investigation of hydrogen bonding
in condensed phase. One can measure bands such as those due to water and solutions
easily with high reproducibility. One can also reduce the effect of interface which
often yields a serious issue in IR and ATR–IR spectroscopy. Using these advantages,
Ikehata et al. [55] explored miscibility of solutions. They investigated the thermal
phase behavior of triethylamine (TEA)–water mixtures which show phase separation
with the lower critical solution temperature (LCST) type by NIR spectroscopy. They
paid attention on a band shift of the first overtone of the C–H stretching modes
of TEA and made a phase diagram of the mixtures. They originally thought that