13 Overview of Application of NIR Spectroscopy to Physical …
305
Morisawa and Suga studied the effect of intermolecular interactions on intensity
of the overtones of OH stretching vibrations of methanol, methanol–d 3 , and tertbutanol-d 9 in n–hexane [31]. The authors determined the relative intensities of the
free and hydrogen–bonded OH for the fundamental and overtone bands (ν 01 , ν 02 , ν 03 ,
and ν 04 ). The obtained results suggest that variation in the dipole moment function of
the OH group generated by hydrogen bond formation induces changes in the transient
dipole moment. Dong et al. examined interactions in ethanol/water mixture by using
NIR spectroscopy [32]. The application of a curve-fitting method enabled the identification of six different kinds of water species in the alcohol–poor region (<10%).
The connection of the excess spectra and 2DCOS for mixtures with higher alcohol
content (10–100%) revealed that the maximum of the alcohol–water interactions
occur at an alcohol concentration of 40%. Further increase of the alcohol content
resulted in self-association of ethanol molecules at the expense of the water–alcohol
associates.
The structure of water has been also studied by NIR spectroscopy together with
PCA, 2DCOS, and MCR. The studies are described in the other chapters of this book.
13.3 Anharmonic Effects in Vibrational Spectroscopy
NIR spectroscopy is a powerful tool to investigate the anharmonicity in molecular
vibrations and its impact on vibrational spectra. Considerable attention has been paid
to the interplay of hydrogen bonding and anharmonic effects [12]. It is known that the
formation of a hydrogen bond induces a change in anharmonicity in the vibrations
of the respective bonded groups. However, an intensive discussion of the extent of
this change in anharmonicity took place over the years. For some hydrogen–bonded
complexes, anharmonicity leads to a decrease in the fundamental excitation of an
X–H bond, whereas the wavenumbers of overtones displays an increase. However, in
many cases, it has been difficult to obtain quantitative insight into the anharmonicity
and the associated coupling constants since experimental methods have a limited
potential in this regard. Therefore, progress achieved in this area is strongly connected
with quantum chemical calculation studies. Over the last decades, numerous investigations have been aimed at different types of hydrogen–bonded complexes. The
anharmonicity of a moderately strong hydrogen–bonded HCN:::HF complex is relatively well studied. It was determined that a moderate increase by 27 cm
−1 (from 90
to 117 cm
−1 ) in the anharmonic constant of the ν(HF) mode occurs in the HCN:::HF
complex as compared to non-bonded HF [12]. Investigations of alcohols enabled to
draw comparative data on the change of anharmonicity induced to stretching and
bending OH vibrations upon formation of hydrogen-bonded OH:::O complexes. The
conclusion was drawn that the anharmonicity of the high–frequency stretching vibration is amplified and the magnitude of this effect is correlated to the strength of the
hydrogen bond [12].
305
Morisawa and Suga studied the effect of intermolecular interactions on intensity
of the overtones of OH stretching vibrations of methanol, methanol–d 3 , and tertbutanol-d 9 in n–hexane [31]. The authors determined the relative intensities of the
free and hydrogen–bonded OH for the fundamental and overtone bands (ν 01 , ν 02 , ν 03 ,
and ν 04 ). The obtained results suggest that variation in the dipole moment function of
the OH group generated by hydrogen bond formation induces changes in the transient
dipole moment. Dong et al. examined interactions in ethanol/water mixture by using
NIR spectroscopy [32]. The application of a curve-fitting method enabled the identification of six different kinds of water species in the alcohol–poor region (<10%).
The connection of the excess spectra and 2DCOS for mixtures with higher alcohol
content (10–100%) revealed that the maximum of the alcohol–water interactions
occur at an alcohol concentration of 40%. Further increase of the alcohol content
resulted in self-association of ethanol molecules at the expense of the water–alcohol
associates.
The structure of water has been also studied by NIR spectroscopy together with
PCA, 2DCOS, and MCR. The studies are described in the other chapters of this book.
13.3 Anharmonic Effects in Vibrational Spectroscopy
NIR spectroscopy is a powerful tool to investigate the anharmonicity in molecular
vibrations and its impact on vibrational spectra. Considerable attention has been paid
to the interplay of hydrogen bonding and anharmonic effects [12]. It is known that the
formation of a hydrogen bond induces a change in anharmonicity in the vibrations
of the respective bonded groups. However, an intensive discussion of the extent of
this change in anharmonicity took place over the years. For some hydrogen–bonded
complexes, anharmonicity leads to a decrease in the fundamental excitation of an
X–H bond, whereas the wavenumbers of overtones displays an increase. However, in
many cases, it has been difficult to obtain quantitative insight into the anharmonicity
and the associated coupling constants since experimental methods have a limited
potential in this regard. Therefore, progress achieved in this area is strongly connected
with quantum chemical calculation studies. Over the last decades, numerous investigations have been aimed at different types of hydrogen–bonded complexes. The
anharmonicity of a moderately strong hydrogen–bonded HCN:::HF complex is relatively well studied. It was determined that a moderate increase by 27 cm
−1 (from 90
to 117 cm
−1 ) in the anharmonic constant of the ν(HF) mode occurs in the HCN:::HF
complex as compared to non-bonded HF [12]. Investigations of alcohols enabled to
draw comparative data on the change of anharmonicity induced to stretching and
bending OH vibrations upon formation of hydrogen-bonded OH:::O complexes. The
conclusion was drawn that the anharmonicity of the high–frequency stretching vibration is amplified and the magnitude of this effect is correlated to the strength of the
hydrogen bond [12].