298
M. A. Czarnecki et al.
13.1 Introduction
As discussed in the other chapters of this book, recent progress in instrumentation,
in particular interest of instruments based on Fourier transform (FT) measurements
significantly improved the accuracy of NIR spectra both in terms of wavenumbers
and absorbance scale. The possibility of recording high accuracy spectra permitted
to study fine effects and distinguished very small differences between individual
spectra. Also, the rapid development of computational hardware and software stimulated progress in NIR spectroscopy and theoretical calculations of anharmonic spectra
became an important tool for the understanding and interpretation of NIR spectra [1–
4]. All these circumstances opened new possibilities and areas of applications in NIR
spectroscopy. One of the most important fields of these applications is physical chemistry, which covers a variety of topics like molecular structure, intra- and intramolecular interactions (in particular hydrogen bonding), solvent effects, clustering, phase
transitions, solution, kinetic studies, and so on [5].
The NIR region is very unique since it provides information not accessible from
the other spectral regions [5]. This specificity results from the anharmonicity of
molecular vibrations and the nonlinearity in the change of the dipole moments. Both
phenomena influence the positions and intensities of bands originating from vibrational modes of different molecular fragments. Therefore, NIR spectra are a rich
source of information on molecular structure and interactions. Since the overtones
and combination modes are forbidden in the harmonic approximation, the corresponding bands are much weaker as compared with the fundamental ones. For these
reasons, NIR spectroscopy is a very powerful tool for studies of highly absorbing
samples like bulk materials, pure liquids, or even aqueous solutions. NIR spectra of
bulk liquids can be recorded in commercially available cells of 1–10 mm width (pathlengths). Typically, these cells are made of quartz, which is water resistant. Hence,
NIR spectroscopy can be employed for study of bulk water and aqueous solutions.
However, due to very strong absorption from water, one has to use cells with shorter
pathlengths (<1 mm). As can be seen in Fig. 13.1, both combination bands from bulk
water are accurately recorded in a 0.1 mm quartz cell. In Fig. 13.1 are compared the
spectra of neat 1–propyl alcohol and a 1:1 water/1–propyl alcohol mixture. It appears
that the ν 2 + ν 3 combination band of water is particularly useful for spectroscopic
studies since it is located in the region, which is free from the absorption of the
other overtones and combination bands (5000–5300 cm
−1 ). In contrast, the ν 1 + ν 3
combination band of water (6800–7200 cm
−1 ) is overlapped by the first overtone of
the alcoholic OH and the C–H combination bands [5].
In principle, NIR spectra do not reveal bands originating from carbon–carbon
double and triple bonds. However, bands originating from C–H stretching groups
attached to double or triple bonds are usually blue-shifted [5]. This shift is very well
seen for the first and second overtones of the C–H stretching vibrations of cyclohexane and benzene (Fig. 13.2). This figure illustrates one more important property
of NIR spectra. The first overtones of both compounds have a complex structure,
since it results from the coexistence of the normal and local vibrations [6]. On the
M. A. Czarnecki et al.
13.1 Introduction
As discussed in the other chapters of this book, recent progress in instrumentation,
in particular interest of instruments based on Fourier transform (FT) measurements
significantly improved the accuracy of NIR spectra both in terms of wavenumbers
and absorbance scale. The possibility of recording high accuracy spectra permitted
to study fine effects and distinguished very small differences between individual
spectra. Also, the rapid development of computational hardware and software stimulated progress in NIR spectroscopy and theoretical calculations of anharmonic spectra
became an important tool for the understanding and interpretation of NIR spectra [1–
4]. All these circumstances opened new possibilities and areas of applications in NIR
spectroscopy. One of the most important fields of these applications is physical chemistry, which covers a variety of topics like molecular structure, intra- and intramolecular interactions (in particular hydrogen bonding), solvent effects, clustering, phase
transitions, solution, kinetic studies, and so on [5].
The NIR region is very unique since it provides information not accessible from
the other spectral regions [5]. This specificity results from the anharmonicity of
molecular vibrations and the nonlinearity in the change of the dipole moments. Both
phenomena influence the positions and intensities of bands originating from vibrational modes of different molecular fragments. Therefore, NIR spectra are a rich
source of information on molecular structure and interactions. Since the overtones
and combination modes are forbidden in the harmonic approximation, the corresponding bands are much weaker as compared with the fundamental ones. For these
reasons, NIR spectroscopy is a very powerful tool for studies of highly absorbing
samples like bulk materials, pure liquids, or even aqueous solutions. NIR spectra of
bulk liquids can be recorded in commercially available cells of 1–10 mm width (pathlengths). Typically, these cells are made of quartz, which is water resistant. Hence,
NIR spectroscopy can be employed for study of bulk water and aqueous solutions.
However, due to very strong absorption from water, one has to use cells with shorter
pathlengths (<1 mm). As can be seen in Fig. 13.1, both combination bands from bulk
water are accurately recorded in a 0.1 mm quartz cell. In Fig. 13.1 are compared the
spectra of neat 1–propyl alcohol and a 1:1 water/1–propyl alcohol mixture. It appears
that the ν 2 + ν 3 combination band of water is particularly useful for spectroscopic
studies since it is located in the region, which is free from the absorption of the
other overtones and combination bands (5000–5300 cm
−1 ). In contrast, the ν 1 + ν 3
combination band of water (6800–7200 cm
−1 ) is overlapped by the first overtone of
the alcoholic OH and the C–H combination bands [5].
In principle, NIR spectra do not reveal bands originating from carbon–carbon
double and triple bonds. However, bands originating from C–H stretching groups
attached to double or triple bonds are usually blue-shifted [5]. This shift is very well
seen for the first and second overtones of the C–H stretching vibrations of cyclohexane and benzene (Fig. 13.2). This figure illustrates one more important property
of NIR spectra. The first overtones of both compounds have a complex structure,
since it results from the coexistence of the normal and local vibrations [6]. On the