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Y. Ozaki and Y. Morisawa
2.1 Characteristics and Advantages of NIR Spectroscopy
2.1.1 Characteristics of NIR Spectroscopy
NIR spectroscopy is concerned with both electronic transitions and vibrational transitions [1–7]. However, as the electronic spectroscopy, it is not easy or almost meaningless to discriminate the NIR region from the visible region. The two regions are
seamless in the electronic spectra. In contrast, it is quite straightforward to distingish
the vibrational spectroscopy in the NIR region from that in the IR region because
NIR spectroscopy deals only with bands arising from overtones and combination
modes, while IR spectroscopy involves mainly bands due to fundamentals, although
those originating from overtones and combinations also appear relatively weakly in
the IR region.
One of the most characteristic features of NIR spectroscopy come from the fact
that bands in the NIR region are weak or very weak. Both bands due to electronic
transitions and those originating from vibrational transitions are weak. The overtones
and combination modes arise from so-called forbidden transitions [1–7]. The reason
why the NIR region is valuable from the point of applications is since only the NIR
region offers as a highly transmitting window to radiation.
2.1.2 Characteristics of NIR Bands
Characteristics of bands appearing in the NIR region can be summarized as follows.
Here, we consider NIR vibrational bands, overtones and combinations.
(1) Bands observed in the NIR region are all due to overtones and combinations;
Not only simple combination bands such as v 1 + v 2 but also second order and
third order combination bands such as v 1 + 2v 2 appear. The NIR region contains
many overlapping bands; NIR bands show strong multicolinearlity. Therefore,
assignment of the NIR bands is generally not easy.
(2) The NIR bands become weaker and weaker as the wavelength becomes shorter
since bands due to higher order overtones and the second and third order combinations appear in the shorter wavelength region. Table 2.1 tabulates the wavelength, wavenumber, and relative intensity of bands due to the fundamental,
first, second, and third overtones of CH stretching mode of chloroform. It is
noted that the overtone bands become weak abruptly with the increase in the
order and that the third overtone bands is located in the Vis region.
(3) Most of the bands in the NIR region originate from functional groups containing
a hydrogen atom (e.g., OH, CH, NH). This is partly due to the fact that an
anharmonic constant of an XH bond is large, and partly due to the fact that an
XH stretching vibration has its fundamental in a high frequency region (38002800 cm
−1 ). Hence, NIR spectroscopy is often called “an XH spectroscopic
Y. Ozaki and Y. Morisawa
2.1 Characteristics and Advantages of NIR Spectroscopy
2.1.1 Characteristics of NIR Spectroscopy
NIR spectroscopy is concerned with both electronic transitions and vibrational transitions [1–7]. However, as the electronic spectroscopy, it is not easy or almost meaningless to discriminate the NIR region from the visible region. The two regions are
seamless in the electronic spectra. In contrast, it is quite straightforward to distingish
the vibrational spectroscopy in the NIR region from that in the IR region because
NIR spectroscopy deals only with bands arising from overtones and combination
modes, while IR spectroscopy involves mainly bands due to fundamentals, although
those originating from overtones and combinations also appear relatively weakly in
the IR region.
One of the most characteristic features of NIR spectroscopy come from the fact
that bands in the NIR region are weak or very weak. Both bands due to electronic
transitions and those originating from vibrational transitions are weak. The overtones
and combination modes arise from so-called forbidden transitions [1–7]. The reason
why the NIR region is valuable from the point of applications is since only the NIR
region offers as a highly transmitting window to radiation.
2.1.2 Characteristics of NIR Bands
Characteristics of bands appearing in the NIR region can be summarized as follows.
Here, we consider NIR vibrational bands, overtones and combinations.
(1) Bands observed in the NIR region are all due to overtones and combinations;
Not only simple combination bands such as v 1 + v 2 but also second order and
third order combination bands such as v 1 + 2v 2 appear. The NIR region contains
many overlapping bands; NIR bands show strong multicolinearlity. Therefore,
assignment of the NIR bands is generally not easy.
(2) The NIR bands become weaker and weaker as the wavelength becomes shorter
since bands due to higher order overtones and the second and third order combinations appear in the shorter wavelength region. Table 2.1 tabulates the wavelength, wavenumber, and relative intensity of bands due to the fundamental,
first, second, and third overtones of CH stretching mode of chloroform. It is
noted that the overtone bands become weak abruptly with the increase in the
order and that the third overtone bands is located in the Vis region.
(3) Most of the bands in the NIR region originate from functional groups containing
a hydrogen atom (e.g., OH, CH, NH). This is partly due to the fact that an
anharmonic constant of an XH bond is large, and partly due to the fact that an
XH stretching vibration has its fundamental in a high frequency region (38002800 cm
−1 ). Hence, NIR spectroscopy is often called “an XH spectroscopic