4 Spectral Analysis in the NIR Spectroscopy
69
(4) Spectra-structure correlations
The NIR spectrum of a compound can be compared with those of similar compounds
to make band assignments. For instance, NIR measurements of a series of alcohols or
that of fatty acids allow one to make assignments of bands due to OH, CH 2 , and CH 3
groups. Figure 4.5 compares NIR spectra in the 7500–4000 cm
−1 region of saturated
(stearic acid, arachidic acid, and palmitic acid) and unsaturated (oleic acid, linolenic
acid, and linoleic acid) long-chain fatty acids (0.05 M in CCl 4 ) [12]. By comparison, one can easily discriminate saturated and unsaturated long-chain fatty acids;
particularly see the 5800–5670 cm
−1 region. A common band near 6908 cm
−1 can
be assigned to the first overtone of OH stretching mode. The unsaturated fatty acids
yield characteristic bands near 4663 and 4590 cm
−1 . Grabska et al. have assigned
the 4663 cm
−1 band to the combination of C=C stretching and CH stretching modes
by quantum chemical calculation [12].
(5) Spectral analysis based on perturbation
NIR measurements of perturbation-dependent spectral variations, such as
temperature-dependent, concentration-dependent, and pH-dependent spectra variations, often give valuable information about the band assignments [1, 2, 7]. As a
good example, temperature-dependent NIR spectra changes of octanoic acid in the
pure liquid over a temperature range of 15–90 °C are shown in Fig. 4.6 [13]. Octanoic
acid in the pure liquid forms a cyclic dimer with hydrogen bonds at room temperature but with the temperature increase the dimer dissociates gradually and a free OH
Fig. 4.5 NIR spectra in the 7500–4000 cm −1 region of saturated (stearic acid, arachidic acid, and
palmitic acid) and unsaturated (oleic acid, linolenic acid, and linoleic acid) long-chain fatty acids
(0.05 M in CCl 4 ). Reproduced from Ref. [12] with permission
69
(4) Spectra-structure correlations
The NIR spectrum of a compound can be compared with those of similar compounds
to make band assignments. For instance, NIR measurements of a series of alcohols or
that of fatty acids allow one to make assignments of bands due to OH, CH 2 , and CH 3
groups. Figure 4.5 compares NIR spectra in the 7500–4000 cm
−1 region of saturated
(stearic acid, arachidic acid, and palmitic acid) and unsaturated (oleic acid, linolenic
acid, and linoleic acid) long-chain fatty acids (0.05 M in CCl 4 ) [12]. By comparison, one can easily discriminate saturated and unsaturated long-chain fatty acids;
particularly see the 5800–5670 cm
−1 region. A common band near 6908 cm
−1 can
be assigned to the first overtone of OH stretching mode. The unsaturated fatty acids
yield characteristic bands near 4663 and 4590 cm
−1 . Grabska et al. have assigned
the 4663 cm
−1 band to the combination of C=C stretching and CH stretching modes
by quantum chemical calculation [12].
(5) Spectral analysis based on perturbation
NIR measurements of perturbation-dependent spectral variations, such as
temperature-dependent, concentration-dependent, and pH-dependent spectra variations, often give valuable information about the band assignments [1, 2, 7]. As a
good example, temperature-dependent NIR spectra changes of octanoic acid in the
pure liquid over a temperature range of 15–90 °C are shown in Fig. 4.6 [13]. Octanoic
acid in the pure liquid forms a cyclic dimer with hydrogen bonds at room temperature but with the temperature increase the dimer dissociates gradually and a free OH
Fig. 4.5 NIR spectra in the 7500–4000 cm −1 region of saturated (stearic acid, arachidic acid, and
palmitic acid) and unsaturated (oleic acid, linolenic acid, and linoleic acid) long-chain fatty acids
(0.05 M in CCl 4 ). Reproduced from Ref. [12] with permission