13 Overview of Application of NIR Spectroscopy to Physical …
323
[44]. This conclusion was possible to attain from the spectra simulations only, as the
corresponding experimental spectra were not accessible.
Be´ c et al. [45] explored the effect of isotopic substitution on NIR spectra of ethanol
and its derivatives by applying similar methodology further in detail. NIR spectra of
these molecules are more complex than that of methanol, as a consequence of presence of the methylene group and co-existence of gauche and trans rotamers. Detailed
band assignments for six ethanol isotopomers including CH 3 CH 2 OH, CH 3 CH 2 OD,
CH 3 CD 2 OH, CD 3 CH 2 OH, CD 3 CD 2 OH, and CD 3 CD 2 OD were presented. In addition, the NIR spectra of CH 3 CD 2 OD and CD 3 CH 2 OD were theoretically predicted,
since these samples are not commercially available. This way, the spectra simulations provide information not accessible from experimental studies. The examination
of ethanol and its isotopomers provided an in-depth understanding of the effect of
isotopic substitution on NIR spectra. Table 13.2 summarizes the relative contributions from two (2ν x and ν x + ν y ) and three–quanta (3ν x , ν x + ν y + ν z , and 2ν x
+ ν y ) transitions. The obtained results lead to the conclusion that the contributions
from the CH 3 group appear to be more important than those from the CH 2 group.
The isotopic substitution in the CH 3 group results in the most prominent intensity
changes in the NIR spectra as compared to the changes due to the substitution of the
other atoms. The bands resulting from three–quanta transitions appear to be more
important for isotopomers of ethanol [45] than for derivatives of methanol [44].
Quantum mechanical calculations of NIR spectra proved to be very useful in
explaining several other observations. For instance, the effect of baseline elevation
appearing in the spectra of carboxylic acids is due to a significant red-shift and
broadening of specific combination bands originating from hydrogen–bonded cyclic
dimers. These findings were consistent in the studies of eight different systems [3].
However, further investigations are required to provide a decisive explanation of
the mechanism underlying such selective shifts and broadening. Further, the manifestation of the C = C bond in aliphatic chains of long-chain fatty acids has been
reproduced in simulation as well [51]. NIR spectra are highly sensitive to differences
in the molecular structure as shown for n–hexanol, cyclohexanol, and phenol [52].
These structural differences induce prominent changes in the associated spectra that
can be accurately reconstructed by theoretical approaches.
Spectra simulations are helpful in elucidating similarities and dissimilarities
between the overtone and the fundamental regions. The fundamental bands of
acetonitrile as well as the first, second, and third overtones, together with binary
and ternary combination transitions, have been calculated by Lutz et al. with an
attempt to benchmark a novel, highly correlated treatment of anharmonic spectra
based on the CR–CC(2,3) method [52]. The examination of vibrational spectra of
nitriles by quantum chemical calculations was essential for the successful elucidation of structural information [10]. The combined MIR, Raman, and NIR study of
acetonitrile, acetonitrile–d 3 , and trichloroacetonitrile suggested a distinct influence
of the chemical environment on MIR and NIR spectra [10]. Further evidences of this
effect were provided in a study of MIR and NIR spectra of polycrystalline spectra of
melamine by Grabska et al. [53]. The explanation of different effects of the chemical
environment on MIR and NIR spectra was obtained from the spectra simulation. IR
323
[44]. This conclusion was possible to attain from the spectra simulations only, as the
corresponding experimental spectra were not accessible.
Be´ c et al. [45] explored the effect of isotopic substitution on NIR spectra of ethanol
and its derivatives by applying similar methodology further in detail. NIR spectra of
these molecules are more complex than that of methanol, as a consequence of presence of the methylene group and co-existence of gauche and trans rotamers. Detailed
band assignments for six ethanol isotopomers including CH 3 CH 2 OH, CH 3 CH 2 OD,
CH 3 CD 2 OH, CD 3 CH 2 OH, CD 3 CD 2 OH, and CD 3 CD 2 OD were presented. In addition, the NIR spectra of CH 3 CD 2 OD and CD 3 CH 2 OD were theoretically predicted,
since these samples are not commercially available. This way, the spectra simulations provide information not accessible from experimental studies. The examination
of ethanol and its isotopomers provided an in-depth understanding of the effect of
isotopic substitution on NIR spectra. Table 13.2 summarizes the relative contributions from two (2ν x and ν x + ν y ) and three–quanta (3ν x , ν x + ν y + ν z , and 2ν x
+ ν y ) transitions. The obtained results lead to the conclusion that the contributions
from the CH 3 group appear to be more important than those from the CH 2 group.
The isotopic substitution in the CH 3 group results in the most prominent intensity
changes in the NIR spectra as compared to the changes due to the substitution of the
other atoms. The bands resulting from three–quanta transitions appear to be more
important for isotopomers of ethanol [45] than for derivatives of methanol [44].
Quantum mechanical calculations of NIR spectra proved to be very useful in
explaining several other observations. For instance, the effect of baseline elevation
appearing in the spectra of carboxylic acids is due to a significant red-shift and
broadening of specific combination bands originating from hydrogen–bonded cyclic
dimers. These findings were consistent in the studies of eight different systems [3].
However, further investigations are required to provide a decisive explanation of
the mechanism underlying such selective shifts and broadening. Further, the manifestation of the C = C bond in aliphatic chains of long-chain fatty acids has been
reproduced in simulation as well [51]. NIR spectra are highly sensitive to differences
in the molecular structure as shown for n–hexanol, cyclohexanol, and phenol [52].
These structural differences induce prominent changes in the associated spectra that
can be accurately reconstructed by theoretical approaches.
Spectra simulations are helpful in elucidating similarities and dissimilarities
between the overtone and the fundamental regions. The fundamental bands of
acetonitrile as well as the first, second, and third overtones, together with binary
and ternary combination transitions, have been calculated by Lutz et al. with an
attempt to benchmark a novel, highly correlated treatment of anharmonic spectra
based on the CR–CC(2,3) method [52]. The examination of vibrational spectra of
nitriles by quantum chemical calculations was essential for the successful elucidation of structural information [10]. The combined MIR, Raman, and NIR study of
acetonitrile, acetonitrile–d 3 , and trichloroacetonitrile suggested a distinct influence
of the chemical environment on MIR and NIR spectra [10]. Further evidences of this
effect were provided in a study of MIR and NIR spectra of polycrystalline spectra of
melamine by Grabska et al. [53]. The explanation of different effects of the chemical
environment on MIR and NIR spectra was obtained from the spectra simulation. IR