13 Overview of Application of NIR Spectroscopy to Physical …
315
types of transitions to NIR spectra. This estimation is based on the ratio between
the integrated intensities of the calculated bands, showing approximately only 19%
(methanol) and 27% (ethanol) of the relative contribution to entire NIR region
(10,000–4000 cm
−1 ) originate from three–quanta transitions. Moreover, the three–
quanta bands are numerous but very weak and tend to show a large degree of overlap.
Thus, these bands are far less specific than the two–quanta counterparts. Therefore,
it is sufficient to limit the calculations to the first overtones and binary combinations to predict NIR spectra with good accuracy. This conclusion has important
practical implications, as for larger systems the requirement in terms of computational resources rises rapidly for the prediction of three–quanta transitions. Note that
the results obtained for methanol and ethanol confirm the previous conclusions that
bands resulting from two–quanta transitions are sufficient to explain the majority of
features observed in NIR spectra [3, 4].
The theoretical reconstruction of the spectra enables robust band assignments, and
the potential benefit is already evident in cases of relatively simple molecules such as
methanol (Fig. 13.10) [3]. Similar improvements in the reconstruction of NIR spectra
have been reported for a number of medium–sized molecules in diluted solutions.
For instance, the simulations carried out by Grabska et al. [46] for the isomers of
butyl alcohol (n–butanol, sec–butanol, iso–butanol and tert–butanol) reconstructed
the differences in the respective NIR spectra between 6000 and 4000 cm
−1 and
reproduced the fine structure of NIR spectra in the regions from 5200 to 4600 and
from 4500 to 4000 cm
−1 (Fig. 13.11).
The rotational freedom of the OH group may lead to energetically distinguishable
structures, called “rotational isomers.” The OH group in various rotational isomers
absorbs at different wavenumbers, but the corresponding bands are close to each
other. Hence, the presence of the rotational isomers can be observed only in highresolution spectra of diluted alcohols in inert solvents. MIR, NIR, and DFT studies of
butyl alcohols in dilute CCl 4 solutions (0.01 M) revealed the presence of various rotational isomers in n–, iso- and sec–butanol (Fig. 13.12) [48]. The trans conformer is
more favorable than its gauche counterpart, and as a result absorbs at lower wavenumbers. These studies revealed a minor effect of C–C dihedral rotation on the position
of the first and second overtone of the OH group. The position of the OH bands
due to rotational isomers primarily results from the order of the alcohol, while the
relative population of a particular rotational isomer depends on the steric effects of
the groups in α and β positions in relation to the OH group.
Highly accurate simulations are capable of reproducing the effect of conformational isomerism manifested also in the other NIR bands [46]. Isomers of butyl alcohol
have very different conformational flexibility depending on the structure of the main
chain. The number of stable rotational conformers is 14, 9, 5, and 1 for n–butanol,
sec–butanol, iso–butanol, and tert–butanol, respectively. As shown in Fig. 13.11, the
spectra of conformers differ noticeably throughout the entire NIR region, not only in
vicinity of the 2ν(OH) band. To reproduce NIR spectrum in detail, it is necessary to
calculate the spectra of each form co-existing in the sample and mix them in accordance to the relative Boltzmann population. Grabska et al. [46] have performed a
detailed conformational search for all butyl alcohols and estimated populations of
315
types of transitions to NIR spectra. This estimation is based on the ratio between
the integrated intensities of the calculated bands, showing approximately only 19%
(methanol) and 27% (ethanol) of the relative contribution to entire NIR region
(10,000–4000 cm
−1 ) originate from three–quanta transitions. Moreover, the three–
quanta bands are numerous but very weak and tend to show a large degree of overlap.
Thus, these bands are far less specific than the two–quanta counterparts. Therefore,
it is sufficient to limit the calculations to the first overtones and binary combinations to predict NIR spectra with good accuracy. This conclusion has important
practical implications, as for larger systems the requirement in terms of computational resources rises rapidly for the prediction of three–quanta transitions. Note that
the results obtained for methanol and ethanol confirm the previous conclusions that
bands resulting from two–quanta transitions are sufficient to explain the majority of
features observed in NIR spectra [3, 4].
The theoretical reconstruction of the spectra enables robust band assignments, and
the potential benefit is already evident in cases of relatively simple molecules such as
methanol (Fig. 13.10) [3]. Similar improvements in the reconstruction of NIR spectra
have been reported for a number of medium–sized molecules in diluted solutions.
For instance, the simulations carried out by Grabska et al. [46] for the isomers of
butyl alcohol (n–butanol, sec–butanol, iso–butanol and tert–butanol) reconstructed
the differences in the respective NIR spectra between 6000 and 4000 cm
−1 and
reproduced the fine structure of NIR spectra in the regions from 5200 to 4600 and
from 4500 to 4000 cm
−1 (Fig. 13.11).
The rotational freedom of the OH group may lead to energetically distinguishable
structures, called “rotational isomers.” The OH group in various rotational isomers
absorbs at different wavenumbers, but the corresponding bands are close to each
other. Hence, the presence of the rotational isomers can be observed only in highresolution spectra of diluted alcohols in inert solvents. MIR, NIR, and DFT studies of
butyl alcohols in dilute CCl 4 solutions (0.01 M) revealed the presence of various rotational isomers in n–, iso- and sec–butanol (Fig. 13.12) [48]. The trans conformer is
more favorable than its gauche counterpart, and as a result absorbs at lower wavenumbers. These studies revealed a minor effect of C–C dihedral rotation on the position
of the first and second overtone of the OH group. The position of the OH bands
due to rotational isomers primarily results from the order of the alcohol, while the
relative population of a particular rotational isomer depends on the steric effects of
the groups in α and β positions in relation to the OH group.
Highly accurate simulations are capable of reproducing the effect of conformational isomerism manifested also in the other NIR bands [46]. Isomers of butyl alcohol
have very different conformational flexibility depending on the structure of the main
chain. The number of stable rotational conformers is 14, 9, 5, and 1 for n–butanol,
sec–butanol, iso–butanol, and tert–butanol, respectively. As shown in Fig. 13.11, the
spectra of conformers differ noticeably throughout the entire NIR region, not only in
vicinity of the 2ν(OH) band. To reproduce NIR spectrum in detail, it is necessary to
calculate the spectra of each form co-existing in the sample and mix them in accordance to the relative Boltzmann population. Grabska et al. [46] have performed a
detailed conformational search for all butyl alcohols and estimated populations of