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Principles of NIR SpectroscopyAs has been previously mentioned, NIR spectroscopy is concerned with both electronic and vibrational transitions. 1−15 It should be noted that it is rather difficult to differentiate electronic transitions in the Vis region and those in the NIR region. What one can say, however, is that most of the electronic transitions observed in the NIR region are d-d

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The temperature-dependent near-infrared (NIR) spectral variations of n- and tert-butyl alcohols in the pure liquid state have been studied by generalized two-dimensional (2D) correlation analysis. The Fourier transform (FT) NIR spectra were recorded in the 6000−11000 cm-1 region over the temperature ranges of 20−85 and 25−75 °C for n- and tert-butyl alcohols, respectively. In the 2D NIR correlation spectra of both alcohols appear the bands due to the OH stretching vibrations of the monomer (first and second overtones), cyclic dimer, and linear hydrogen bonds in acyclic and cyclic polymers. Moreover, the spectra of tert-butyl alcohol reveal the bands that can be attributed to the free terminal OH groups in open chain species (7047 cm-1), bend OHO bonds (6520 cm-1), and a band at 6610 cm-1. The last one, not reported previously, was assigned to the small cyclic associates (trimers or tetramers). Our results evidenced that in the pure liquid state the population of rotational isomers is determined mostly by the accessibility of the OH proton. This is why the bands due to the less stable rotamers are much more intense than the energetically favorable ones in neat n- and sec-butyl alcohols. Owing to smaller population of the free OH groups, as compared to the sec-butyl alcohol, the rotational isomerism was not observed for n-butyl alcohol in the pure liquid phase. Yet, this effect was clearly seen in diluted solution of n-butyl alcohol. The analysis of the asynchronous 2D correlation spectra has proved that the variations in the population of the associated species and monomers do not occur at the same rate. The changes for the monomers are slower than that for any other species; however, the population of this species increases faster at higher temperatures. At elevated temperatures the rapid increase in the population of the cyclic dimers is reduced by simultaneous dissociation into the monomers. It has been demonstrated that the stability of the cyclic dimers is diminished upon branching. Our results show the relationship between the chemical structure of alcohols and dynamic properties of the hydrogen bonding.

The first paper in a series devoted to self-association in neat butanols presents the results of two-dimensional (2D) near-infrared (NIR) correlation analysis of temperature-induced spectral variations of sec-butanol. By taking advantage of resolution enhancement in the 2D correlation spectra, it was possible to identify spectral features due to vibrations of the free and associated OH groups in the first-overtone region. On the basis of a few assumptions, band assignments of the various types of OH bonds have been proposed. The monomer band (near 7100 cm−1) can be resolved into three components; two of them are due to a rotational isomerism (7089 and 7116 cm−1), and the third one is attributed to the free terminal OH groups in linear polymers (7055 cm−1). The presence of the 7055 cm−1 band implies that the intensity of the monomer peak cannot be used as a measure of the concentration of the monomer species (except in very diluted solutions). Thus, previous estimations of equilibrium constants and thermodynamic parameters associated with hydrogen-bond dissociation have been subject to unacceptable error. At higher temperatures, a new band near 6550 cm−1 becomes visible. This band originates from bended OHO bond, mostly in the cyclic polymers. In order to obtain more detailed information on the complex mechanism of the thermal dissociation of hydrogen-bonded sec-butanol in the pure liquid phase, the entire experimental temperature range was divided into narrower ranges, and then 2D correlation analysis was performed for smaller data sets. It has been shown that the variations of population of the polymeric species and the cyclic dimers are faster than the corresponding changes for the monomers. At elevated temperatures an appreciable dissociation of the cyclic species takes place.