Resolution enhancement and band assignments for the first overtone of OH stretching modes of butanols by two-dimensional near-infrared correlation spectroscopy.: 2.: Thermal dynamics of hydrogen bonding in n- and tert-butyl alcohol in the pure liquid states

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 degrees C for n- and tert-butyl alcohols, respectively. In the 2D NIR correlation spectra of both alcohols appear the bands due to the OK 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 nand 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.