Hydrogen bonding and phase transitions of a group of alcohols derived from 2,2-dimethylpropane

Abstract

A group of alcohols with the general formula (CH 3 ) 4− x C(CH 2 OH) x ( x = 1,2,3,4), all of which are forming plastic crystalline phases, were investigated by vibrational spectroscopy and thermal analysis in order to determine their hydrogen bonds in the different crystal phases and the melts. Additional structural information was obtained from IR spectra of solutions in carbon tetrachloride as well as in low temperature argon matrices. The alcohols have strong hydrogen bonds forming layers in the crystals. At the phase transition from the crystal to the plastic crystalline state the OH stretching bands shift to higher wavenumbers and a considerable band broadening occurs in the vibrational spectra. Further heating gives no additional increase in the half bandwidth of the melt spectra. The observations from the vibrational spectra agree with the enthalpies, which are high for the phase transitions between the anisotropic crystals and the plastic phase crystals (depending on the number of OH groups), but very low for the melting. The data reveal that the hydrogen bonds become weaker in the plastic crystalline phases and in the melts. The larger rotational freedom in the plastic crystalline state and the melts leads to more than one spectroscopically distinguishable conformer for all the investigated alcohols except 2,2-dimethyl-1-propanol.