Hierarchy of hydrogen bonding among constitutional isomers of hexanol

Abstract

The strength and thermal stability of the hydrogen bonding influence the bulk properties of associating liquids, such as higher alcohols. Temperature-induced decay of the hydrogen bonds translates into uncommon trends in the heat capacity of the liquid, frequently exhibiting vast plateaus or distinct maxima. Experimental isobaric heat capacities of sixteen aliphatic hexanols in their liquid phase, resulting from Tian-Calvet calorimetry in the temperature range of 260–340 K, are presented. This temperature range is extended to 380 K with a power-compensation DSC for thirteen compounds. Among the studied compounds, a maximum on the temperature dependence of the heat capacity is explicitly observed below 380 K for six species. Upon derivation of residual heat capacities, this maximum gets amplified and can be recognized for 13 species (below 400 K). Molecular dynamics simulations of pure samples of all hexanol isomers are performed to interpret the observed trends. The hydrogen-bonding hierarchy is established computationally in terms of structural and energetic descriptors of bulk liquids. The strength of the hydrogen bonds is directly related to the extent of steric hindrance imposed on the hydroxyl moieties by the side alkyl chains in the bulk liquid. The hexanol isomers predicted computationally to form the weakest hydrogen bonds in the liquid match those, for which the non-monotonous trends of the heat capacities were observed. The position of the heat capacity maximum is sensitive to a subtle variation of the hydrogen bonding strength and shifts to higher temperatures upon strengthening of the hydrogen bonding for less branched and primary hexanol isomers.