Abstract
Supersonic jet FTIR spectra of the OH stretching vibrations in complexes of mono-, di- and trifluoroethanol with water are presented. In contrast to the non-fluorinated ethanol case, the fluorinated alcohols are all shown to act as O-H···O hydrogen bond donors towards water. This is found to be mostly a consequence of the intramolecular electron-withdrawing effect of the fluorine atoms and, with decreasing importance for increasing fluorination, due to the attractive intermolecular contact between one of the dangling water OH groups and the fluorine atoms. The findings provide a stepwise rationalization for the hydrophobic properties of the pharmaceutically important trifluoromethyl group.
Highlights
Supramolecular design rests on differentiation and control of intermolecular interactions such as hydrogen bonds.[1]
By studying the simplest model systems of their kind, useful insights for more complex structures can be obtained.[2,3,4,5,6]. Such a simple model system is the mixed dimer of ethanol and water,[7] in which the water molecule is known to act preferentially as the hydrogen bond donor
We focus on mixed dimers based on the most stable fluoroethanol monomers in which the OH group is in a gauche conformation relative to the alkyl chain
Summary
Supramolecular design rests on differentiation and control of intermolecular interactions such as hydrogen bonds.[1]. It is predicted to be a better hydrogen bond donor than water,[8] but the former superiority prevails.[7] The experimental proof of this donor–acceptor preference is quite recent: a Raman spectroscopic study[7,9] has shown that in the cold ethanol–water dimer, water acts as a donor and ethanol switches from its preferred trans structure to a gauche conformation This preference in conformation and coordination is predicted by many quantum-chemical calculations,[7,10,11,12,13] which show that the energy penalty for the inverted complex with water as an acceptor is not very large, on the order of 3–4 kJ molÀ1. The origins and consequences of this switch are analyzed in detail and may help to understand anomalies in bulk mixtures of water with fluorinated alcohols.[17]
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