The effect of fluorine substitution in alcohol–amine complexes

The effect of fluorine substitution on the hydrogen bond strength in alcohol–amine molecular complexes was investigated, with a combination of vapour phase infrared spectroscopy and theoretical calculations. The complexes were combined from methanol (MeOH), ethanol (EtOH) and trifluoroethanol (TFE) as the hydrogen bond donor, and either dimethylamine (DMA) or trimethylamine (TMA) as the acceptor. The fundamental OH-stretching vibration involved in hydrogen bonding was measured for all complexes, as well as the weak second NH-stretching overtone for the DMA complexes. Equilibrium constants for complex formation were determined by combining a calculated intensity and the measured integrated absorbance. The observation of two transitions in the alcohol–DMA complexes provides an opportunity for two independent determinations of the equilibrium constants. Molecular interactions between the monomers were elucidated by Natural Bond Orbital, Atoms in Molecules and Non-covalent Interactions analysis. We find that the alcohol–amine complexes with TFE as the hydrogen bond donor form stronger hydrogen bonds and that secondary interactions between the monomers increase from MeOH to EtOH to TFE. TFE is a stronger acid than EtOH and MeOH making the OH bond weaker, and the OH-stretching frequency is redshifted in TFE relative to EtOH. This redshift is small in the monomers but significantly enhanced upon complexation.