ABSTRACT
UV/Vis absorption and NMR spectroscopy titrations have been used to investigate the formation of complexes between cations and neutral H-bond acceptors in organic solvents. Complexes formed by two different H-bond acceptors with fifteen different cations were studied in acetone and in acetonitrile. The effects of water and ion pairing with the counter-anion were found to be negligible in the two polar solvents employed for this study. The data were used to determine self-consistent H-bond donor parameters (α) for a series of organic and inorganic cations; guanidinium, primary, tertiary and quaternary ammonium, imidazolium, methylpyridinium, lithium, sodium, potassium, rubidium and caesium. The results demonstrate the transferability of α parameters for cations between different solvents and different H-bond acceptor partners, allowing reliable prediction of cation recognition properties in different environments. Lithium and protonated nitrogen cations form the most stable complexes, but the α parameter is only 5.0, which is similar to the neutral H-bond donor 3-trifluoromethyl,4-nitrophenol (α = 5.1). Quaternary ammonium is the weakest H-bond donor investigated with an α value of 2.7, which is comparable to an alcohol. The α parameters for alkali metal cations decrease down the group from 5.0 (Li+) to 3.5 (Cs+).
ABSTRACT
A combination of high-throughput NMR titration experiments, UV-Vis absorption titrations and data collected from the literature on 1:1 H-bonded complexes has been used to characterise the H-bond properties of non-polar organic solvents: alkanes, perfluorocarbons, aromatic and halogenated organic solvents. The results are analysed in the context of the electrostatic solvent competition model, which assumes that solvent effects on intermolecular interactions can be interpreted based on the exchange of specific functional group contacts, with minimal involvement of the bulk solvent. For solvents where the H-bond parameters have been measured as solutes in carbon tetrachloride solution, the H-bond parameters measured here for the same compounds as solvents are practically identical, i.e. solute and solvent H-bond parameters are directly interchangable. For the very non-polar solvents, alkanes and perfluorocarbons, the experimental H-bond parameters are significantly larger than expected based on calculated molecular electrostatic potential surfaces. This suggests an increase in the relative importance of van der Waals interactions when electrostatic effects are weak, but there is no detectable difference between the solvation properties of cyclic and linear alkanes, which have different van der Waals interaction properties.
