Gas Chromatographic Study of Solute Hydrogen Bond Basicity

The purpose of the present work was to develop a scale of relative hydrogen bond basicity for a wide variety of solutes by means of their retention in gas chromatography. We used a powerful hydrogen bond donor (4-dodecyl-α,α-bis-(trifluoromethyl)benzyl alcohol) as an active hydrogen bond donor phase and a related ether (4-dodecyl-α,α-bis(trifluoromethyl) benzyl methyl ether) as a chemically similar but hydrogen bond inert reference stationary phase. The results are compared to a free energy based scale for the formation of 1:1 hydrogen bond complexes. In general, agreement is good, but a number of systematic discrepancies are found. FT-IR studies show that complexes with stoichiometries higher than 1:1 can be formed even for species as simple as THF in the presence of excess donor. Our results indicate that the use of hydrogen bond basicity scales based on the free energy of formation of hydrogen bond complexes to the rationalization of solvation-related phenomena must be used with discretion, at least in solvents which are very strong hydrogen bond acids.