FT-IR and DFT studies of the proton affinity of small aminal cages.

The proton affinity scale of small aminal cages was investigated using experimental and theoretical methodologies. The formation constant (K(f)) was determined for 1:1 hydrogen-bonded complexes between p-fluorophenol (PFP) and some aminal cage type (B) in CCl(4) at 298 K using FT-IR spectrometry. Then, the total interaction energy (E(PFF***B)), the energy of protonation (E(HB+)), the HOMO-LUMO GAP values and the Fukui index were calculated using the DFT/B3LYP/6-31 G(d,p) level of theory as theoretical descriptors. The values of the formation constant and energy changes vary with the tetrahedral character of the nitrogen lone pair. Good correlation between experimental and theoretical scales was observed, evidence for the existence of a relationship between the total energy of interaction calculated by structural parameters and the proton affinity in this series.