AB-INITIO CALCULATIONS OF SUPRAMOLECULAR RECOGNITION MODES - CYCLIC VERSUS NONCYCLIC HYDROGEN-BONDING IN THE FORMIC-ACID FORMAMIDE SYSTEM

We present a theoretical investigation of the bonding between formic acid and formamide in various geometrical arrangements as a model for a motif that occurs in supramolecular 1:2 complexes of dicarboxylic acids with dimethylformamide. We calibrate our theoretical model using the known energetics of the formic acid dimer as a benchmark. In these calculations we study the influence of basis set, in particular the basis set superposition error, as well as the correlation contributions to the hydrogen bond, and thus arrive at a theoretical model that provides an estimated accuracy of around 5 kJ/mol for the binding energy of the hydrogen bonds in complexes similar to the formic acid dimer. We apply the model to our target system as well as to some geometrical arrangements of the formamide dimer, which to our knowledge have not been studied so far in the literature. The calculations show that the weakest form of hydrogen-bonding interaction, substantiated by the C-H...O bond in a cyclic complex of formic acid with formamide, still contributed a significant amount of 10-15 kJ/mol to the complex interaction energy. Thus we find a clear indication that the formyl proton may indeed participate in hydrogen bonding, despite of a long bond length of up to 250 pm. We interpret this result in relation to the structure of supramolecular complexes of 1,1'-binaphthyl-2,2'-dicarboxylic acid and 9,9'-spirobifluorene-2,2'-dicarboxylic acid.