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 a cid and formamide in various geometrical arrangements as a model for a motif that occurs in supramolecular 1:2 complexes of dicarboxylic aci ds with dimethylformamide. We calibrate our theoretical model using th e known energetics of the formic acid dimer as a benchmark. In these c alculations we study the influence of basis set, in particular the bas is set superposition error, as well as the correlation contributions t o the hydrogen bond, and thus arrive at a theoretical model that provi des 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 ar rangements of the formamide dimer, which to our knowledge have not bee n studied so far in the literature. The calculations show that the wea kest form of hydrogen-bonding interaction, substantiated by the C-H... O bond in a cyclic complex of formic acid with formamide, still contri buted a significant amount of 10-15 kJ/mol to the complex interaction energy. Thus we find a clear indication that the formyl proton may ind eed 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 an d 9,9'-spirobifluorene-2,2'-dicarboxylic acid.