Weak hydrogen bonds in complexes pairing monohalomethanes with neutral formic acid

Ab initio calculations are used to analyze the interaction between formic acid and CH3X, for X equal to each of F, Cl, and Br. Seven minima are located on each potential energy surface, four of which contain HCOOH in its syn rotamer, and three in anti. All minima are cyclic in that they contain more than one H-bond. The three most strongly bound contain a OH⋯X bond, along with CH⋯O, and the other four minima contain CH⋯X and CH⋯O interactions. The most stable minima for each X are computed to have binding energies in the range of 17-24 kJ/mol. Alterations of the covalent bond lengths within each subunit, and vibrational frequency shifts, coupled with electronic charge shifts, reveal fundamental features of these complexes, and the nature of the interactions. The OH⋯X bond is the strongest of those examined here, followed by CH⋯X and CH⋯O.