Ab initio calculation of cubic force constants in formamide monomer, formamide dimer, formic acid dimer and their D, D2 and D4 derivatives. An estimation of fermi resonance and intra–intermolecular coupling

We report preliminary results of the ab initio calculations in the 4-31G basis set of cubic force constants in normal modes for the HCONH2 and DCONH2 monomers, and a series of cyclic dimers (HCONH2)2, (HCOND2)2, (HCOOH)2, and (HCOOD)2. These include diagonal cubic constants and nondiagonal constants, describing Fermi resonances between X—H(D) stretching modes and overtones of other vibrations, and couplings between intra- and intermolecular modes. Results allow us to interpret experimental Fermi resonance observed in the infrared spectrum of the HCONH2 monomer. Diagonal anharmonicities improve an agreement between experimental and calculated frequencies for the dimers. The calculation shows that the coupling between the X—H(D) and X …. Y stretching vibrations is substantially stronger in the formic acid dimers than in the formamide dimers.