STRUCTURE, ENERGETICS, AND FORCE-FIELDS OF THE CYCLIC FORMAMIDE DIMER - MP2, HARTREE-FOCK, AND DENSITY-FUNCTIONAL STUDY

MP2, Hartree-Fock, and local and gradient-corrected density functional theory calculations of the structure, proton-transfer energies, interaction enthalpies, basis set superposition errors, and harmonic force fields were carried out for the cyclic formamide dimer. Calculated intermolecular force constants (FCs) and variations in the FCs of formamide caused by hydrogen bonding were compared among different basis sets and among the methods mentioned above. Special attention has been paid to correcting systematic errors in the force fields, calculated by lower methods. The uniform or differential scaling can be efficiently used for correcting frequencies of intermolecular vibrational modes, whereas it is not recommended for correcting frequency and zero-point vibrational energy differences between the noninteracting molecules and the dimer. The local density approximation was found to yield lengths of hydrogen bonds short by 0.2 Angstrom, to overestimate interaction enthalpies and force constant variations due to hydrogen bonding significantly, and to underestimate the barrier height for proton transfer. The HF/4-31G intermolecular force constants and geometries of hydrogen bonds agreed well with the MP2/6-31G(d,p) and B-LYP/6-311++G(d,p) results. This method also provided reasonable values of the force constant variations caused by hydrogen bonding. As far as higher-level results including electron correlation effects are concerned, the gradient-corrected functional with basis sets containing diffuse functions provided interaction enthalpies, intermolecular geometries, and force constant variations close to the MP2/6-31G(d,p) results, while being more cost effective.