INELASTIC NEUTRON-SCATTERING STUDY OF HYDROGEN-BONDED SOLID FORMAMIDEAT 15-K

The inelastic neutron scattering spectra for formamide (HCONH2) and it s N-deuterated isomer (HCOND2) have been measured. The spectra are int erpreted on the basis of a vibrational harmonic normal-mode analysis u sing a variety of models. The models are a single molecule, a formamid e dimer, a tetramer resembling the unit cell of crystalline formamide, and a cluster of 36 formamide molecules. Force fields of the first th ree model systems were computed ab initio using the B3LYP DFT function al and the 6-31G* basis set. The tetramer force field was then applie d to the cluster of 36 molecules. Most of the observed vibrational ban ds could be assigned by comparing the relative INS intensities and fre quencies calculated for the theoretical models with the measured spect ra. Reasonable agreement between the calculation and the experiment wa s achieved on this basis. The results confirm the presence of the stro ng hydrogen bonding in the formamide lattice. However, the previously proposed model for the hydrogen bonding in N-methylacetamide involving a dynamical proton exchange between the amidic (... OCNH ....) and im idolic (... HOCN ...) forms is not consistent with our findings.