Molecular conformation of isolated and hydrogen bonded N,N′-diformohydrazide: an ab initio study

The molecular conformation of N,N′-diformohydrazide has been studied by ab initio molecular orbital calculations including electron correlation corrections up to the MP4/6-31 G*//MP2/6-31G* level. The calculations indicate that the stable structures of the molecule are non-planar at nitrogen and show twisted conformation about the N–N bond. Structures and rotational barrier heights have been compared with those of N,N′-dimethylhydrazine, calculated at uniform levels of theory. A simulation of the crystal field has been proposed to investigate the effects of intermolecular hydrogen bonding on the structural properties of N,N′-diformohydrazide. Our crystal model, studied by HF/4-31G* calculations, shows that hydrogen bonding causes not only a concerted change in the length of the C–N, CO and N–H bonds, but also a significant conformational change. In agreement with the neutron diffraction structure, the crystal molecule of our model is predicted to be planar. The overall geometrical perturbations due to self association reveal an increase of π-conjugation between the nitrogen and carbonyl groups which can rationalize the conformational change. The effect of intermolecular hydrogen bonding on the vibrational spectrum is also discussed.