Ammonium Cyanate: a DFT Study of Crystal Structure, Rotational Barriers and Vibrational Spectrum

The crystal structure, rotational barriers and vibrational spectrum of ammonium cyanate have been studied by DFT calculations. The results show that, in the most stable structure, the ammonium ion is oriented such that each N-H bond points towards the N atoms of a cyanate anion giving rise to N-H...N hydrogen bonding, rather than N-H...O hydrogen bonding. The N-C and C-O bond lengths suggest that the structure of the anion in the crystal is best described as N-=C=O. These structural features are in agreement with recent results from neutron diffraction. At the transition state for rotation of the ammonium cation about an N-H bond, the cation is displaced and distorted from its equilibrium configuration. The barrier to the rotation of the ammonium cation about the (4) over bar axis is found to be larger than the minimum barrier to rotation about an N-H bond, suggesting that the latter is the preferred rotational mode.