The Structure of Nitromalonamide: A Combined Neutron-Diffraction and Computational Study of a Very Short Hydrogen Bond

Abstract

Nitromalonamide has a remarkably short intramolecular hydrogen bond with an O−O distance of 2.391(3) Å, which according to some expectations would indicate a symmetric hydrogen bond. However, the enol hydrogen is confirmed by neutron diffraction to have an asymmetric position between the two oxygen atoms in the otherwise quite symmetrical molecule. The O−H distances are 1.14(1) Å and 1.31(1) Å. Analysis of the atomic displacement parameters of the enol hydrogen and comparison with those of similar systems indicate that the hydrogen atom resides in a single well potential. The experimental structure is compared to geometry optimized structures obtained from high level ab initio computations and, except for the position of the enol hydrogen, generally good agreement is obtained when correlation is taken into account. The significant differences between experimental and ab initio O−H bond lengths are ascribed to dynamical and intermolecular effects. Extremely small proton transfer barriers of 0.6 kJ/mol at the MP2/cc-pVTZ and 1.2 kJ/mol at the B3LYP/cc-pVTZ levels of theory were calculated. The enol hydrogen is found to vibrate freely between the two oxygen atoms, without the molecule passing through a well refined transition state structure. A simple model of the crystal environment explains the asymmetry of the hydrogen bond as resulting from intermolecular hydrogen bonding.