Molecular structure, gas-phase acidity and basicity of N-hydroxyurea
The geometries of various tautomers and rotamers of N-hydroxyurea (aminoformohydroxamic acid), its anions and protonated forms were optimized at the Becke3LYP DFT level using the 6-311+G(d,p) basis set. The calculations showed that the molecule of neutral acid should exist in several forms very close in energy. The hydroxamic tautomer is about 42 kJ mol-1 more stable than the hydroximic structure. The most stable conformer of N-hydroxyurea is non-planar with the OH hydrogen atom out of the plane of heavy atoms. In the structure of the anion the intramolecular hydrogen bond stabilizes the structure and makes the most stable conformations more planar. The HON- anion is more stable than the O-anion, hence N-hydroxyurea in the gas phase is an N-acid. N-hydroxyurea is a weak acid with calculated acidity of about 1470 kJ mol-1. The hydroxyamide tautomer of N-hydroxyurea is an oxygen base in the gas phase. However, the effective polarization of the cations of extended planar hydroximic tautomers causes the unusual increase of the stability of N-cations. Hence the hydroximic tautomer should be protonated with almost the same probability. The proton affinity of N-hydroxyurea was computed to be -856.0 kJ mol-1.