Molecular structure, gas-phase acidity and basicity of N-hydroxyurea

The geometries of various tautomers and rotamers of N-hydroxyurea (aminofor mohydroxamic acid), its anions and protonated forms were optimized at the B ecke3LYP DFT level using the 6-311 + G(d,p) basis set. The calculations sho wed that the molecule of neutral acid should exist in several forms very cl ose in energy. The hydroxamic tautomer is about 42 kJ mol(-1) more stable t han 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 t he structure of the anion the intramolecular hydrogen bond stabilizes the s tructure and makes the most stable conformations more planar. The HON- anio n 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 14 70 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 ex tended planar hydroximic tautomers causes the unusual increase of the stabi lity of N-cations. Hence the hydroximic tautomer should be protonated with almost the same probability. The proton affinity of N-hydroxyurea was compu ted to be -856.0 kJ mol(-1).