Tuning of character of the N-O bond in HONO from covalent to protocovalent by different types of intramolecular interactions.

Quantum-chemical calculations of the H-O-N=O molecule in the equilibrium and transition states and the complexes of the HONO with BH3, study of the intramolecular interactions using NBO theory, and investigation of the electron distribution on the basis of topological analysis of the ELF function clearly indicate the influence of the n-π* conjugation and n-σ* hyperconjugation interactions on a par with exchange repulsion of lone pairs the character of the N-O bond. It is shown that repulsion between lone pairs of oxygen and nitrogen atoms causes the elongation of the N-O bond only but character of this bond remains covalent. The interaction between lone pair of the terminal oxygen atom and antibonding orbital of the N-O bond (n-σ* hyperconjugation) coincides with influence of repulsion and reinforces it changing the character of the N-O bond from covalent to protocovalent. In contrary, the n-π* conjugation interaction between lone pairs of the bridged oxygen atom and π-orbital of the N=O double bond leads to the strengthening of the N-O bond making it more covalent.