Computational characterization of the hydroxylamino (NHOH) group

Abstract

AbstractWe have examined computationally some properties of the hydroxylamino group, N(H)OH, in different molecular environments. Geometries were optimized at the B3LYP/6‐31G** level and used to calculate B3LYP/6‐311G(3df,2p) energy differences and HF/6‐31G* electrostatic potentials V(r) and local ionization energies $\bar I{\rm (}{\bf r}{\rm )}$. V(r) and $\bar I{\rm (}{\bf r}{\rm )}$ were evaluated on the molecular surfaces defined by the 0.001 au contours of the electronic densities. Two important factors in determining the N(H)OH structure are lone pair repulsion and possible intramolecular hydrogen bonding involving the remainder of the molecule, which can affect the pyramidal character of the nitrogen. The nitrogen and oxygen lone pairs produce regions of negative potential, those of the oxygen being stronger. These should function as hydrogen bond acceptors, although not as well as NH3 and H2O. The nitrogen lone pairs, although less negative, are expected to be the more basic, often roughly similar to pyridine. The amine and hydroxyl hydrogens tend to have quite positive potentials, and should be potent in hydrogen bonding. Depending upon the remainder of the molecule, the hydroxyl and the amine protons can range from essentially no acidity to weak to quite strong. The hydroxylamino group is estimated to have an average polarizability of 2.38 Å3 and as a substituent on aromatic rings to be activating and ortho, para‐directing, with Hammett δp = −0.37. Copyright © 2007 John Wiley & Sons, Ltd.