Isomerization of HNO to HON in the singlet state assisted by amino acid residues and/or water molecules

The effects of amino acid residues in the presence or absence of water molecules on the isomerization of the singlet state of HNO/HON have been systematically investigated at the B3LYP/6‐311++G** level of theory. The structural characteristics, proton transfer (PT) mechanisms, and the corresponding thermodynamic and kinetic parameters, have been discussed, respectively. All the optimized complexes have been characterized by the ring structures through the intermolecular H‐bonds. The origin of the increase in N? H stretching frequency (blue shifts) occurring in the reactants has also been investigated using the natural bonding orbital (NBO) analyses, which is mainly attributed to the decrease of the electron densities in the antibonding orbital of the N? H bonds as well as the increase of the polarization of the N? H bond. All the PTs proceed with the concerted mechanisms since no ionic intermediates have been located during PT processes. At the same time, the cooperative effects of amino acid residues and water molecules on the selected PT processes have been observed, where the PTs assisted solely by the selected residues cannot occur without the participation of the water molecule. Overall, the introductions of one or two water molecules are more favorable for the isomerization of HNO assisted by the amino acid residues. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008