INVESTIGATION OF XNO AND XON (WHERE X = CL OR BR) AND THEIR PROTONATED ISOMERS

Ab initio molecular electronic structure methods have been used to examine nitrosyl chloride (ClNO) and nitrosyl bromide (BrNO), their hypohalite isomers (i.e., ClON and BrON), and all possible singly-protonated forms of these species. Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for each parent structure and protonated isomer at the self-consistent-field (SCF) double zeta plus polarization (DZP) level of theory. Molecular structures also have been optimized with the DZP basis set, utilizing the single and double excitation configuration interaction method (CISD). Our study has located stable minima corresponding to the nitrogen hypohalites XON (where X = Cl or Br), neither of which has been observed experimentally. In addition, we found that, for all of the species under study, protonation at the nitrogen is energetically favored over protonation at the oxygen atom. Protonation at the halogen atom of either XNO structure, however, results in the formation of stable trans and cis isomers. Similar protonation of XON results in a rearrangement to the halogen-protonated XNO form. By contrast, fluorine protonation of FNO or FON yields only one structure. Among the protonated isomers obtained, the trans halogen-protonated species lie lowest energetically relative to their parent structures. It is also found that, in terms of total energies, all protonated forms of XNO lie lower than those of XON. Moreover, protonation is found to preferentially stabilize XNO relative to XON.