Characterization of BrNO2, cis-BrONO, and trans-BrONO. Implications for Atmospheric Chemistry

Abstract

The titled compounds are investigated using large one-particle basis sets in conjunction with the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T). Accurate geometries, dipole moments, harmonic vibrational frequencies, and IR intensities are determined. Agreement with the available experimental data (which is limited to fundamental vibrational frequencies) is very good except for the Br?N stretching mode in BrNO₂. Convincing theoretical and experimental arguments are presented which indicate that the experimental assignment for the Br?N fundamental is in error. Accurate isomerization energies are determined at the CCSD(T) level using spdfg one-particle basis sets. Comparison of ab initio results for the titled compounds with previously published ab initio results from the fluorine and chlorine analogs elucidates several trends involving equilibrium geometries, bonding characteristics, and relative energies. Heats of formation of all three titled compounds are evaluated and used to show that BrNO₂ and cis-BrONO are sufficiently thermally stable to exist in the Earth's stratosphere. Possible stratospheric formation and destruction mechanisms are discussed.