CHARACTERIZATION OF BRNO2, CIS-BRONO, AND TRANS-BRONO - IMPLICATIONS FOR ATMOSPHERIC CHEMISTRY

The titled compounds are investigated using large one-particle basis s ets in conjunction with the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected t riple excitations, denoted CCSD(T). Accurate geometries, dipole moment s, 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 argu ments are presented which indicate that the experimental assignment fo r the Br-N fundamental is in error. Accurate isomerization energies ar e determined at the CCSD(T) level using spdfg one-particle basis sets. Comparison of ab initio results for the titled compounds with previou sly published ab initio results from the fluorine and chlorine analogs elucidates several trends involving equilibrium geometries, bonding c haracteristics, and relative energies. Heats of formation of all three titled compounds are evaluated and used to show that BrNO2 and cis-Br ONO are sufficiently thermally stable to exist in the Earth's stratosp here. Possible stratospheric formation and destruction mechanisms are discussed.