15min:

J. ORPHAL, Institute of Environmental Physics and Institute of Remote Sensing, University of Bremen, P. O. Box 330440, D-28334 Bremen, Germany; D. SCHEFFLER, B. REDLICH AND H. WILLNER, Institute of Physical Chemistry and Institute of Anorganic Chemistry, University of Hannover, Callinstr. 9, D-30167 Hannover, Germany.

Nitryl bromide (BrNO₂) is a molecule of atmospheric interest. It is probably formed by heterogeneous reactions on polar stratospheric clouds and on sea-salt aerosol particles in the marine troposphere, and by gas-phase reactions of bromine atoms with NO₂. BrNO₂ is chemically unstable and difficult to synthesize. From the spectroscopic point of view, BrNO₂ is a heavy asymmetric top with small rotational constants and some low-energy fundamental vibrations, leading to a rather congested spectrum in the mid-infrared. Recently, we have presented the first high-resolution infrared spectrum of gaseous BrNO₂ in the region of the b -type ₄ bands around 6 µm. The ground-state rotational constants of BrNO₂ were determined from the infrared spectra using ground-state combination differences. Although it is possible to estimate the molecular structure using the ground-state rotational constants of ⁷⁹BrNO₂ and ⁸¹BrNO₂, the small displacement of the center-of-mass between these isotopomers causes a strong correlation between the Br-N bond length and the O-N-O angle. Therefore, high-resolution absorption spectra of the a -type ₁ bands of Br¹⁴NO₂ and Br¹⁵NO₂ were recorded with a spectral resolution of 0.002 cm^-1 using the Bruker IFS-120HR FTS at the University of Hannover. The bands were analyzed using a Watson-type A -reduced Hamiltonian in the I^r representation. The analysis confirms the rotational constants derived from line assignments in the ₄ bands, and allows for an accurate determination of the ground-state substitution structure of BrNO₂.