DETERMINATION OF THE STRUCTURE OF HBr DBr

Abstract

We have investigated the structure and dynamics of the weakly bound complex of the four bromine isotopomers of HBr DBr using pulsed-jet Fourier transform microwave spectroscopy. In the hydrogen halide dimers, allowed transitions are across the inversion doublet caused by the geared interchange of the donor and acceptor of the hydrogen bond. While in the classic study of $(HF)_{2}^{1}$, it was observed that this splitting results in an inversion frequency in the microwave region of the spectrum, in $(HCl)_{2}$ and $(HBr)_{2}$ the lower inversion barrier results in an inversion frequency in the infrared. In order to investigate the hydrogen bromide dimer with the high precision allowed by an FT microwave experiment, we have "quenched" the inversion motion by substituting a deuterium for one of the hydrogens. The Br-Br distance in the complex is $4.136 {\AA}$. Using measurements of the nuclear quadrupole coupling constants of the bromine nuclei, the wide amplitude bending motions of the hydrogen-bonded deuterium can be determined as can the wide amplitude bending angle of the non-bonding hydrogen atom. Preliminary analysis suggests that the Br---Br-D angle undergoes excursions of $29^{\circ}$ about zero, and that the H-Br--Br angle oscillates $36^{\circ}$ about its $90^{\circ}$ equilibrium angle.