THE MICROWAVE SPECTRA AND STRUCTURE OF THE 1,4-DIFLUOROBENZENE$\cdots$WATER DIMER

$^{a}$H.S. Gutowsky, T. Emilsson, and E. Arunan, J. Chem. Phys. 99, 4883 (1993). $^{b}$P. Tarakeshwar, Kwang S. Kim, B. Brutschy, J. Chem. Phys. 110, 8501 (1999). $^{c}$Cheolhwa Kang and David W. Pratt, $55^{th}$ International Symposium on Molecular Spectroscopy, RB06 (2000).Author Institution: Department of Chemistry and Biochemistry, Universit""at Kiel; Institut f""ur Physikalische Chemie, Universit""at KielWe report the microwave spectra of the 1,4-difluorobenzene$\cdots$water dimer in the frequency region 3-20 GHz which were recorded using a pulsed molecular beam Fourier transform microwave spectrometer with a confocal Fabry-Perot resonator. This dimer of a doubly fluorinated organic ring compound with water is of special interest as different structures may result from various types of intermolecular interaction. In the case of the benzene$\cdots$water dimer, a microwave study yielded a structure where the water is located above the ring corresponding to a $\pi$-type $interaction^{a}$. However, ab initio calculations performed for 1,4-difluorobenzene$\cdots$water by $Brutschy and co-workers^{b}$ predict in addition to this $\pi-type$ structure a slightly more stable so-called $\sigma-type$ structure which results from the formation of two H-bonds (water hydrogen and fluorine, water oxygen and benzene hydrogen) and leads to a planar configuration of the dimer. Our results support the theoretical predictions and are in good agreement with the experimental results from fluorescence excitation spectroscopy which were presented earlier by $C. Kang and D.W. Pratt^{c}$. The structure, refined rotational constants and the results of the analysis of the observed splittings of the rotational transitions will be presented