Knowledge Bank

We 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 $interactiona$. However, ab initio calculations performed for 1,4-difluorobenzene$\cdots$water by $Brutschyandco-workersb$ 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 $C.KangandD.W.Prattc$. The structure, refined rotational constants and the results of the analysis of the observed splittings of the rotational transitions will be presented.