Microwave spectra and structure of an isoxazole-water complex

The rotational spectrum of a hydrogen-bonded isoxazole-water complex has be en measured between 6-18 GHz with a pulsed nozzle Fourier transform microwa ve spectrometer. In addition to isoxazole-H2O, the complexes with HDO and D 2O as well as isoxazole-N-15-H2O have been investigated in order to determi ne the structure of the complex. Rotational constants, quartic centrifugal distortion constants and quadrupole coupling constants, where applicable, h ave been fitted to the measured transition frequencies of the isotopomers. Structural data, which have been deduced from the planar moments of inertia and the quadrupole coupling constants of the isotopomers, have established conclusively that water binds to nitrogen in the ring plane of isoxazole. Ab initio calculations have revealed that complexes with a hydrogen-bond to nitrogen or to oxygen are both stable. The complex with water attached to nitrogen has been found to be more strongly bound than that with water atta ched to oxygen. Small splittings of the rotational transitions of the two c omplexes with H2O have been interpreted as being the result of an internal rotation of water with respect to isoxazole.