The microwave spectrum, structure, and internal motion of the cyclopropane-water complex

The microwave spectrum of the cyclopropane-water complex has been observed by Fourier-transform microwave spectroscopy. Only a-type transitions were observed; they were split into doublets of unequal intensity separated by 0.5-2 MHz. The stronger and weaker transitions were fit separately to rotational constants A = 19950 (100) MHz, B = 2518.845 (5) MHz, C = 2340.321 (5) MHz and A = 19945 (160) MHz, B = 2518.843 (7) MHz, C = 2340.876 (7) MHz, respectively. The water is hydrogen bonded to the center of an edge of the cyclopropane; the hydrogen bond is nearly linear (bond center-H-O angle = 180 (5)-degrees) with the O and hydrogen-bonded H in the CCC plane. The hydrogen bond length (H-to-edge distance) is 2.34 angstrom. The position of the free H is uncertain. Nuclear spin statistics indicate that the splittings in the spectrum arise from a high-barrier internal motion of the water subunit which exchanges the two hydrogen atoms; a nearly free rotation of the water about the hydrogen bond is postulated to explain anomalous dipole moment and nuclear quadrupole coupling results. The mu(a) dipole moment of the complex is determined as mu(a) = 1.209 (10) D for the strong state and mu(a) = 1.241 (10) D for the weak state. The nuclear quadrupole coupling constants for C3H6.HDO and (C3H6.H2O)-O-17 were determined.