Knowledge Bank

The microwave spectrum of the cyclopropane-water complex has been observed by Fourier transform microwave spectroscopy. Only $a$-type transitions were observed in the spectrum; they were split into doublets of unequal intensity separated by 0.5 to 2 MHz. The stronger and weaker transitions were fit separately to rotational constants $A=19950(100)$ MHz, $B=2158.845(5)$ MHz, $C=2340.321(5)$ MHz and $A=19945(160)$ MHz, $B=2158.843(7)$ 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 $angle=180(5)0$) with the O and bonded H in the CCC plane. The H to edge distance is 2.35 {\AA}. The position of the free H is uncertain. Nuclear spin statistics and isotopic data indicate that the splittings in the spectrum arise from an internal motion of the water subunit which exchanges the two hydrogens.""