Structure and Internal Motions of a Multifunctional Alcohol-Water Complex: Rotational Spectroscopy of the Propargyl Alcohol•••H2O Dimer

We have studied the rotational spectra of the propargyl alcohol (PA)-water complex using a pulsed-nozzle Fourier transform microwave spectrometer. A hydrogen-bonded ring structure is observed. The propargyl alcohol acts as an H-bond donor to form a strong O-H center dot center dot center dot O bond with H2O, and H2O donates back an H-bond to the acetylenic moiety, forming a weak O-H center dot center dot center dot pi bond. Splittings of the rotational transitions were observed, which are indicative of internal motions of the H2O fragment. The two lowest-energy conformers differ only in the position of the nonbonded hydrogen of H2O. Several isotopic substitutions were carried out to ascertain the position of the nonbonded hydrogen of H2O. Rotational spectroscopy helps to assign the observed structure to one, though it would be vibrationally averaged with a shallow potential along some coordinates, which could interchange the two conformers. These results are compared with earlier results on several alcohol-water complexes to understand the donor-acceptor capabilities of the OH groups in alcohol-water complexes. An empirical correlation between pK(a) and H-bond donor ability has been observed.