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···O bond with H₂O, and H₂O donates back an H-bond to the acetylenic moiety, forming a weak O–H···π 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.