Microwave spectroscopic and theoretical studies on the phenylacetylene···H₂O complex: C-H···O and O-H···π hydrogen bonds as equal partners

Abstract

Rotational spectra of five isotopologues of the title complex, C₆H₅CCH···H₂O, C₆H₅CCH···HOD, C₆H₅CCH···D₂O, C₆H₅CCH···H₂¹⁸O and C₆H₅CCD···H₂O, were measured and analyzed. The parent isotopologue is an asymmetric top with κ = -0.73. The complex is effectively planar (ab inertial plane) and both 'a' and 'b' dipole transitions have been observed but no c dipole transition could be seen. All the transitions of the parent complex are split into two resulting from an internal motion interchanging the two H atoms in H₂O. This is confirmed by the absence of such doubling for the C₆H₅CCH···HOD complex and a significant reduction in the splitting for the D₂O analog. The rotational spectra, unambiguously, reveal a structure in which H₂O has both O-H···π(π cloud of acetylene moiety) and C-H···O (ortho C-H group of phenylacetylene) interactions. This is in agreement with the structure deduced by IR-UV double resonance studies (Singh et al., J. Phys. Chem. A, 2008, 112, 3360) and also with the global minimum predicted by advanced electronic structure theory calculations (Sedlack et al., J. Phys. Chem. A, 2009, 113, 6620). Atoms in Molecule (AIM) theoretical analysis of the complex reveals the presence of both O-H···π and C-H···O hydrogen bonds. More interestingly, based on the electron densities at the bond critical points, this analysis suggests that both these interactions are equally strong. Moreover, the presence of both these interactions leads to significant deviation from linearity of both hydrogen bonds.