STRUCTURE AND DYNAMICS OF THE H2O-HCN DIMER

This report extends an earlier microwave study of the H2O-HCN weakly bonded dimer by Legon [Proc. R. Soc. London, Ser. A 396, 405 (1984)]. We have resolved the H-H (H2O) hyperfine structure (hfs) in rotational transitions of H-2 O-(HCN)-N-15 and the O-17 hfs in H-2 O-17-(HCN)-N-15, using a modified Balle/Flygare Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the sample source. Also, the rotational constants of H-2 O-(HCN)-C-13 have been determined. The hfs, particularly that O-17, and a substitution analysis, are used to clarify the dynamics of the dimer. The analyses support a pseudoplanar, H-2 O-HCN, C2-upsilon structure in which the H-2 O and HCN experience in-plane and out-of-plane bending vibrations of modest on average amplitude. The out-of-plane H-2 O bend is 20-degrees and the in-plane is perhaps half that. The bending of the HCN is isotropic, with an amplitude of 9.4-degrees in both directions. The molecular mechanics for clusters (MMC) model was used to explore the potential energy surface (PES) for the weak-bonding coordinates. The calculated equilibrium structure differs greatly from the experimental, with the H-2 O rotated out of plane by 60-degrees in one direction and the HCN by 20-degrees in the other (cis). The difference is shown by the O-17 hfs and its dependence upon the H-2 O bending to be caused by the zero-point vibrational averaging of the structure, which extends over the shallow symmetric double minimum in the PES. The interaction energy is large (- 1590 cm-1), but the PES is relatively flat in the bending coordinates over large regions between the equilibrium minima, making the vibrational averages differ substantially from the equilibrium values.