Structure and dynamics of the H2O–HCN dimer

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Abstract
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 H2O–HC15N and the 17O hfs in H217O–HC15N, using a modified Balle/Flygare Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the sample source. Also, the rotational constants of H2O–H13CN have been determined. The hfs, particularly that of 17O, and a substitution analysis, are used to clarify the dynamics of the dimer. The analyses support a pseudoplanar, H2O –HCN, C2v structure in which the H2O and HCN experience in-plane and out-of-plane bending vibrations of modest on average amplitude. The out-of-plane H2O bend is 20° and the in-plane is perhaps half that. The bending of the HCN is isotropic, with an amplitude of 9.4° 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 H2O rotated out of plane by 60° in one direction and the HCN by 20° in the other (cis). The difference is shown by the 17O hfs and its dependence upon the H2O 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.

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