Determination of properties of hydrogen-bonded dimers by rotational spectroscopy and a classfication of dimer geometries

Abstract

Rotational spectroscopy is a rich source of information about the molecular geometry, the potential-energy function and the electric-charge distribution of simple hydrogen-bonded dimers in the gas phase. Techniques for the observation of such spectra are now available and have led to a considerable amount of information for a wide range of dimers. We outline two techniques that have been used and then review the various molecular properties that can be derived from the observed spectroscopic constants. We indicate how vibrational ground-state rotational and centrifugal distortion constants can lead to information about the molecular geometry and potential-energy function and how observations of the Stark effect and nuclear quadrupole hyperfine structure allow conclusions about charge redistribution on dimer formation. We also show how important information can be obtained by the study of rotational spectra of dimers in vibrationally excited states. Two specific examples, HCN ⋯ HF and N₂⋯ HF, are examined in detail before a general discussion of results for a number of dimer species is presented in which geometries, force constants and dissociation energies are compared systematically. We show that, although the observed geometries correspond to broad potential energy minima, it is nevertheless possible to propose a simple rule which accounts for the preferred equilibrium conformation.