Structure Determination of Weakly Bound Complexes

Abstract

Radiofrequency and microwave spectra have been obtained for hydrogen bonded complexes such as (H₂O)₂, NH₃-H₂O and NH₃-H₂S with the molecular beam electric resonance method. The purpose of these experiments is to understand the structural factors influencing the formation of hydrogen bonds and to provide accurate data for models and ab initio calculations of hydrogen bonding. The information available from this work includes rotational and centrifugal distortion constants, electric dipole moments, and nuclear spin-spin and quadrupole coupling hyperfine constants. The geometries of these complexes are calculated principally from the rotational constants with some angular information from the hyperfine interaction results. Deuterium substitution data is particularly useful for increasing the number of structural parameters which can be determined. The large amplitude vibrational motions of the monomers in weakly bound complexes complicate this procedure in two ways. Firstly, interchange of indistinguishable nuclei by tunneling processes can lead to large splittings in the rotational spectra, as observed for (H₂O)₂, and to more subtle effects such as the absence of K-doubling and resulting linear Stark effects in NH₃-H₂S. Secondly, even if these effects are absent or have been accounted for, the spectroscopic constants used in a structure calculation reflect vibrational averages, typically for several isotopically distinct species.