Molecular Structures of Free Boron Clusters

Abstract

This chapter deals with gas-phase molecular structure determinations of neutral boranes and heteroboranes employing the techniques of gas-phase electron diffraction (GED) and/or modern quantum chemical calculations. Such calculations were useful for computing various observables in order to facilitate the analysis of the electron diffraction data. Additionally, microwave spectroscopy was utilized for the two thiaboranes (in conjunction with the University of Oslo). Unless otherwise stated, the samples used for the work described throughout this chapter originated from the Institute of Inorganic Chemistry of the Academy of Science of the Czech Republic, v.v.i., Řež while the GED studies were performed mainly in the School of Chemistry at the University of Edinburgh. The structurally characterized boron clusters belong to the range of structural motifs, from closo to nido, which obey the so-called Wade’s rules. Examples of boranes that do not obey Wade’s rules were also studied, as were selected macropolyhedral clusters and metallaboranes. Finally, in order to gain an insight into electron density distribution, analyses of the experimental dipole moments were carried out for a few examples. Whereas the earlier GED studies of boranes and carbaboranes ignored the calculated vibrational effects because of a lack of force fields for these clusters, the current electron diffraction investigations of boranes and various types of heteroboranes used calculated force fields to good effect. They revealed an interesting feature: the amplitudes of vibration for bonded and non-bonded cage distances are very similar, which is at odds with various empirical rules suggesting that amplitudes of vibration should be roughly proportional to the corresponding internuclear distances.