Hypervalence & Aromaticity

Promotion on 2 June 2008

Hypervalence and aromaticity are two fundamental chemical concepts that deal with the propensity of a system to localize or delocalize bonds. Typically, so-called hypervalent and aromatic molecules have highly symmetrical structures with equal-sized bonds in contrast to the geometries of non-hypervalent and antiaromatic species which are usually asymmetric with alternating short and long bonds. S. C. A. H. (Simon) Pierrefixe investigated how hypervalence and aromaticity can be understood and represented within the electronic structure framework of Kohn-Sham molecular orbital theory.

His objective is to develop a transparent physical model that enables to understand the nature of (non-) hypervalence and (non-) aromaticity. In other words, he want to scrutinize which feature in the bonding mechanism is essential for determining whether an atom can form a hypervalent structure (or not) or whether a π-conjugated ring adopts an aromatic geometry with delocalized double bonds (or not).

The thesis work of Simon gradually builds-up as he investigates the bond-localizing propensity of H3- and Li3- model structures, via the archetypal aromatic molecule of benzene until comparing larger and larger aromatic and anti-aromatic heterocyclic structures. This leads him to develop a Molecular Orbital theory based model that catches up with Valence Bond (VB) theory regarding the treatment and understanding of the phenomenon of hypervalence and aromaticity. He concludes that the interplay of electronic and steric factors plays a role in the question whether an atom has the capability to form stable hypervalent structures with its substituents while the question of whether a species is aromatic or anti-aromatic is a purely electronic problem.

PhD promotion of
Simon C. A. H. Pierrefixe
Prof. Dr. E. J. Baerends
Dr. F. M. Bickelhaupt
2 June 2008
Main building of the VU University Amsterdam
Boelelaan 1081, Amsterdam