Lu Xu, Thom Dunning, David Woon (University of Illinois – Urbana/Champaign, USA)
The tetravalence of carbon is the foundation of organic chemistry. Pauling introduced the concept of spn hybrids to rationalize the tetravalence of carbon: sp3 hybridization was used to explain carbon’s ability to form the four single covalent bonds in CH4; sp2 and sp hybridizations were used to explain C2H4 and C2H2, respectively. This ad hoc model rationalizes the geometry of these carbon hydrides. However, it does not explain how the bonds form and the reasons for the geometries and bond strengths of the CHn and C2Hn molecules. The recoupled pair bond model is based on high-accuracy quantum chemical calculations and offers a systematic and accurate explanation on the bonding in carbon hydrides (and many others). This model in being implemented in the University of Illinois bonding module. It teaches students how to build molecules from their constituent atoms and understand the impact of bonding on molecular structure, energetics, and properties.