All triple bonds are not the same

Chemists consider dinitrogen inert because of its strong nitrogen-nitrogen triple bond. But acetylene is reactive, despite its even stronger carbon-carbon triple bond. Molecular orbital theory characterizes both triple bonds as having one σ and two π bonds, each containing a pair of electrons. A new study explores those triple bonds using generalized valence bond theory and finds differences in their electronic structure that could explain the molecules’ reactivities (J. Phys. Chem. A 2016, DOI: 10.1021/acs.jpca.6b03631). Lu T. Xu and Thom H. Dunning Jr. of the University of Illinois, Urbana-Champaign, found that the bonding structure of dinitrogen closely matches the traditional view from molecular orbital theory. But acetylene’s electronic structure includes a significant contribution from excited C–Hs that have three electrons with unpaired spin on each of the carbon atoms. That contribution likely affects acetylene’s reactivity. “Electron spin is usually only explicitly considered when describing radicals and excited states,” Dunning says. But, he says, acetylene shows that how spins couple can be important in ground states too.