title: The Quadruple Bonding in C2 Reproduces the Properties of the Molecule.
abstract: Ever since Lewis depicted the triple bond for acetylene, triple bonding has been considered as the highest limit of multiple bonding for main elements. Here we show that C2 is bonded by a quadruple bond that can be distinctly characterized by valence-bond (VB) calculations. We demonstrate that the quadruply-bonded structure determines the key observables of the molecule, and accounts by itself for about 90% of the molecule's bond dissociation energy, and for its bond lengths and its force constant. The quadruply-bonded structure is made of two strong π bonds, one strong σ bond and a weaker fourth σ-type bond, the bond strength of which is estimated as 17-21 kcal mol(-1). Alternative VB structures with double bonds; either two π bonds or one π bond and one σ bond lie at 129.5 and 106.1 kcal mol(-1), respectively, above the quadruply-bonded structure, and they collapse to the latter structure given freedom to improve their double bonding by dative σ bonding. The usefulness of the quadruply-bonded model is underscored by "predicting" the properties of the (3)Σ+u state. C2's very high reactivity is rooted in its fourth weak bond. Thus, carbon and first-row main elements are open to quadruple bonding!
authors.affiliation: Institute of Chemistry and, The Lise Meitner-Minerva Center for Computational Quantum Chemistry, Hebrew University of Jerusalem, 91904, Jerusalem, Israel. sason.shaik@gmail.com.
authors.forename: Sason
authors.initials: S
authors.lastname: Shaik
authors.affiliation: Institute of Chemistry and, The Lise Meitner-Minerva Center for Computational Quantum Chemistry, Hebrew University of Jerusalem, 91904, Jerusalem, Israel.
authors.forename: David
authors.initials: D
authors.lastname: Danovich
authors.affiliation: UPMC Université Paris 06, CNRS UMR 7616, Laboratoire de Chimie Théorique, C. 137, 4 Place Jussieu, 75252, Paris Cedex 05, France.
authors.forename: Benoit
authors.initials: B
authors.lastname: Braida
authors.affiliation: Laboratoire de Chimie Physique, UMR CNRS 8000, Bat. 349, Université de Paris Sud, 91405, Orsay Cédex, France. philippe.hiberty@u-psud.fr.
authors.forename: Philippe C
authors.initials: PC
authors.lastname: Hiberty
keywords: C2; bond dissociation energy; bond order; bonding; force constants; quadruple bonds; valence bonds