Computational Study on Electronic Structure, Atomic Charges Distribution and Frontier Molecular Orbitals of Butadiene: General Features for Diels-Alder Reaction

Abstract

The electronic structure, Mulliken atomic charges distribution, and frontier molecular orbitals of the industrially important 1, 3-butadiene are investigated using computationally cheap yet decent first principles density functional theory (DFT) method. The DFT calculations confirm the planar molecular shape of the butadiene with two pairs of double bonds facing opposite directions of a sigma covalent bond where the steric effects from the two methylene groups are also significantly minimized. The inhomogeneous partial atomic charges distribution in the terminal and nonterminal carbon atoms approximated by the Mulliken population analysis method validates the existence of conjugated pi-bond regions in the butadiene system. Such bridging regions of the interjacent single bond are also reconfirmed by the DFT derived electron density surface maps of the HOMO and LUMO, where former orbital is localized mostly across the double bonded regions and the latter is across the single bonded regions; further reassures the pi-bonded regions of the butadiene as chemically more active sites. Again, the DFT predicted features of the butadiene-HOMO/LUMO as asymmetric/symmetric with single/double node/s respectively ascertains that the butadiene molecule stereochemically favors Diels-Alder chemical reactions while reacting with suitable dienophiles. This insight not only underscores the importance of electron density surface maps of the frontier molecular orbitals, their occupancy, symmetries, and stereoisomerism to Diels-Alder reactions but also strengthens computing ability of the DFT model.