Symmetry properties of Molecular Orbital’s

 

Symmetry properties of Molecular Orbital’s:

·       Molecular orbital’s of Ethene

            In alkenes molecular orbital’s of ethene the carbon atoms are sp2 hybridized. The double bond between two carbon atoms is comprised of a σ-bond and π-bond. The former (C-C σ-bond) is formed by overlap of two sp2 orbital’s. The overlapping result’s in two π-molecular orbitals. In this case bonding π-orbital’s is formed by the overlap of in phase (same phase) of ‘p’ orbital and other π* anti bonding orbital’s arises from the interference between two p-orbital’s of opposite phases. These are designated by ψ1 and ψ2 respectively. 

Molecular orbital diagram of ethene

·        Molecular orbital’s of 1,3 butadiene:      

    In butadiene there are four p-orbital’s (ignore the σ-skeleton) and their combination gives four orbital’s having wave function ψ1, ψ2,ψ3,ψ4 with different energies of these ψ1 and ψ2 π-molecular orbital is bonding and ψ3 and ψ4 are antibonding molecular orbital’s. Molecular orbital is bonding if the number of bonding interactions is greater than number of nodes between nuclei and molecular orbital’s is antibonding, if the number of bonding interaction is less than the number of nodes between the nuclei.

 Molecular orbital diagram of 1,3 butadiene 

    In case of butadiene out of the four molecular orbitals is ground state two vic. Ψ1 & ψ2 are lowest energies ψ2 is the highest occupied molecular orbital (HOMO) and ψ3* is the lowest unoccupied molecular orbital  (LUMO). Both HOMO and LUMO are referred to as Frontiers molecular orbital and are used in analysis of pericyclic reactions. On absorption of a photon of proper wavelength (in the UV range) an electron is promoted from ψ2 to ψ3* which then becomes a new HOMO.

Molecular Orbital’s of 1,3,5-hexatriene

            The six p-electron of 1,3,5-hexatriene are accommodated in the first three molecular π-orbital’s  ψ1,ψ2,ψ3 which are bonding orbital’s and the remaining three higher energy molecular π-orbital’s (ψ4,ψ5,ψ6) are antibonding orbital’s which are remain unoccupied in the ground state. That six π-electrons are accommodated in ψ1,ψ2 and ψ3 the bonding orbital’s.

Molecular Orbital digram of 1,3,5-hexatriene

The bonding or antibonding interactions of Frontier Molecular Orbital’s (FMO) determine whether reactions are thermally or photo chemically allowed or forbidden.