Molecular Orbital Theory PDF

Molecular Orbital Theory (MOT) Postulations  MOT is developed by F. Hund and R. S. Mullikan to describe the structure and properties of different molecules.  MOT is expressed through the LCAO (Linear Combination of Atomic Orbitals) approach.  Atomic orbitals are invo...

Molecular Orbital Theory (MOT) Postulations  MOT is developed by F. Hund and R. S. Mullikan to describe the structure and properties of different molecules.  MOT is expressed through the LCAO (Linear Combination of Atomic Orbitals) approach.  Atomic orbitals are involved in the LCAO process to form molecular orbitals.  The total number of molecular orbitals formed will always be equal to the total number of atomic orbitals involved.  There exist three different types of molecular orbitals:  A) bonding molecular orbitals,  B) anti-bonding molecular orbitals.  C) Non-bonding molecular orbitals.  Anti-bonding molecular orbitals will always have higher energy than the atomic orbitals from which they were formed.  Bonding molecular orbitals will always have lower energy than the atomic orbitals from which they were formed.  The electrons are filled into molecular orbitals in the increasing order of orbital energy (from the orbital with the lowest energy to the orbital with the highest energy).  According to the Schrodinger wave equation, atomic orbital can be expressed as wave function (ψ).  For example, if we take molecule AB, Atomic orbitals of AB molecule are represented as ψA and ψB. Mathematically ΨMO = ψA ± ψB Bonding Molecular Orbital(BMO) ΨBMO A molecular orbital that is formed by the additive combination of atomic orbitals is known as Bonding Molecular Orbital. Ψ BMO = ψA + ψB BMOs will always have lower energy than corresponding atomic orbitals. BMOs are formed by the overlapping similar phases of orbitals. Anti-Bonding Molecular Orbital(ABMO) Ψ*ABMO A molecular orbital that is formed by the subtractive combination of atomic orbitals is known as Anti Bonding Molecular Orbital. Ψ*ABMO = ψA - ψB ABMOs will always have higher energy than corresponding atomic orbitals. ABMOs are formed by the overlapping of dissimilar phases of orbitals.  Electron in an atomic orbital is influenced by on nucleus. Electron in a molecular orbital is influenced by two nuclei.  Molecular orbitals are filled with electrons according to the Aufbau principle, Hund’s rule and Pauli’s principle.  Like atomic orbitals, molecular orbitals also accommodate two electrons with opposite spin. MOED Diagrams: Increasing energy order of various molecular orbitals. σ1s2 σ*1s2 σ2s2 σ*2s2 π2py2 = π2pz2 σ2px2 π*2py2 = π*2pz2 σ*2px2 Bond order: The total number of bonds present between two atoms in a diatomic molecule is called bond order. (or) Nb= Number of electrons present in bonding molecular orbitals. Na = Number of electrons present in anti-bonding molecular orbitals.  As the bond order increases the stability of the molecule increases.  Zero, -ve bond orders do not exist.  Molecules with fractional bond orders (0.5, 1.5, 2.5..etc) are less stable.  Bond order 1,2,3…indicates that the molecule is stable and contains single bond, double bond, and triple bond respectively. Magnetic behavior:  Molecules with paired electrons are diamagnetic in nature.  Molecules with unpaired electrons are paramagnetic in nature.