MOLECULAR ORBITAL THEORY[MOT]:

THE MOLECULAR ORBITAL THEORY  describes the valance electrons as associated with all the nuclei concerned. The nuclei in equilibrium positions in the the stable molecule and electrons associated in equilibrium the nuclei can be described by the wave functions.

The energy states of electrons can be described in the combined state or molecular orbitals.The molecular orbitals are multi-centred or delocalized. They are filled with the required number of electrons (each molecular orbital is usually filled with two electrons). Molecular orbital may be obtained by the linear combination of the atomic orbitals(LCAO method).

 

The molecular orbitals are assumed to process the following characteristics:

  1. Each electron in the molecule is described by a wave function fie. The value of fie is such that the value of fie square at any point represents the probability of finding the electrons in unit volume around that point.The wave functions are called molecular orbitals.These molecular orbitals are polycentric so that the electron moves in the filed of all the nuclei.
  2. Each molecular orbital have its own energy.
  3. Each molecule have a definite spin and paulo’s exclusion principle is observed.
  4. The appropriate form of the wave equation is quite complicated and cannot be used for exact solution except for hydrogen.Thus approximations are necessary. One of the approximations is that when an electron comes in the vicinity of the one nucleus.The force arising on it is due to the nucleus and its other electrons.Both the wave equation and the molecular consists of series of superposed self-consistent orbitals(LCAO)
  5. The greater the overlap of atomic orbitals are obtained.
  6. The energy of molecular orbitals is least when the combining atomic orbitals have equal or almost equal energy state.
  7. Each molecular wave function corresponds to a definite energy value.The sum of the individual energies of the molecular orbitals after correction represents the total energy of the molecule.

Molecular-orbital-theory-diagram

    Non bonding orbitals;

If an atomic orbital does not form a bonding or anD-bonding orbital, it is called a non-bonding orbital. There are two common reasons that atomic orbitals form non-bonding orbitals:

1) they do not have a similar energy to an orbital on another atom or

2) construe Due interference with one lobe of an orbital is cancelled by destruc Dve interference with another lobe of that orbital. The diagrams below show the hydrogen fluoride situation.

Orbital Symmetry of MOLECULAR ORBITAL THEORY:

           The MO picture for a molecule gets complicated when many valence AOS are involved.We can simplify the problems enormously by noting that orbitals of different symmetry with respect to the molecule do not interact.

Orbital-Symmetry-in-molecular-orbital-theory

AO’s must have the same nodal symmetry (as defined by the molecular symmetry operations), or their overlap is zero.
For example, in the HCl molecule, the molecular symmetry (bonding) axis is z, as shown in figure .
Cl

3pBecause these two orbitals have different symmetries, the Cl  3py orbital is non

bonding and doesn’t interact with the HH 1s. The same is true of the  Cl 3px orbital. px

 
The  pxpyand py orbitals have ππ symmetry (nodal plane containing the bonding axis) and are labelled πnbπnb    in the MO energy level diagram.

3pz1 In contrast, the  H1s and ClCl3pz orbitals both have σσ symmetry, which is also the symmetry of the clay pot shown in . Because these orbitals have the same symmetry (in the point group of the molecule), they can make the bonding and anti bonding combinations shown in the previous section.

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