RULES OF QMOT

1.Consider valence orbitals only

exp:- these orbitals are the outermost orbitals of a atom and they take part in chemical reaction.

2. Form completely delocalised mos as linear combination of s and p AOS.

exp:- we need to combine i.e add and subtract the atomic orbital.(u will understand this better when we start building molecules).

3. MOS must be either symmetric or antisymmetric with respect to the symmetry operations of the molecule.

4. compose MOS for the structures  of high symmetry and then produce orbitals for related but less symmetric structures by systematic distortion of the orbitals for higher symmetry.

exp:- WALSH DIAGRAM

5.Molecules with similar molecular structures,such as ch3 and nh3 ,have more or less same mos,only difference being the number of electron that occupy mo.

6. the total energy is the sum of the molecular orbitals energies of the individual valence electrons.

7.if the two highest energy mos of a given symmetry derive primarily from different kinds of AOS ,then mix the 2 MOS to form hybrid orbitals.

exp:- you will understand this when i will explain pyramidal methyl

8. when 2 orbitals interact the lower energy orbitals is stabilised and the higher energy orbital is destabilised .the out phase or anti-bonding interaction between the 2 starting orbitals always raise energy more than the corresponding in phase lowers the energy.

9.when two orbitals interact the lower energy orbitals mixes into itself the  higher energy orbital one in bonding way ,while the higher energy orbital mixes itself with lower energy orbitals in anti bonding way

exp:- in phase mixing has greater portion of the lower energy orbital and out phase mixing has higher energy orbital.

10. the smaller energy gap between 2 orbitals the greater the mixing.

11. the more electronegative  elements have lower energy AOS.

12.A change in geometry of a molecule will produce a large change in the energy of a  particular MO if the geometry change results in changes in AO overlap that are large.

exp:- as stated in the earlier rule i.e greater the overlap greater the interaction ,therefore energy changes with change in overlap.

if u have any difficulty in understanding any particular rule comment down below will reply as soon as possible.