In the previous video we have studied the molecular orbital diagram of Beryllium di hydride and water molecule. Both the molecules are similar with only change in central atom. One is having beryllium and other is oxygen also the bond angle changes as beryllium di hydride is linear with bond angle 180 deg and water is angular with 104.5 deg. This change in bond angle changes the bond stability also and the energy of each bond changes as the bond angle changes.
The diagrams which show the change in energy of molecular orbitals with variation in bond angle are called as Walsh correlation diagrams. The name is derived from the name of British chemist Arthur D. Walsh who published an extensive account involving the use of these diagrams in 1953.
Here we show the molecular orbital diagram of beryllium di hydride and water molecule again. Both the molecules contains four molecular orbitals, sigma s, sigma z and sigma x and sigma y. In case of beryllium di hydride, 2px and 2py remains as non bonding while in case of water only 2py remains as non bonding orbital. Shape of each molecular orbital also changes as the angle changes which are shown next to each orbital.
It can be seen that the sigma s and sigma z orbital of beryllium dihydride are formed by overlap along the axis, while 2px and 2py orbitals remain non bonding. In case of water the two hydrogen lobes are coming from the bottom so it overlaps with 2s, 2pz and 2px orbitals and 2py remains non bonding.
Now lets discuss the changes in energy of these orbitals.
If we plot the energy in y axis and the bond angle in x axis. The sigma s orbitals of linear and angular molecules can be shown as in fig. Fig shows that bonding orbital sigma s is slightly stabilised by the bending process. This is because, s orbital of central atom is spherical and hence there is no significant effect on the extent of overlapping if Hydrogen orbitals comes from linear or angular position. But in case of angular molecule, there is slight increase in 1s 1s orbital overlap of two hydrogen atoms. That means two hydrogen atoms are also close to each other and overlap between them decreases energy and stabilises the sigma s bond.
However molecular orbital, sigma z gets destabilised as the molecule changes from liner to angular. Incase of linear molecule the overlap is across the overlap axis while in case of angular molecule it is lateral to the overlap axis. This creates situation of less overlap as compared to linear molecule, thus energy increases and the bond is destabilised.
And finally 2px and 2py orbitals which remains non bonding in linear molecule have the highest energy, but in case of angular molecule there is overlap between orbitals of the two hydrogen atoms laterally as they are approaching from the bottom of the molecule, that means in x direction. This decreases the energy of sigma x orbitals and stabilises the molecule.
You must have seen in the molecular orbital diagram of water, that sigma z is below sigma x molecular orbital, this is because the bond angle is only 104.5 and not 90 degree. So we can say that the molecule tries to stabilise all his molecular orbitals by adjusting its bond angle, where all the molecular orbitals are stabilised to the maximum extent.
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30 сен 2024
