Question

Among the molecules given below, identify those which are microwave
active. Justify your answer.
I2, HBr, CO2, CS2, NO​

Answer 1

Out of all the molecules given, only NO ( nitric oxide) and HBr ( Hydrogen bromide) are microwave active.

The important factor for a molecule to be microwave active is dipole moment. A molecule should possess a permanent dipole moment for it to get a rotational spectrum.

Homonuclear molecules are not microwave active so I2 is inactive, CO2 and CS2 have net zero dipole moment so only HBr and NO qualify as microwave active molecules.

Answer 2

"According to the gross selection rule, the molecule must have a permanent dipole moment in which transitions are allowed in the microwave active.

In CO2 dipole moment is 0, so it is microwave inactive.

In I2 dipole moment is 0., it is inactive.

In Hbr it doesn’t contain permanent dipole moment, hence it is microwave inactive.

NO is micro active as it contains permanent dipole moment. NO is a heteronuclear diatomic.

"