Question

For an HCl molecule find the rotational quantum numbers of two neighbouring levels whose energies differ by 7.86 meV. The nuclei of the molecule are separated by the distance of 127.5 pm.

Answer 1

The axis of rotation passes through die centre of mass of the HCl molecule. The distances of the two atoms from the centre of mass are

The energy difference between two neighbouring levels whose quantum numbers are J & J - 1 is

 

Hence J = 3 and the levels have quantum numbers 2 & 3,