1.

For an `HCI` 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 distacne of `127.5p m`.

Answer» The axis of rotation passes through the centre of mass of the `HCl` molecule.The distance of the two atoms from the centre of mass are
`d_(H)=(m_(cl))/(M_(Hcl))d, =(m_(H))/(m_(Hcl))d`
Thus `I=` moment of interita about the axis
`(4)/(2) m_(H)d_(H)^(2)+m_(cl)d_(cl)^(2)=(m_(H)M_(cl))/(m_(H)+m_(cl))d^(2)`
The energy difference between two neighbouring levels whose quantum numbers are `J&J-1` is
`( ħ^(2))/(2I).2J=( Jħ^(2))/(I)= 7.86 meV`
Hence `J=3` and the levels have quantum numbers `2 & 3`.


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