Water molecule absorbing radiation (or not?)

[mikeph]

Hi

My current understanding is of a water molecule able to absorb radiation by a transition, a change in the vibration or rotation. But if we consider that the molecule has three axes of rotation, surely one of these axis can be parallel to the molecular dipole, so shouldn't it be impossible for rotation about this axis to be altered by absorption of radiation?

Say the dipole is up-down oriented, then any change in the molecule's spin about this axis doesn't affect the dipole, so this quantum number should always be in the ground state?

What's wrong with this picture?

Thanks

 

[DrClaude]

This is indeed correct. The rotation of polyatomic molecule is described by three quantum numbers, $J$, $M_J$, and $K$. For water, the selection rule $\Delta K = 0$ applies because the coupling of the dipole moment with the electromagnetic field cannot apply a torque that would change that quantum number.