# Chemical bonding and dipole moment

We know that ammonia undergoes amine inversion. Why doesn't the dipole moment decrease in ammonia, since the direction of the dipole changes to the opposite direction every time there's inversion?

Shouldn't the effective dipole moment become zero?

• Come to think of it, the effective dipole moment of anything is essentially zero, because the molecules rotate all the time, so the direction of dipole changes to the opposite every now and then. – Ivan Neretin Nov 6 '16 at 18:00
• Probably due to the preservation of symmetry - not a true "inversion". See this Wikipedia article on molecular symmetry: "Similarly, ammonia (NH3) has two equivalent pyramidal (C3v) conformations which are interconverted by the process known as nitrogen inversion. This is not an inversion in the sense used for symmetry operations of rigid molecules, since NH3 has no inversion center. Rather it is a reflection of all atoms about the centre of mass (close to the nitrogen), which happens to be energetically feasible for this molecule. – Todd Minehardt Nov 7 '16 at 1:02
• The 'inversion' in ammonia is also called an 'umbrella' motion as seen when one is opened the wrong way on a windy day; C3v changes to D3h to C3v to pointing the other way and back again. Inversion in the symmetry sense is replacing x, y, z by -x, -y, -z. The 'effective' or long time average dipole should really be zero as you suggest because during the vibration, the H atoms more from one side of the N atom to the other, so the dipole goes to zero at halfway in the motion and has the opposite sense every half cycle. A dipole assumes the molecule has its canonical structure. – porphyrin Nov 7 '16 at 8:54

• Of course a molecule can have a dipole, e.g. $\ce{HCl}$ but in the gas phase (or solution) thermal motion in the ensemble of molecules has them pointing in all possible directions so the overall experimentally measured dipole is zero. Applying an external field breaks the symmetry and the average dipole can be measured and from this the true one. In small molecules in gas phase quantum effects are more obvious & restrict orientation. You can quite easily measure the inversion frequency of ammonia by observing its spectrum and its about $0.7 \pu{cm^{-1}}$ – porphyrin Nov 8 '16 at 13:53