I Xt - Understanding Dipole Moment Components in Rigid and Non-Rigid Molecules

[Konte]

Hi everybody,

I want to ask a question about the dipole moment vector $\vec{d}$, with components expressed in the molecule-fixed axis. By permuting identical nuclei that compose a rigid molecule, I know how $d_x$, $d_y$, $d_z$ change.
But when dealing with non-rigid molecule, I no longer know how to direct the molecule fixed- axis?

Thank you everybody.

Konte

 

[Jilang]

Could you work with expectation values?

 

[Konte]

Could you work with expectation values?

How? What does you mean?

Konte.

 

[blue_leaf77]

In molecular mechanics, what is meant by rigid is usually the assumption that the molecule only rotates in space without any vibration or at least vibrates with very small amplitude. In reality of course the molecule vibrates and hence the concept of rigid molecule no longer applies however for some low vibrational quantum numbers where the vibrational eigenfunction is more localized around the equilibrium position, you can take the equilibrium position to determine the axis.

 

[DrDu]

Look out for "Sayvez conditions".

 

[Konte]

Thank you guys for your answer.