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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: AussieKenDoll on August 20, 2020, 01:12:51 AM

Title: What is theselection rule for a vibrational mode of a molecule to be IR active?
Post by: AussieKenDoll on August 20, 2020, 01:12:51 AM
is it ΔV=±1 or what?
What is ΔV?
Title: Re: What is theselection rule for a vibrational mode of a molecule to be IR active?
Post by: Corribus on August 20, 2020, 08:57:37 AM
This is something you can just look up in a textbook or on google.
Title: Re: What is theselection rule for a vibrational mode of a molecule to be IR active?
Post by: AussieKenDoll on August 20, 2020, 09:07:19 AM
Yea I know that The mode in question must involve a changing dipole moment during the vibration. This can involve a bending process as well as a stretching process.
But can I show this statement mathematically as ΔV=±1  instead of that explanation?
Title: Re: What is theselection rule for a vibrational mode of a molecule to be IR active?
Post by: Corribus on August 20, 2020, 10:49:02 AM
These are two completely different conditions - the requirement of a change in dipole is not the same as the requirement for a change in the quantum number of ± 1. Both requirements can be derived by evaluating the transition moment integral, but that's a fairly advanced mathematical exercise. The dipole change condition is easily shown by symmetry considerations.

You can see a little more here (https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Fundamentals_of_Spectroscopy/Selection_rules_and_transition_moment_integral), although this link doesn't provide the actual derivation.

Note that the Δν = ±1 selection rule is only strictly valid in the harmonic approximation. In real systems that have anharmonic contributions, overtones with Δν = ±2, etc., become weakly allowed.