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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: xshadow on November 05, 2020, 07:11:50 AM

Title: active vibrational mode IR/RAMAN doubt
Post by: xshadow on November 05, 2020, 07:11:50 AM

HI

I have a doubt about the vibrational modes...

If I have a centrosymmetric molecule can I have a vibrational mode that is not active both in RAMAN and IR??
Because I know that for centrosymmetric molecules If a normal mode is active in IR shouldn't be  in raman (and vicevresa)

But if I discover for a centrosymmetric molecule that a certain vibrational mode isn't IR actvie can i say aoutomatically that that mode is active in RAMAN?

thanks!

Title: Re: active vibrational mode IR/RAMAN doubt
Post by: Corribus on November 05, 2020, 10:06:21 AM

https://en.wikipedia.org/wiki/Rule_of_mutual_exclusion

Title: Re: active vibrational mode IR/RAMAN doubt
Post by: xshadow on November 05, 2020, 12:46:32 PM

Quote from: Corribus on November 05, 2020, 10:06:21 AM

https://en.wikipedia.org/wiki/Rule_of_mutual_exclusion

I found the answer!
""This does not mean that a vibrational mode which is not Raman active must be IR active: in fact, it is still possible that a mode of a particular symmetry is neither Raman nor IR active""

Thanks!