Multiwfn forum / Blue-shift instead of expected red-shift in XB-bonded complexes

Re: Blue-shift instead of expected red-shift in XB-bonded complexes

dummy@example.com (saeed_E) — Mon, 22 Feb 2021 15:06:45 +0000

Dear Tian,
Thank you very much for your valuable guidance.

Sincerely,
Saeed

Re: Blue-shift instead of expected red-shift in XB-bonded complexes

dummy@example.com (sobereva) — Mon, 22 Feb 2021 12:14:43 +0000

I don't have a clear answer. Please try to google something like "blue-shift halogen-bond" to see if there are existing relevant studies.

Blue-shift instead of expected red-shift in XB-bonded complexes

dummy@example.com (saeed_E) — Mon, 22 Feb 2021 08:17:29 +0000

Dear Tian,

In some cases of XB-bonded complexes such as R-F...Y or R-Cl...Y (R: highly electron-withdrawing group, Y: Lewis base), after formation of complex the R-F (R-Cl) distance is contracted instead of an expected elongation! In other words, while we expect a red-shift happens for the R-F stretching frequency, a blue-shift is observed instead; that is, R-F distance in R-F...Y complex is shorter than the respective value in the isolated R-F.

Could you please let me know your valuable rationalization regarding such an unexpected contraction happened in R-F distance?

Sincerely,
Saeed