Typical motif in case of hydrogen bonds is charge transfer from electron rich acceptor Y to (usually) anti-bonding X-H σ* orbital. If so, the X-H bonding distance increases, bond weakens, stretching frequency decreases. In IR spectrum, it is visible as red shift of X-H peak and that particular peak intensifies.

Usually the peak intensity has to do with the amount of bonds it represents in the studied material.

Why does peak intensify in this case since it represents the same X-H bond, and the number of bonds is unchanged? I would've guessed it would, on contrary, decrease, since proton exchange between X and Y may occur and sometimes X-H...Y would become X...H-Y. What am I missunderstanding here?

  • $\begingroup$ Rather than intensifies it broadens, isn't? However, they are no more the same bond, ie the oscillator strength is different $\endgroup$ – Alchimista Sep 3 '19 at 9:50
  • $\begingroup$ @Alchimista iupac recommendation for hydrogen bond says "increases", as multiple other sources. I actually never read a source that says broadens. "The length of the X–H bond usually increases on hydrogen bond formation leading to a red shiftin the infrared X–H stretching frequency and an increase in the infrared absorption cross-section for the X–H stretching vibration." Could you make "they are no more the same bond, ie the oscillator strength is different" a bit more clear? $\endgroup$ – voldermot Sep 3 '19 at 13:11
  • $\begingroup$ The point is that isn't anymore the same bond as you put it using iupac. Cross section increases and intensity does but then you shouldn't take the peak height - intensity being the integral under the curve. A broad bell like band can be overall more intense that a taller sharp one. The broadening is due to the fact that you may have a collection of slightly different hydrogen bonds. Concentration of a specific bond and its own oscillator strength both count on determining the spectrum appearance, ie H-bond is not the best for applying a linear Abs vs conc relationship $\endgroup$ – Alchimista Sep 17 '19 at 10:15

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