Continuum in H‑bond and Other Weak Interactions (X–Z···Y): Shift in X–Z Stretch from Blue Through Zero to Red

Abstract

We review the variation of X–Z stretching frequency in the formation
of Z-bonds (X–Z···Y, Z = H, or other main group element). Majority
of hydrogen bonds (H-bonds) occur between a strongly polar X–H bond
and an electron pair donor (H-bond acceptor) Y and show a red shift
in the X–H stretch. In very weak H-bonds, there are several instances
where the X–H bond is blue shifted. We show that X–H shifts span a continuum
going from red through zero to blue; there is nothing improper or
anti about H-bonds that are blue shifted. Other Z-bonds bring more factors
in deciding the blue and red shift. When negative hyperconjugation
in the uncomplexed monomer outweighs the charge transfer from incoming
electron-rich species Y during the formation of X–Z···Y (Z = halogen,
chalcogen, etc.), it leads to blue shifting X–Z bond. In the absence of
negative hyperconjugation in the monomer, X–Z bond length elongation
(red shift) is almost always observed on complexation with Y. In an
X–Z···Y interaction, the detailed nature of the fragment X has the major
influence on the way Z and Y control bond length elongation/contraction.
The general trend in the magnitude of blue shift amongst various weak
interactions was observed to be in the following order: halogen > chalcogen
> hydrogen > pnicogen bonding.