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.