Electronic basis of improper hydrogen bonding: A subtle balance of hyperconjugation and rehybridization

The X-H bond length in X-H..Y hydrogen bonded complexes is controlled by a balance of two main factors acting in opposite directions. "X-H bond lengthening" due to n(Y)-->sigma*(H-X) hyperconjugative interaction is balanced by "X-H bond shortening" due to increase in the s-character and polarization of the X-H bond. When hyperconjugation dominates, X-H bond elongation is reflected in a concomitant red shift of the corresponding IR stretching frequency. When the hyperconjugative interaction is weak and the X-hybrid orbital in the X-H bond is able to undergo a sufficient change in hybridization and polarization, rehybridization dominates leading to a shortening of the X-H bond and a blue shift in the X-H stretching frequency.