Electronic Basis of Improper Hydrogen Bonding: A Subtle Balance of Hyperconjugation and Rehybridization

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Abstract
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)→σ*(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.