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J Am Chem Soc. 2003 May 14;125(19):5973-87.

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

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  • 1Contribution from the Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, USA. alabugin@chem.fsu.edu


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

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