Electronic basis of improper hydrogen bonding: a subtle balance of hyperconjugation and rehybridization.
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.
Contribution from the Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, USA. alabugin@chem.fsu.edu
Igor V
IV
Alabugin
Mariappan
M
Manoharan
Scott
S
Peabody
Frank
F
Weinhold