Chemical origin of blue- and redshifted hydrogen bonds: Intramolecular hyperconjugation and its coupling with intermolecular hyperconjugation

Upon formation of a H bond Y ⋯H-XZ, intramolecular hyperconjugation n(Z )→σ^*(X-H) of the proton donor plays a key role in red- and blueshift characters of H bonds and must be introduced in the concepts of hyperconjugation and rehybridization. Intermolecular hyperconjugation transfers electron density from Y to σ^*(X-H) and causes elongation and stretch frequency redshift of the X-H bond; intramolecular hyperconjugation couples with intermolecular hyperconjugation and can adjust electron density in σ^*(X-H); rehybridization causes contraction and stretch frequency blueshift of the X-H bond on complexation. The three factors—intra- and intermolecular hyperconjugations and rehybridization determine commonly red- or blueshift of the formed H bond. A proton donor that has strong intramolecular hyperconjugation often forms blueshifted H bonds.