Quantification of hyperconjugative effect on the proton donor X-H bond length changes in the red- and blueshifted...

A whole dataset containing 55 hydrogen bonds were studied at the MP2/aug-cc-pVTZ level of theory. The changes of geometries and stretching vibrational frequencies show that there are 31 redshifted and 24 blueshifted hydrogen-bonded complexes. Natural bond orbital analysis was carried out at the B3LYP/aug-cc-pVTZ level of theory to obtain the electron densities in the bonding and antibonding orbitals of the proton donor X-H bond, which are closely associated with its bond length. Based on their relationship, a generally applicable method considering both the electron densities in the bonding and antibonding orbitals of the proton donor X-H bond has been developed to quantitatively describe the hyperconjugative effect on the X-H bond length changes in these hydrogen-bonded complexes.