Energetic Origins of the Hydrogen-Bond Redshift: IQA Partitioning of Normal Mode Force Constants

The redshift of the D-H (where D represents the H donor group containing F, O, N) bond stretch normal coordinate is notable evidence of a hydrogen-bonded system. Upon the formation of the hydrogen bond, electron density is transferred from the acceptor moiety of the complex to the hydrogen atom donor, causing an elongation of the D-H bond and a reduction of the force constant. Within the orbital paradigm of chemistry, the redshift of the D-H stretch frequency is caused by the donation of electrons from the base lone pair to the antibonding, σ*, orbital of the D-H bond. The increased electron population of the antibonding orbital is, therefore, responsible for the decrease in the force constant and, consequently, for the redshift. In this work, we present a description of the H-bond redshift in terms of the electron density, substituting the molecular orbital theory interpretation with the Quantum Theory of Atoms in Molecules with the Interacting Quantum Atoms (IQA) energy decomposition scheme. The results herein suggest that the energetic origin of the redshift depends on the acid structure. When H₂O acts as a H donor, the redshift is mostly determined by the intratomic and exchange-correlation, whereas the redshift of the HF stretch is caused primarily by the Coulomb contribution. The H acceptor molecule modulates the amount of variation in the above-mentioned cases. The better the acceptor, the greater the variation in the IQA contribution to the force constant.