A comprehensive analysis of hydrogen bond interactions based on local vibrational modes

Local stretching modes for 69 different DH single bonds and 58 H···A H‐bonds are calculated at the ωB97X‐D/aug‐cc‐pVTZ level of theory to describe the changes in donor D and acceptor A upon forming the hydrogen‐bonded complex. The intrinsic strength of the DH and AH interactions is determined utilizing the properties of a well‐defined set of local, uncoupled vibrational modes. The local mode stretching force constant k a ( HA ) provides a unique measure of bond strength for both covalently and electrostatically bonded complexes. Generally applicable bond orders are derived, which can be related to the binding energies of the hydrogen bonded complexes. Although the red shifts in the DH stretching frequencies can be used to detect hydrogen bonding, they are not sufficient to assess the strength of hydrogen bonding. It is demonstrated that the calculated BSSE‐corrected binding energies of hydrogen bonded complexes are related to the sum of bond order changes caused by hydrogen bonding. The covalent character of charge assisted hydrogen bonds is explained. Because local mode frequencies can also be derived from experimental normal mode frequencies, a new dimension in the study of hydrogen bonding is gained.