Valence Bond Theory Allows a Generalized Description of Hydrogen Bonding

[Image: see text] This paper describes the nature of the hydrogen bond (HB), B:---H–A, using valence bond theory (VBT). Our analysis shows that the most important HB interactions are polarization and charge transfer, and their corresponding sum displays a pattern that is identical for a variety of energy decomposition analysis (EDA) methods. Furthermore, the sum terms obtained with the different EDA methods correlate linearly with the corresponding VB quantities. The VBT analysis demonstrates that the total covalent-ionic resonance energy (RE(CS)) of the HB portion (B---H in B:---H–A) correlates linearly with the dissociation energy of the HB, ΔE(diss). In principle, therefore, RE(CS)(HB) can be determined by experiment. The VBT wavefunction reveals that the contributions of ionic structures to the HB increase the positive charge on the hydrogen of the corresponding external/free O–H bonds in, for example, the water dimer compared with a free water molecule. This increases the electric field of the external O–H bonds of water clusters and contributes to bringing about catalysis of reactions by water droplets and in water-hydrophobic interfaces.