Electrostatic-covalent hydrogen bond model

Dispersion is not the only short-range force that needs to be added to the electrostatic interactions. For example, hydrogen bonding is not 100% electrostatic but includes covalent aspects as well, and exchange repulsion is not included in classical electrostatics at all. An accurate model should take account of all the ways in which short-range forces differ from the electrostatic approximationwith the bulk value for the dielectric constant. 

E. R. Lippincott The proposed model is certainly empirical. However, the internuclear potential function used for the terms V1 and F2 may be derived from a quantum mechanical model which lends support to their use in such a treat-ment of hydrogen bond systems. Professor Pauling is quite right in suggesting that the terms Vx and F2 may include some electrostatic contribution, since it is known that the internuclear potential function used correlates properties fairly well for partial polar bonds. Nevertheless the fact that additional terms of the electrostatic type are not needed to describe a number of the important properties of hydrogen bond systems, suggests that the covalent, repulsion and dispersions energy contributions are more important than the electrostatic contribution. 

Would you agree that hydrogen bonds (e.g. the attraction between two water molecules) might be modelled in MM as weak covalent bonds, as strong van der Waals or dispersion forces, or as electrostatic attractions Is any one of these three approaches to be preferred in principle ... 

However, there are reasons to believe that more is involved in hydrogen bonding than simply an exaggerated dipole-dipole interaction. The shortness of hydrogen bonds indicates considerable overlap of van der Waals radii and this should lead to repulsive forces unless otherwise compensated. Also, the existence of symmetrical hydrogen bonds of the type F . .. H F cannot be explained in terms of the electrostatic model. When the X—Y distance is sufficiently short, an overlap of the orbitals of the X—H bond and the electron pair of Y can lead to a covalent interaction. According to Eq. (2-8), this situation can be described by two contributing protomeric structures, which differ only in the position of the proton ... 

Within the last 5 years a number of quantum mechanical calculations have been carried out, in particular for hydrogen-bonded systems such as HgO, NHg, and HF as solvents. Contrary to electrostatic models, MO calculations allow for the possibility of covalent bond formation and, consequently, constitute a fundamentally better approach to ion-molecule interactions. Some interesting results of recent model calculations, although qualitative in nature, are discussed in Section V. 

The hydrogen bond can be considered electrostatic or ionic in character, since the classical concepts of chemical bonding allow hydrogen to form only one covalent bond. This model does not, however, account for all the properties of the hydrogen bond it is appealing to consider the bond covalent in some cases. For example, the distance of a hydrogen bond such as —H—O O— (1 A for O—H and 1.8 A for H - O) is smaller than the van der Waals radius (2.6 A) between H and O atoms. 

Kollman PA. The role of the electrostatic potential in modeling hydrogen bonding and other non-covalent interactions. In Politzer P, Truhlar DG, eds. Chemical Applications of Atomic and Molecular Electrostatic Potentials. New York Plenum, 1981 243-255. 

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