Lone-Pair Directionality of

Vedani, A., Dunitz, J.D. Lone-pair Directionality of H-bond Potential Functions for Molecular Mechanics Calculations the Inhibition of Human Carbonic Anhydrase II by Sulfonamides. 

Much of our current knowledge of intermolecular interactions has been derived from the study of crystal structures. For example, it was not until an extensive survey of 1509 N—H---0=C hydrogen bonds in CSD structures that the O lone pair directionality of this key interaction was first recognized [31]. Another example is the detailed study of hydrogen bond geometries in 15 globular proteins carried out by Baker and Hubbard [32] and these hydrogen bond definitions, derived nearly 30 years... 

Lone-pair directionality of the halogen bond. For amines, phosphines, arsines and pyridines, the direction of the halogen bond (that is, of the vector X- B) is very close to the axis of the sp or sp non-bonding electron pair of nitrogen, phosphorus or arsenic. 

An important observation is that experimental structures may be reproduced satisfactorily with and without the angular terms involved in the hydrogen bonding interaction1481. Thus, irrespective of the fundamental reason for the deviation from linearity, the angular dependence may be modeled accurately either by van der Waals repulsion or by lone-pair directionality. 

Figure 9. Visualization of the lone pair directionality (complementarity) of three tetraazamacrocyclic ligands L5, L6 and L7.19...

A possible approximation to be used for the cls function can be chosen considering two ideas. In contrast to the directionality and saturability characteristic for organic covalent bonds, those formed by metal ions do not possess these properties. Thus there is no need to invoke the HO formation on the metal ion. At the infinite separation limit, the cls wave function must flow to the antisymmetrized product of the lone pair geminals of eq. (2.61). The latter is in fact a single determinant function with all lone pair HOs doubly filled. With these arguments, we arrive at the conclusion that the single determinant (HFR) wave function is an appropriate form... 

The lone-pair electrons of oxygen are conventionally assigned to sp3 orbitals, which suggests some tetrahedral directionality. However, these electron distributions are diffuse and easily polarized. So this directionality, if it exists, might be dependent on the configuration of the group to which the oxygen is bonded, as between and 0=C, for example... 

Thylor R, Kennard O, Vereichel W (1983) Geometry of the N-H—0=C hydrogen bond. 1. Lone-pair directionality. J Am Chem Soc 105 5761-5766... 

Vedani A, Dunitz JD (1985) Lone-pair directionality in hydrogen bond potential functions for molecular mechanics calculations. The inhibition of human carbonic anhydrase II by sulfonamides. J Am Chem Soc 107 7653-7658... 

The question of the exact geometry of hydrogen bonds (distances, angles, lone-pair directionality) has been reviewed [194]. 

A general equation can be derived that describes the variation in direction of the valence electron density about the nucleus. The distortion from sphericity caused by valence electrons and lone-pair electrons is approximated by this equation, which includes a population parameter, a radial size function, and a spherical harmonic function, equivalent to various lobes (multipoles). In the analysis the core electron density of each atom is assigned a fixed quantity. For example, carbon has 2 core electrons and 4 valence electrons. Hydrogen has no core electrons but 1 valence electron. Experimental X-ray diffraction data are used to deri e the parameters that correspond to this function. The model is now more complicated, but gives a better representation of the true electron density (or so we would like to think). This method is useful for showing lone pair directionalities, and bent bonds in strained molecules. Since a larger number of diffraction data are included, the geometry of the molecular structure is probably better determined. 

A. Vedani and J. D. Dunitz,/. Am. Chem. Soc., 107, 7653 (1985). Lone-Pair Directionality in Hydrogen Bond Potential Functions for Molecular Mechanics Calculations The Inhibition of Human Carbonic Anhydra.se II by Sulfonamides. 

The results and conclusions summarized here concerning lone-pair directionality in doubly hydrogen-bonded amide systems, based on molecular packing and potential energy calculations, match those of Thylor, Kennard and Versichel [108,151], who used a completely different approach. But the results presented here for single hydrogen-bond acceptors in trans amides do not show a pronounced preference for lone-pair directionality (0 130°). 

Lone-pair directionality. Journal of the American Chemical Society, 105, 5761-5766. 

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