AIM atomic charges

There are two other problems with AIM atomic charges. One is the magnitude of the calculated atomic charges for polar bonds, the calculated internal redistribution is often" significantly-larger than commonly accepted values. This may qualitatively be-... 

The theory of atoms in molecules of R. F. W. Bader and coworkers provides another, more sophisticated approach to atomic charges and related properties. Jerzy CiosJowski has drawn on and extended this theory, and he is responsible for the AIM faciJityin Gaussian. 

The AIM facility in Gaussian can be used to predict a variety of atomic properties based on this theory. We will use it to compute atomic charges and bond order for the ally cation. 

Run an AIM=BondOrders calculation for allyl cation at the MP2/6-31G(d) model chemistry. What are the predicted atomic charges and bond orders for this molecule ... 

As its name implies, AIM enables us to calculate such properties of atoms in a molecule as atomic charge, atomic volume, and atomic dipole. Indeed it shows us that the classical picture of a bond as an entity that is apparently independent of the atoms, like a Lewis bond line or a stick in a ball-and-stick model, is misleading. There are no bonds in molecules that are independent of the atoms. AIM identifies a bond as the line between two nuclei. 

Clearly the concepts of ionic and covalent character have only an approximate qualitative significance. They cannot be defined and therefore measured in any quantitative way. Although they are widely used terms and have some qualitative usefulness if used carefully they have caused considerable misunderstanding and controversy. The AIM theory does, however, provide properties that we can use to characterize a bond quantitatively, such as the bond critical point density and the atomic charges. It seems reasonable to assume that the strength of a bond depends on both these quantities, increasing as pb and the product of the atomic charges increase. 

The concepts of ionic and covalent character of a bond are vague and ill defined. The well-defined AIM-derived quantities such as the integrated atomic charges and the bond critical point density provide a quantitative characterization of bonding (4). 

An alternative physical observable that has been used to define partial atomic charges is the electron density. In X-ray crystallography, the electron density is direedy measured, and by comparison to, say, spherically symmetric neutral atoms, atomic partial charges may be defined experimentally, following some decisions about what to do with respect to partitioning space between the atoms (Coppens 1992). Bader and co-workers have adopted a particular partitioning scheme for use with electronic structure calculations that defines the atoms-in-molecules (AIM) method (Bader 1990). In particular, an atomic volume is... 

Studies have also been carried out which are more specifically aimed at charge transfer on an atomic scale and deal with the atomic situation within the lattice. This is especially so in the case of binary oxides. Many authors assume that, in these systems, both types of cation participate in electron transfer. The reactivity of the binary oxides is then explained by the hypothesis that the cation on the active site obtains an electron supply from the second type. 

Thus pure MM (MM by itself) can t calculate UV spectra, the shapes and energies of molecular orbitals, and electron distribution and derivative properties of this, like atomic charges, dipole moments, and more arcane molecular features like bond paths (associated with atoms-in-molecules theory, AIM [1]). 

Fig. 10. Atomic charges for TNDAP - upper number from AIM analysis, lower from kappa refinement.

As mentioned above, practical applications of the Klopman relationship (Equation 6.50) often require further simplifications, particularly in cases where the relevant MO properties are known only for one of the two reaction partners. The latter is the typical situation in QSAR investigations that aim to elucidate the impact of molecular reactivity characteristics on the toxic potential of chemical agents. While the Coulomb interaction term can be reduced to calculate net atomic charges (see above), a possible candidate to replace the orbital interaction term of Equation 6.50 by a one-molecule property is Fukui s delocalizability, as was pointed out over 35 years ago (Cammarata, 1968 Cammarata and Rogers, 1971). 

Fig. 26. Perspective view of the (010)-surface of the vanadium pentoxide with different mutual arrangements of neighboring pyramidal [VOs] units (Panel a). Panels b and c correspond to neutral, stoichiometric surface cluster including two layers of the (010) surface pyramids (126 atoms) in Panel b, which illustrates the SINDO AIM net-charge distribution, the bipyramidal subsystems I and II are shown (see Table 1) Panel c represents the AIM FF distribution diagram...

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