Charges on atoms in molecules

The values are called the net atomic populations and the overlap population. Chemists speak of the charges on atoms in molecules, and Mulliken s second contribution was to propose a method of partitioning the overlap population between contributing atoms. He proposed that the overlap populations be divided equally between participating atoms, so giving the gross atomic populations of... 

Plus (+) and minus (-) signs are often used to indicate the presence of formal charges on atoms in molecules. Assigning formal charges to specific atoms is a bookkeeping technique that makes it possible to keep track of the valence electrons around an atom and offers some clues about chemical reactivity. 

Several alternatives to the Mulliken population have been presented that attempt to provide more rigorous estimates of the charges on atoms in molecules or clusters although not all have been applied in chemisorption and catalysis. We quote the Natural Bond Order analysis, and the elegant topological analysis of the electron density or of the electron localization function, ELF, introduced by Becke and Edgecombe. " ELF analysis has... 

Baumer, L., Sala, G. and Sello, G. (1989). Residual Charges on Atoms in Qrganic Structiu-es Molecules Containing Charged and Backdonating Atoms. Tetrahedron Comput.MethodoL, 2, 105-118. 

It is important to note that the calculated charges for atoms in molecules depends critically on the method used. For illustrative examples, see ... 

Formal charges do nnt represent actual charges on atoms in a molecule. In the O3 molecule, for example, there is no evidence that the central atom bears a net -1-1 charge or that one of tlie terminal atoms bears a — 1 charge. Assigning formal charges to the atoms in the Lewis structure merely helps us keep track of the electrons involved in bonding in the molecule. 

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 theory of atoms in molecules defines chemical properties such as bonds between atoms and atomic charges on the basis of the topology of the electron density p, characterized in terms of p itself, its gradient Vp, and the Laplacian of the electron density V p. The theory defines an atom as the region of space enclosed by a zero-/lMx surface the surface such that Vp n=0, indicating that there is no component of the gradient of the electron density perpendicular to the surface (n is a normal vector). The nucleus within the atom is a local maximum of the electron density. 

The difference in these patterns probably reflects that the hydrate entropies are related simply to the net positive charge on the cationic species (i.e., +2 for Pu022) while the hydrolysis reaction is the result of interaction of a water molecule with the metal atom itself — i.e., Pu in Pu022. If this is a valid explanation, the hydrolysis order indicates that the charge on Pu in Pu022 is actually between +3 and +4 and probably about +3.3. 

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