Electron distribution of molecules

R. J. Boyd and L.-C. Wang, J. Comput. Chem., 10, 367 (1989). The Effect of Electron Correlation on the Topological and Atomic Properties of the Electron Distributions of Molecules. 

DFT calculations describe the electron density p at a point in a particular field, designated n(r). The external potential operating on this field, symbolized by -u(r), is generated by the atomic nuclei. The electron distribution is specified by p(r), which is the measure of electron density per unit volume at any point r. Integration over space provides the information needed to describe the structure and electron distribution of molecules. The calculation involves the construction of an expression for the electron density. The energy of the system is expressed by the Kohn-sham equation. ... 

The genera] expansion method, using a linear combination of several determinants, outlined in the last chapter is far too general both in overall structure and in computational demands to form the basis for either a set of concepts for the understanding of molecular electronic structure or for a practical approach to the computation of the energetics and electron distributions of molecules. 

The above result (24), valid also for a = o and/or h = o, allows us to express the first- and second-order Coulombic interactions in terms of densities and transition densities for the separate molecules. Such interactions are also conveniently described using potentials the potential energy of an electron at in the field of the unperturbed electron distribution of molecule B is... 

Studies of Hoffmann and Anderson [86, 87] as well as Baetzold [88] apply the extended Hiickel method to the calculation of the electron distribution of molecules interacting with metal slabs. 

Most aspects of chemistry are strongly influenced, if not dominated, by the properties of the peripheral regions of electron distributions of molecules. Molecular surfaces defined by isodensity contours or by some approximate model are exceptionally suitable for the description of some of the most es-... 

The induction energy describes the stabilization which occurs when the electron distribution of molecule B relaxes under the influence of the unperturbed electrostatic field from molecule A, and vice versa. The dispersion energy describes the stabilization which results when the motion of the electrons in the two molecules is correlated, rather than uncorrelated as it is in the unperturbed description. This interpretation is more readily apparent if we express the perturbation in a different form. 

There are many simations in which the electron distribution of molecules suffers important changes some examples are chemical reactions, where bonds are formed or broken, or electron excitations, where large charge redistribution takes place. It is well known that a classical description (through force fields) does not reproduce adequately the charge flows that accompany these processes and the use of quanmm... 

Electrostatic Potential. A function describing the energy of interaction of a point positive charge with the nuclei and fixed electron distribution of a molecule. 

Fig. 7.2 indicates the electron distribution of HO of carbon monoxide which largely localizes at the carbon atom 79>. This orbital resembles a lone-pair AO on the carbon atom and leads to the expectation that the carbon atom would behave as the electron-donating centre. As a matter of fact, the CO molecule coordinates with a metal cation by M—C—O type linkage (M represents a metal cation) in various metal carbonyl compounds. It is of interest to remark that the total electron population of the CO molecule has been shown by recent reliable calculation 80> to be rich on the oxygen atom in place of the carbon atom. 

An important consequence of the nonaxially symmetric electron distribution of the oxygen atom in an angular OX ligand is that B(OX)4 molecules do not have a regular tetrahe-... 

Dispersion forces. These are weak attractions caused by instantaneous fluctuation of the electron distribution in molecules and even atoms. They were first posed by Fritz London whose focus was on helium liquefaction. Such London forces fall off with the sixth power of the distance of separation. Any individual fluctuation creates a +/— local charge and that instantaneous dipole can interact with other such instantaneous dipoles nearby. The important... 

Two atoms of the same electronegativity will share electrons equally in a pure covalent bond therefore, any molecule that contains atoms of only one element, like H2 or CI2, has pure covalent bonding. Two atoms of different electronegativities, however, will have either the distorted electron distribution of a polar bond or the complete electron transfer of an ionic bond. Table 5-6 interprets the bonding between two elements as a function of the difference in their electronegativity. 

Second, molecular mechanics calculations reveal nothing about bonding or, more generally, about electron distributions in molecules. As will become evident later, information about electron distributions is key to modeling chemical reactivity and selectivity. There are, however, important situations where purely steric effects are responsible for trends in reactivity and selectivity, and here molecular mechanics would be expected to be of some value. 

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