Electron distribution within a molecule

The description of electronic distribution and molecular structure requires quantum mechanics, for which there is no substitute. Solution of the time-independent Schrodinger equation, Hip = Eip, is a prerequisite for the description of the electronic distribution within a molecule or ion. In modern computational chemistry, there are numerous approaches that lend themselves to a reasonable description of ionic liquids. An outline of these approaches is given in Scheme 4.2-1 [1] ... 

The usual image of a molecule invoked in contemporary chemistry is a curious combination of quantum mechanical and classical mechanical models. Whereas it is well accepted that even a crude description of the electron distribution within a molecule must rely on quantum mechanics leading, for example, to various... 

The electronegativity of fluorine can also have pronounced effects on the electron distribution within a molecule bearing other substituents and affect the dipole moment of the molecule, the overall reactivity and stability of the compound and acidity or basicity of the neighboring groups. Fluorine can also participate in hydrogen bonds (50,51) or function as a ligand for alkali metals because of the available three non-bonded electron pairs (52). 

Dispersion force (Section 2.13) A noncovalent interaction between molecules that arises because of constantly changing electron distributions within the molecules. 

Formal charge is primarily useful as a bookkeeping device for electrons, but it also gives a rough guide to the charge distribution within a molecule. 

The reliability of results obtained by molecular dynamic simulations strongly depends on the pair-potential functions employed. If molecules are not strictly spherical, the choice of structure models for the molecules becomes an essential factor determining the reliability of results. A brief discussion of various models will be given. Also discussed are the electron distribution within a water molecule and potential functions... 

The absorption spectrum occurs from the ground state. Therefore, it will characterize the electronic distribution in this state. Fluorescence and phosphorescence occur from excited states, and so they are the mirrors of electronic distribution within the excited states, S for fluorescence and T for phosphorescence. Any modification of the electronic distribution in these states, such as in the presence of a charge transfer, will modify the corresponding spectrum. One such example is the reduction of cytochromes. The addition of an electron to the ground state, for example, modifies the electronic distribution within the molecule affecting the absorption spectrum. 

Toxicity, as with all forms of biological activity, is a result of the molecular structure of the chemical concerned. Given that fact, the computational chemist is presented with a problem that is, at least theoretically, soluble. The tools that have been applied so successfully to rationalizing biological activity in terms of chemical structure can also be used for correlating toxicity with various structural parameters.24 Such structural descriptors may be physicochemical values,25 functions of molecular size and shape, molecular connectivity, and numbers of atoms, or they may be quantum-chemical parameters relating to electronic distribution within the molecule.26-27... 

In conclusion, the shielding constant of a nucleus in a particular molecule is not only determined by the electronic distribution within the molecule, but also by the nature of the surrounding medium. The observed shielding constant, (Tobsd> is the sum of the shielding constant for the isolated molecule, Uo, and a contribution medium, arising from the surrounding medium, according to... 

However, real molecules are quantum mechanical objects and they do not have a finite body defined in precise geometrical terms and a finite boundary surface that contains all the electron density of the molecule. The peripheral regions of a molecule can be better represented by a continuous, 3D electronic charge density function that approaches zero value at large distances from the nuclei of the molecule. This density function changes rapidly with distance within a certain range, but the change is continuous. The fuzzy, cloud-like electronic distribution of a molecule is very different from a macroscopic body [251], and no precise, finite distance can be specified that could indicate where the molecule ends. No true molecular surface exists in the classical, macroscopic sense. 

Electron paramagnetic resonance (EPR) spectroscopy is the technique of choice for the study of paramagnetic species. It can, in many cases, provide a detailed picture of the electronic distribution within the molecule of the examined free radical or radical ion through the knowledge of the spin density distribution, which in a broad sense reflects the distribution of the unpaired electron within the... 

The reductive electrochemistry of several Ni complexes of unsaturated dithiolate ligands has been examined. On the basis of the electrochemical redox potentials, EPR spectral evidence, and SCF calculations, these reduction products are best formulated as Ni complexes for dithiocarbamate and 1,2-dithiolene ligands, and as Ni stabilized ligand-radical anions for dithiodiketonate species. It is often difficult to assign electron-density distributions within a molecule, particularly with delocalized ligands such as dithiolenes. 

The Dipole Moment. A dipole results from the presence of an unsymmetrical distribution of electron density within a molecule, due either to a formal charge separation, such as in amino acids, or due to differences in the electronegativities of the atoms forming a covalent bond, as in carbonyl compounds, water, and alcohols. An isolated, neutral atom, of course, cannot have an unsymmetrical electron distribution therefore atoms cannot be dipolar in nature. That is not to say, of course, that they cannot be polarized, or have their electron cloud distorted by an external electric field, but that subject is considered later. The dipole momenl, of a molecule is defined as... 

Experimental measurements of atomic and molecular properties typically probe one of two parameters how much energy is present in a particular type of motion, and where the particles—electrons and/or nuclei—can be found. Spectroscopy determines energies, and a variety of experiments determine location. Conductivity measurements, for example, can provide one measure of where electron charge is distributed within a molecule. 

Knowledge of the g values and the detailed hyper-fine interactions (A values) allow us to help identify radical species, and these parameters contain information about the electron distribution within the molecule. Radicals are often present as intermediates during a reaction consequently their identification will give information concerning the reaction mechanism and measurement of how their concentration changes with time will give kinetic data. 

The advent of inexpensive, fast computers has allowed chemists to develop methods for displaying the electron distribution within molecules. This distribution is obtained, in principle, by solving the Schrodinger equation for a molecule. Although the solution can be obtained only by using approximate methods, these methods provide an electrostatic potential map, a way to visualize the charge distribution within a molecule. 

Two molecular properties—the dipole moment (see Section 10-7) and polarizability (see Section 9-7)—are essential for describing the physical basis of attractive intermolecular forces. These properties are used to describe the distribution of electron density within a molecule. Before discussing different types of intermolecular interactions, weTl review some of the points we made earlier about these two molecular properties. 

Most organic compounds are electrically neutral they have no net charge, either positive or negative. We saw in Section 2.1, however, that certain bonds within a molecule, particularly the bonds in functional groups, are polar. Bond polarity is a consequence of an unsymmetrical electron distribution in a bond and is due to the difference in electronegativity of the bonded atoms. 

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