Electronic distribution atomic natural charges

The concept of natural orbitals may be used for distributing electrons into atomic and molecular orbitals, and thereby for deriving atomic charges and molecular bonds. The idea in the Natural Atomic Orbital (NAO) and Natural Bond Orbital (NBO) analysis developed by F. Weinholt and co-workers " is to use the one-electron density matrix for defining the shape of the atomic orbitals in the molecular environment, and derive molecular bonds from electron density between atoms. 

Natural resonance theory (NRT) allows these conflicting pictures of the oxyan-ion electron distributions to be tested quantitatively.149 Table 3.36 compares the geometries, NRT bond orders, atomic charges, and d-orbital occupancies for a representative variety of first- and second-row XOm"+ species,... 

C6o solubility in pyridine is identical to that in benzene. Pyridine has a pronounced "aromatic" nature. zi-electron distribution in a pyridine molecule is identical to that in benzene. Pyridine has six mobile 7i-bonds, one of them is formed by an unshared pair of -electrons of a nitrogen atom. Pyridine can be nitrated. A nitro group enters the P-position. Because carbon with the highest electron density is a center for electrophilic substitution, one can make a logical assumption that the reaction center for charge-transfer interaction between pyridine molecules and C6o is also in the P-position or, what is equivalent, in the ortho-position relative to a nitrogen atom (Table 6). 

Because of the continuous variation of the electron distribution in molecules or crystals, the counting of electron charge is hopelessly dependent on the nature of the distribution function over which the integration is taken. Clearly, the charge of an atom in a molecule or crystal is not an observable it is arbitrary. 

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... 

Further information about the nature of the reactivity of the superoxide anion towards the imidazolium cation was attained by determining the electron distribution in organic cations such as imidazolium, pyrrolidinium and quaternary ammonium (Fig. 18.3) calculated by Gaussian98 with 6-31G/UHF. The atoms with highest positive charge will be susceptible to attack by the reactive superoxide anion. 

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