Sigma charge calculation

Sigma Charge Calculation As a first attempt to provide the... 

Figure 2. The substructures used for calculation of the environment and sigma charge descriptors for the PAH study...

The product function retrieves the partial atomic sigma charge of reaction center 3 and assigns it to the variable qsigp3. Finally, the reactivity is calculated using the partial sigma charges of both reaction centers. The rule returns OK if no error occurred and provides the reactivity value for further evaluation of reaction probability. 

For each bond of aldol, calculations were made by the methods developed in our group for the bond dissociation energy, the sigma charge distribution and the amount of resonance stabilisation of the charges generated by polar breaking of that bond. The two ways of heterolysis of a bond are shown with the points 4 and 7. Point 4 corresponds to the dissociation of OH , whereas point 7 represents the loss of OH. ... 

The various atomic charges and bond overlap populations of CH2 = NH are shown below and confirm our expectations based on consideration of sigma conjugative effects. However, an anomaly is noted in the case of the 4—31G calculation of the C—Hb and C—Hc overlap populations. Since the STO—3G optimization leads to a longer C—Hc bond, as predicted, the anomaly represents most likely a computational artifact. 

One could perhaps think of an approach consisting of straightforward calculations of the NMR shifts of the heavy atoms, a task that can be performed with accuracy thanks to methods developed by a number of authors [165,169,250,298]. Evidently, this would stiU leave us with the problem of assigning the charges of the hydrogen atoms attached to the various atoms, paying particular attention to those attached to atoms other than carbon but that problem could surely be overcome. Alternative examples exist for sigma systems [34-37] they could perhaps offer useful hints, but we prefer to keep an open mind on that matter and try harder for simpler solutions. 

Fig. 5.9. Charge distribution in sigma complexes E—C6H6+ (refined model, on the right calculation for E = H).

The quantum mechanical indices described above are obtained from calculated molecular wavefunctions. The quality of the wave-function and, consequently, of the indices depends entirely on the formalism and the level of approximation one uses. Because the molecular wavefunction is often described as a linear combination of atomic orbitals (LCAO), one can easily obtain some of the indices mentioned above. These include the atomic net charges, sigma and i charges, frontier electron densities, Euqmq and LEMO t ie superdelocalizability parameters. Within the LCAO approximation one can assign the reactivity indices to specific atoms or bonds in the molecule. These indices reflect the stationary reactivity properties of the atoms and bonds in the molecule as described by charges or orbital energies and can therefore serve only as indicators of the reactivity. 

Over the years we came up with methods to calculate bond dissociation energies, charge distribution in sigma-bonded systems, charge distribution in pi-bonded systems, the inductive effect, the resonance effect, the hyperconjugation effect and the polarisability effect. ... 

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