Partial charge distribution

In the present work, we shall investigate the problem of the amount of correlation accounted for in the DF formalism by comparing the molecular electrostatic potentials (MEPs) and dipole moments of CO and N2O calculated by DF and ab initio methods. It is indeed well known that the calculated dipole moment rf these compounds is critically dependent on the level of theory implemented and, in particular, that introduction of correlation is essential for an accurate prediction [13,14]. As the MEP property reflects reliably the partial charges distribution on the atoms of the molecule, it is expected that the MEP will exhibit a similar dependence and that its gross features correlate with the changes in the value of dipole moment when switching from one level of theory to the other. Such a behavior has indeed been reported recently by Luque et al. [15], but their study is limited to the ab initio method and we found it worthwhile to extend it to the DF formalism. Finally, the proton affinity and the site of protonation of N2O, as calculated by both DF and ab initio methods, will be reported. 

Analysis of the Hansch descriptors illustrates that increasing the lipophilicity property for this series of compounds will increase the bioactivity in a more dramatic fashion than increasing the Tafts steric parameter. Increasing the water accessible surface area (MOE descriptor) will increase the bioactivity of the compounds. But there is more than just adding lipophilicity and water accessible surface area to the molecules to increase binding affinity. A comparison of the substituents to the bioactivities illustrates that the bulkier X and Y groups increase bioactivity in conjunction with atomic partial charge distribution of the substituents. 

Although what was stated in Section 5.1.3 is indeed completely correct, one might think that the analysis of the partial charge distribution in the aromatic substrate could allow, too, an at least qualitatively correct prediction of the reactivity and the regioselectivity in Ar-SE reac-... 

Extrapolating from the arguments presented in Problem 2(h) of Chapter 1, the partial charge distribution of 1,3-dimethoxybenzene is as shown below. 

Cycloaddition reactions are a very important class of pericyclic reactions in which two unsaturated molecules join by converting two tt-bonds into two new a-bonds between their termini. Although cycloaddition reactions are concerted (no intermediate species are formed), the two new bonds in a few cases may be formed in an asynchronous fashion. Depending on partial charge distribution in both reactants, the formation of one bond may lead to the development of the other. 

Consider a system composed of a collection of molecules and ions, with partial charges distributed according to the details of the force field, in a cubic volume V = Let s begin by considering the electrostatic potential... 

Figure 3 illustrates a remarkably simple correlation between lattice enthalpies and the formula unit volume for A X" salts recognized by Mallouk et al. (30) as being within the usual errors of rather sophisticated evaluations from known structures and partial charge distributions. The empirical relationship... 

SOM FA is a grid-based approach that does not use a probe to determine interaction energies. Instead, each grid point is assigned the shape or —> molecular electrostatic potential (MEP) value (a) shape is represented by binary values equal to 1 for points inside the van der Waals envelope and zero otherwise (b) electrostatic potential values at grid points are calculated from partial charges distributed across the atom centers [Robinson, Winn et al., 1999]. 

For transition metals, the participation of d orbitals must also be taken into account, and values have been tabulated for the first row series [87] At first sight it seems that equations (1), (2-4) and (6) are all what we need to compute partial charge distributions. This is not the case and the main problem lies in the evaluation of the chemical hardnesses. This can be easily understood by applying (1) and (2-4) to a simple diatomic molecule A-B where B is more electronegative than A. If we apply the electronegativity equalization principle XA = XB — 0 )> it comes with qA = —5jj = q ... 

Concerning the calibration of electronegativities and hardnesses, we must refer to some known partial charge distributions and see what values should be used for the %s and atomic radius parameters to reproduce... 

Figure 6. Partial charge distributions in the crystalline hydrogen-bonded...

Figure 7. Partial charge distributions in the crystalline hydrogen-bonded Na2Si02(0H)2.5H20 phase, (a) Extraction of the naked [Si02(0H)2]7 ion. (b) Extraction of a small portion of the naked [Si02(0H)2] infinite chain, (c) Full network structure.

Figure 11b show a skewed conformation for such a trimer found in the mineral kilchoanite that is best formulated as Cae( Si04)( S130jq) [154]. Besides this skewed trimer an isolated monomer is found which displays a partial charge distribution very similar to that found in orthosilicate structures. The calcium-atoms have nearly their Ml ionic charge but in contrast to the... 

Figure 11. Partial charge distributions in linear polysilicates, (a) [Si3O10]s- trimer found in Na2CasSi3O10. (b) [Si04] - monomer and [Si3O10]8- trimer found in kilchoanite Ca5Si40i4. (c) [Si04]4" monomer and [Si40i3] " tetramer found in Ag2Si207.

Figure 12 Partial charge distributions in cyclic trimers [SD Os] " found in polysilicates minerals, (a) Benitoite BaTiSi 09. (b) Wollastonite 03381309 (charges given are those on the Fl la network-extracted trimer). (c) Wadeite K2ZrSi309.

Figure 14. Partial charge distributions in cyclic hexamers found in...

Figure 15. Partial charge distributions in cyclic polysilicates found in minerals, (a) muirite BajQ(Ca,Mn,Ti)4Sig024(Cl,OH,0)i2.4H2o (P /mmm with R = 6.5%) and (b) eudialyte. Naj2CagFe3Zr3Si2406g(Cl,0H)g (Rim with R = 15%). Charges given are those on the network-extraeted oligomers.

Figure 16. Partial charge distributions in various polycyclic polysilicates, (a) Tetrahedral tetramer of Td symmetry [Si40io] - (b) Prismatic hexamer [Si O]5] " found in the synthetic compound [Ni(en)3]Si205.8,7 H2O (P63 with R = 9.1%). (c) Cubic octamer [Si802o] found in the mineral ekanite ThKiCaNaE ig O20 (P lmcc with R = 5.8%). (d) Prismatic dodecamer [Sii203o] found in the mineral milarite KCa2(Be2Al)Sii2O30.H2O (P6lmcc with R = 2.9%).

Henry, M. (1994) Partial Charge Distributions in Crystalline Materials through... 

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