Charge distribution calculations

Alkylideneallyl cations can be described as resonance hybrids of 1-vinyl-substituted vinyl cations and allenylmethyl cations, and thus contain two reactive sites (the sp- and sp2-hybridized carbons) for nucleophilic addition (Scheme 1) (7,2). Hybridization affects the electronic and steric character of these reaction sites. The electronic property was deduced from the l3C NMR chemical shifts of alkylideneallyl cations measured under superacidic conditions (3) and also from the charge distribution calculated (4). The charge distributions are affected by substituents on the cation the sp2 carbon is more positive than the sp carbon when two methyl groups are introduced at the sp2 carbon. 

A similar SCF calculation of ferrocene has been made by Shustorovich and Dyatkina (73) in which Slater functions were used for the iron orbitals. These calculations gave an exactly opposite charge distribution to that of Dahl and Ballhausen, owing to the more contracted metal orbitals used by the latter authors. Because values of overlap integrals of the type S (2pa3da) and S(2p7T3d7T) calculated by the latter authors are almost identical with those calculated directly from the Watson functions (74), it seems that the charge distribution calculated by Dahl and Ballhausen is the correct one. 

The ring proton chemical shifts (8 values) of l,2,4-triazolo[l,5-a]pyrimi-dine derivatives are in the order H-7 > H-5 > H-2 > H-6 (64CPB204).The charge densities determined from proton chemical shifts showed a remarkably good correspondence with the charge distributions calculated by the simple Htlckel Molecular orbital (HMO) method (64CPB204). HMO calculations for all possible tautomeric forms of the isomeric triazolopyrimidin-5(7)-ones were also performed (88M341). 

FIGURE 5. Molecular structures and charge distributions calculated at the SCF level for H2Si=S (1), ClHSi=S (126), Cl2Si=S (127) and Cl2Si=0 (128). SEN = shared electron numbers. See C. Ehahardt and R. Ahlrichs, Theor. Chim. Acta, 68, 231 (1985)... 

There have been several molecular orbital treatments of the structure and reactivity of oxazole these are summarized by Turchi and Dewar (75CRV389). Table 1 lists the a, it and net charge distributions calculated by the all-valence ab initio method, the net charges obtained by the MINDO/3 method and the bond lengths calculated by the latter method. 

Cyclodehydration of a-(thioaroylthio)- or a-(thiocarbamoylthio)carboxylic acids (96) with acetic anhydride in the presence of catalytic amounts of boron trifluoride etherate, or with a mixture of acetic anhydride and triethylamine (1 1), leads to the 2-aryl- or 2-amino-l,3-dithiolium-4-olates (97) listed in Table VIII.Chemical properties, dipole moments, NMR, IR, and UV spectra, and charge distribution calculations are consistent with a mesoionic structure. " ... 

Schulte, J. H., Gleiter, R., Rominger, F. Regiochemistry of S-Alkyl-Substituted Alkynes in Pauson-Khand Reactions. Is a Correlation with X-ray Data and Charge Distribution Calculations of the Co2(CO)6-Alkyne Complexes Possible Org. Lett. 2002,4, 3301-3304. 

Thus, comparison of the charge distribution calculated by CNDO/2, the pKa values and the partition coefficients shows that both NM-X and NM-N are excellent models for a study of conformation-activity relationships of methamphetamine. 

NMR, IR, and UV spectra, and charge distribution calculations are consistent with a mesoionic structure. ... 

A recent alternative to group-contribution activity-coefficient estimation methods is based on interactions between surface charge distributions (determined by quantum-mechanical calculations) of molecules in solution. The solvation model used for the charge-distribution calculation is known as COSMO the most widely used method based on this technique is called COSMO-RS [47]. 

The susceptibility of the atoms of sugar molecules to attack by acid and base catalysts may be associated with electronic density, as well as with stereomeric factors. Zhdanov and coworkers259 have calculated the electronic charges of the atoms of various types of carbohydrate molecules by an adaptation of the molecular-orbital method. The charge distributions calculated for aldopentopyranose and aldopento-furanose molecules are as follows ... 

An atomic charge distribution, calculated theoretically using one of the available population analysis schemes, although arbitrary and method dependent, is a useful tool to study the electronic density distribution. The performed calculations indicate in most cases very little charge dispersion from the core ions to the ligand space. In the case of the N0 (H20>2 complex the calculated electron loss fi om NO amounts to 0.03 electron [66]. The similarity of the measured photoelectron spectra of NO and its complexes... 

These resonance structures are the nitrogen and oxygen analogs of the allyl cation. The effect of this tt delocalization is to attenuate the polar destabilization by these substituents. These interactions are reflected in MO energies, bond lengths, and charge distributions calculated for such cations (review Section 3.4.1). 

Referring to the charge distribution calculated by Ohno et al. and the structural model Fig. 28, we have... 

Many attempts were made to explain lipophilicity by related properties, e.g. by solubility, solvent-accessible surface, and charge distributions calculated from semiempirical methods ([190, 272 — 284] and references cited therein) while some of the results allow a better understanding of the intrinsic nature of lipophilicity, none of these alternative approaches have led to a reliable log P prediction system so far. 

The most striking changes produced by heteroatoms are usually in the charge distributions. Calculations for pyrrole using the above parameters give the following charges ... 

Tne energies (without and with apprcpriate Q terms) and charge distributions calculated for VII-XI are presented in Fig. l((W ). 

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