Hydrocarbons charge distribution

Fig. 1.6. Charge distributions in strained cyclic hydrocarbons in comparison with cyclohexane. Data are from K. B. Wiherg, R. F. W. Bader, and C. D. H. Lau, J. Am. Chem. Soc. 109 1001 (1987).

For nonaltemant hydrocarbons the energies of the bonding and antibonding orbitals are not equal and opposite and charge distributions are not the same in... 

These techniques proved to be successful for the majority of the hydrocarbons, but they failed for some compounds such as phenanthrene, terphenyl, or quater-phenyl. The failure has been interpreted by the effect of the electric field on the charge distribution [190]. Actually, these molecules have strongly anisotropic polarizabilities. In a more recent study, it was demonstrated that the isomeric ratio also depends on the counterion [191]. 

Non-alternant hydrocarbons possess uneven charge distributions in the ground state, even when no substituents are present, and again comparisons of qr and separately with experimental data may produce... 

The negative charge distribution in the adducts was evaluated on the basis of the chemical shift values and found to be nearly 75% concentrated on the nitrogen atom. In the reactions of l-ethoxycarbonyl-l,4-dihydropyridines with organosodium and organopotassium compounds, the resulting metal-associated (j-adducts were not soluble in aliphatic hydrocarbons alone but were made so by addition of 18-crown-6. This behavior would support ionic structures for the potassium and sodium adducts. 

Interestingly, the interactions between zeolites and unsaturated chlorocarbons like trichloroethylene (TCE) are found to be strikingly different from those between zeolites and unsaturated hydrocarbons (i.e. ethylene and benzene). Both our simulations and our spectroscopic results on the adsorption of TCE in faujasites show that interactions between the n electrons and the cations, which dominate in the case of hydrocarbons, are replaced by interactions between the chlorine atoms and the cations [18]. Figure 3 shows typical positions of TCE in NaY zeolite as predicted by energy minimizations. This is a consequence of the different charge distribution in hydrocarbons and halocarbons. 

A semi-empirical molecular orbital method for the correlation of charge distributions with 13C shifts in amino acids was described [95]. Plotting of chemical shift parameters versus charge density changes of a-carbon atoms relative to the corresponding atoms in the parent hydrocarbons permits prediction of the chemical shifts of the a-carbons with an accuracy of about 10%. However, the slope (280 ppm per electron) in Fig. 5.12 is... 

Andersson developed a semi-empirical model for the charge distribution around the (V=0) bonds in V205, V6013, and V02.73 The surfaces of the lower oxides were treated, upon the basis of ESCA results discussed on p. 107, as being in an oxidized state, which is proposed to be the case under the usual conditions in (amm) oxidation reactions. The main result is that 02-in the form of (V=0) groups is responsible for the catalytic oxidation of hydrocarbons. 

In most isocyclic hydrocarbons (alternating hydrocarbons, see below) the charge distribution is uniform, so that qk = 1 for all atoms. Then there occurs in place of these quantities the so-called self-polarizability ... 

Compared with the pronounced solvent-induced chemical shifts observed with ionic and dipolar solutes, the corresponding shifts of nonpolar solutes such as tetrame-thylsilane are rather small cf. Table 6-6. A careful investigation of chemical shifts of unsubstituted aromatic, as well as alternant and nonalternant, unsaturated hydrocarbons in aliphatic and aromatic non-HBD solvents by Abboud et al. has shown that the differential solvent-induced chemical shift range (relative to benzene as reference) is of the order of only —1.4...+1.0 ppm (positive values representing downfield shifts) [405]. The NMR spectra of these aromatic compounds have been shown to be sensitive to solvent dipolarity and polarizability, except in aromatic solvents, for which an additional specific aromatic solvent-induced shift (ASIS see later) has been found. There is no simple relationship between the solvent-induced chemical shifts and the calculated charge distribution of the aromatic solute molecules. This demonstrates the importance of quadrupoles and higher multipoles in solute/solvent interactions involving aromatic solutes [405]. 

Fio. 3.7. Planar projections of molecular graphs of hydrocarbon molecules generated from theoretical charge distributions. Bond critical points are denoted by black dots. Structures 1 to 4 are normal hydrocarbons from methane to butane, 5 is isobutane, 6 is pentane, 7 is neopentane, and 8 is hexane. The remaining structures are identified in Table 3.2. The structures depicted in these diagrams are determined entirely by information contained in the electronic charge density. 

It can be demonstrated that alternate hydrocarbons in their ground states have all a = 1 (ref. 124). Part of their special stability is attributed to this uniformity of charge distribution. 

Proceeding as we did for the allyl radical, it is easily seen that the electron charge distribution is uniform (one n electron onto each carbon atom, alternant hydrocarbon) and the spin density is zero, as expected for a state with S = Ms = 0 since the two bonding MOs are fully occupied by electrons with opposite spin. The delocalization (or conjugation) energy for linear butadiene is ... 

The contents of Part 1 is based on such premises. Using mostly 2x2 Hiickel secular equations, Chapter 2 introduces a model of bonding in homonuclear and heteronuclear diatomics, multiple and delocalized bonds in hydrocarbons, and the stereochemistry of chemical bonds in polyatomic molecules in a word, a model of the strong first-order interactions originating in the chemical bond. Hybridization effects and their importance in determining shape and charge distribution in first-row hydrides (CH4, HF, H20 and NH3) are examined in some detail in Section 2.7. 

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