Resonance transitions, electron density

Electrophilic substitution reactions of unsubstituted quinoxaline or phenazine are unusual however, in view of the increased resonance possibilities in the transition states leading to the products one would predict that electrophilic substitution should be more facile than with pyrazine itself (c/. the relationship between pyridine and quinoline). In the case of quinoxaline, electron localization calculations (57JCS2521) indicate the highest electron density at positions 5 and 8 and substitution would be expected to occur at these positions. Nitration is only effected under forcing conditions, e.g. with concentrated nitric acid and oleum at 90 °C for 24 hours a 1.5% yield of 5-nitroquinoxaline (19) is obtained. The major product is 5,6-dinitroquinoxaline (20), formed in 24% yield. 

Since the time of the quantum-mechanical calculations by Longuet-Higgins, many attempts have been made to calculate tt-electron densities, resonance energies, dipole moments, and optical transitions both by the LCAO-MO and the valence bond method.However, no agreement has been reached on the importance of pd-hybridization of the sulfur atom. This is considered by some workers an essential... 

It must be emphasized once again that neither by the resonance nor by the field effect are any electrons actually being donated or withdrawn, though these terms are convenient (and we shall use them). As a result of both effects, the electron-density distribution is not the same as it would be without the effect (see pp. 17, 42). One thing that complicates the study of these effects on the reactivity of compounds is that a given group may have an effect in the transition state which is considerably more or less than it has in the molecule that does not react. 

In practice, the NBO program labels an electron pair as a lone pair (LP) on center B whenever cb 2 > 0.95, i.e., when more than 95% of the electron density is concentrated on B, with only a weak (<5%) delocalization tail on A. Although this numerical threshold produces an apparent discontinuity in program output for the best single NBO Lewis structure, the multi-resonance NRT description depicts smooth variations of bond order from uF(lon) = 1 (pure ionic one-center) to bu 10n) = 0 (covalent two-center). This properly reflects the fact that the ionic-covalent transition is physically a smooth, continuous variation of electron-density distribution, rather than abrupt hopping from one distinct bond type to another. 

The electron density centered at M is the only central contributor at the nuclear position M, as in this case the nucleus coincides with the field point P, which is excluded from the integrals. For transition metal atoms, the central contributions are the largest contributors to the properties at the nuclear position, which can be compared directly with results from other experimental methods. The electric field gradient at the nucleus, for instance, can be measured very accurately for certain nuclei with nuclear quadrupole resonance and/or Mdssbauer spectroscopic methods, while the electrostatic potential at the nucleus is related to the inner-shell ionization energies of atoms, which are accessible by photoelectron and X-ray spectroscopic methods. 

It must be emphasized that these cross sections are only valid for an electron excitation into free-electron like final states (conduction band states with parabolic band shape) and not for resonance transitions as f — d or p - d excitations. If too low excitation energies (< 10 eV, see Table 1) are used in UPS, the final states are not free-electron like. Thus the photoemission process is not simply determined by cross-sections as discussed above but by cross-sections for optical transitions as well as a joint density of states, i.e. a combination of occupied initial and empty final states. 

Nuclei in molecules are affected by the local electron density and their chemical shift, the ratio of the frequency at which an NMR transition occurs to the strength of the main field as measured by the frequency at which protons resonate, is given by ... 

Fig. 15.20 Resonance forms for a transition metal carbene complex. Form (a) shows metal-carbon double bond character which results from donation of metal d electron density to an empty p orbital of carbon. Form (b) shows oxygen-carbon double bond character which results from donation of oxygen p electron density to an empty p orbital of carbon Form (W provides the dominant contribution.

There may be a relatively large charge separation, e.g. in MoSj- and almost equal net charges on the metal and sulfur atoms, e.g. in [Mo2S(CN),2]6-. 78a MO calculations on the latter ion show that there are k MOs delocalized over three centers. Resonance Raman studies further indicate that the delocalization extends over the whole linear N—C—Mo—S—Mo—C—N system. The tt(S) -> t/(Mo) donation induces a decrease of electron density on the sulfur and is, therefore, responsible for an unusual charge transfer transition Mo-+S (band at 27100cm-1), quite the reverse assignment to that in examples with terminal sulfide where a considerable p contribution also has to be anticipated (see Table l).3,4... 

The ligand field strength of the ligands is between that of the dithiocarbamates and water.49 IR studies show characteristic bands near 1250, 1100, 1020 and 550 cm-1.93 The contribution of the resonance form (85) in transition metal complexes is less than that of the analogous structure in the dithiocarbamates.60 The electron density on the metal is not very high, which accounts for the fact that abnormal high oxidation states are exceptional and a strong interaction of bases with the square planar nickel (and some other metal) xanthates is found.94... 

The finding that a similar order of reactivity holds for electrophilic substitution requires a modification of this interpretation. The proposed resonance structure involving inductive relay of electron density to the carbonium center is not possible with the benzenium ions. In the transition state for the substitution, as approximated by the benzenium ion, resonance is permissible only through bicyclic structures ... 

Figure 37 Plasma shift AE (scaled by Z-2) for 2s2 So - 2s2p 1PJ resonance transitions as a function of the electron density (n in cm-3). Reprinted with permission from [248] 2007, IOP Publishing Limited...

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