Pi-electrons

Figure Bl.23.5. Schematic illustration of tlie TOE-SARS spectrometer system. A = ion gun, B = Wien filter, C = Einzel lens, D = pulsing plates, E = pulsing aperture, E = deflector plates, G = sample, PI = electron multiplier detector with energy prefilter grid and I = electrostatic deflector.

Figure C3.2.9. Both nearest neighbour and nonnearest neighbour coupling interactions mediate superexchange between tire temrinal pi-electron groups of rigid dienes witlr saturated bridging units. From [31],...

Inherently Conducting Polymers. Conducting polymers are polymers with a pi-electron backbone capable of passing an electrical current. These polymers generally are not sufficiently conductive as neat polymers but require the inclusion of an oxidi2ing or reducing agent (dopant) to render them conductive. 

As the preceding section correctly suggests, aromatic rings a-hound in compounds that show biologic activity. The reasons for this are many the role of the pi electrons in some form of charge transfer complex ranks among the more Important. There ire few monocyclic alicyclic compounds known that are used as medicinal agents. 

To relate the pi electron ring current diagram in Figure 5 to the response surfaces of Figure 2, we note that the diamagnetic pi electron response for a field applied perpendicularly to the molecular plane at the tt/3 field point... 

The susceptibility or mixing coefficients, pj and pj , depend upon the position of the substituent (indicated by the index, /) with respect to the reaction (or detector) center, the nature of the measurement at this center, and the conditions of solvent and temperature. It has been held that the p/scale of polar effects has wide general applicability (4), holding for substituents bonded to an sp or sp carbon atom (5) and, perhaps, to other elements (6). The or scale, however, has been thought to be more narrowly defined (7), holding with precision only for systems of analogous pi electronic frameworks (i.e., having a dependence on reaction type and conditions, as well as on position of substitution). 

It should be noted here that the units of the F-nmr shifts compared to the log (kiko) values are such that sets 15, 16 and 17 are very highly weighted in the determination of the Or scale. In the statistical sense, this weighting is justified by the relatively high precision of the F-nmr shifts. In the physical sense, however, it is not clear that the high weighting is justified. Although F is the weakest pi electron donor of the united-atom-like first-row pair donor... 

The data sets (above) with the large values of X are physical property measurements which presumably involve predominantly the pi electrons. For example, the F - and C -nmr shifts are dominated by paramagnetic contributions resulting from unbalanced pi orbitals (24). Ionization equilibria, on the other hand, are characterized by a much higher blend of polar effects (a blend which traditionally has been defined as X = 1.00 for the ionization of benzoic acid, H2O, 25 ). 

X values clearly reflect the poorer transmission of pi delocalization effects from the meta than para position. For the select meta sets of Table XI, for example, X" is typically about. 4, whereas for corresponding reactions, X is around unity. Evidence is meager with respect to the ortho position, but it appears that in accord with the classical ideas of pi electron transmission in the benzene ring, generally X > X > X"" for corresponding reactions (cf. subsequent discussion). 

There is a considerable presumption that steric twisting from coplanarity, which diminishes effective transmittal of pi-electron effects, has appreciable effect on X. For example, the X = 1.10 for the phthalide saponification rate (no. 28 of Table III) compared with X=. 89 for the corresponding benzoate saponification rate is probably a reflection of the rigid coplanarity conditions imposed in the former structure. However, the reactions were carried out in differing compositions of H2 0-Et0H solvent, so that a solvent effect may also be involved. 

The inclusion of p-substituted fluorobenzene F-nmr shifts in the basis sets suggests that weakly interacting Y pi electron donor groups are also permitted. 

Y is a strong interacting pi electron acceptor. In addition to carbonium and oxocarbonium ion centers, examples of Y groups from Table XIII include Nj and several electrophilic substitution transition states (cf. sets 10,21 and 23) of the type... 

Y is a strongly pi electron donor group. As previously noted in the results section, examples of Y from Table VI include centers of high pi electron charge density at carbon, sulfur, nitrogen, and oxygen. Also included in Table VI are examples of nucleophilic substitution transition states (cf. reactions 21 and 22) of the type... 

It is interesting to note in the latter connection that nucleophilic substitution transition states in which there apparently is not strong delocalization of pi electron density into the substituent tend to fall into the <7 type (cf. reactions 18 and 19 of Table VIII). In set 18, there are two ortho nitro groups which apparently take up much of the pi charge (thus it is unavailable to X), whereas in set 19, the positive piperidinium center may cause (perhaps with assistance from the NH hydrogen bonding permitted by the aprotic solvent) the... 

The dominant state in the adduct formation is clearly the adduct. Further, the shifts for the uncomplexed ketone are best fitted by the ajj(BA)> whereas those for the adduct state are best fitted by, in accord with structural expectations. Because the adduct state is dominant in the complex formation equilibrium, the A values are also best fitted by the Or scale. The values for the individual states (summarized in Table XVIII) for this reaction (as well as other similar examples for BF3, BBr3 and adducts) are consistent with the idea that increasing pi electron demand at the reaction center increases the -Pr... 

HMO calculations of the pi electronic energies of the radicals were done using the values of coulomb and bond integrals suggested by Streitwieser (5). The only exception to these integral values was for the case of a heteroatom (with lone electron pair) bonded to the radical center carbon. The bond integrals for this case were chosen to be one-half the values suggested by Streitwieser ... 

Ethylene can be viewed as a nucleophile because it has a pair of pi-electrons that can be shared with another atom... 

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