Ir-Electrons

A decreasse in a net charge at C-5 with, as a corollary, a slight increase in ir-electronic density at that position. 

TABLE II1-4. correlation BETWEEN ir ELECTRON DENSITIES AND CORRECTED CHEMICAL SHIFTS (108,110)... 

Wood furniture (ir-, electron beam-, and uv-curable coatings)... 

When hydroxypteridines are considered, it must be borne in mind that these compounds exist principally in the pteridinone forms, containing thermodynamically stable amide functions, and consequently have low reactivity. Their stability towards acid and alkali correlates well with the number of electron-donating groups which apparently redress the deficit of ir-electrons located at the ring nitrogen atoms. Quantitative correlations can be seen in the decomposition studies of various pteridinones (Table 7). These results are consistent with the number of the oxy functions and their site at the pteridine nucleus. The... 

The stability of isothiazole derives from the fact that it has an aromatic delocalized ir-electron system. The NMR chemical shifts, which depend, inter alia, on ring currents, and the high stability of the molecular ions in mass spectrometry, are typical of aromatic compounds, and X-ray measurements confirm the partial double bond character of all the bonds of the ring. 

Saccharin does not comply with the normal 4n + 2)ir-electron rule for aromaticity, but in view of the fact that it has been shown earlier to have a degree of ir-electron delocalization through the sulfur atom, and for convenience of classification of its chemical reactions, it will be considered to be aromatic in the subsequent sections dealing with its chemistry. 

A complete mechanistic description of these reactions must explain not only their high degree of stereospecificity, but also why four-ir-electron systems undergo conrotatory reactions whereas six-Ji-electron systems undergo disrotatory reactions. Woodward and Hoifinann proposed that the stereochemistry of the reactions is controlled by the symmetry properties of the HOMO of the reacting system. The idea that the HOMO should control the course of the reaction is an example of frontier orbital theory, which holds that it is the electrons of highest energy, i.e., those in the HOMO, that are of prime importance. The symmetry characteristics of the occupied orbitals of 1,3-butadiene are shown in Fig. 11.1. 

Hiickel realized that his molecular- orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons. He pointed out that cycloheptatrienyl cation, also called twpylium ion, contained a completely conjugated closed-shell six-ir electron system analogous to that of benzene. 

In pynole, on the other hand, the unshared pah belonging to nitrogen must be added to the four tt electrons of the two double bonds in order to meet the six-ir electron requhement. As shown in Figure. 6b, the nitrogen of pynole is 5/r -hybridized and the pair of electrons occupies a p orbital where both electrons can participate in the aromatic tt system. 

Section 24.13 On being heated, allyl aryl ethers undergo a Claisen rearrangement to form o-allylphenols. A cyclohexadienone, formed by a concerted six-ir-electron reorganization, is an intermediate. 

Theoretical studies of the relative stabilities of tautomers 14a and 14b were carried out mostly at the semiempirical level. AMI and PM3 calculations [98JST(T)249] of the relative stabilities carried out for a series of 4(5)-substituted imidazoles 14 (R = H, R = H, CH3, OH, F, NO2, Ph) are mostly in accord with the conclusion based on the Charton s equation. From the comparison of the electronic spectra of 4(5)-phenylimidazole 14 (R2 = Ph, R = R3 = H) and 2,4(5)-diphenylimidazole 14 (R = R = Ph, R = H) in ethanol with those calculated by using ir-electron PPP method for each of the tautomeric forms, it follows that calculations for type 14a tautomers match the experimentally observed spectra better (86ZC378). The AMI calculations [92JCS(P1)2779] of enthalpies of formation of 4(5)-aminoimidazole 14 (R = NH2, R = R = H) and 4(5)-nitroimidazole 14 (R = NO2, R = R = H) point to tautomers 14a and 14b respectively as being energetically preferred in the gas phase. Both predictions are in disagreement with expectations based on Charton s equation and the data related to basicity measurements (Table III). These inconsistencies may be... 

It is notable that pyridine is activated relative to benzene and quinoline is activated relative to naphthalene, but that the reactivities of anthracene, acridine, and phenazine decrease in that order. A small activation of pyridine and quinoline is reasonable on the basis of quantum-mechanical predictions of atom localization encrgies, " whereas the unexpected decrease in reactivity from anthracene to phenazine can be best interpreted on the basis of a model for the transition state of methylation suggested by Szwarc and Binks." The coulombic repulsion between the ir-electrons of the aromatic nucleus and the p-electron of the radical should be smaller if the radical approaches the aromatic system along the nodal plane rather than perpendicular to it. This approach to a nitrogen center would be very unfavorable, however, since the lone pair of electrons of the nitrogen lies in the nodal plane and since the methyl radical is... 

I Cyclooctatetraene has eight it electrons and is not aromatic. The ir electrons are localized into four double bonds rather than delocalized around the ring, and the molecule is tub-shaped rather than planar. 

Although five equivalent resonance structures can be drawn for all three species, Huckel s rule predicts that only the six-ir-electron anion should be aromatic. The four-77-electron cyciopentadienyl carbocation and the five-7r-electron cyciopentadienyl radical are predicted to be unstable and antiaromatic. 

FIGURE 3.22 As a result of resonance m between two structures like the one. g shown in Fig. 3.21 (corresponding to 3 resonance of the two Kekule structures), 3 the ir-electrons form a double doughnut- j. shaped cloud above and below the plane of the ring. w... 

However, because the delocalized ir-electrons constitute such a stable arrangement, the hydrogen atom shown can be pulled from the ring as H+ by a bromine atom in the FeBr4 complex in that way, delocalization is regained ... 

The curve corresponding to this equation is also shown in Fig. 1. It is seen that it lies below the curve representing resonance between a single bond and a ry (or ir ) double bond by the amount 0.026 A. at 0.5, and by 0.045 A. at the point for a pure double bond, n — 1.0, these differences representing the effect of the extra resonance stabilization when two ir electron pairs are involved. Values corresponding to the curve are given in Table I. 

The meso carbon atom should present a carbenium (Cs+) structure with a low 77 electron density in the ground state, in the excited state this carbon possesses the carbeniate structure (C8-) with a high ir electron density (119). An electron-donating group in such a position should stabilize the ground state and rise the excited state to the highest level hypsochromic shift results as a whole. 

With radicals of the benzyl type, 11 through 18, the dimerization equilibrium depends on the relative magnitudes of the energy gain arising from the C-C bond formation and of the ir-electron energy loss which results from a reduction of the conjugated system. 

In the case of p-fluorobenzonltrlle this effect appears to Induce a change In orientation of the adsorbed molecule. At lower potentials It Is oriented flat on the surface, bonding by the ir-electron system, but at higher potentials It sits up and bonds via the nitrogen (34). 

Contrary to naive expectation, an extended ir-electron system snch as that in the original silvery polyacetylene film does not imply perfect bond conjngation or perfectly like bonds, or conduction along the chain It only implies a degree of charge-density delocalization. Such a material has the electronic structure of a... 

Nigo et al. claim to have prepared Ni(Et2 /c)2Br2 106), whereas Jensen reports the preparation of Ni(Et2spectral data, we believe these products to be Ni(Et2 itc)3Br and Ni(Et2 xc)3Br 100), respectively, contaminated with NiBr2-... 

The relative response of the electron-capture detector to some haloalkylacyl derivatives is sumi rized in Table 8.17 [451]. In general terms, the monochloroacetyl and chlorodifluoroacetyl derivatives provide a greater response than the trifluoroacetyl derivatives. Increasing the fluorocarbon chain length of the fluorocarbonacyl derivatives increases t ir electron-capture detector response without inconveniently increasing their retention times. The heptafluorobutyryl and pentafluoropropionyl derivatives are considered to be the best compromise between detector sensitivity and volatility for most applications. 

Fig. 12 The experimental geometries of cyclopropane- -HC1 and cyclopropane- -C1F (drawn to scale) and the Coulson-Moffitt pseudo-ir-electron model of cyclopropane. See Fig. 1 for key to the colour coding of atoms...

Fig. 4. Helium(I) photoelectron spectrum of 1, showing ionization of Si=Si ir-electrons at 7.53 eV.54...

Noteworthy is the "B resonance in 103, which falls in the range 58-60 ppm, very little different from the signal of trivinylborane (55 ppm) therefore, any extensive ir-electron delocalization and density on boron can be ruled out. Thus both in chemical and physical properties, such systems as 103 can be considered antiaromatic. 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

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