7r-Electron density

M.o. theory has had limited success in dealing with electrophilic substitution in the azoles. The performances of 7r-electron densities as indices of reactivity depends very markedly on the assumptions made in calculating them. - Localisation energies have been calculated for pyrazole and pyrazolium, and also an attempt has been made to take into account the electrostatic energy involved in bringing the electrophile up to the point of attack the model predicts correctly the orientation of nitration in pyrazolium. ... 

Beyer synthesis, 2, 474 electrolytic oxidation, 2, 325 7r-electron density calculations, 2, 316 1-electron reduction, 2, 282, 283 electrophilic halogenation, 2, 49 electrophilic substitution, 2, 49 Emmert reaction, 2, 276 food preservative, 1,411 free radical acylation, 2, 298 free radical alkylation, 2, 45, 295 free radical amidation, 2, 299 free radical arylation, 2, 295 Friedel-Crafts reactions, 2, 208 Friedlander synthesis, 2, 70, 443 fluorination, 2, 199 halogenation, 2, 40 hydrogenation, 2, 45, 284-285, 327 hydrogen-deuterium exchange, 2, 196, 286 hydroxylation, 2, 325 iodination, 2, 202, 320 ionization constants, 2, 172 IR spectra, 2, 18 lithiation, 2, 267... 

Bagal (1974) studied the influence of substituents on the ground and first excited states of arenediazonium ions. With regard to compounds in which mesomeric structures such as 4.1b are important, these authors are skeptical about the validity of the PP method. Later, Bagal et al. (1982) used CNDO/2. The calculated 7r-electron densities at all the nitrogen and carbon atoms were similar to those in the earlier PP results. 

If the fl-effect were operative in the ground state of a vinylic borane (31a), the effects would be to enhance the 7r-electron density of the a-carbon (31b) and to reduce C=C bond character (31c). 

Charge 7r-electron density Charge 7r-electron density... 

The importance of alkali metal binding with available 7r-electron density in the formation of CIPs was also demonstrated by Niemeyer in the structural elucidation of the first monomeric non-solvated lithium cuprate, [(2,6-Mcs2(LI L)2CuLi] 450, formed from the reaction of 2 equiv. of (2,6-Mcs2Gf,I L)Li with /-BuOCu in pentane.447 The complex crystallizes as two different independent molecules in which the C-Cu-C angles differ (171.1° and 173.8°) as does the mode of coordination to the Li cations C pso and rf to one pendant Ph in molecule 1, with an additional rf interaction to a second Ph group in molecule 2. In the second molecule, the Li site is 10% occupied by a Cu ion. 

It is often of interest to compare the 7r-electron densities in bonds rather than on atoms. Although cross terms above were assumed to vanish because of the HMO zero-overlap assumption, they are actually not zero, especially not between a.o. s on nearest neighbours. The coefficients of the cross terms can hence be interpreted as a measure of overlap in the bonds. On this basis bond order is defined as pim = Yljnjcijcmj- For the 1-2 bond in butadiene pl2 = 2chc2i + 2c2iC22 = 0.88 = P34. p23 = 0.40. These values showthat the delocalized bond contributes more strongly to bind the end pairs than the central pair and gives a simple explanation of the experimental fact that the end bonds are shorter than the central bond. 

Since the correlation of 170 chemical shifts of nitrobenzenes and nitrostyrenes is very good and nitrobenzene chemical shifts have been related to the -electron density on oxygen57, it appears that the chemical shift of the nitrostyrenes is dependent upon the 7r-electron density on the nitro oxygen also. 

Ionic or dipolar valence-bond resonance structures of nitrobenzene (53a,b) and aniline (54a,b) illustrate the difference of 7r-electron density in the ring of these two compounds85. 

The enthalpy of methylphenols was about 2.0 kcal mol- and that of chlorophenols varied from 2.0 to 2.4 kcal mol -1 in the case of pentachlorophe-nol, indicating that the retention difference depended not upon the size but on the 7r-electron density.39 A similar result was obtained for alkylated and halogenated aromatic acids, whose enthalpies were nearly equal, but whose retention factors were different.40 The AH values may depend on the type of stationary phase used and the water content of the eluent.41... 

The replacement of the —CH= group in an alternant aromatic hydrocarbon with a heteroatom gives rise to the alternation of the 7r-electron density at the ring atoms. As a result, the aromaticity should, according to the Julg criterion, Eq. (21), be somewhat reduced. Indeed, the values of A found from the data of MNDO calculations on benzene (22), boraben-zene (33), and pyridine (34) are, respectively, 1.0,0.939, and 0.971. Homo-desmotic stabilization energy calculations indicate (Table II) that pyridine is, in its aromatic character, only slightly inferior to benzene. The values... 

Of course, because of the linear inverse relationship between a and tt charges, the correlation with total charges also implies the existence of individual linear correlations with (T and tt populations, namely, upheld shifts for increasing 7r-electron densities, just as in the case of aromahc hydrocarbons. The latter result also follows from Patiser-Parr-Pople calculahons of tt charge densities reported by Witanowski et al. [161]. AU of these considerahons apply equally well to the series including pyridine, 1,3-diazine, 1,4-diazine, and 1,3,5-triazine, as revealed by their spectra in DMSO soluhon [162] (Fig. 6.8). 

Hiickel MO calculations of the 7r-electron density for pyrazolo[3,4-d]-pyrimidine 290 reveal N-3 to be the most electron rich (69CJC1129). The same conclusion was reached with simple linear combination of atomic orbitals (LCAO) calculations (291). LCAO data for electron densities on pyrazolo[4,3-LCAO calculations exaggerate electronegativities of nitrogen atoms (see 293)... 

Moreover, these results demonstrate that the stabilizing interaction of a positively charged phosphorus atom with the 7r-electron density of the unsaturation is not the main driving force for the isomerization of the double bond into the a, -position. Therefore, the isomerization ratio generally depends on the actual structure of the alkenyl chain (reactions 38 and 39). 

Calculations of electron density and of bond orders for compounds 1-3 have been performed, using the SCF-LCAO-MO method of Roothaan, as modified by Pariser, Parr, and Pople.224 The 7r-electron densities are shown for each of the three molecules in Fig. 1 in the case of compound 3, values... 

Of the compounds 117-119, only the last two have been studied experimentally (see, e.g., refs. 129-131) all have been investigated theoretically. A prediction of reactivity based on 7r-electron densities and superdelocalizabilities16 is not unambiguous. Some properties of 119 and its relatively low dipole moment are discussed in ref. 129. 

To illustrate the presently adopted meaning of the term aromaticity, let us cite an example 1- and 2-benzothiopyrylia are unstable (non-aromatic) in media containing OH- ions in spite of their relatively high DEjm value owing to the presence in the molecules of positions with low 7r-electron densities. On the other hand, under the usual conditions of electrophilic substitution these compounds appear... 

Fig. 10. The plot of 7r-electron densities (q) against atom-atom polarizabilities (ir ).7...

The pK values have been determined for several pyridine-like heterocycles.162,162° 1626 Their relation to the difference (AW) in -electronic energies of models of the conjugated acids (1FN+) and of the free bases (FFN) has been investigated.7 Values for 1FN+, Wx, AW, 7r-electron densities at the nitrogen atom in the free base (q), and localization energies (A,) of the position in the parent hydrocarbon in which there is a nitrogen atom in the heterocycle are given in Table VIII. 

B. Indices of Chemical Reactivity Table X presents the following data for models of molecules XV-XXV 7r-electron densities (q), atom-atom polarizabilities (77 ), free valences (F, Nmhx = /3), and exact superdelocalizabilities (Se,8r, and Sn). Table XI gives Wheland s atom-localization energies for a few molecules. Bond orders for molecules XV-XXV are compiled in Table XII. 

Considerable progress has been achieved in the application of the NQR spectroscopy173 concerning 14N. The experimentally determined 7r-electron densities at the nitrogen atoms in pyridine, pyrazine, sym-triazine, and phenazine have been compared with those calculated by the MO method on various levels of sophistication. It seems that the experimental results are best explained by the HMO data. 

The acid strengths of the quinoxaline monocarboxylic acids can be correlated with the electron densities at the carbon atoms attached to the carboxyl group. Quinoxaline 2-carboxylic acid has a pAa of 2.88, compared with 4.03 and 3.64 for the 5- and 6-carboxylic acids. The calculated 7r-electron densities at C-2, C-5 and C-6 in the parent heterocycle are 0.866, 0.938, and 0.933, and thus the carboxyl group attached to C-2, the ring carbon that carries least electronic charge, is the most easily ionized.246... 

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