7t electron density

MO calculations at the 6-3IG level have been done on both acrolein and aminoethylene. The resulting MOs were used to calculate charge distributions. Figure 1.26 gives the 7t-electron densities calculated for butadiene, acrolein, and aminoethylene. Inclusion of the hydrogen and a orbitals leads to overall charges as shown. These charge distributions result from a polarization which is counter to the n polarization. 

One possible explanation for the abnormal results noted above comes from considering the 7t-electron density distribution when + M substituents are present. This is indicated on the structure (XII) such that a solid constituent p orbital is a region of higher... 

According to Dewar (41) the metal-to-olefin bond in such complexes consists in part of the overlap of the 7T-electron density of the olefin with a cr-type acceptor orbital of the metal atom and in part of the back-donation of electrons from filled metal or other d-n-pn hybrid oribitals into the antibonding orbitals on the carbon atoms. 

The oxygen disposed of in molecular form on heating might be adsorbed by radicals in the surface or by defects in the basal planes possessing a high 7T electron density. 

The high-resolution C spectrum has also been measured, and the following values obtained chemical shift (in acetone) 62.4 ppm upheld from CSg (130.9 ppm downfield from tetramethylsilane) J (geminal) CH, 205 Hz J (vicinal) CH, 13.4 Hz. The chemical shift value is in accord with an empirical equation based on the number and position of the nitrogen atoms in several five-membered heterocyclics, and also reflects the 7T-electron density of the system as calculated by the extended Hiickel method - and by the simple MO method. spectra of v-triazole and of its 1- and 2-methyl derivative have also been obtained. ... 

It is well known that metal carbenes can be classified as Fisher and Schrock carbenes. The classification is mainly based on the n electron density distribution on the M = C moiety (Scheme 4.2). On the basis of the n electron density distribution, carbene complexes of the Fisher-type (E) are normally electrophilic at the carbene carbon while carbene complexes of the Schrock-type (F) are nucleophilic at the carbene carbon. Similarly, metal vinylidenes could also be classified into the two types Fisher-type (G) and Schrock-type (H). The majority of isolated metal vinylidenes belong to the Fisher-type. On the basis of the 7t electron density distribution shown in... 

Calculations for 7t-electron density of pyrazolo[3,4- pyrimidines and their tautomers were discussed in CHEC-II <1996CHEC-II(7)431>. 

In contrast to the chemical properties of enolates, azaenolates of imines exhibit a marked thermodynamic preference for the syn configuration at the C-N partial double bond (syn effect)2 due to the repulsion between the lone pair density at nitrogen and the 7t-electron density at the carbon. Thus, lithiation of imines derived from ketones occurs with strong regioselectiv-ity at the least substituted carbon, followed by a regioselective reaction with electrophiles exclusively at the carbon of the ambident azaalkyl moiety. 

Substitutions may be electrophilic or nucleophilic according to the reagent used. Electrophilic chlorination or bromination usually takes place at position 3 and /or 5 where a relatively high 7t-electron density is located in all pyran-like heterocycles. In a second process, electrophilic attack occurs only after deprotonation of a starting substrate with a strong base substitution in the 2-, 4-, or 6-positions is typical. Nucleophilic substitution usually takes place if a pyran or thiopyran has in the 2-, 4-, or 6-position a chlorine, methoxy, or an amine leaving group. 

A well understood case is that of quinoline reaction at position 2 is kinetically favored as compared with reaction at position 4, but the adduct from the latter is thermodynamically more stable. This situation, where the site of attack leading to the more stable adduct is the y position, is analogous with those regarding the formation of Meisenheimer adducts from benzene and pyridine derivatives and RCT nucleophiles. Presumably, with quinoline kinetic control favors the position that is more strongly influenced by the inductive effect of the heteroatom. The fact that position 2 of quinoline is the most reactive toward nucleophilic reagents is probably related to the lower 71-electron density at that position.123 However, the predominance of the C-4 adduct at equilibrium can be better justified by the atom localization energies for nucleophilic attachment at the different positions of quinoline. Moreover, both 7t-electron densities and atom localization energies indicate position 1 of isoquinoline to be the most favored one for nucleophilic addition. 

According to MO calculations, 41 is a closed-shell molecule, the maximum 7t-electron density being at the central carbon atom.9,47 Thus aza substitution to cycl[3,2,2]azine (1) gives considerable stabilization. 

Recent refinements on the atom-atom potential method include the development of accurate anisotropic model intermolecular potentials from ab initio electron distributions of the molecules. The non-spherical features in these charge distributions reflect features of real molecules such as lone pair and 7t-electron density, and therefore are much more effective at representing key interactions such as hydrogen bonding. 

Figure 26 Plot of the 7t-electron density for the degenerate S0 and Sj states at the conical intersection structure (Cl). The arrows indicate the number of electrons migrated from the CH2CHCH— allyl fragment to the —CHCHNH2 fragment. (From Ref. 37.)...

A further ring nitrogen atom in the 5-position in 2,4-diamino-1,3,5-triazine leads to a considerable reduction in basic strength (pKa = 3 91) and the site of protonation remains N-l (or the equivalent N-5) (Roth and Strelitz, 1969). The effect of 6-substituents on p/fa correlates with Hammett s om -constants, and it is claimed that calculations of 7T-electron densities and nmr spectra indicate protonation at N-3, rather than at N-l as in the unsubstituted compound (Marimoto, 1966). 

Donor/acceptor-substituted phosphole 22 exhibits classical properties, namely the phosphorus atom has a pyramidal geometry and the aromatic character of the heterole is similar to that of cyclopentadiene <2000JOC2631>. Due to the push-pull substitution pattern, significant delocalization of the endocyclic 7t-electron density over the entire system... 

Semi-empirical calculations of quantum-mechanical parameters such as free valence, 7T-electron density distribution or localization energy also predict that addition at the less substituted end of the double bond is favoured (Sato and Cvetanovic, 1959 Jennings and Cvetanovi6, 1961 Binks and Szwarc, 1958). However the calculations are not accurate enough to account satisfactorily for the differences found for pentene-2 and indeed the different treatments predict opposite effects. 

Preparation of the apofiavoprotein (1) and its reconstitution with and N-enriched flavins and subsequent NMR analysis yields information about the 7t-electron density of the atoms of the isoalloxazine ring in the different redox states (47). Reconstitution with chemically modified ( artificial ) flavins can be of help in substrate structure-activity relationship studies (23) and provide information about the solvent accessibility of the active site (48). 

Zaror [497] compared the uptakes of four phenols on a commercial activated carbon at a pH of 2 and reported both relatively low uptakes (<0.1 mmol/g at 0.3 mmol/L) and relatively small differences, in spite of the substantial differences in the nature of the substituents. Wang et al. [439] reported that the uptakes of p-nitrophenol, p-chlorophenol, and phenol were consistent with the notion that materials of high molecular weight are adsorbed to a more considerable extent than those of low molecular weight for compounds of similar chemical constitution Traube would be happy to learn that a century later [498] his rule lives on and that, under certain (perhaps very limited) conditions, the complexities discussed elsewhere in this review can be ignored. A clear example of the inapplicability of Traube s rule, however, is the study of Mostafa et al. [416] the authors did not attribute this to electrostatic effects or to changes in 7t- electron density (see below) but to the difference in the ability of hydrogen bond formation of the different phenols. ... 

Values for the tt-electron densities in imidazole and two related ions are given in Figure 1. The 7T-electron densities in the conjugate acid of imidazole are greater at the 2-position... 

A 7T electron density larger than 1 at the para position. 

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