Aromatic compounds electronic structure

The MNDO calculations on sila-, germa-, and stannacyclopentadienyli-denes have shown that whereas for cyclopentadienylidene (272) the energies of the antiaromatic 47t- and the aromatic 6ir-electron structures are close in value (89UK1067), in the (273)-(275) series the 67r-electron structures are quite noticeably destabilized (Table XXIII). Unlike (272), the electronic ground state of compounds (273)—(275) correspond to minima on the PES. These results point to the diminished role of antiaromatic destabilization in the 47r-electron structure (273)—(275), as opposed to (272). It should therefore be expected that these molecules would be more stable than (272). This has indeed been confirmed by our calculation on the heats of the isodesmic reaction (85) (Table XXIII). 

Thiophene is an aromatic compound. Its structure can be assumed to be derived from benzene by replacement of two annular CH groups with sulfur. The sulfur atom in this five-membered ring acts as an electron-donating heteroatom by contributing two electrons to the aromatic sextet and thiophene is thus considered to be an electron-rich heterocycle. 

An ionic 2-chloro-l,3,2-diazaphospholene (72) was prepared from a silicon precursor (73). Since the saturated analogue 74 was covalent, the phosphenium ion 72 is probably stabilised by having an aromatic 6n electron structure. The phosphadiazonium compound 75 with a sterically hindered phenol or aniline gave the phosphenium ions 76 this constitutes a new preparative route to phosphenium ions. A series of phosphenium ions (77), stabilised by two intramolecular dative P-N bonds, has been prepared, and the X-ray crystal structure of one (77, X = H, Y = PFg) determined. ... 

Many semiempirical methods have been created for modeling organic compounds. These methods correctly predict many aspects of electronic structure, such as aromaticity. Furthermore, these orbital-based methods give additional information about the compounds, such as population analysis. There are also good techniques for including solvation elfects in some semiempirical calculations. Semiempirical methods are discussed further in Chapter 4. 

In TT-complexes formed from aromatic compounds and halogens, the halogen is not bound to any single carbon atom but to the 7r-electron structure of the aromatic, though the precise geometry of the complexes is uncertain. The complexes with silver ions also do not have the silver associated with a particular carbon atom of the aromatic ring, as is shown by the structure of the complex from benzene and silver perchlorate. ... 

Aromatic compound (Section 113) An electron delocalized species that is much more stable than any structure wntten for It in which all the electrons are localized either in cova lent bonds or as unshared electron pairs... 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

Resonance is an extremely useful concept that we ll return to on numerous occasions throughout the rest of this book. We ll see in Chapter 15, for instance, that the six carbon-carbon bonds in so-called aromatic compounds, such as benzene, are equivalent and that benzene is best represented as a hybrid of two resonance forms. Although an individual resonance form seems to imply that benzene has alternating single and double bonds, neither form is correct by itself. The true benzene structure is a hybrid of the two individual forms, and all six carbon-carbon bonds are equivalent. This symmetrical distribution of electrons around the molecule is evident in an electrostatic potential map. 

Chemists sometimes represent the two benzene resonance forms by using a circle to indicate the equivalence of the carbon-carbon bonds. This hind of representation has to be used carefully, however, because it doesn t indicate the number of tt electrons in the ring. (How many electrons does a circle represent ) In this book, benzene and other aromatic compounds will be represented by a single line-bond structure. We ll be able to keep count of tt electrons this way but must be aware of the limitations of the drawings. 

All the known porphyrin isomers are typical benzoid aromatic compounds which show distinctly porphyrin-like characteristic electronic absorption spectra.13 Also the complexation properties for metal ions, NH tautomerism and the NMR spectra are quite similar to the parent porphyrin structure. 

Benzocyclopropene is an intriguing example in which the electronic structure of benzene is greatly perturbed by the fusion of the smallest alicyclic ring, cyclopropene, to the aromatic system. Benzocyclopropene thus arouses theoretical interest and the high strain energy (approximately 68 kcal./mole)3 associated with the compound suggests unusual chemical reactivity. A review article has recently appeared.4... 

The idea that the properties of many organic compounds, especially the aromatic compounds, cannot be simply correlated with a single valence-bond structure, but require the assignment of a somewhat more complex electronic structure, was developed during the period 1923 to 1926 by a number of chemists, including Lowry, Lapworth, Robinson, and Ingold in England, Lucas... 

Electronic Effects in Metallocenes and Certain Related Systems, 10, 79 Electronic Structure of Alkali Metal Adducts of Aromatic Hydrocarbons, 2, 115 Fast Exchange Reactions of Group I, II, and III Organometallic Compounds, 8,167 Fluorocarbon Derivatives of Metals, 1, 143 Heterocyclic Organoboranes, 2, 257... 

Compared with monocyclic aromatic hydrocarbons and the five-membered azaarenes, the pathways used for the degradation of pyridines are less uniform, and this is consistent with the differences in electronic structure and thereby their chemical reactivity. For pyridines, both hydroxylation and dioxygenation that is typical of aromatic compounds have been observed, although these are often accompanied by reduction of one or more of the double bonds in the pyridine ring. Examples are used to illustrate the metabolic possibilities. 

Of course, in reality new chemical substances are not synthesized at random with no purpose in mind—the numbers that have still not been created are too staggering for a random approach. By one estimate,1 as many as 10200 molecules could exist that have the general size and chemical character of typical medicines. Instead, chemists create new substances with the aim that their properties will be scientifically important or useful for practical purposes. As part of basic science, chemists have created new substances to test theories. For example, the molecule benzene has the special property of aromaticity, which in this context refers to special stability related to the electronic structure of a molecule. Significant effort has gone into creating new nonbenzenoid aromatic compounds to test the generality of theories about aromaticity. These experiments helped stimulate the application of quantum mechanical theory to the prediction of molecular energies. 

C-H and N-H bond dissociation energies (BDEs) of various five- and six-membered ring aromatic compounds (including 1,2,5-oxadiazole) were calculated using composite ab initio CBS-Q, G3, and G3B3 methods. It was found that all these composite ab initio methods provided very similar BDEs, despite the fact that different geometries and different procedures in the extrapolation to complete incorporation of electron correlation and complete basis set limit were used. A good quantitive structure-activity relationship (QSAR) model for the C-H BDEs of aromatic compounds... 

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