Benzene electronic structure

Cooper D L, J Gerratt and M Raimondi 1986. The Electronic Structure of the Benzene Molecule. Nature 323 699-701. 

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. ... 

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

Write a resonance form for guanine in which the six membered ring has an electronic structure analogous to benzene Show all unshared pairs... 

Hiickel models of molecular electronic structure enjoyed many years of popularity, particularly the r-electron variants. Authors sought to extract the last possible amount of information from these models, perhaps because nothing more refined was technically feasible at the time. Thus, for example, the inductive effect was studied. The inductive effect is a key concept in organic chemistry a group R should show a - -1 or a —I effect (according to the nature of the group R) when it is substituted into a benzene ring. 

McWeeny, R., and Peacock, T. E., Proc. Phys. Soc. A70, 41, The electronic structure and spectra of some nitrogen hetero-benzenes."... 

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... 

In phenol, aniline, and other compounds in which an atom with unshared electron pairs is attached directly to the benzene ring, structures such as = 0 H or especially... 

Three years ago it was pointed out2 that observed values of interatomic distances provide useful information regarding the electronic structures of molecules and especially regarding resonance between two or more valence bond structures. On the basis of the available information it was concluded that resonance between two or more structures leads to interatomic distances nearly as small Us the smallest of those for the individual structures. For example, in benzene each carbon-carbon bond resonates about equally between a single bond and a double bond (as given by the two Kekul6 structures) the observed carbon-carbon distance, 1.39 A., is much closer to the carbon-carbon double bond distance, 1.38 A., than to the shrgle bond distance, 1.54 A. 

Benzene has often been used as a test system for vibrational calculations using a variety of different electronic structure algorithms. The molecule exhibits regular hexagonal planar symmetry with six carbon atoms joined by a bonds and six remaining p-orbitals which overlap to form a delocalised n electron over all six carbon atoms. Table 1 shows comparisons of several different methods for benzene. 

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

Coordination compounds of dianionic dithiolene (S2C2 R2) and benzene-1,2-dithiolene (bdt = (S2C6H4) and their derivatives have been studied since the 1960s by Mossbauer spectroscopy [87] and other techniques. Nevertheless, many aspects of their electronic structure remained uncertain for a long time. The five-coordinate ferric complexes with two equatorial dithiolene ligands exhibit intermediate spin and show the Mossbauer parameters = 0.25-0.38 mm s and A q = 1.6-3.2 mm s For example, [Fe° mnt)2/ y] with two mnt ligands (=S2C2(CN)2) and an... 

Four-coordinate, planar iron(II)-dithiolate complexes also exhibit intermediate spin. The first example described was the tetraphenylarsonium salt of the square-planar bis(benzene-l,2-dithiolate)iron(II) dianion, (AsPh4)2[Fe(II)bdt2], which showed 5 = 0.44 mm s and AEq = 1.16 mm s at 4.2 K [157]. The electronic structure of a different salt was explored in depth by DFT calculations, magnetic susceptibility, MCD measurements, far-infra red spectroscopy and applied-field Mossbauer spectroscopy [158]. 

Fig. 11. Top molecular orbital energies for precursor, structure C (broken lines) and for bridged intermediate, structure D (full lines). Bottom bridging energy (AE) for N =0 (full line) and N = 1 (broken line), where N is the number of electrons transferred from the carbon residue to the platinum. The energies are plotted as functions of the 7rC3-to-platinum overlap integral (S). The energy unit 0 [ is the absolute value of the exchange integral between a pair of p1 orbitals in benzene. For structures C and D, cf. reaction (7). After J. R. Anderson and N. R. Avery, J. Calal. 7, 315 (1967).

The molecular structure of the [anisole, IC1] complex is analogous to the [benzene, Cl2] complex - the earliest known example of a charge-transfer complex for which the molecular structure has been established by X-ray crystallography226 and the electronic structure theoretically predicted.227... 

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. 

The electronic structure of the nitrone group, except for the main A structure, includes four canonical B-E structures. In the case of aromatic derivatives, it is necessary to consider the conjugation with the benzene ring (structures F and G)... 

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