Benzene, plane structure

Tetrachlorohydroquinone (45) is closely related to chloranil and might be expected to show overcrowding effects similar to those found in chloranil by Ueda. The crystal structure of tetrachlorohydroquinone has been investigated by Sakurai (1962) using nuclear quadrupole resonance and X-ray diffraction techniques. The crystal makes use of the molecular centre of symmetry and the asymmetric unit consists of half a molecule. The analysis shows that the C—O bond deviates by 0-9° from the aromatic plane and that the two adjacent C—Cl bonds are also bent out of the benzene plane, in the same direction, through 0-8°. (This is reminiscent of Harding and Wallwork s findings for chloranil in the hexamethylbenzene-chloranil complex.) Sakurai... 

Crystalline 1 1 complexes of a few arenes with halogen molecules have been isolated and structurally characterized. As an example, the structure of the n complex C between benzene and bromine is given in Fig. 17 (264). In chains of alternate benzene and Br2 species the bromine atoms are localized on the sixfold symmetry axis of the benzene ring. The bromine-bromine distance of 2.28 A is nearly the same as that observed in the free molecule. The distance from each bromine atom to the adjacent benzene plane is 3.36 A ( 0.30 A less than the sum of the van der Waals radii). The extent of charge transfer is estimated to be 0.06 electrons in the electronic ground state (265), indicating the weakness of n bonding in this and other comparable complexes. 

The resonance energy is a maximum for a plane structure, so that the electron configurations will be as planar as possible. We shall see that in most cases it is the electrons in p functions, the so-called -re electrons, that cause the multiplicity of the electron configurations, compare benzene. Now the determinative quantity (3 is very closely connected with the overlapping of the wave functions. 

According to condition 2 (p. 197) the arrangement of the atoms in the KekuLE configuration must be the same as that in the stationary state in benzene, thus a plane structure with six equal C—C distances of 1.39 A. The most complete proof has now been furnished both by electron diffraction and by the interpretation of Raman and infra-red spectra that this plane regular hexagon structure is correct. 

Excimer formation is observed quite frequently with aromatic hydrocarbons. Excimer stability is particularly great for pyrene, where the enthalpy of dissociation is A// = 10 kcal/mol (Fbrster and SeidI, 1965). The excimers of aromatic molecules adopt a sandwich structure, and at room temperature, the constituents can rotate relative to each other. The interplanar separation is 300-350 pm and is thus in the same range as the separation of 375 pm between the two benzene planes in 4,4 -paracyclophane (13), which exhibits the typical structureless excimer emission. For the higher homologues, such as 5,5 -paracylophane, an ordinary fluorescence characteristic of p-dialkyl-benzenes is observed (Vala et al., 1965). 

The trans form is symmetrical and therefore is expected to have zero electric moment. It is found experimentally that the compound which the chemists had previously selected as the tram form does in fact have zero moment, whereas the cis form has a moment of about 1.74 X 10-18 e.s.u. (The unit 10 18 e.s.u. is sometimes called a Debye unit.) Strong evidence for the plane structure of benzene is also provided by electric-moment data, and many other problems of interest to chemists have been attacked in this way. 

Now consider the reaction of H2 loss from benzene, and suppose that the TS structure involves the loss of adjacent H atoms. If the TS structure is a planar benzene ring with two adjacent H atoms leaving in the benzene plane, the TS symmetry number will be 2. Thus the reaction degeneracy for the reaction... 

In contrast to the orthorhombic benzene I structure in which the molecules adopt an approximately cubic close-packed arrangement, the molecular arrangement in benzene II is more like hexagonal close packing in other words, this crystal contains definite layers of molecules (in the projection plane... 

Another investigation of isomers, the halogen derivatives of benzene, has been published by Pierce (57), who finds a plane structure for the Cg-ring with the halogen atoms in or near the plane. This result has also been obtained by Kaiser (45), who finds po—c= 1 4 2A. 

The dipole method has afforded valuable evidence also on the structure of simple aromatic parent substances. Benzene, diphenyl, naphthalene and higher condensed ring systems are recognized as plane structures. 

A third possible symmetry element is the center of symmetry. If there exists a point within the molecule such that for every atom a straight line can be drawn through the point connecting this atom with an equivalent atom at an equal distance from the point, then this point is called a center of symmetry. Carbon dioxide (OCO, linear), benzene (plane hexagon), ethylene, and SFe (octahedral) each possesses a center of symmetry on the basis of the present view of its structure. Methane, although it has a carbon atom at the center of gravity of the molecule, does not possess a center of symmetry. Molecules which do possess a center of symmetrj may have an atom at the center (SFe) or they may not (benzene). 

Why do cone-l Cr(CO)3 and cone-l 2Cr(CO)3 adopt such an unusual bis-roof conformation Careful examination of Figs. 2 and 3 reveals two structural characteristics of these complexes. The first characteristic is related to the position of Cr metal on the benzene ring. In conventional arene-tricarbonylchromium complexes Cr metal occupies the centro-position (Cg) on the benzene ring and the Cr-Cg line is perpendicular to the benzene plane [26-28]. In cone-1 Cr(CO)3 and cone-l Cr(CO)3, on the other hand, Cr atom shifts to the m- or p-position side in benzene A of cone-1 Cr(CO>3, for example, the distances from Cr metal to 3-, 4- and 5-carbons are 2.20,2.21 and 2.24 A, respectively... 

HMO theory is named after its developer, Erich Huckel (1896-1980), who published his theory in 1930 [9] partly in order to explain the unusual stability of benzene and other aromatic compounds. Given that digital computers had not yet been invented and that all Hiickel s calculations had to be done by hand, HMO theory necessarily includes many approximations. The first is that only the jr-molecular orbitals of the molecule are considered. This implies that the entire molecular structure is planar (because then a plane of symmetry separates the r-orbitals, which are antisymmetric with respect to this plane, from all others). It also means that only one atomic orbital must be considered for each atom in the r-system (the p-orbital that is antisymmetric with respect to the plane of the molecule) and none at all for atoms (such as hydrogen) that are not involved in the r-system. Huckel then used the technique known as linear combination of atomic orbitals (LCAO) to build these atomic orbitals up into molecular orbitals. This is illustrated in Figure 7-18 for ethylene. 

What is the MM3 enthalpy of formation at 298.15 K of styrene Use the option Mark all pi atoms to take into account the conjugated double bonds in styrene. Is the minimum-energy structure planar, or does the ethylene group move out of the plane of the benzene ring ... 

Figure 19.29 The structure of [Cr( ) -C6H6)(CO)3] showing the three CO groups in staggered configuration with respect to the benzene ring the Cr-O distance is 295 pm and the plane of the 3 O atoms is parallel to the plane of the ring.

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