Benzene, molecular geometry

Here /, are the three moments of inertia. The symmetry index a is the order of the rotational subgroup in the molecular point group (i.e. the number of proper symmetry operations), for H2O it is 2, for NH3 it is 3, for benzene it is 12 etc. The rotational partition function requires only information about the atomic masses and positions (eq. (12.14)), i.e. the molecular geometry. 

Comparison (Tables 7-9) shows that 47 and 48 are similar in their host properties, but they are not equivalent in hehavior. Thus, host compound 48 is more qualified to select according to spatial aspects (see benzene derivatives) and, as a rule, it also forms the thermally more stable inclusions. This may be attributed to the rigid molecular geometry of the spirane 48, whereas the biaryl 47 allows sterical adaptation to different guests via the flexible hinge to a certain degree. 

Although several phenyl derivatives of the lanthanides and actinides have been characterized, only one re-arene complex of the / transition metals is known to date. This is the uranium(III) benzene complex, U(AlCl4)s CeHe 153), prepared by the combination of uranium tetrachloride, aluminum trichloride and aluminum powder in refluxing benzene, the Fischer-Hafner method [154). The molecular geometry of the complex is shown in Fig. 18. 

Although the initial crystallographic study of the simple heptiptycene 79 did not afford a structural model, 80 recently the structure of the crystalline 1 1 heptiptycene-chloro-benzene clathrate, in which the solvent molecule was packed in the channels between ribbons of the heptiptycene, was ascertained. 81 The calculated molecular geometry via Hartree-Fock (6-31G(D)) and local density methods compared well with the X-ray data. 

NMR chemical shifts can be measured with high accuracy and, therefore, the reliability of IGLO chemical shifts enables the determination of molecular geometries. For example, it was found by Ottosson and Cremer [45] that the experimental shift value of 111 ppm [9] corresponds to a Si-C(benzene) bond length of 2.29 A. Such a distance is indicative of covalent bonding and, therefore, documents the loss of silylium ion character in the benzene complex. 

All resonance structures must have identical geometries. Otherwise they do not represent the same molecule. For example, the following structure (known as Dewar benzene) is not a resonance form of benzene because it is not planar and has two less ir electrons. Because molecular geometry is linked to hybridization, it follows that hybridization also is unchanged for the atoms in resonance structures. (Note If it is assumed that the central bond in this structure is a bond, then it has the same number of electrons as benzene. However, in order for the p orbitals to overlap, the central carbon atoms would have to be much closer than they are in benzene, and this is yet another reason why Dewar benzene is an isolable compound rather than a resonance form of benzene.)... 

Domenicano, A., Vaciago, A., and Coulson, C. A. Molecular geometry of substituted benzene derivatives. I. On the nature of ring deformations induced by substitution. Acta Cryst. B31, 221-234 (1975). 

Aromatic hydrocarbons are structurally related to benzene or made up of benzene molecules fused together. These molecules are called arenes to distinguish them from alkanes, alkenes, and alkynes. All atoms in arenes lie in the same plane. In other words, aromatic hydrocarbons are flat. Aromatic molecules have electrons in delocalized % orbitals that are free to migrate throughout the molecule. See Skill 6.1a for a description of the molecular geometry of arenes. 

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