Aromatic compounds bond lengths

Several aromaticity indices (bond lengths, bond orders, Jug and Francois s aromaticity index) indicate that, despite the nonplanarity of the five-membered ring in 2,5-diphenylthiophene-l-oxide (108), this compound is intermediate in aromaticity between the corresponding thiophene 107 and the nonplanar 1,1-dioxide 109 (Scheme 48).149 The theoretical calculations were supported by experimental electrochemical data.150... 

Structures tendency toward bond-length equalization and planarity (if applicable). Bond-length equalization cannot be used as the only criterion for aromaticity because some bond-equalized systems are not aromatic. Moreover, bond-length equalization due to 7t-electron delocalization is found not only in cyclic systems but also in highly conjugated acyclic compounds. 

The NMR spectrum of this compound shows a diamagnetic ring current of the type expected in an aromatic system. X-ray crystal structures of 1 and its carboxylic acid derivative 2 are shown in Fig. 9.2. Both reveal a pattern of bond lengths very similar to that in naphthalene (see p. 534). ... 

X-ray crystallographic studies are available on the ethoxy compound 1858 and on N,N-dimethylcyclopent[c]azepin-3-amine(19).59 The former is a fully conjugated, planar, 14k, non-benzenoid aromatic system, while the latter is virtually planar and, on the basis of bond length measurements, is best represented as the dipolar mesomer 19B. 

Conjugation of the 7t-electrons of the carbon-carbon double bond with the LUMO sulfur 3d-orbitals would be expected to stabilize the Hiickel 4n -I- 2 (n = 0) array of n-electrons in the thiirene dioxide system. No wonder, therefore, that the successful synthesis of the first member in this series (e.g. 19b) has initiated and stimulated several studies , the main objective of which was to determine whether or not thiirene dioxides should be considered to be aromatic (or pseudo-aromatic ) and/or to what extent conjugation effects, which require some sort of n-d bonding in the conjugatively unsaturated sulfones, are operative within these systems. The fact that the sulfur-oxygen bond lengths in thiirene dioxides were found to be similar to those of other 802-containing compounds, does not corroborate a Hiickel-type jr-delocalization... 

The observed planarity and bond length equalization in 1,3,2-diazaphospholenium cations likewise suggest that these compounds have substantial n-electron delocalization and possess possibly aromatic character. Several studies were undertaken to quantify the degree of n-delocalization by computational calculations using the interpretation of population analyses, ELF calculations, evaluation of magnetic criteria [nucleus independent chemical shift (NICS) values], and the... 

The units by which crystallographers describe interatomic distances are Angstrom units (A = 10 8 cm.). Normal values for carbon-carbon interatomic distances are 1.34 A for a double bond (as in ethylene) and 1.54 A (as for-diamond) for a single bond. In a truly aromatic compound (such as benzene) the C-C bond length, as mentioned above, is 1.39 A. C-C-C angles are 109.5° for a tetrahedral carbon atom (sp3) and 120.0° for a trigonal carbon atom (sp2). 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

Aromaticity, defined as a structural feature, was used as a predictive tool for compounds that had not been prepared previously, whereas reactivity, bond length, or magnetic criteria had to await the isolation of a compound and its experimental investigation. Only recently has the development of quantum-chemical methods reached the point where one can predict with sufficient accuracy the magnetic properties, the bond lengths, and the reactivity patterns of aromatics. The multidimensional character and the definition and measurement of aromaticity generated confusion and conflicts.43 A recent review discussed the multidimensional character of aromaticity and theoretical and experimental approaches to aromatic structures and their predictions, and references are indicated extensively.66... 

---