Structures of Conjugated Hydrocarbons

We would suspect, and we find, to use 1,3-butadiene as an example, that the torsion barrier about the central bond is much lower than that in ethylene. The torsion barriers about the end bonds of butadiene, on the other hand, are quite high, but still somewhat lower than that of ethylene. As the bond order is reduced from 1.0 (in ethylene) to a smaller number as in the bonds in butadiene (0.96 and 0.26 for the short and long bonds, respectively, the V2 coefficient describing the barrier is also reduced. The V2 term is, in fact, gradually replaced by a smaller V3 term, which dominates if the bond order falls down near to zero, and the double bond goes over to a single bond. If we wish to have the force field describe not only ethylene, benzene, and butadiene, but also such things 

Let us now consider the structures of some conjugated hydrocarbon molecules as they are calculated with the MM4 force field. Table 5.1 shows structural data on the simple molecules ethylene, benzene, and tran -butadiene. Thus, we see that we can first of all calculate to within experimental error the structures of our simple standard molecules (Table 5.1), including the rotational barriers of ethylene and butadiene, which are not included in the table.  

We then applied the force field from those molecules to several somewhat more complicated cases, such as 1,3-cyclohexadiene (Table 5.2), and, as expected, we were also able to calculate the structure of those molecules to within experimental error. 

TABLE 5.1. Structures of Ethylene, Benzene, and trans-Butadiene  

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Individual formal valence structures of conjugated hydrocarbons are excellent substrates for research in chemical graph theory, whereby many of the concepts of discrete mathematics and combinatorics may be applied to chemical problems. The lecture note published by Cyvin and Gutman (Cy-vin, Gutman 1988)) outlines the main features of this type of research mostly from enumeration viewpoint. In addition to their combinatorial properties, chemists were also interested in relative importance of Kekule valence-bond structures of benzenoid hydrocarbons. In fact, as early as 1973, Graovac et al. (1973) published their Kekule index, which seems to be one of the earliest results on the ordering of Kekule structures These authors used ideas from molecular orbital theory to calculate their indices... 

Finally, we consider the application of the symmetry rule to the prediction of the geometrical structures of the excited states of conjugated hydrocarbons. On the basis of the same approximations as we... 

A limited amount of information is available on the formation of metabolites of aromatic hydrocarbons in aquatic invertebrates however, some studies have elucidated the structures of conjugated and non-conjugated derivatives produced in these organisms. The bioconversion of naphthalene by the spider crab (Maia squinado) has been studied by Corner et al. (45). These workers identified... 

Photochemistry of conjugated hydrocarbons can be rationalized by the common electronic and molecular structure of the surface crossing between a covalent excited state and the ground state. 

Jerman-Blazic Dzonova, B. and Trinajstic, N. (1982). Computer-Aided Enumeration and Generation of the Kekule Structures in Conjugated Hydrocarbons. Computers Chem., 6,121-132. 

The main difficulty with the RE concept is the hypothetical nature of the reference structure its choice being somewhat arbitrary. The RE index of conjugated hydrocarbons, calculated in the standard way. 

Coulson, C. A., Longuet-Higgins, H. C. (1947). The Electronic Structure of Conjugated Systems. II. Unsaturated Hydrocarbons and their Hetero-Derivatives. Proceedings of the Royal Society of London A Mathematical, Physical and Engineering Sciences, 792(1028), 16-32. 

Randic M (1976b) Enumeration of the Kekuld Structures in Conjugated Hydrocarbons. J Chem Soc Faraday Trans 2 72 232... 

The effects of the extension of the conjugated s rans (mainly in condensed benzenoid rings, where the reductions of a double bond or a reduction in the side chain are related to the structure of the hydrocarbon), are treated by quantum chemical methods.d)... 

Already in the seminal paper by Dewar and Longuet-Higgins [15], it was established that the Ai -regularity does not hold for all polycyclic conjugated n-electron systems, and that corrections for the parity of Kekule structures need to be taken into account. This became the concept of algebraic structure count [22, 23], which was later shown to be not applicable to all non-altemant conjugated hydrocarbons [24, 25]. The fortunate fact is that all Kekule structures of benzenoid hydrocarbons have the same parity [20], which has the consequences that all Kekule-structure-based models work best for benzenoid systems. Also in this... 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

Many completely conjugated hydrocarbons can be built up from the annulenes and related structural fragments. Scheme 9.2 gives the structures, names, and stabilization energies of a variety of such hydrocarbons. Derivatives of these hydrocarbons having heteroatoms in place of one or more carbon atoms constitute another important class of organic compounds. 

Benzo[c]phenanthridine alkaloids are widespread in Papaveraceae, Fumariaceae, and Rutaceae. Fagaridine (118), the structure of which had to be revised, is a derivative of the unknown 5-methyl-benzo[c]phenan-thridine-8-olate (119) which is isoconjugate with the 2-methyl-chrysene anion (Scheme 43). Thus, Fagaridine is a member of class 1 of conjugated heterocyclic mesomeric betaines, which are isoconjugate with odd alternant hydrocarbon anions. 

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