Non-alternating hydrocarbons

An important distinction for conjugated hydrocarbons is the classification into alternant and non-alternant hydrocarbons. Alternant hydrocarbons are those like ethene, hexatriene, benzene and naphthalene where we can divide the carbon atoms into two sets called starred and unstarred , such that no member of one set is directly bonded to another member (Figure 7.4). 

Non-alternant hydrocarbons possess uneven charge distributions in the ground state, even when no substituents are present, and again comparisons of qr and separately with experimental data may produce... 

R. Zahradnik and C. P rk nyi, Collection Czech. Chem. Commun. 30, 196 (1965) (thiophenes) and Collection Czech. Chem. Commun., in press (analogues of non-alternant hydrocarbons). 

Equally interesting is the situation in the second class of compounds studied (analogues of non-alternant hydrocarbons), which is best divided into two sub-groups analogues of the tropylium ion and analogues of azulene. The empirical correlation of experimental and theoretical excitation energies studied requires a further subdivision into compounds with one heteroatom (e.g. thiopyrylium ion) and two heteroatoms, either adjacent (e.g., 1,2-dithiolium ion) or non-adjacent (e.g., 1,3-dithiolium ion). Experimental and theoretical data are presented in Table VII. Table VIII summarizes data for the derivatives of dithiolia. Figure 15 shows the absorption curves of 1-benzo-... 

In Eq. (6), DEexp (cf. ref. 2) is given in kcal/mole and DEth in /3 units. Data for non-alternant hydrocarbons and quinones have not been included. The data for pyridine2 and quinoline2,33 obey the equation satisfactorily. On the other hand, the theory predicts too high values for the diazines (pyridazine, pyrimidine, and pyrazine) which have recently been studied experimentally.33... 

It should be noted, however, that planarity had not been excluded for corannulene until the X-ray analysis was performed. The molecule is a non-alternant hydrocarbon because it contains an odd-membered ring. Hence, the it-electron density distribution cannot be uniform in the ground state. One of such polar structures is the double Hiickel aromatic structure 66, c consisting of peripheral 14n and central 6ir systems. The contribution of resonance as shown in 6 c will be at its maximum when the whole molecule is planar like coronene (8). Notwithstanding, the planar corannulene will have huge angle strain. If we assume that all C-C bonds are 1.40 A in length and the... 

It is well known that non-alternant hydrocarbons are more difficult to treat with hmo theory, and one of the problems in these systems is the presence of finite n charge densities (hmo electron densities not equal to 1), which are usually overestimated by single determinant methods (Haddon Raghavachari 1983). As a result of the presence of 5MRs the fullerenes are also non-alternant hydrocarbons , but because of its high symmetry C60 is the only non-alternant hydrocarbon with zero charge density at each carbon atom (apart from the hypothetical fullerene C20). 

The free valence number therefore provides a good guide to reactivity in alternant hydrocarbons unfortunately this approach cannot easily be extended to compounds of other types. It can be used only when the contribution to 6E due to changes in the qt is the same for different aromatic systems and this is so only if coulomb integrals a, are the same for all the atoms present. Equation (33) shows that this is not so for molecules containing heteroatoms the ionization potential Wt is different for different atoms. Equation (33) shows that non-alternant hydrocarbons such as azulene must also be excluded here the... 

The rate of success falls whenever these conditions are not met. For example, condition (1) is not always satisfied in aromatic substitution reactions. The FOs of polycyclic aromatic hydrocarbons are not well separated from the other MOs,66 so subjacent orbital control may intervene. Particular care should be taken with non-alternant hydrocarbons because they tend to react under charge control. This is even truer in heteroaromatic systems (cf. Exercise 15, p. 119, and Exercise 16, p. 121), where Hiickel... 

B. DIPOLE MOMENT, NON-ALTERNATING HYDROCARBONS, COMPARISON OF V.B. AND M.O. METHODS, HYPERCONJUGATION... 

Methylenecyclopropene (triafulvene) is the simplest member of the cross-conjugated, non-alternant, hydrocarbon family known as the fulvenes. Strong polarization of the tc-system might be expected for such a structure and extensive efforts have been made... 

If the number of starred and unstarred C atoms is not the same, the difference corresponds to the occurrence of non-bonding tt m.o.s. The tt molecular orbitals for many alternate and non-alternate hydrocarbons have been tabulated (see, for example, ref. 13)... 

Acyclic hydrocarbon Linear hydrocarbon Alternant hydrocarbon Non-alternant hydrocarbon Topological matrix Topological MO MO energy level... 

Hydrocarbons which can be represented by bipartite graphs are called alternant hydrocarbons (AH). The concept of AH s was first introduced by Coulson and Longuet-Higgins 52>. Hydrocarbons which can be represented by non-bipartite graphs are called non-alternant hydrocarbons (NAH). We use both terms (AH = bipartite graph, NAH = non-bipartite graph) which are, in fact, the graph theoretical and chemical expressions of the same concept. 

In Section II the notion of alternant and non-alternant hydrocarbons was introduced, using a graph colouring approach. Although this idea looks interesting, it is rather difficult to see whether there is any physical concept behind the colouring process. In the present section we show the extraordinary importance of separating AHs from NAHs, and in subsequent sections a number of applications will be discussed. 

Fig. 7. The direction of dipole moments in some non-alternant hydrocarbons...

Conjugated hydrocarbons that do not contain an odd-membered ring are called alternant hydrocarbons (AHs). The distinction between alternant and non-alternant hydrocarbons (NAHs) provides a very important classification of conjugated hydrocarbons, especially with regard to excited states. In AHs, the unsaturated C atoms can be assigned to two sets, the starred ( ) and the unstarred (o) set, such that no atoms of the same set are bound to each other. This is not possible for NAHs (Figure 4.18). 

Fig. 6-2 The carbon-atom connectivity of the non-alternant hydrocarbon, fluoranthene.

We should begin by asking about one of the mistakes, in particular, which Huckel theory would make when, in a hydrocarbon, for example, the charges are not all uniform as we have seen, in 6.5, this situation would obtain in a non-alternant hydrocarbon. In such a case, the particular error in question is associated with the Coulomb term for atom r. The reader will recall that this was defined in ( 2.5) to be the integral... 

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