Conjugated systems, alternant hydrocarbons

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... 

Of course, a close stmctural relationship between radical cations and parent molecules is not likely to hold generally, but it is a fair approximation for alternant hydrocarbons. Deviations have been noted some stilbene radical cations have higher-lying excited states without precedent in the PE spectrum of the parent for radical cations of cross-conjugated systems (e.g., 1) already the first excited state is without such precedent. These states have been called non-Koop-manns states. Alkenes also feature major differences between parent and radical cation electronic structures. 

Alternant hydrocarbons (AH) are defined as conjugated systems in which the atoms can be divided into two sets, starred and unstarred, in such a way that no atoms of tike parity are directly linked. Benzene is typical. 

Synthesis of the closely related acyclic (19) and macrocyclic (20) polyradicals has recently been reported (Figure 5.1).1231 The -conjugated carbanions (e.g., the calix[4]-arene-based tetraanion and the related calix[3]arene-based trianion) were synthesized and studied.1241 Oxidation of these tetra- and tri-anions gave the corresponding tetra- and tri-radicals, respectively. It has been shown in closely related systems that it is not the shape or overall geometric symmetry of the molecules, but rather it is the juxtaposition of the carbenic centers within the jt-cross-conjugated structure, that is most important in determining the spin multiplicity of the alternant hydrocarbon molecule.1251... 

We shall only give a scheme according to which the energy, the bond order and the free valency of an arbitrary conjugated system can be deduced in a simple approximate way (with open chain or only even membered rings, alternating hydrocarbon, p. 285). 

It is useful to divide the hydrocarbons with a conjugated system of bonds into the common, alternating hydrocarbons with exclusively rings with an even num-... 

The molecule of this substance contains thirteen double bonds. It is seen that eleven of these double bonds are related to one another in a special way—they alternate regularly with single bonds. A regular alteration of double bonds and single bonds in a hydrocarbon chain is called a conjugated system of double bonds. The existence of this... 

If one considers only hydrocarbons, and more especially the so-called alternant hydrocarbons, i.e. first of all the conjugated polyenes and the aromatic hydrocarbons of the benzene series, the greater part of their physical properties, ionization potentials, lower electronic transitions etc., can be interpreted qualitatively and often quantitatively in terms of the electronic structure of the n system alone. As the number of n electrons is small with respect to the total number of electrons of the molecule, a considerable simplification of the quantum-mechanical problem is obtained. However, it must be noted immediately that the assumptions of a complete a—n separation and of a rigid a frame are not sufficient to eliminate the a electrons completely from the theory because the n electrons of an unsaturated molecule are not attracted by bare nuclei, but are subject to an effective potential containing Coulomb and exchange contributions from the a electrons. 

---