Cations, alternant hydrocarbons

Demethylvasconine (85) (9-methoxy-5-methyl-phenanthridin-8-olate) presented in Scheme 31 was found in Crinum kirkii (95P1291) (Amaryllidaceae). Although published as cation, no information about the anion of this alkaloid is given. Its relationship to other alkaloids of this class, however, makes a betainic structure more than likely and this is confirmed by a comparison of the NMR data of 85 with the cationic and betainic alkaloids presented in Table III. This betaine is isoconjugate with the 2-methylphenanthrene anion and thus defined the alkaloid as a member of class 1 (odd alternant hydrocarbon anions). Whereas substitution of the isoconjugate phenanthridinium moiety at the 1-position with an anionic fragment results in zwitterions (cf. Section III.D), the phenanthridinium-2-olate is a mesomeric betaine. 

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. 

As mentioned for the relationship between the PE spectrum of a parent molecule and the electronic spectrum of its radical cation, any close correspondence between the electronic spectra of anions and cations or their hyperfine coupling patterns holds only for alternant hydrocarbons. The anions and cations of nonalternant hydrocarbons (e.g., azulene) have significantly different hyperfine patterns. Azulene radical anion has major hyperfine splitting constants (hfcs) on carbons 6, and 4,8 (flH = 0-91 mT, H-6 ah = 0-65 mT, H-4,8 ah = 0-38 mT, H-2) in contrast, the radical cation has major hfcs on carbons 1 and 3 (ah = 1.065 mT, H-1,3 Ah = 0.152 mT, H-2 ah = 0.415 mT, H-5,7 ah = 0.112 mT, H-6). °°... 

Parent Skeleton An Alternant Hydrocarbon According to HMO calculations, the as yet unknown compound 116 seems capable of existence as an anion, radical, or cation since it has a... 

However, QMe (or B) may vary with type of radical, especially charge type, and also with the nature of the a-atom. Thus, methyl hyperfine splittings in alternant hydrocarbon anion- and cation-radicals, for which Hiickel MO theory predicts identical spin populations at the methylated positions, typically differ by a factor of at least two.15... 

To illustrate the effect of solvation on temporary anions we will consider the naphthalene molecule. This molecule is particularly interesting because it is an alternant hydrocarbon (14), and for such molecules, the pairing theorem (15) predicts that the anion and cation spectra should be identical. This theorem is valid for both Huckel and PPP model Hamiltonians, but is not valid for ab initio or CNDO calculations. It has been found (1 ) to be true to a good approximation ( /0.1 eV) in organic glasses (16). The ETS spectra allows an examination of the validity of this... 

An alternant hydrocarbon is a bichromatic system in which the cores are classified into two parities, starred and unstarred, and no cores of like parity are directly linked. Perturbation theory suggests that a heteroatom gathers the electron density from the carbon atoms of the opposite parity, as shown by the electron densities of alternant aromatic nitrogen cation... 

Figure 2.24. Orbital energy levels of alternant hydrocarbon ions a) anions and cations of odd-alternant systems, and b) radical anions and radical cations of even-alternant systems in the HMO approximation and c) in the PPP approximation.

If the transition considered is the HOMO LUMO transition of an alternant hydrocarbon, then first-order theory predicts that inductive perturbation will have no effect at all, because for = fo as a consequence of the pairing theorem. Small red shifts are in fact observed that can be attributed to hyper conjugation with the pseudo-7t MO of the saturated alkyl chain.290 On the other hand, alkyl substitution gives rise to large shifts in the absorption spectra of radical ions of alternant hydrocarbons whose charge distribution is equal to the square of the coefficients of the MO from which an electron was removed (radical cations) or to which an electron was added (radical anions), and these shifts are accurately predicted by HMO theory.291... 

Examples of monocation dimer formation among aromatic hydrocarbons have been confined mostly to alternant hydrocarbons, and the dimer can be regarded as an association of two closed shell molecules which have lost an electron. Recently Paskovich and Reddoch (1972) made a new class of monocation dimers, in which an electron is missing from two associated open shell molecules. Oxidation of phenalene by oxygen led to the phenalenyl radical and, it is thought, to the phenalenyl cation, association of which gave the monocation dimer (93). 

The fourth approach is to use derivatives of odd-alternant hydrocarbons, such as the phenalenium radical, in which the Hiickel resonance energy is the same for the anion, the radical, and the cation, because the HOMO is a non-bonding MO [43,44]. However, the radicals seem too reactive. 

We know (see p. 60) that an alternant hydrocarbon (AH) has a self-consistent field so that = 0 at all atoms therefore if we remove an electron from the NBMO to get a benzyl cation, the pasitive charge will be distributed sole over those atoms whose orbital coefficients are not zero for the NBMO. The same will be true if we add an electron to the radical and make the benzyl anion. The NBMO coefficients are clearly of signal importance since their values determine the calculated distribution of the odd electron in the radical and the charges in the cation and anion. For the benzyl radical the NBMO may be rendered schematically as follows ... 

The electronic absorption spectra of the aromatic alternant hydrocarbon anions have been interpreted under the assumption that the ions exist as free solvated particles in the solutions and accordingly no allowance has been made for the possible presence of cation effects. During the last few years evidence has been presented that such effects may induce marked charges in the spectra (37, 97-104). This has also been established by magnetic resonance investigations (81). 

Charged alternant hydrocarbons occur as pairs of negatively and positively charged ions, and the pairing theorem relates the states of the former to the states of the latter. While at the standard PPP level of theory the absorption spectra of the members of a pair are predicted to be identical, and indeed experimentally almost exactly are, their MCD spectra are expected to be mirror images of each other and indeed nearly are (e.g., diphenylmethyl anion and cation). ... 

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