Alternant hydrocarbons excited states

In several instances, Mannich-type cyclizations can be carried out expeditiously under photochemical conditions. The photochemistry of iminium ions is dominated by pathways in which the excited state im-inium ion serves as a one-electron acceptor. The photophysical and photochemical ramifications of such single-electron transfer (SET) processes as applied to excited state iminium ions have been expertly reviewed. In short, one-electron transfer to excited state iminium ions occurs rapidly from one of several electron donors electron rich alkenes, aromatic hydrocarbons, alcohols and ethers. Alternatively, an excited state donor, usually aromatic, can transfer an electron to a ground state iminium ion to afford the same reactive intermediates. Scheme 46 adumbrates the two pathways that have found most application in intramolecular cyclizations. Simple alkenes and aromatic hydrocarbons will typically suffer addition processes (pathway A). However, alkenic and aromatic systems with allylic or benzylic groups more electrofugal than hydrogen e.g. silicon, tin) commonly undergo elimination reactions (pathway B) to generate the reactive radical pair. 

Pople, J. A. The electronic spectra of aromatic molecules. II A theoretical treatment of excited states of alternant hydrocarbon molecules based on self-consistent molecular orbitals. Proc. Phys. Soc. (London) A 68, 81—89 (1955). 

Since its discovery by Chandross and to this day, peroxy-oxalate chemiluminescence has been controversial because of its enormous complexity in view of the many alternative steps involved in this process. The principal mechanistic feature of the peroxy-oxalate chemiluminescence pertains to the base-catalyzed (commonly imidazole) reaction of an activated aryl oxalate with hydrogen peroxide in the presence of a chemiluminescent activator, usually a highly fluorescent aromatic hydrocarbon with a low oxidation potential . A variety of putative high-energy peroxide intermediates have been proposed for the generation of the excited states . In the context of the present chapter, it is of import to mention that recent work provides experimental evidence for the intervention of the 1,2-dioxetanedione 18 (Scheme 11) as the high-energy species responsible for the chemiexcitation. Furthermore, clear-cut experimental data favor the CIEEL mechanism as a rationalization of the peroxy-oxalate chemiluminescence . 

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. 

Which of these two states has the lowest energy and what is the transition intensity to the two states. These simple properties of excited states of alternant hydrocarbons remain approximately valid in more accurate theories, at least for the lower excited states. 

Figure 1.6. Schematic representation of first-order configuration interaction for alternant hydrocarbons. Within the PPP approximation, conHgurations corresponding to electronic excitation from MO 4>i into and from MO., into are degenerate. The two highest occupied MOs (i =, k = 2) and the two lowest unoccupied MOs (f = r and k = 2 ) are shown. Depending on the magnitude of the interaction, the HOMO- LUMO transition

Figure 6.18. Excited-state barriers a) orbital correlation diagram for a thermally forbidden conversion of an alternant hydrocarbon b) the corresponding configuration and state correlation diagram for the case that Ihe HOMO— LUMO excitation does not represent the longesl-wavelength absorption.

The excited-state r-bond orders of an alternant hydrocarbon are given by... 

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). 

As will be seen from this account, Pople s simplification of Roothaan s self-consistent theory was aimed at clarifying the description of the ground state, particularly of alternant hydrocarbons.65 It was not concerned, in the first instance, with the characteristic problems which arise when one is discussing excited electronic states. However, Pople and Hush63 developed from it a theory of the ionization potentials and electron affinities of aromatic hydrocarbons, based on a consideration of the Hartree-Fock eigenvalues Ef. According to Koopmann s theorem, the ionization potential of a closed shell should approximate to the... 

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