Closed-shell hydrocarbons

The parameter set used in the calculations on closed-shell hydrocarbons appears to be appropriate also in treatments of open-shell systems (34, 58-60). Moreover, it has been found to give reasonable results for radicals in ground as well as in excited states (61). 

Figure II. Typical scheme of MO energy levels of a closed-shell hydrocarbon.

We introduce here for the conjugated hydrocarbon radicals the same classification which we used for the closed-shell hydrocarbons 55). We divide the hydrocarbons into two large groups alternant and nonalternant. Further classification concerns the even and odd systems, and the presence of cycles in the skeleton. The phenyl substituents are... 

As far as the present review is concerned, we shall only be concerned with the primary process consisting in the ejection, by a photon of energy hv, of an electron e" from a closed-shell hydrocarbon C H in its electronic ground state (cf configuration 15). Tlie product is a hydrocarbon radical ion in one of its doublet electronic states... 

This section consists of a compilation of references pertaining to He(Ia) and He(IIa) gas-phase PE spectra of saturated, closed-shell hydrocarbons C H (Table 1). (With few exceptions, such PE spectra consist of broad, strongly overlapping, and usually unresolved bands. For this reason, only a few PE spectra will be shown explicitly in Section V.)... 

As can be seen from Table 1, errors in the AHf of cations are slightly larger than equivalent errors in neutral molecules. As a result, any calculations involving cations must be considered with more caution than calculations involving uncharged jpecies. Results of another recent study indicate that results obtained from MINDO/3 calculations on cations are comparable with MNDO for 26 closed-shell hydrocarbon cations, the average MINDO/3 error was 11 kcal/mol compared to 12 kcal/mol using MNDO. Conversely, for 32 hydrocarbon radical cations, the MINDO/3 error was 16 kcal/mol versus 9 kcal/mol for MNDO. 

The Hiickel description of aromaticity was based in part on benzene, a cyclic fully conjugated hydrocarbon having (4n -l- 2) -electrons (ff = I) in the closed shell (ring). 

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons He pointed out that cycloheptatrienyl cation also called tropyhum ion contained a completely conjugated closed shell six tt electron sys tern analogous to that of benzene... 

Modern reviews of BDEs in closed-shell molecules measured by using mass spec-trometric methods have been provided by Ellison and co-workers. ° Additional evaluations have been provided by Ervin and DeTuri. A list of important fundamental hydrocarbon BDEs is given in Table 5.2. 

Masuhara H, Shioyama H, Saito T et al (1984) Fluorescence quenching mechanism of aromatic hydrocarbons by closed-shell heavy metal ions in aqueous and organic solutions. J Phys Chem 88 5868-5873... 

A quite simple picture is obtained in the ET case if we go from many-particle theory to orbitals via Koopmans theorem. This treatment is correct in the limit case when donor and acceptor exchange electrons using well-separated MO s on donor and acceptor. The simplest example is the case studied by McConnell [7] (fig.l). Two identical n systems are connected via a hydrocarbon chain, which acts as a bridge. ET is possible in an open shell system. We may assume that either the whole ET system is either a negative ion or a positive ion and that the corresponding neutral molecule has closed shells. In the former case the electron occupies 7t LUMO. In the latter case there is a hole in rt HOMO. 

A similar mechanism (Eq. 4) is operative in reactions of saturated hydrocarbons with closed-shell oxidizing electrophiles (E = Haln+, NOz+, etc.) where the H-trans-fer from a C-H bond is accompanied by an ET through the linearly H-coupled fragment (H-coupled electron transfer) [13]. The hydrocarbon part of the transition structures resembles the respective radical cation (that for the reaction of adamantane with Cl7+ is shown (6) in Scheme 2) [13]. 

The neo classification divides all benzenoids into normal (n), essentially disconnected (e) and non-Kekuleans (o), where the n and e systems cover all the Kekuleans. Cyvin and Gutman [26] have advocated for this classification by saying From the point of view of the enumeration of Kekule structures the classification. . . [neo]. . . seems to be a rather appropriate one [94,87] . However, the distinction between Kekulean (closed-shell, non-radicalic) and non-Kekulean (radicalic) benzenoid hydrocarbons was made long before the explicit definition of the neo classification. This practice started with the first (substantial) enumeration of benzenoids in the chemical context by Balaban and Harary [13]. 

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