Degenerate dications

Both the H and C NMR spectra of structure 23 indicate fivefold symmetry, even at low temperatures, for example, -15O C, with no significant line broadening. This leaves only two alternatives for the structure of the dication The nonclassical structure 23 or a set of rapidly equilibrating degenerate dications (234) (Scheme 5.14) with an activation energy, for the exchange, of <5 kcal mol". 

Olah et al. (1966) reported the synthesis and degenerate rearrangements of pentaphenylethyl cation [96] from the precursor [97]. However, Tomboulian and Stehower (1968) showed that this precursor actually has structure [98] and not [97]. This led Olah et al. (1969) to the conclusion that the ion that they had originally observed was the dication [99] and not [96]. Ion [99] had previously been synthesized independently by Volz and Volz de Lecea (1964) and Hart et al. (1963). 

The first singlet excitation of the hypothetical unsubstituted cyclobutadiene dication C4H4 (6a) corresponds to an electronic transition from the lowest molecular orbital into the degenerate nonbonding n MOs Similarly,... 

Table 1 Double-ionizations of the benzene molecule to singlet dication states, predicted in the standard enhanced ADC(2) approximation [5] and in the diagonal approximation described in the text. In this and subsequent tables, Term indicates the term symbol for a transition. Character the sum of the squares of the normalised transition eigenvector coefficients associated with the dominant basis configuration, and AE the small symmetry-breaking energy splitting in degenerate irreducible representations introduced by the diagonal approximation (see text)...

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