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A spectroscopic study of the charge-transfer complex of anthracene and sym-trinitrobenzene Lower, Stephen Kent


Complexes of aromatic hydrocarbons (donors) with electron acceptors such as 1,3,5-trinitrobenzene (TNB) are stabilized principally by resonance between a dative and a "no-bond" wave function: ϕN = a ψ օ (DA) + b ψ ₁ (D⁺A⁻) By means of second-order perturbation theory it can be shown that, providing certain symmetry requirements are met, the energy WN corresponding to ϕN will be less than that of the free separated donor and acceptor. The resulting molecular complexes have a stability of 2-10 kcal, and usually possess a colour which is associated with a transition between the ground state WN and an excited state WE. These charge-transfer spectra have previously been studied only in solution, and have been useful sources of thermodynamic data. In order to learn more of the transition itself, it is necessary to study the spectrum of the crystal, where the molecules are held in fixed and (sometimes) known positions. The procedure by which the polarized crystal spectrum of the anthracene-TNB complex was obtained is briefly described. Comparison of the observed spectrum with the partially-known crystal structure has shown 1) The supposition that the transition dipole moment is perpendicular to the planes of the aromatic rings is supported; 2) Vibrational structure can exist in a charge-transfer band; this is the first reported observation of it; 3) The band is split into two oppositely-polarized components, differing in energy by about 200/cm. An attempt is made to explain this splitting in terms of Davydov's "weak-coupling" model of a crystal, in which degenerate molecular states are presumed to become crystal states whose degeneracy depends on the symmetry of the associated unit-cell wave functions. Some preliminary steps are described which should eventually lead to the detailed calculation of this effect in the present case.

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