Neutral and Charged Biradicals, Zwitterions, Funnels in S₁, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision

Abstract

A knowledge of the geometries at which excited molecules return to the electronic ground state (So) is essential for the understanding of the structures of photoproducts. Particularly good candidates are geometries corresponding to local minima on the S, (lowest excited singlet) and TI (lowest triplet) surfaces, as well as So-Sl conical intersections (funnels). Given sufficient effort, such geometries can nowadays be found numerically for small enough molecules. Still, it is interesting to ask whether more approximate, but also more general, statements can be made concerning the geometries at which the So and S, surfaces closely approach each other. Since many of these are biradicaloid geometries, it is logical to examine the properties of biradicals and related species at some length. After reviewing the two-electron two-orbital model for molecules at biradicaloid geometries, we formulate the conditions under which the So and S, surfaces touch. The results obtained for the simple model are supported by a b initio large-scale configuration interaction (CI) calculations for the twisting of ethylene in the polarizing field of a positive charge and for the twisting of charged double bonds and n-donor-to-n-acceptor single bonds, and by similar calculations for "push-pull" perturbed cyclobutadienes, some of which are predicted to have nearly degenerate So, S,, and TI states. The likely consequences of these results for the detailed description of the mechanisms of cisstruns isomerization, the formation of twisted internal charge-transfer (TICT) states, proton translocation, and possibly of the initial step in vision, as well as for the understanding of the regiospecificity of singlet photocycloaddition, are summarized.

The restricted active space followed by second-order perturbation theory method: Theory and application to the study of CuO2 and Cu2O2 systems

“…[40][41][42][43][44][45][46][47][48][49][50][51][52] Wave function theories restricted to a single determinant are poorly suited to the description of such species since singlet biradicals are intrinsically two determinantal. 53,54 Moreover, even when oxidation states more likely to be characterized as closed shell in nature are considered, e.g., ͓LCu͑III͔͒ 2 ͓O͑2−͔͒ 2 , computational studies have found that large contributions from dynamical correlation effects influence relative isomer energetics. [40][41][42][47][48][49]51,52 In this section, we apply the RASPT2 method to two problems previously studied in considerable detail using a wide range of theoretical models ͑see Fig.…”

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

“…1,13 Some chemists prefer the name "biradical", 14 but others call molecules with two positive charges "dications" and molecules with two negative charges "dianions". Therefore, calling molecules that, at least potentially, have two unpaired electrons "diradicals" has the virtue of a consistency.…”

A three-state effective Hamiltonian for symmetric cationic diarylmethanes

“…64 In the limit where |B is completely dominated by the subspace spanned by |lb 2 r and |l 2 r 2 , the one-electron density matrix would be a multiple of the identity in the subpace spanned by the left and right orbitals l and r. This is the defining characteristic of a biradical. 69 This suggests that it is inappropriate to think of the singlet pairing in state |B as indicative of a "long bond." This is consistent with the conclusions of an analysis of the valence-bond electronic structure of allyl ions.…”