Triplets theory

A stereochemical origin of the Code has been put forward that is based on the physicochemical correlation of amino acids and their codons. This idea is also called the escaped triplet theory and is supported by experimental evidence. 

Yarns M (1998) Origins of the genetic code the escaped triplet theory. Annu Rev Biochem 74 179-198... 

The perturbations in this case are between a singlet and a triplet state. The perturbation Hamiltonian, H, of the second-order perturbation theory is spin-orbital coupling, which has the effect of mixing singlet and triplet states. 

Wong S K, Hutchinson D A and Wan J K S 1973 Chemically induced dynamic electron polarization. II. A general theory for radicals produced by photochemical reactions of excited triplet carbonyl compounded. Chem. Phys. 58 985-9... 

A which is not observed in individual solutions of the two enones at the same concentrations and may thus be indicative of a complex formation. However, the ratio of isomeric cyclobutane products resulting from such photocycloadditions is generally seen to be a quite sensitive function of steric effects and of the properties of the reaction solvent, of the excited state(s) involved (in some cases two different excited triplet states of the same enone have been found to lead to different adducts) and of the substituents of the excited enone and substrate. No fully satisfactory theory has yet been put forth to draw together all the observations reported thus far. 

G1 theory does badly with ionic molecules, with triplet-state molecules such as O2 and S2 and with hypervalent molecules. Gaussian-2 (G2) theory eliminates some of these difficulties by making the following three changes ... 

Hall, G. G., Proc. Roy. Soc. [London) A213, 113, The molecular orbital theory of chemical valency. XI. Bond energies, resonance energies and triplet state energies. ... 

P-Q-R triplets 225 P-R doublet 225, 249, 250 P-R exchange 135, 256 partial dipole moment operator 231 perturbation theory 5-6, 64-9 accuracy 78-9... 

Jablonski (48-49) developed a theory in 1935 in which he presented the now standard Jablonski diagram" of singlet and triplet state energy levels that is used to explain excitation and emission processes in luminescence. He also related the fluorescence lifetimes of the perpendicular and parallel polarization components of emission to the fluorophore emission lifetime and rate of rotation. In the same year, Szymanowski (50) measured apparent lifetimes for the perpendicular and parallel polarization components of fluorescein in viscous solutions with a phase fluorometer. It was shown later by Spencer and Weber (51) that phase shift methods do not give correct values for polarized lifetimes because the theory does not include the dependence on modulation frequency. 

In 1982 the present author discovered cyclic orbital interactions in acyclic conjugation, and showed that the orbital phase continuity controls acyclic systems as well as the cyclic systems [23]. The orbital phase theory has thus far expanded and is still expanding the scope of its applications. Among some typical examples are included relative stabilities of cross vs linear polyenes and conjugated diradicals in the singlet and triplet states, spin preference of diradicals, regioselectivities, conformational stabilities, acute coordination angle in metal complexes, and so on. 

The orbital phase theory has been developed for the triplet states [19]. The orbital phase continuity conditions (Scheme 4) were shown to be applicable. We describe here, for example, the triplet states of the TMM and BD diradicals, with three a spin electrons and one 3 spin electron. The a and 3 spins are considered separately (Scheme 8). 

Stability of diradicals is important for photochemical reactions. Spin multiplicity of the ground states is critical for the molecular magnetic materials. The relative stability of singlet (triplet) isomers and the spin multiplicity of the ground states (spin preference) [48] has been described to introduce the orbital phase theory in Sects. 2.1.5 and 2.1.6. Applications for the design of diradicals are reviewed by Ma and Inagaki elsewhere in this volume. Here, we briefly summarize the applications. 

Orbital phase discontinuity in singlet state. In contrast to the triplet state, orbital phase continuity conditions cannot be satisfied simultaneously (denoted by the dashed line in Fig. 6c) in the singlet. Thus, the singlet 1,3-diradical suffers from the orbital phase discontinuity. According to the orbital phase properties, the triplet states of TMM (1) and TM (2) were predicted to be more stable than their singlet states by the orbital phase theory [29, 31]. 

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