Triplet ground state steric effects

According to the orbital phase theory, the Gloss s diradical 62 is predicted to have a triplet ground state due to the same orbital-phase topology as the TM (2). In derivatives of 62, electronic and steric effects of various substituents as well as ring strains in the cyclic diradicals have successfully been applied to modulate the... 

Silicon substituted silylenes attract the chemists interest for a special reason. According to ab initio calculations 85 137 176], substituents acting as cr-donors should induce a relatively large red-shift of the UV maximum or, in other words, the n(Si) —> 3p(Si) transition energy should be relatively small. Therefore these species are potential candidates for the long-sought triplet ground state silylenes, especially if this electronic effect is supported by a steric one. Nevertheless, reports on the matrix isolation of silicon substituted silylenes are comparatively scarce. 

Photooxetane formation is quite inefficient, a fact which usually points to the presence of an intermediate which can partially revert to ground state reactants. Cleavage of the diradical must be responsible for some of the inefficiency in oxetane formation 129>. However, in the past few years convincing evidence has appeared that a CT complex precedes the diradical iso.isi). The two most telling pieces of evidence are the relative reactivities of different alkenes 130> and the absence of any measurable secondary deuterium isotope effect on quenching rate constants 131>. Relative quenching rates of sterically un crowded olefins are proportional both to the ionization potentials of the donor olefins 130> and to the reduction potentials of the acceptor ketones 131>, as would be expected for a CT process. Inasmuch as n,n triplets resemble electron-deficient alkoxy radicals, such substituent effects would also be expected on direct radical addition of triplet ketone to olefin. However, radical addition would yield an inverse isotope effect (in, say, 2-butene-2,3-d2) and would be faster to 1,1-dialkylethylenes than to 1,2-dialkylethylenes, in contrast to the actual observations. 

Still, in many cases photoderacemization of racemic mixtures has been successful. Irie et al. announced [54] the detection of the effect, but a detailed study of this system was published only as a patent [55] in CH2CI2 solutions of 1,l -binaphthyl they found values of [a]345 =-JX75-and 4- 0.7 for 1- and r-cpl with a 270-300 nm band of a 500 W Xe source at room temperature in ca. 5 min. Here two aromatic units are connected by a single bond. These systems are realizations of Kuhn s coupled oscillator model of optical activity [56]. The steric overlap provides chirality, ground state enantiomerization has AIv 92 kJ mol 1 [57], and the triplet state enantiomerizes fast, with 8 kJ mol 1. 

Furthermore, when the cis and trans isomers are not very different with respect to the steric effect, and the cis isomer is almost the same as, or slightly higher in energy than, the trans isomer in the ground state as well as in the triplet state, c and t are in equilibrium and the deactivation may take place from both c and t (Fig. 6). Thus, the two-way isomerization takes place as an adiabatic process as described later. 

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