Preferred triplet structure

In fact, barrelene rearranges by the di-7t-methane pathway to semibullvalene only when acetone sensitized direct irradiation produces cyclooctatetraene as the major photoproduct. A number of similar examples exist, each demonstrating the general phenomenon of preferred triplet multiplicity for di-7r-methane rearrangements in rigidly constrained systems, i.e. structures which prohibit free rotation about the Ti-bonds. ... 

The preceding orbital phase predictions of some topological units (like 1, 4-6) can be easily extended to more complex cychc diradicals [29], as shown in Fig. 12. On the basis of TMM sub-structure (1), diradicals 8-11 are predicted to be phase continuous in their triplet states. Such a triplet preference in their ground states is in agreement with calculation results and available experiments, as listed in Table... 

The classification into Kekule and non-Kekule diradicals is mainly based on the difference in their resonance structures. From the proceeding discussions, however, such a classification does not closely relate to the relative stabilities and spin preference of TT-conjugated diradicals. For example, some non-Kekule diradicals, such as 1 and 8, prefer a triplet ground state, but some others (like 7) have a singlet ground... 

Figure 2 shows the initial equilibrated intercalated state. The DNA sequence is d(GCGCACGTGCGC)2- The intercalated structure s geometry is that of the B-DNA except for the 5th to 7th base pairs (A5 to G7), which stay close to the crystal structure used for the starting conformation, with a root mean square deviation of 3.2 A for all the heavy atoms of those three basepairs. In this sequence, daunomycin is in contact with the strongest binding triplet sequence (A/T)CG [18,19]. Moreover, due to the chosen sequence of the DNA, the intercalation site is flanked by the same sequence of base-pairs in either direction, eliminating any related orientational preference of... 

The efficiency of the initiation effect of triplet oxygen depends on the hydrocarbon structure, i.e., on the strength of the attacked C —H bond. For instance, the ratio of the rate constants of the reaction of oxygen with formaldehyde and methane is 1.3 x 109 at 100 °C [8]. This indicates that intermediates of oxidation may be more sensitive towards oxidation than the original substrate which may contribute to the appearance of heterogeneous regions where the oxidation takes place preferably. 

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