Triplet path

The symmetrical insertion of 0(3P) into H2 results in a barrier of approximately 335 kJ mol-1 and yields a linear metastable intermediate. For distances of approach of 0(SF) to H2 less than 3.0 a.u., the triplet insertion path is unstable to an unsym-metrical motion of the nuclei which leads to OH(aII) and H(2S). The energy minimum on the symmetric triplet path is therefore, a saddle point and does not represent a stable nuclear configuration for triplet water. 

We believe that essentially the same processes are important in the photolyses of 3-ter(-butyldiazirine and pentamethylenediazirine. In the former case the 11% of 1,1-dimethylcyclopropane found by extrapolation to zero pressure would be formed by the triplet path. There are good reasons to believe that this triplet molecule would be much longer lived than the corresponding vibrationally excited species (see Placzek and Rabinovitch and references cited by them). 

In a final step, the electron jumps in a very fast process directly to the emitter molecule and it results an excited emitter. This process may occur as singlet or triplet path (S-path, T-path) depending on the initial spin orientation of the electron-hole pair. The corresponding time constants are of the order of one picosecond (see next section). The population of Sn and Tn states, as shown in Fig. 3, is only depicted as an example. Subsequently, the system will exhibit the usual behavior of an optically excited emitter mole-... 

The final steps of the mechanisms described above can also be discussed in a slightly different way, to illustrate the occurrence of specific singlet and triplet paths. The situation of a lacking electron in the ground state of the doped emitter molecule (dopant D) and of additional electron charge density on nearest neighbor matrix molecules M can be characterized by dopant-... 

The free energies of reaction and activation have been calculated by the semiempirical molecular orbital method for the oxidation of ammonia and mono-, di- and trimethylamine by singlet and triplet oxygen atoms as models for oxidation by cytochrome P-450 [56]. For the non-radical oxidation (closed-shell path), the results indicate a two-step addition-rearrangement mechanism leading to both N-hydroxy and N-methoxy products via N-oxide intermediates. In the triplet path both a-C- and N-oxidation are competitive. N-oxidation via an addition mechanism seems to be favored over the H-abstraction mechanism, but no stable N-oxide radical intermediate is found on the triplet surface. 

According to the quantum chemical calculation for this reaction, there are singlet and triplet paths involving a complex CH3OOOOH, which are related to the formation of singlet and triplet O2 in reaction (5.79) (Zhou et al. 2006). 

Both reactions proceed via triplet excited species and, to some extent, are controlled by whether the ti-tt (path A) or n-rr states are involved. The di-rr-methane rearrangement pathway is restricted to 4-aryl- or 4-vinylcyclohexenones. At the most basic level of... 

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

The delocalization-polarization mechanism in the singlet state is more complicated than that in triplet. Similar to the triplet state, there also exists a cyclic - G- T - E- T - configuration or -7t-p-7t -q- (-o-p-o -q-) orbital interaction in the singlet (Fig. 6). In the singlet state, however, the radical orbital q is an electron-accepting orbital (A) for the a-spin electron (rather than the donating orbital in triplet). Thus, there is an additional path of a-spin electron delocalization, - G- T - Tj- T - or... 

Fig. 14 The heteroatom-containing 1,3-diradicals, where the triplet stabilization is depressed by the strengthening of p-Jt -q (denoted by bold lines) and weakening of p-JC-q wavy lines) interaction path...

A carbonyl chromophore in a macromolecule can participate in a variety of photochemical processes that can have as end result the degradation of the polymer via processes like the Norrish Type I or Type II reaction, the triggering of a chain reaction leading to peroxidation, the transfer of energy to another chromophore or, it can also behave as an energy sink if a suitable, non-degradative path, is available to the triplet state. 

Fig. 3 Transient spectra obtained upon the application of a 200-fs laser pulse to a solution of stilbene (S) and chloranil (Q) in dioxane. (a) The fast decay ( 20 ps) of the contact ion-radical pair S+ , Q generated by direct charge-transfer excitation (CT path), (b) The slow growth ( 1.6 ns) of the ion pair S+ Q due to the diffusional quenching of triplet chloranil (A path) as described in Scheme 13. Reproduced with permission from Ref. 55.

On the other hand, reactions in which the return to So occurs from a "non-spectroscopic minimum (Fig. 3, path g) are probably the most common kind. The return is virtually always non-radiativef). This may be the very first minimum in Si (Ti) reached, e.g., the twisted triplet ethylene, or the molecule may have already landed in and again escaped out of a series of minima (Fig. 3, sequence c, e). For instance, triplet excitation of trans-stilbene 70,81-83) gives a relatively long-lived trans-stilbene triplet corresponding to a first spectroscopic minimum in Ti. This is followed by escape to the non-spectroscopic , short-lived phantom twisted stilbene triplet, corresponding to a second and last minimum in Ti. This escape is responsible for the still relatively short lifetime of the planar nn triplet compared to nn triplet of, say, naphthalene. A jump to nearby So and return to So minima at cis- and trans-stilbene geometries complete the photochemical process ). 

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