Cross mechanism

Selection of an entrainer such that although the desired products lie in different regions, some type of boundai y-crossing mechanism is employed. 

They concluded that the reactivity of carbenes toward CO2 is determined by their philicity (more nucleophilic carbenes are more reactive) and that carbene spin state interestingly has little effect. Kovacs and Jackson have suggested that this reactivity pattern may be explained by a nonequilibrium surface crossing mechanism. ... 

Figure 16-9. Schematic representation of the photochemical events after light absorption once isoallox-azine has been promoted vertically to Sj. (a) Population of Tn in the vicinity of Franck-Condon region by intersystem crossing mechanism, (b) Internal conversion toward Tt mediated by the (TN/Tj )CI leading ultimately to the 3(Tnr )min structure. Energies are in kcalmol-1. Qj and Q2 are reaction coordinates. (Reproduced from Ref. [83] with permission from the American Chemical Society)...

As a second example of intersystem crossing mechanism in biochromophores we include here the case of the DNA pyrimidine nucleobases, starting by the uracil molecule [91]. In previous sections we presented a model for the rapid internal conversion of the singlet excited rationalizes the ultrafast decay component observed in these systems, both in the gas phase and in solution. Despite the short lifetimes associated to this state, which is the main contributor to the photophysics of the system, formation of photodimers PyroPyr has been observed for the monomers in solution, as well as in solid state, for oligonucleotides, and DNA [92], Since the sixties, the determination of the mechanism of the photoinduced formation of cyclobutane dimers has been the subject of numerous studies [92, 93-97], One of the most classic models that has been proposed for the photodimerization of Pyr nucleobases in solution invokes photoexcitation of a molecule to a singlet state followed by population of a triplet state by an intersystem crossing mechanism... 

The quenching of 0(XZ)) by N2, reaction [4], can be a very efficient source of vibrationally excited nitrogen [260] even though the curve-crossing mechanism may not permit more than one or two quanta of N2 vibrational energy per Of1/)) quench [258, 259]. Hunten and McElroy [56] have pointed out that whereas 0(XZ>) is readily quenched by energy transfer to N2, O S) is not, even though it is nearly resonant with v = 16. 

Table 2 summarizes the a factors for various substituted DBK s. Furthermore, this table shows, as is expected, that the application of an external magnetic field reduces the amount of isotope enrichment. Figure 8 and Fig. 9 illustrate the effect of the application ofan external magnetic field on the extent of the cage effect and on the efficiency of isotope enrichment. The results can be understood in terms of a change in the relative contributions in the intersystem crossing mechanism of the hyperfine and Zeeman mechanisms14). 

Morita, A., Kato, S., Theoretical Study on the Intersystem Crossing Mechanism of a Diradical in Norrish Type II Reactions in Solution, J. Phys. Chem. 1993, 97, 3298 3313. 

Unfortunately, the computational studies differ in quantitative detail regarding the importance of the mechanisms that involve either Glu 165 or His 95 as the acid-base catalysts to catalyze interconversion of the tautomeric enediolate intermediates. Friesner and coworkers concluded that the transition state for proton abstraction from DHAP is the highest point on the energy diagram after formation of the enediolate anion intermediate, the calculations predict that the barrier for the criss-cross mechanism catalyzed by Glu 165 is - 3 kcal moU lower than that for classical mechanism involving catalysis of tautomerization of the enediolate intermediates by His 95, so the criss-cross mechanism is predicted to be the favored mechanism. In contrast, Cui and Karplus concluded that transition state energies for tautomerization of the enediolate anion intermediates via an enediol intermediate are isoenergetic for both the classical and criss-cross mechanisms. 

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