Triplet temperature dependence

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

This key paper was followed by a flurry of activity in this area, spanning several years." " "" A variety of workers reported attempts to deconvolute the temperature dependence of carbene singlet/triplet equilibria and relative reactivities from the influence of solid matrices. Invariably, in low-temperature solids, H-abstraction reactions were found to predominate over other processes. Somewhat similar results were obtained in studies of the temperature and phase dependency of the selectivity of C-H insertion reactions in alkanes. While, for example, primary versus tertiary C-H abstraction became increasingly selective as the temperature was lowered in solution, the reactions became dramatically less selective in the solid phase as temperatures were lowered further. Similar work of Tomioka and co-workers explored variations of OH (singlet reaction) versus C-H (triplet reaction) carbene insertions with alcohols as a function of temperature and medium. Numerous attempts were made in these reports to explain the results based on increases in triplet carbene population... 

Fig. 6.18. Temperature dependence of 02 emission (f) and triplet oxygen release (2) during linear heating of VgOn sample in vacuum.

Fig. 6.19. Temperature dependence of triplet oxygen absorption (f) and singlet oxygen emission (2) during the heating of VgOu sample in 0.1 Torr O2.

The relative reactivity of cyclopentadiene and ds-dichloroethylene toward triplet cyclopentadiene was found to be greater than 20 1 while that for cyclopentadiene and trans-dichloroethylene is less than 5 1. Thus the trans isomer is about four times more reactive toward the triplet cyclopentadiene than the cis isomer. An interesting temperature dependence of the product distribution of this reaction has been reported (Table 10.8). The data in Table 10.8 indicate that the relative amount of 1,4 addition [products (39) and (40)] is much more sensitive to temperature than 1,2 addition [products (35)—(38)], especially for the trans-olefin. The data also indicate that some rotation about the CHC1-CHC1 bond occurs in intermediate radicals derived from both cis- and trans-dichloroethylene. However, rotational equilibrium is not established at ring closure since the ratios of ds-dichlorocyclobutanes... 

A new triplet diradical is detected by ESR from the photolysis of 2-nitrobiphenyl106 (equation 85). The spectrum shows a temperature dependence which implies that the observed triplet state is a ground state. 

The structural stability of mixed-metal hemoglobin hybrids also has allowed us to study low-temperature electron transfer in this system. We first reported the temperature dependence of triplet-state quenching within the [ (ZnP), Fe (H20)P] hybrids, which we attributed to the ZnP Fe P ET reaction [7d]. The rate constant dropped smoothly as the temperature was lowered from room temperature to 200 K. Below this temperature the rate constant remained roughly constant with a tunnelling rate constant of kt 9 s (Fig. 7). 

Fig. 7. Temperature dependence of the triplet-state quenching rate constant (k,) for the [a(Zn), PiFe +HjO)] hybrid. Adapted from Ret [7d]...

Fig. 10 Theoretical curves [equation (12)] representing the temperature dependence of the signal intensities due to a triplet species that has a singlet manifold in equilibrium. The A -values represent the energy gap of the two states.

In the case of the pseudo-me/o-isomer (m-[38]), signals due to a quintet state were not observed. Instead, a set of signals due to the thermally populated triplet Tj appeared at 25 K. The temperature dependence data were analysed in terms of an equation similar to (13) [3 exp —AEjRT) in place of 5 exp (-iAEIRT) in the numerator] to give AE j = 98cm" ( = 280 cal mol" ). The behaviour of p-[38] was more like o-[38], although the former was more reactive and did not allow a satisfactory temperature-dependence study. 

In all these cases the triplet correlation is expressible in terms of the pair correlations and the temperature dependence of the correlation is predictable, knowing the ligand-ligand interactions. This is, in general, not the case for systems with indirect correlations, discussed in the following sections. 

The deuterium kinetic isotope effect for intramolecular CH insertion of the nitrene (87), generated by photolysis of the corresponding azide, is 14.7 0.3 at 20 °C and is consistent with the H-abstraction-recombination mechanism from the triplet state. The temperature dependence of the kinetic isotope effect suggests that quanmm mechanical tunnelling is important in this process. 

A molecular structure, similar to that of hexasilylated benzene derivative 229, was obtained from tetracyclic hexasilylbenzene 230 by Kira, Sakurai and coworkers, where the six silicon centres are incorporated in three five-membered ring systems (Scheme 80) °. In dimetalated compound 231, two lithium centres, coordinated by a quinuclidine ligand each, are capping the phenyl ring plane from both sides in the solid state. Moreover, it could be found that compound 231 has a thermally accessible triplet state, investigated by temperature-dependent ESR spectroscopy °°. 

This law means that the intensity becomes smaller as the temperature is raised. Deviations from linearity could indicate a temperature-dependent equilibrium between a triplet and a singlet. Conversely, a linear Curie plot is taken as evidence for a triplet ground state far below the singlet, but such observations require that the carbene be stable to a certain extent at elevated temperature. 

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