Spin-state mixing process effect

The origin of postulate (iii) lies in the electron-nuclear hyperfine interaction. If the energy separation between the T and S states of the radical pair is of the same order of magnitude as then the hyperfine interaction can represent a driving force for T-S mixing and this depends on the nuclear spin state. Only a relatively small preference for one spin-state compared with the other is necessary in the T-S mixing process in order to overcome the Boltzmann polarization (1 in 10 ). The effect is to make n.m.r. spectroscopy a much more sensitive technique in systems displaying CIDNP than in systems where only Boltzmann distributions of nuclear spin states obtain. More detailed consideration of postulate (iii) is deferred until Section II,D. 

Critical to the observation of magnetic field effects in solution is the ability of the RP to interconvert between triplet (nonreactive) and singlet (reactive) spin states. Having established the various interactions present in the RP, we can now consider how such a mixing process might take place. 

However, the Ne(2s2) state has some triplet character due to mixing with Ne(2s4). It is of considerable interest to investigate the restrictive effect of an overall spin-change in any electronic process. It has been noted that, of the inert gases, only Xe induces collisional deactivation of 0(2 lD) to 0(2 3P), which may be due either to chemical interaction, or to the spin-orbit coupling in the heavy atom. Most gases will induce the spin forbidden transition... 

From the best fits, the kinetic rate constants, and kb, and the ratio of the radiative rate constants, A/B, of the resonantly coupled spin levels could be obtained. The lifetimes of the triplet sublevels in the various chelates [56] are collected in Table 2. Evidently, the sublevel lifetimes are on the millisecond time scale, and about three orders of magnitude shorter than the phosphorescence lifetimes of the free ligand molecules (bpy Tp = 0.8 s phen Tp=1.4 s [60, 61]). For [Rh(bpy)3] (0104)3 the radiative rate constants are in the ratio T Ty. Tz= 10 1 2, showing that the sublevel is the most active in the radiative as well as non-radiative processes [62]. The shortening of the triplet state sublevel lifetimes for the Rh(III)-chelates as compared to the triplet sublevel lifetimes of the free ligand molecules is reminiscent of the heavy atom effect as, for example, observed for halonaphthalenes [33-35]. In the latter, the mixing of states with UTT and Vtt excitations is enhanced by SOC within the heavy atom. In... 

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