Spin conversion

The theoretical method employed is based on and largely similar to the theory of electron-transfer reactions in solution [123,124,125]. Thus the intramolecular spin conversion may be described as a transition between an initial manifold of states [f((r, qc)ZK,( c)[Pg.94]

Assuming that the spin conversion is a nonadiabatic process, the macroscopic rate constant may be expressed, following Levich [125], in terms of the thermally averaged transition probability, the averaging being extended over the initial vibronic levels, as ... 

Following Eq. (75), the rate constant for spin conversion may be expressed as a product of the electronic matrix element V and the nuclear Franck-Condon... 

Fig. 14. Dependence of the quantity (fito/gy l V )k where k is the rate constant for spin conversion on halkgT as calculated from the single-mode approximation Eq. (79) solid lines) and the full expression Eq. (77) for the value q = 0.3 of the solvent reorganization parameter dashed lines). Data are given for p = — 2.0, 0, -I- 2.0 and for the value of the coupling parameter S = 15. According to Ref. [117]...

Fig. 15. Dependence of the quantity hw/gf where k is the rate constant for spin conversion on the coupling parameter S. Data are given for various values of ha>/kgT. According to Ref [117]...

The calculations are based on the assumption of octahedral symmetry of the complex. Essentially two types of spin conversion may be distinguished (i) in the configurations and d , a single electron is transferred between the and 6g orbitals thus producing a first term in Eq. (72) which is different from zero (ii) in the configurations and d , two electrons are involved in the transition. Here, it turns out that the first term in Eq. (72) vanishes, whereas the second term is different from zero. 

Iron(III) Complexes. The measured spin conversion rates are /clh = 1-6 x... 

Cobalt(II) Complexes. Spin conversion rates greater than 10 s have been estimated for [Co(terpy)2](PFg)2 [29]. Although Ar 0.04 A is available for the [Co(terpy)2] complex [132] there is a large uncertainty concerning the magnitude of electronic coupling. Therefore, no attempt has been made to calculate the rate constant. 

In solution studies, the modification of the equilibrium nuclear configuration appears primarily as a change of the partial molar volume A V° of the two spin states. The presently available values of AV° for spin conversions in solution are collected in Table 16. There is no apparent difference between the values for iron (II) and iron (III) spin transition compounds, the variation being... 

The high-spin/low-spin interconverison in a Ni11 complex of the cyclam derivative (639) bearing a luminescent naphthalene substituent has been used as a fluorescent molecular thermometer.161 The Ni11 tends to quench fluorescence of the proximate naphthalene subunit, but the two spin states exert a different influence on the emission properties. Emission is temperature dependent, since the high spin —> low spin conversion is endothermic, i.e., a temperature increase favors formation of the low-spin form. 

In the theory of deuteron spin-lattice relaxation we apply a simple model to describe the relaxation of the magnetizations T and (A+E), for symmetry species of four coupled deuterons in CD4 free rotators. Expressions are derived for their direct relaxation rate via the intra and external quadrupole couplings. The jump motion between the equilibrium positions averages the relaxation rate within the same symmetry species. Spin conversion transitions couple the relaxation of T and (A+E). This mixing is included in the calculations by reapplying the simple model under somewhat different conditions. The results compare favorably with the experimental data for the zeolites HY, NaA and NaMordenite [6] and NaY presented here. Incoherent tunnelling is believed to dominate the relaxation process at lowest temperatures as soon as CD4 molecules become localized. 

Photo-induced LS to HS spin conversion has been observed for the polycrystalline samples of the bpe derivative. Irradiation at 10 K with orange light (600 nm) induces a quantitative transformation of the LS ground state... 

The iron(II) complexes of the hexadentate 2-pyridylmethyl derivatives of bipy (94) and (95) are spin cross-over compounds, whose light-induced high-spin to low-spin conversion has been monitored in solution by laser flash photolysis. Single exponential kinetics (A h l = 6.7 x 10 s , at 273 K) were observed for [Fe(94)] ", but for [Fe(95)] " kinetics were biphasic, with the spin-conversion step (ku i = 2.5 x 10 s ) followed by a slower step (k = 3.7 x 10 s ) involving rearrangement of the pyridylmethyl pendant arms. ... 

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