Two-step transition

The transitions of type (c) are defined by two transition temperatures, one for decreasing (Ti/2j), and one for increasing temperature (Ti/2 ). Two-step transitions (Fig. Id), first reported in 1981 for an iron(III) complex of 2-bromo-salicylaldehyde-thiosemicarbazone [21], are relatively rare and have their origins in several sources. The most obvious is the presence of two lattice sites for the complex molecules. There are several examples of this [22]. In addition, binuclear systems can give rise to this effect, even when the environment of each metal atom is the same—in this instance the... 

In 1977 Driessen and van der Voort identified an extremely abrupt spin crossover with Tm of 213 K for [Fe(isoxazole)6](Cl04)2 [71]. Although various spectroscopic techniques have been employed to study this spin transition, the structural features of this compound at the time could not be determined, due to its extreme sensitivity to decomposition [71]. The same applies to the tetrafluoroborate salt that also displays a spin crossover, but in this instance a two-step transition was observed [71]. 

Keywords Spin crossover Dinuclear complexes Two-step transition Plateau Magnetic field Mossbauer spectroscopy... 

The important result of the LIESST experiments in (bt, S) is that the pho-toinduced species are not only [HS-LS] but also [HS-HS] pairs. The appearance of [HS-LS] species should be interpreted in terms of a synergy between intramolecular and intermolecular cooperative interactions which energetically stabilise the mixed pairs. However, the time dependent measurements (Fig. 14) reveal that [HS-HS] pairs are unstable and revert with time to both [HS-LS] and [LS-LS] configurations [17]. This observation is important in the comparative analysis of the two-step transition in (bt, S) (see Sect. 6). 

The two-step transition is obtained when the intra-sublattice interactions are ferromagnetic (JA, JB > 0) and the inter-sublattice interactions are antiferromagnetic (JAB < 0). The two-step character of the conversion is provided by Tg = TB for 17 1 > 7B and the plateau width is (Tg — Tg). 

These equations can be solved by an iterative procedure. However, the situation is complicated by the fact that the solutions in the critical region, where a two-step crossover can occur, may depend upon the initial values of the parameters at the beginning of the iteration. The results of modelling are displayed in Fig. 9.18. For typical values of interaction constants the conversion curve obeys a two-step transition, the second step being smoother than the first step. 

Fig. 37. Optimized geometries computed at MC-SCF/STO-30 (no superscript) and MC-SCF/4-31G (asterisk) (a) iso-xazole, (b) concerted transition state, (c) two-step transition state corresponding to the formation of the first bond.

Reedijk and co-workers have studied the iron(II) spin-state crossover in [Fe(teec)6]X2, where X is BF4, CIO4, or PFg, and teec is the monodentate l-(2-chloroethyl)tetrazole ligand. Depending upon the anion and the rate of precipitation, these complexes exhibit differing, partial or complete, spin-state crossover behavior with gradual or sharp, one- or two-step, transitions both with and without hysteresis. 

After the two-step transition described above, visible light is emitted by the transition from Eex, el)S> Eem, vib) to Eg el)(S> Ei vib). Since the excitation (I) is a conventional adiabatic process, its probability of occurrence is more than 10 times that of the first-step excitation. Furthermore, the probability of visible light emission by the infrared excitation is governed only by the phonon-assisted excitation of the first step because the second-step transition easily saturates. This is the origin... 

It was shown on theoretical grounds by Conway and Bockris that the two-step transition should have a lower overall energy barrier than the single-step process with a two-electron exchange in one transition state. Hence, it could be expected that the prevailing mode of reaction will involve two single-electron transitions. 

We recognize this is the formula for matrix multiplication. Hence the matrix of two-step transition probabilities is given by... 

Often a fictitious "virtual level" is introduced to describe the two-photon transition by a symbolic two-step transition E. - (Fig.8.40). 

Not only the stoichiometry but also compression or substitution of the element R affects the transition very much. A sharp two-step transition as is shown in figs. 47 and 49 is observed only in the Y-compound. There are reasons to believe that Ho- and Er-compounds will exhibit the transition if they are stoichiometric (see next subsection). Unfortunately, these compounds exist only in an oxygen-deficient form (to X = 0.09 in Ho- and x = 0.015 in Er-compounds) when synthe-... 

Fig. 52. Examples of the temperature dependence of the resistivity with no, one- or a two-step transition.

---