Laser perturbation temperature-jump

The spin state lifetimes in solution of the complexes II and III have been measured directly with the laser Raman temperature-jump technique189). Changes in the absorbance at 560 nm (CT band maximum) following the T-jump perturbation indicate that the relaxation back to equilibrium occurs by a first-order process. The spin-state lifetimes are r(LS) = 2.5 10 6 s and r(HS) =1.3 10 7 s. The enthalpy change is AH < 5 kcal mol-1, in good agreement with that derived from x(T) data in Ref. 188. The dynamics of intersystem crossing processes in solution for these hexadentate complexes and other six-coordinate ds, d6, and d7 spin-equilibrium complexes of iron(III), iron(II), and cobalt(II) has been discussed by Sutin and Wilson et al.u°). 

In summary, the necessary condition for temperature jump experiments is that the equilibrium for the chemical system of interest changes with a change in temperature. The advantages of temperature jump experiments are that the perturbation is achieved by a change in a property of the solvent, a fast time resolution can be achieved, as short as picoseconds when using lasers, and a time domain over more than 6 orders of magnitude can be probed with the same technique. The disadvantage of the technique... 

Monitoring of events following perturbations can be achieved in much shorter times by photolysis. A variety of monitoring techniques have been linked to both methods (Table 3.7). It is valuable to obtain kinetic data by more than one method, when possible. The measurement of spin-change rates have, for example, been carried out by a variety of rapid-reaction techniques, including temperature-jump, ultrasonics and laser photolysis with consistent results (Sec. 7.3). 

An alternative method of overcoming the time delay of mixing is to use a relaxation method. An equilibrium mixture of reagents is preincubated and the equilibrium is perturbed by an external influence. The rate of return, or relaxation, to equilibrium is then measured. The most common procedure for this is temperature jump (Figure 4.6).13 A solution is incubated in an absorbance or fluorescence cell and its temperature is raised through 5 to 10°C in less than a microsecond by the discharge of a capacitor (or, in more recent developments, in 10 to 100 ns by the discharge of an infrared laser). If the equilibrium involves an... 

Consideration of the thermodynamics of a representative reaction coordinate reveals a number of interesting aspects of the equilibrium (Fig. 5). Because the complex is in spin equilibrium, AG° x 0. Only complexes which fulfill this condition can be studied by the Raman laser temperature-jump or ultrasonic relaxation methods, because these methods require perturbation of an equilibrium with appreciable concentrations of both species present. The photoperturbation technique does not suffer from this limitation and can be used to examine complexes with a larger driving force, i.e., AG° 0. In such cases, however, AG° is difficult to measure and will generally be unknown. 

A third simple and generally applicable method is temperature jump, to perturb a pre-existing chemical or structural equilibrium. However, limitations on thermal conductivity mean that thermal equilibration in response to an external heat pulse is slow [22, 24], This points to the use of IR lasers that excite the crystal directly (unpublished results of Volz, K., Spirgatis, A. and Moffat, K.). 

The two basic requirements of relaxation experiments are The sufficiently fast disturbance of the equilibrium and the specific observation of the transient helix-coil transition. Stationary relaxation methods, such as sound absorption and dielectric relation, do not fulfill the second requirement. The application of electric field-jump and laser temperature-jump perturbations in combination with optical rotation detection not only satisfies both requirements, but as a bonus it provides also for the use of a large variety of solvents and ionic strengths. 

The first of these two choices is limited by the threshold of dielectric breakdown of most liquids 10 V cm ). To avoid arcing and plasma formation in both electric field-jump and laser temperature-jump [31] experiments, one has to stay below the threshold, which is already closely approached in the current practice. Therefore, the main interest is to increase the sensitivity through repetitive perturbation and averaging the output signals. 

---