Temperature jump Joule heating

The tautomerisation of the N(9)H- and N(7)H-isomers of adenine (93) has been studied in aqueous solution using the temperature-jump technique with Joule heating (Dreyfus et al., 1975). The equilibrium constant K = [N(7)H]/[N(9)H] has a value of 0.28 at 20°C. The reciprocal relaxation time... 

Temperature jump [25] with Joule heating was used in an early study of rapid outer-sphere electron-transfer reactions [22] between polypyridine metal complexes and haxachloro- and hexabromometalates, Eq. 25,... 

The p-jump method has several advantages over the t-jump technique. Pressure-jump measurements can be repeated at faster intervals than those with t-jump. With the latter, the solution temperature must return to its ini-lial value before another measurement can be conducted. This may take 5 min. With p-jump relaxation, one can repeat experiments every 0.5 min. One can also measure longer relaxation times with p-jump than with t-jump relax-mion. As noted earlier, one of the components of a t-jump experiment is It heat source such as Joule heating. Such high electric fields and currents can destroy solutions that contain biochemical compounds. Such problems lIo not exist with the p-jump relaxation method. 

Fig. 7.8 Experimental setup with a temperature-jump cell designed for Joule heating by means of an electrical discharge.

Activation parameters are more and more frequently being reported as important features of temperature jump relaxation method kinetic studies. This is true in spite of the fact that the restricted temperature range accessible to a Joule heating temperature jump apparatus and the usual 10% uncertainties in measured relaxation times are not favorable characteristics for achieving precise values of AH and AS. ... 

However, if the system is simple and only one process occurs or if the field-induced changes are longlived and relax with time constants large compared to the field duration. Joule heating temperature jump spectrometers may be used. ... 

Considerations of cost and of workshop facilities are also important. Commercial instriiments are well developed for stopped-flow, pressure-jump, temperature-jump (by joule heating), fluorescence, flash photolysis, pulse radiolysis, n.m.r. and e.s.r. Apparatus can also be built for most of these, given the necessary man-hours and workshops. 

Summarizing it should be mentioned that as compared to the Joule heating temperature jump the pressure jump technique offers a wider choice of solvent composition, an extended time range and shorter time intervals between repeats. And finally it has been shown that for organic solvents the temperature jump due to the adiabatic pressure jump causes an equilibrium shift sufficiently large for kinetic measurements [9]. 

A Joule heating temperature jump (T-jump) relaxation method study (8) of the kinetics of complexation of monovalent cations in methanol by dibenzo-30-crown-10 particularly intrigued us. Chock had found the rate of complexation of several monovalent cations (Na, K+,NHi +,etc.) to be almost diffusion controlled and essentially too fast for precise determination by T-jump equipment then available to him. He also noted an even faster relaxation process that was completely inaccessible. This latter relaxation process Chock ascribed to a conformational change of the dibenzo-30-crown-10 between two ligand conformers one of which is more suitable for complexing the cation. Such an inference is entirely consistent with known, rapid conformational equilibria in solutions of valinomycin (2), for example. 

Kinetic studies were performed using a Joule-heating temperature-jump apparatus. Temperature jumps of about 4-5°C were normally applied. Heating times were 5 ysec in the 0.2 M KNO3 solution. This orientational effect was canceled out by choosing the suitable direction of the polarizer which is inserted between the cell and the monochrometer. Kinetic experiments were carried out at 20°C with a thermostated cell and the others at room temperature. The relaxation was followed spectro-photometrically at 430 nm and the spectrophotometric data were analyzed at 444 nm. 

The temperature jump equipment comprised a Hartley Measurements, joule heating temperature jump assembly with both oscilloscope output and a Varian trace superposition noise averaging facility for increasing signal to noise ratio, also equipped with an analogue to digital (paper tape) converter for data storage. The relaxation effect was monitored by fast kinetic spectrophoto-... 

A Joule heating T-jump apparatus with a heating time below 8 xs and an adjustable jump of temperature up to 7°C is commercially available from U.K. Hi-Tech Scientific Ltd. (U.K.) Figure 2.3 shows a block diagram of the setup. 

---