Temperature jump

The temperature jump method is a technique used in chemical kinetics for the measurement of very rapid reaction rates. It is one of a class of chemical relaxation methods pioneered by the German physical chemist Manfred Eigen in the 1950s. In these methods, a reacting system initially at equilibrium is perturbed rapidly and then observed as it relaxes back to equilibrium. In the case of temperature jump, the perturbation involves rapid heating which changes the value of the equilibrium constant, followed by relaxation to equilibrium at the new temperature.

The heating usually involves discharging of a capacitor (in the kV range) through a small volume (

The fractional extent of the reaction (i.e. the percentage change in concentration of a measurable species) depends on the molar enthalpy change (ΔH°) between the reactants and products and the equilibrium position. If K is the equilibrium constant and dT is the change in temperature then the enthalpy change is given by the Van 't Hoff equation:

: {\Delta H^o} = {RT^2}.\frac{d \ln K}{dT}

where R is the universal gas constant and T is the absolute temperature. When a single step in a reaction is perturbed in a temperature jump experiment, the reaction follows a single exponential decay function with time constant (\tau) equal to a function of the forward (ka) and reverse (kb) rate constants. For the perturbation of a simple equilibrium A B which is first order in both directions, the reciprocal of the time constant equals the sum of the two rate constants :1/\tau = k_a + k_b

The two rate constants can be determined from the values of (\tau) and the equilibrium constant :K = k_a / k_b, yielding two equations for two unknowns.

In more complex reaction networks, when multiple reaction steps are perturbed, then the reciprocal time constants are given by the eigenvalues of the characteristic rate equations. The ability to observe intermediate steps in a reaction pathway is one of the attractive features of this technology.

Related chemical relaxation methods include pressure jump,