nonArrhenius

Non-Arrhenius refers to the situation in chemical kinetics where reaction rate constants do not follow the Arrhenius equation over temperature. In many processes the Arrhenius form k = A exp(-Ea/RT) predicts a straight line when ln k is plotted against 1/T. When the plot shows curvature, the system exhibits non-Arrhenius behavior, indicating that simple single-step activation does not capture the temperature dependence.

Causes of non-Arrhenius behavior include changes in the mechanism with temperature, such as different rate-determining steps or competing pathways becoming favorable at various temperatures. Other factors are a temperature-dependent pre-exponential factor, formation of intermediates, strong coupling to molecular motions, solvent or matrix effects, phase transitions or glassy dynamics, and quantum tunneling at low temperatures. Changes in heat capacity between the ground state and the transition state can also produce curvature as Ea effectively varies with temperature.

Several models are used to fit non-Arrhenius data. The Vogel-Fulcher-Tammann (VFT) equation, k = A exp[-B/(T − T0)], describes temperature-dependent rates in viscous or glass-forming systems. The Williams-Landel-Ferry (WLF) equation is popular in polymer science near the glass transition. Modified Arrhenius forms allow Ea or A to vary with temperature, or more complex transition-state theories incorporate temperature-dependent parameters.

Understanding non-Arrhenius behavior is important for accurate kinetic analysis, extrapolation, and mechanism interpretation in fields ranging from catalysis and enzymology to materials science and atmospheric chemistry.