Arrhenius group

Here ae is the effective thermal dififusivity. The nondimensional group y is called the Arrhenius group, and represents a nondimensional activation energy for the chemical reaction. 

Parameter defined by Equation 7.28 parameter defined in Table 7.3 Arrhenius group defined by E/R T. 

The experimental studies of a large number of low-temperature solid-phase reactions undertaken by many groups in 70s and 80s have confirmed the two basic consequences of the Goldanskii model, the existence of the low-temperature limit and the cross-over temperature. The aforementioned difference between quantum-chemical and classical reactions has also been established, namely, the values of k turned out to vary over many orders of magnitude even for reactions with similar values of Vq and hence with similar Arrhenius dependence. For illustration, fig. 1 presents a number of typical experimental examples of k T) dependence. 

The measured dependence of kn(T) and T) consists of an Arrhenius region ( = 9.6 kcal/mol) going over to the low-temperature plateau below IlOK, where k 10 s . The isotope effect grows as the temperature drops, kn/ko — 20 at T = 100 K (fig. 15). Tunneling is promoted by the torsional vibrations of the OH and CH groups, as well as the oxy-group bending vibration. 

The effeet of temperature satisfies the Arrhenius relationship where the applieable range is relatively small beeause of low and high temperature effeets. The effeet of extreme pH values is related to the nature of enzymatie proteins as polyvalent aeids and bases, with aeid and basie groups (hydrophilie) eoneentrated on the outside of the protein. Einally, meehanieal forees sueh as surfaee tension and shear ean affeet enzyme aetivity by disturbing the shape of the enzyme moleeules. Sinee the shape of the aetive site of the enzyme is eonstrueted to eoirespond to the shape of the substrate, small alteration in the strueture ean severely affeet enzyme aetivity. Reaetor s stirrer speed, flowrate, and foaming must be eontrolled to maintain the produetivity of the enzyme. Consequently, during experimental investigations of the kineties enzyme eatalyzed reaetions, temperature, shear, and pH are earefully eontrolled the last by use of buffered solutions. 

Methyl- and 2,6-dimethylpyridine as catalysts with sterically hindered a-com-plexes give greater isotope effects (k2n/k2D up to 10.8). Such values are understandable qualitatively, since the basic center of these pyridine derivatives cannot easily approach the C-H group. The possibility of tunneling can be excluded for these reactions, as the ratio of the frequency factors 4h 4d and the difference in activation energies ED—EU (Arrhenius equation) do not have abnormal values. 

Contrary to the results obtained with carbon tetrachloride solvent and entirely in accord with the postulate that the effect arises from the steric hindrance to solvation, the rates of cleavage of ArSnR3 compounds in methanol decrease on increasing the size of the group R. This is shown by the rate coefficients in Table 265, though it is difficult to draw any conclusion from the Arrhenius parameters... 

Arrhenius parameters The pre-exponential factor A (also called the frequency factor) and the activation energy Ea. See also Arrhenius equation. aryl group An aromatic group. Example —C6H5, phenyl. 

---