Reaction rate tunneling

Relationships between reaction rate and temperature can thus be used to detect non-classical behaviour in enzymes. Non-classical values of the preexponential factor ratio (H D i 1) and difference in apparent activation energy (>5.4kJmoRi) have been the criteria used to demonstrate hydrogen tunnelling in the enzymes mentioned above. A major prediction from this static barrier (transition state theory-like) plot is that tunnelling becomes more prominent as the apparent activation energy decreases. This holds for the enzymes listed above, but the correlation breaks down for enzymes... 

Figure 22. Thermal reaction rate constant of Hj. Solid line-without tunneling. Dotted hne-with tunneling. Taken from Ref. [9].

It is worthwhile to first review several elementary concepts of reaction rates and transition state theory, since deviations from such classical behavior often signal tunneling in reactions. For a simple unimolecular reaction. A—>B, the rate of decrease of reactant concentration (equal to rate of product formation) can be described by the first-order rate equation (Eq. 10.1). 

In contrast to classical overbarrier reactions, QMT can occur from the lowest vibrational quantum levels without thermal activation. Under these circumstances at the lowest temperatures, the degree of tunneling, and hence the reaction rate, is independent of temperature. At some point as the temperature is raised, higher vibrational levels become populated. As illustrated in Figure 10.1, the effective barrier is narrower for excited vibrational levels, and hence tunneling becomes more facile, leading to an increase in rate. Finally, as temperatures are raised further, classical reaction begins to compete, and usually dominates at room temperature (but, not always). 

Hie possibility that a particle with energy Jess than the barrier height can penetrate is a quantum-mechanical phenomenon known as the tunnel effect. A number of examples are known in physics and chemistry. The problem illustrated here with a rectangular barrier was used by Eyring to estimate the rates of chemical reactions. ft forms the basis of what is known as the absolute reaction-rate theory. Another, more recent example is the inversion of the ammonia molecule, which was exploited in the ammonia maser - the fbiemnner of the laser (see Section 9.4,1). 

Reaction Rates, Chemical, Collision Theory of (Widom). Reaction Rates, Chemical, Large Tunnelling Corrections 5 353... 

Quantum Theory of the DNA Molecule (Lowdin) Tunnelling Corrections, Large, in Chemical Reaction Rates 8 177... 

The kinetic model for proton transfer based upon transition state theory that incorporates a tunneling contribution to the overall reaction rate assumes that tunneling occurs near the region of the transition state (pathway a in Scheme 2.5). There is, however, another possibility for the reaction path for proton transfer. In lieu of thermally activating the vibration associated with the proton-transfer coordinate to bring it into the region of the transition state, the proton may instead... 

Here it was found that the tunneling factor k is very close to unity. However this result is uncertain because the magnitude of k is sensitive to the choice of the potential energy surface which is not as well established for reaction 6.20 as it is for 6.6. For that matter, learning whether a given reaction rate is significantly influenced by tunneling either on the basis of theory or experiment is not a trivial problem as will be pointed out in further discussion. 

Although a tunneling mechanism does not necessarily lead to a plateau in the temperature dependence of the chemical reaction rate, it can be believed that the existence of such a plateau means the validity of a tunneling mechanism. 

Z A proton tunneling calculation involving a realistic energy barrier was made (and had a significant effect in increasing the reaction rate). 

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