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Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

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8.4 The specific unimolecular reaction rate constants () 193<br />

• The Kassel formula describes experimental data qualitatively well. However, instead<br />

of the full value of =3 − 6, one usually has to use an effective number<br />

eff . As suggested by Troe, eff can be estimated from the specific heatofthe<br />

reactant molecules via<br />

=<br />

µ <br />

<br />

<br />

<br />

= <br />

<br />

<br />

µ<br />

2 ln <br />

<br />

using the known expression for ,<br />

Y 1<br />

=<br />

1 − exp (− )<br />

=1<br />

<br />

≈ eff (8.87)<br />

(8.88)<br />

• Often, eff is equal to about one half of the number of oscillators,<br />

eff ≈<br />

(3 − 6)<br />

2<br />

(8.89)<br />

I<br />

Figure 8.15: Specific rate constants: RRK formula.<br />

8.4.2 Rice-Ramsperger-Kassel-Marcus (RRKM) theory<br />

RRK theory fails in the following points:<br />

• Itcanbeusedonlywith as an effective parameter ( eff ).<br />

• It leads to the wrong results close to threshold, where → 0 (see the conditions<br />

above).<br />

• It is difficult to derive for oscillators with different frequencies.<br />

• RRK theory is essentially a classical theory. It does not know about zero-point<br />

energy.

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