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

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7.3 Thermodynamic interpretation of transition state theory 164<br />

7.3.2 Applications of thermodynamic transition state theory<br />

a) Comparison of ∆S ‡ -values for different types of transition states<br />

I<br />

Bimolecular reactions:<br />

A+BC→ ABC ‡ (7.97)<br />

y<br />

∆ ‡ 0 (7.98)<br />

or even<br />

∆ ‡ ¿ 0 (7.99)<br />

Furthermore, for chemically otherwise similar molecules<br />

(nonlinear molecule) (linear molecule) À (cyclic molecule) (7.100)<br />

Thus,<br />

¯<br />

¯∆ ‡ (nonlinear molecule)¯¯ ¯¯∆ ‡ (linear molecule)¯¯ ¿ ¯¯∆ ‡ (cyclic molecule)¯¯<br />

(7.101)<br />

For example:<br />

⎛<br />

A ··· B<br />

∆ ‡ ⎜<br />

⎝<br />

¯<br />

y<br />

...<br />

⎞ ‡ ⎛<br />

‡ ⎞ ⎛<br />

⎟ A ··· B<br />

<br />

¯<br />

C<br />

⎠¯¯¯¯¯¯¯ ¯∆‡ ⎝ A ··· B ··· C ⎠¯<br />

⎞ ‡ ¯ ¿<br />

∆ ‡ ⎜ ⎟<br />

⎝<br />

. .<br />

¯ C ··· D<br />

⎠¯¯¯¯¯¯¯<br />

(7.102)<br />

cyclic ¿ linear nonlinear (7.103)<br />

I<br />

Unimolecular reactions:<br />

ABC → ABC ‡ (7.104)<br />

• For isomerization reactions with a tight transition state:<br />

∆ ‡ ⎛<br />

⎝<br />

• For bond fission reactions with a loose transition state:<br />

‡<br />

B<br />

⎞<br />

Á Â ⎠ . 0 (7.105)<br />

A – C<br />

∆ ‡ ¡ AB ···C ‡¢ 0 (7.106)<br />

y<br />

loose tight (7.107)

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