Chbe 551 lecture 20
Download
1 / 24

CHBE 551 Lecture 20 - PowerPoint PPT Presentation


  • 118 Views
  • Uploaded on

CHBE 551 Lecture 20. Unimolecular Reactions. Last Time Transition State Theory. Transition state theory generally gives preexponentials of the correct order of magnitude. Transition state theory is able to relate barriers to the saddle point energy in the potential energy surface;

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'CHBE 551 Lecture 20' - jena-sykes


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Chbe 551 lecture 20

CHBE 551 Lecture 20

Unimolecular Reactions


Last time transition state theory
Last Time Transition State Theory

  • Transition state theory generally gives preexponentials of the correct order of magnitude.

  • Transition state theory is able to relate barriers to the saddle point energy in the potential energy surface;

  • Transition state theory is able to consider isotope effects;

  • Transition state theory is able to make useful prediction in parallel reactions like reactions (7.27) and (7.29).



Why does transition state theory fail
Why Does Transition State Theory Fail?

  • Ignores the effect of energy transfer on the rate

  • Consider a stable molecule AB. How can AB  A + B

  • If you start with a stable molecule, it does not have enough energy to react.

    • Need a collision partner so AB can accumulate enough energy to react.

    • Energy accumulation ignored in TST


Lindeman approximation
Lindeman Approximation

  • Assume two step process

    • First form a hot complex via collission

    • Hot complex reacts

  • Steady State Approximation Yields


Comparison to data for ch 3 nc ch 3 cn
Comparison To Data For CH3NC  CH3CN



Why the difference
Why The Difference?

  • Bimolecular collision lasts ~10-13 sec

    • Molecule must be in the right configuration to react

  • Hot unimolecular complex lasts

    ~10-8 sec

    • Even if energy is put in the wrong mode, the reaction still happens


Rrk model
RRK Model

  • Assume correction to TST by

  • Qualitative, but not quantitative prediction


Rrkm model
RRKM Model

  • Improvement to RRK model proposed by Rudy Marcus (ex UIUC prof).


Derive equation
Derive Equation

  • Consider

  • Excite molecule to above the barrier then molecule falls apart

  • Derive Equation for reverse reaction

  • At Equilibrium


Derivation continued
Derivation Continued

  • From Tolman's equ

  • Pages Of Algebra


Chbe 551 lecture 20
Note

  • Reactants have a fixed energy ~laser energy

  • Products have a fixed energy too, but since they have translation, the products can have vibrational+ rotation energy between the top of the barrier and E*


Substituting and assuming energy transfer fast
Substituting, And Assuming Energy Transfer Fast

  • N(E*) E* is the number of vibrational modes of the reactants with an vibrational energy between E* and E* +  E*

  • G+(E*) is the number of vibrational modes of the transition state with a vibrational energy between E‡ and E* independent of whether the mode directly couples to bond scission.


Next separate vibration and rotation
Next Separate Vibration and Rotation

where GVT is the number of vibrational states at the transition state, with an energy between E‡ and E*. NV(E*) is the number of vibrational states of the reactants with an energy between E* and E* +E ; qR‡ is the rotational partition function for the transition state and qR* is the rotational partition function for the excited products.







Does rrkm always work
Does RRKM Always Work?

  • Assumes fast dynamics compared to time molecule stays excited

A comparison of the experimental rate of isomerization of stilbene (C6H5)C=C(C6H5) to the predictions of the RRKM model



Summary
Summary

  • Unimolecular reactions have higher rates because configurations that do not immediately lead to products still eventually get to products

  • RRKM – rate enhanced by the number of extra states

    • Close but not exact – still have dynamic effects


Query
Query

  • What did you learn new in this lecture?