Molecular geometry

  • #1

Main Question or Discussion Point

Hey! How can we compute the molecular geometry for molecules and further use the VSEPR since electrons are continouly in motion, of which i actualy expected to hav temporary geometries ie. Oscilatin once an not a fixed or rigid which shows as if the system is static!
 

Answers and Replies

  • #2
chemisttree
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Fluxional compounds do just as you say. They are in a constant state of change. Most amines are examples of this. More stable compounds are in constant motion as well(above absolute zero) but the motion is motion about a certain geometry. The average structure is the one usually shown since it is difficult to draw a vibrating thingy every time you discuss something.
 
  • #3
Thanks i now get it! But what do you actually mean by fluxional compounds. gec am interested

also do u min that the geometries we actualy predict cary large probability of occurence? Oh what did you actualy mean that it oscilate round the predicted geometry!
 
  • #4
chemisttree
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The latter for most compounds. By fluxional I mean that some compounds interconvert between different 'shapes'. Examples of these are the organometallic compounds such as bis(cyclopentadienyl)mercury (II) that exists both as the bis-monohapto and bis-pentahapto complexes. In the bis-monohapto complex the mercury is sigma bonded once to two carbon atoms... one on each of the two cyclopentadiene groups. In the bis-pentahapto complex, the mercury is bonded to every carbon in both cyclopentadienyl groups. That's about as fluxional as you can get.
 
  • #5
DrDu
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You are right, molecules don't have a shape on a fundamental level. The shape arises as a new concept upon application of the Born-Oppenheimer approximation.
Basically, we are replacing the molecular system of interest by a model system of particles with defined shape which is easier to describe. The error e.g. in energy from this approximation is small on a chemical scale but may be too large on a spectroscopic scale.
 
  • #6
Hey what does the born approximations say on the molecular expresions
 
  • #7
DrDu
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Just google for "Born Oppenheimer" you should find tons of references. A fascinating but high brow book on the topic is "Chemistry, quantum mechanics, and reductionism : perspectives in theoretical chemistry / Hans Primas".
 

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