Molecular Geometry: Computing & VSEPR Theory

[Godwin Kessy]

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 have temporary geometries ie. Oscilatin once an not a fixed or rigid which shows as if the system is static!

 

[chemisttree]

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.

 

[Godwin Kessy]

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 occurrence? Oh what did you actualy mean that it oscilate round the predicted geometry!

 

[chemisttree]

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.

 

[DrDu]

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.

 

[Godwin Kessy]

Hey what does the born approximations say on the molecular expresions

 

[DrDu]

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".