Molecular geometry

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 hav 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 occurence? 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".