Molecular Geometry: Computing & VSEPR Theory

In summary, fluxional compounds are those that exist in a constant state of change between different shapes.
  • #1
Godwin Kessy
91
0
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!
 
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  • #2
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
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
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
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".
 

Related to Molecular Geometry: Computing & VSEPR Theory

What is molecular geometry?

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. It is determined by the distribution of electron pairs around the central atom and is important in understanding the physical and chemical properties of a molecule.

How is molecular geometry calculated?

Molecular geometry is calculated using VSEPR (Valence Shell Electron Pair Repulsion) theory, which states that electron pairs in a molecule will arrange themselves to minimize repulsion and achieve the most stable arrangement. This can be determined by the number of electron pairs around the central atom and their arrangement, which can be predicted using the VSEPR chart.

What is the significance of molecular geometry?

The molecular geometry of a molecule affects its polarity, bond angles, and overall shape, which in turn influences its physical and chemical properties. For example, a linear molecule will have different properties than a bent molecule due to their different molecular geometries.

Can molecular geometry be predicted for any molecule?

Yes, the VSEPR theory can be applied to predict the molecular geometry of most covalent molecules. However, it may be more challenging for larger and more complex molecules with multiple central atoms.

How does molecular geometry relate to molecular orbitals?

Molecular orbitals are formed by the overlap of atomic orbitals in a molecule and are influenced by the molecular geometry. The arrangement of atoms in a molecule can affect the energy levels and stability of the molecular orbitals, which in turn affects the reactivity and properties of the molecule.

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