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Introduction to fluxionality

It does not take an extensive knowledge of chemistry to understand that as-drawn chemical structures do not give an entirely correct picture of molecules. Unlike drawings, molecules are not stationary objects in solution, the gas phase, or even in the solid state. Bonds can rotate, bend, and stretch, and the molecule can even undergo conformational changes. Rotation, bending, and stretching do not typically interfere with characterization techniques, but conformational changes occasionally complicate analyses, especially nuclear magnetic resonance (NMR).

For the present discussion, a fluxional molecule can be defined as one that undergoes an intramolecular reversible interchange between two or more conformations. Fluxionality is specified as intramolecular to differentiate from ligand exchange and complexation mechanisms, intermolecular processes. An irreversible interchange is more of a chemical reaction than a form of fluxionality. Most of the following examples alternate between two conformations, but more complex fluxionality is possible. Additionally, this module will focus on inorganic compounds. In this module, examples of fluxional molecules, NMR procedures, calculations of energetics of fluxional molecules, and the limitations of the approach will be covered.

Examples of fluxionality

Bailar twist

Octahedral trischelate complexes are susceptible to Bailar twists, in which the complex distorts into a trigonal prismatic intermediate before reverting to its original octahedral geometry. If the chelates are not symmetric, a Δ enantiomer will be inverted to a Λ enantiomer. For example not how in [link] with the GaL 3 complex of 2,3-dihydroxy-N,N‘-diisopropylterephthalamide ( [link] ) the end product has the chelate ligands spiraling the opposite direction around the metal center.

Bailar twist of a gallium catchetol tris -chelate complex. Adapted from B. Kersting, J. R. Telford, M. Meyer, and K. N. Raymond, J. Am. Chem. Soc. , 1996, 118, 5712.
Substituted catchetol ligand 2,3-dihydroxy-N,N‘-diisopropylterephthalamide. Adapted from Kersting, B., Telford, J.R., Meyer, M., Raymond, K.N.; J. Am. Chem. Soc. , 1996, 118, 5712.

Berry pseudorotation

D 3h compounds can also experience fluxionality in the form of a Berry pseudorotation (depicted in [link] ), in which the complex distorts into a C 4v intermediate and returns to trigonal bipyrimidal geometry, exchanging two equatorial and axial groups . Phosphorous pentafluoride is one of the simplest examples of this effect. In its 19 FNMR, only one peak representing five fluorines is present at 266 ppm, even at low temperatures. This is due to interconversion faster than the NMR timescale.

Berry pseudorotation of phosphorus pentafluoride.

Sandwich and half-sandwich complexes

Perhaps one of the best examples of fluxional metal complexes is (π 5 -C 5 H 5 )Fe(CO) 2 1 -C 5 H 5 ) ( [link] ). Not only does it have a rotating η 5 cyclopentadienyl ring, it also has an alternating η 1 cyclopentadienyl ring (Cp). This can be seen in its NMR spectra in [link] . The signal for five protons corresponds to the metallocene Cp ring (5.6 ppm). Notice how the peak remains a sharp singlet despite the large temperature sampling range of the spectra. Another noteworthy aspect is how the multiplets corresponding to the other Cp ring broaden and eventually condense into one sharp singlet.

Questions & Answers

Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
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what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
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s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
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Sanket Reply
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Damian Reply
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abeetha Reply
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Sravani Reply
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Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
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In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
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Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
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