Cause of chemical shifts (ll)

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 Other Factors affecting Chemical shifts. 

·       One is Inductive effect which was stated earlier (in previous post).
·       Hybridisation of C atom:
This is somehow related to inductive effect. There are generally three types of hybridisation of carbon observed in organic compounds( sp3 sp2 and sp). We know that more the ‘s’ character more will be the deshielding (more ‘s’ character shows availability of electron cloud towards more to the nucleus) and more will be chemical shift.
According to above , the order of chemical shift will be:
sp > sp2 > sp3
but this is not actual order , the order is :
sp2 > sp3 > sp
This trend is explained by another term called Magnetic Anisotropy.
·       Magnetic Anisotropy:
The word anisotropy means non spherical environment. For example let us take a C-C single bonded carbon atom. When we see the electron cloud around this C-C bond we found that this is not spherical which create non uniform magnetic field.
Due to which one part of a molecule becomes shielded and another become deshielded. The nucleus which comes under electron cloud region are said to shielded nuclie and another one is deshielded nuclie.

In the above figure the double cone shaped region is shielded region.

                               
                                    Acetaldehyde ( δ (ppm) ~9.7)                                                                                                       
                                       Ethyne (δ (ppm) ~ 2.5)



  • ·       Mesomeric effect or Resonance effect:
    In Resonance effect different number of  resonating structure obtained and in these structures some atom accumulate positive charge and some accumulate negative charge. This accumulation of charge causes in shielding and deshielding of proton attached to them. Ex:-



                                  


·       Stearic Effect or Vander Wall’s Deshielding :
    Generally it is observed where bulky groups were present around Hydrogen nuclei. The electron cloud of bulky group repel each other and hence result in the deshielding of Hydrogen nuclei.


 
  • Hydrogen bonding:
Hydrogen bonding in a molecule causes deshielding of hydrogen nuclei.For example, in aquous solution of ethanol -OH bonded hydrogen is observed in downfield region whereas in D2O solvent it observed in upfield region.

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