Predict Proton NMR

Predict Proton NMR

If you want to learn about proton NMR ,then this is the right place, here you find the method to predict proton NMR , factors influencing proton NMR, solvents used in proton NMR, Chemical shift value table of proton NMR and example of proton NMR.

1.Introduction of proton NMR spectroscopy

2.predict proton NMR 
3.Factors influencing proton NMR
4.Chemical shift table of proton NMR 
5.Solvents used in proton NMR
6.Example of  proton NMR

Introduction of NMR spectroscopy 

In organic chemistry, a method called proton NMR spectroscopy is employed to investigate compounds containing hydrogen. It makes use of the nuclear magnetic resonance principle, which is taken from nuclear physics. Protons have charge that can spin, much like electrons, and this will cause a magnetic dipole moment. The study of matter's physical, chemical, and biological characteristics is done using NMR spectroscopy. It is used by chemists to ascertain the identification and structure of molecules. For diagnostic reasons, medical professionals use magnetic resonance imaging (MRI), a multidimensional NMR imaging technology.

Predict proton NMR spectra:

Assign number to molecules according to nomenclature for heterocyclic compounds(1), then use chemdraw software, chemical shift value, upfield, downfield signal tells the nature and chemical environment of atoms (H) present in the molecule. Chemical shift (⸹) values ranges from zero to ten ppm and TMS is used as standard (its value is near to zero in the sigma scale and 10 in the tau scale because it is highly shielded molecule). Signal nearer to TMS shows shielded protons means electronegative or electron withdrawing group is not present near to this proton and low value means they feel less magnetic field but their spin is opposite to applied magnetic field and they require high amount of applied magnetic field so their signals appear upfield. Downfield signal or high value of sigma means these protons are deshielded and they feel high magnetic field.

Factors influencing proton NMR:

1.Inductive effect:

It occurs due to electronegative atom or dipole moment in the molecule that contain sigma bonds, signal appear upfield.

Its effect decrease as the distance increase from the electronegative atom.

For example,

Inductive effect

Inductive effect

2.Wander walls deshielding:

It decreases with increase in the distance of the groups attach to molecule, larger groups cause repulsion and wander walls deshielding effect becomes prominent in the molecule.

                                                 

     Wander walls deshielding

Due to repulsion proton will become deshielded and signal of proton appear at high value of sigma.

3.Anisotropic effect:

               

                  Anisotropic effect

This effect occurs in shielded protons. Shielded protons have spin opposite to applied magnetic field, so they require high amount of magnetic field to bring them into resonance, signal appear when the applied magnetic field and proton magnetic field becomes equal in direction (this is called resonance).

Solvents used in proton NMR:

•      CCl₄

•      CS₂

•      CDCl₃

•      (CCl₃) C=O

•      DMSO

•      Trifluoroacetic acid

Example

Let’s look at the spectra of benzene,

predict proton NMR

Use chemdraw software for predicting proton NMR

In above example you noted that one peak occurs in the spectra because all the protons have same chemical environment. The downfield shift (7.26 ppm) shows that there are is no E.N atom present in the molecule. All the protons are shielded and require higher applied magnetic field to bring protons in resonance because the direction of magnetic field of ring and applied magnetic field (Bo) are opposite as shown below .so, in this way you predict easily the spectra of any compound.

Direction of Magnetic field

Chemical shift table for proton NMR

 Chemical shift value table

    ⸹ value

                                                              

This is the table that tells us the range of values for different types of organic molecules. TMS is taken as standard and all other values are obtained by comparing with TMS. As in the table Alpha disubstituted aliphatic molecules have sigma value ranges from zero to two and 2-4 for olefinic molecules.

(1)

Nomenclature for Heterocyclic compounds