chemical shift factors affecting-NMR spectroscopy

nmr spectroscopy-chemical shift

1. Chemical Shift ;- The chemical shift δ is expressed quantitatively as the difference in NMR frequencies between absorption signal of sample and a reference (ν X  and ν ref  respectively) relative to the operating frequency, ν  (the NMR frequency of the nuclide at that magnetic field): δ is generally expressed in parts per millions (ppm)

2. For example, an NMR signal that is 300 Hz higher than TMS at an operating frequency of 300 MHz has a chemical shift of:

3. Electronegativity effects A proton is said to be deshielded if it is attached to the electronegative atom or group. Greater is the electronegativity of the atom, greater is the deshielding caused to the proton. Larger is the deshielding of a proton, larger is the chemical shift value (δ). The chemical shift values of CH 4 , CH 3 F, CH 3 Cl, CH 3 Br and CH 3 I protons are 0.33, 4.26, 3.05, 2.68 and 2.16 respectively. Since ‘F’ is the most electronegative atom and hence the chemical shift of CH 3 F, protons is very high, whereas the Iodine being least electronegative among the halogens, show the lower value of chemical shift.

4. The presence of more electron withdrawing groups will produce a greater deshielding and therefore a larger chemical shift,  i.e.

5. Magnetic Anisotropy( due to the presence of pi system)- Magnetic anisotropy means that there is a nonuniform magnetic field. Electrons in π-systems (e.g., aromatics, alkenes, alkynes, carbonyls, etc.) interact with the applied magnetic field, which induces a magnetic field that causes the anisotropy. As a result the nearby protons can cause either shielding or deshieleding depending upon the orientation of proton with respect to applied magnetic field. As a result, the nearby nuclei will thus experience three different fields: (1) the applied field, (2) the shielding field of the valence electrons, and (3) the field due to the π system. Depending on the position of the nucleus in this third field, it can be either shielded (smaller δ) or deshielded (larger δ), which implies that the energy required for the frequency of the absorption will change.

6. In benzene, loops of pi electrons are so oriented cylindrically over the aromatic ring. These loops of electrons are induced to circulate in the presence of external magnetic field producing current. The induced magnetic field opposes the external magnetic filed in the center whereas it aligns the external magnetic field outside the ring. These experience greater deshielding effect The chemical shift value is 7.2ppm

7. In alkenes the pi electrons are so oriented that the plane of double bond is at right angles to the applied field. The pi electrons are induced in such a way that it which reinforce the applied magnetic field, thereby deshieleding of the protons. The chemical shift value is 5.8ppm

8. In acetylene, the electronic circulation around triple bond takesplace in such a way that the protons lies parallel to the external magnetic field. The induced magnetic filed opposes the applied magnetic field strength. Thus protons shielded and hence resonance occurs at higher applied field . The chemical shift value is ~2.5 ppm

9. CCl4:- being aprotic and cheap, is an ideal solvent for proton NMR spectra. Its use is limited to nonpolar compounds. It is very hydrophobic and any moisture in a sample is dissolved in this solvent will yield a turbine solution and thus impair homogeneity of the solution and consequently degrade resolution. This solvent can also be used for acid sensitive compounds. Deutero methanol(MeOH d6):- it is a polar solvent and is suitable for salts and extremely polar compounds. It has high affinity for water and is alomost imopssible to keep the sample dry.

10. Peak area:- In NMR spectrum various peaks represents equivalent sets of protons. The area of the peak or size tells the number of protons present in the compound in a particular environment. Greater the number of protons greater will be the energy absorbed and greater is the peak area . he area of the peak is proportional to the number of atoms that it represents. So in an H NMR, the integration of a peak gives the area of the peak and this area gives us a measure of the number of H atoms it represents, i.e. the number of H of that type. Example :- The  1 H NMR spectrum of a compound with one methyl group (CH 3 ), one methylene (CH 2 ), and one methine (CH) will have 3 peaks with peak ratios of 3:2:1.