Factors Influencing Chemical shifts of NMR active nuclei

NMR Chemical shifts result from surrounding environment nuclei

NMR Chemical shifts result from surrounding environment of  nuclei

NMR active nuclei are seldom found in isolation and even if they were to exist in isolation the NMR information revealed would be of little value to the NMR spectroscopist. The shielding and deshielding influence of electrons in the neighbourhood results in differences in resonance frequencies.

The effective magnetic field becomes less than the applied magnetic field due to the shielding by electrons in the neighbourhood so the applied field requires an increase to bring about resonance. The converse is true when neighbouring electrons shield the nucleus. In effect shielding and deshielding result from different chemical environments and resonance frequencies can be different on account of the surrounding electronic environment of the nuclei.

Chemical Shift

It is clear that NMR spectrum cannot be obtained on isolated nuclei. It therefore is necessary to use a suitable standard which can define the degree of shielding or deshielding of nuclei in different chemical environments. Tetramethylsilane (CH_3)_4Si is an ideal reference for reporting chemical shifts due to different groups.

Chemical shift is a dimensionless quantity but its magnitude is extremely small in comparison to the applied magnetic field or frequency. Therefore the observed value is multiplied by 10^6 and reported in parts per million (ppm). Conventionally the chemical shift scale ranges from 0 to 12 ppm. TMS is conventionally assigned 0 ppm and the values increase to the left along the x-axis.

Influencing factors on chemical shifts

Electronegativity

Electronegative atoms present in molecules tend to draw the electron density towards themselves and deshield the nucleus. An increase in electronegativity of the surrounding groups will result in decrease of the electron density and lead to an increase in chemical shift value due to the shielding of the nucleus.

Anisotropy

Anisotropy refers to the property of the molecule where a part of the molecule opposes the applied field and the other part reinforces the applied field. Chemical shifts are dependent on the orientation of neighbouring bonds in particular the π bonds. Examples of nucleus showing chemical shifts due to π bonds are aromatics, alkenes and alkynes. Such anisotropic shifts are useful in characterizing the presence of aromatics or other conjugated structures in molecules.

Hydrogen bonding

Hydrogen bonding results from the presence of electronegative atoms in neighbourhood of protons .The resulting deshielding leads to higher values of chemical shifts. This confirms the presence of hydrogen bonding in the molecules.

Chemical shifts of NMR active protons and other nuclei serve to provide a wealth of structural information on molecules.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

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