Chapter 15, Problem 74IP

Interpretation Introduction

Interpretation: The reason for the difference in chemical shift value of two methyl groups in the given structure to be interpreted.

Concept Introduction:

${}^{\text{1}}\text{H}\text{NMR}$: The ${}^{\text{1}}\text{H}\text{NMR}$ spectrum gives information on the different electronic environment of protons. The number of signal (proton types) generated in ${}^{\text{1}}\text{H}\text{NMR}$ are predicted by performing symmetry operations (rotation or reflection symmetry).

The ${}^{\text{13}}\text{C}\text{NMR}$ spectrum gives information on the different electronic environments of carbon. As like ${}^{\text{1}}\text{H}\text{NMR}$, the number of signals generated in ${}^{\text{13}}\text{C}\text{NMR}$ are predicted by performing symmetry operations (rotation or reflection symmetry). Only chemical shift values are reported in the spectrum but not the multiplicity and integration values because the coupling between two neighboring ${}^{\text{13}}\text{C}{-}^{\text{13}}\text{C}$ nuclei are weakly involved due to the low abundance of ${}^{\text{13}}\text{C}$ isotopes of carbon atom.

Anisotropic effects: Placing a compound in the strong magnetic field causes the pi-electrons to circulate and this flow of electrons creates an induced magnetic field. This result is diamagnetic anisotropy, which means the different region of space in the compound experience different magnetic field strengths.

Shielding effect: If the protons are aligned towards or near to the newly generated induced magnetic effect opposes the external magnetic field (protons experience the weak magnetic field) and termed as shielding effect reflects in the low chemical shift values.