Chapter 23, Problem 3Q

Interpretation Introduction

Interpretation:

Structure and the chemical shift of all the carbon nuclei in ${\text{C}}_{\text{8}}{\text{H}}_{\text{10}}$ should be estimated. Also, signals should be assigned.

Concept introduction:

Nuclear magnetic resonance spectroscopy is applied for the identification of the structure of molecules. The energy in the radiofrequency region is suitable for NMR. Nuclear magnetic resonance results from the spin of the nucleus of an atom. The value of I is obtained using the atomic number and the mass number of an atom. The non-zero magnetic moment of an isotopic nucleus is detectable by the NMR technique.

Any nucleus with both an even atomic number and the mass number has 0 nuclear spins. There are a total of $\left(2\text{I}+1\right)$ energy levels that are allowed for a nuclear spin (I). In the absence of a magnetic field, energy levels are equal in energy that is degenerate.

However, energy levels become non-degenerate in the presence of a magnetic field.

Deuterated chloroform $\left({\text{CDCl}}_{\text{3}}\right)$ is a common NMR solvent generally used because it dissolutes a broad range of organic compounds and is inexpensive.

The total signal intensity of each set of proton is given by the height of each set of steps. The integration value defines the relative number of each kind of proton in the molecule.

In NMR spectrum, the intensity of signals is plotted against the magnetic field or frequency. Nuclei that are non-equivalent show only one peak in the NMR spectrum. However, protons absorb at different frequencies that are non-equivalent.

An increase in the electron density that surrounds the nucleus shields it from the applied field. This results in a net decrease in the field experienced by the nucleus. The value of the observed chemical shift of the signal therefore decreases, and, on a typical NMR spectrum, the signal moves to the right, which is called an upfield shift because, at a constant frequency, a slightly higher applied magnetic field is required for resonance to occur. De-shielding is the effect of a decline in the electron density around a nucleus which leads to shifting in the peaks of a chemical shift towards left in the NMR spectrum that results in an increase in delta values, hence downshift.