Chemical shift refers to the difference in resonant frequency between a nucleus and a reference standard, caused by differences in shielding.

Nuclei in different chemical environments will have slightly different resonant frequencies.

δ

Parts per million (ppm)

The electrons surrounding a nucleus act to shield it from the external magnetic field. Nuclei that are more shielded by surrounding electrons will experience a weaker magnetic field. This results in a lower resonance frequency and a smaller chemical shift value for more shielded nuclei.

For example, electronegative atoms like oxygen draw electron density away from directly bonded carbon atoms, decreasing their shielding and increasing their chemical shift.

TMS gives a single peak with all 12 hydrogen atoms resonating at the same frequency. This peak is defined as 0 ppm, so TMS provides a reference point to measure other chemical shifts.

The x-axis represents the chemical shift (δ), measured in ppm.

The y-axis represents the energy absorbed from the radio signal.

Chemical shift increases in value from right to left. 

This means the TMS peak at 0 ppm is always found at the far right of the chemical shift scale.

A proton NMR spectrum reveals:

1. The number of proton environments.
2. The types of proton environment present.
3. The relative numbers of each type of proton.
4. The number of non-equivalent protons adjacent to a given proton.

The number of peaks in the ¹H NMR spectrum corresponds to the number of unique proton environments in the molecule.

The chemical shift range that a peak appears in indicates the type of proton environment. Different ranges correspond to different functional groups.

Spin-spin splitting patterns reveal the number of non-equivalent protons on adjacent carbon atoms.

For a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern = n + 1

For example, a peak split into three (i.e. a triplet) indicates there are two hydrogens on the neighbouring carbon atom. 

A deuterated solvent is one in which the ¹H atoms have been replaced by ²H atoms.

The ²H isotope of hydrogen is called deuterium (D).

The ¹H atoms in a regular solvent produce signals that can interfere with the spectrum of the sample being analysed.

Deuterium does not produce any signal in the frequency ranges used for ¹H, so this interference is prevented and a higher resolution of the sample's spectrum can is obtained.

CDCl₃ is a commonly used deuterated solvent in NMR spectroscopy.

-OH and -NH peaks are often broad and variably shifted due to hydrogen bonding.

1. To identify -OH and -NH proton signals, compare two ¹H NMR spectra: one without D₂O and one with D₂O added to the sample.
2. When D₂O is added, the -OH and -NH protons exchange with deuterium, which does not produce signals in the ¹H NMR range.
3. As a result, the -OH and -NH peaks present in the first spectrum will disappear in the second spectrum, allowing you to identify these exchangeable protons.

The chemical environment of an atom refers to its position within a molecule, encompassing all atoms, functional groups, and bonds directly connected to the nucleus.

¹H NMR spectra are more complex than ¹³C NMR spectra due to the presence of spin-spin coupling between hydrogen atoms which causes NMR peaks to split into sub-peaks.

The relative peak areas are proportional to the relative numbers of equivalent protons producing each signal.