Theory and information stored within the 1H NMR spectra

Observations in ¹H NMR

Observation

• Number of signals

Information

• Number of ¹H environments

Observation

• Chemical shift (position of signal)

Information

• Type of ¹H environment

Observation

• Integration

Information

• Number of ¹H of each type

Observation

• Peak splitting (coupling)

Information

• Number of adjacent ¹H

Number of ¹H NMR signals

The magnitude of delta E, and therefore energy of radiowaves emitted, is dependent on the size of the magnetic field experienced by the nucleus

Multiple peaks tells us that not all the ¹H nuclei are experiencing the same magnetic field

• Result of the electron surrounding the ¹H nuclei

• ‘Electric current’ from these surrounding electrons affects the magnetic field the protons experience

• ‘Shields’ the nucleus meaning that the effective magnetic field strength is lower than the applied field - this will change delta E

Number of ¹H NMR peaks

Electron density around the proton ‘shield’ it from the magnetic field - making the nuclei ‘experience’ smaller magnetic field and delta E will change

Hydrogens bonded to electron withdrawing atoms/groups in a molecule such as paracetamol have a lower share of electrons and their nuclei experience a higher magnetic field moving them downfield (higher ppm)

Low e^- density around the proton = deshielded = higher magnetic field experienced = higher ppm value

¹H NMR shifts - general trends

¹H NMR shifts - data supplied in the exam

Integrations in ¹H NMR

The y axis describes the intensity of the signal = amount of radiowaves absorbed at each point on the x axis

Each hydrogen contributes to the signal equally, therefore the area under each peak relates directly to the number of hydrogen atoms in the sample that are in that environment

The area under the curve can be expressed as the integral - as shown in red here