High Resolution NMR (DP IB Chemistry: HL)

• In the first part of NMR spectroscopy, we have seen that the nuclei of H atoms behave as tiny magnets and can interact with an applied magnetic field
• Only atoms with odd mass numbers show signals on NMR spectra and have the property of nuclear spin

Table showing nuclei which posses spin

• They can align themselves with the external magnetic field (lower energy state) or against the external field (higher energy state)
• Energy from the radio frequency end of the electromagnetic spectrum can excite the nuclei and cause them to ‘flip’ between a lower and higher energy state - this is resonance
• Samples are irradiated with radio frequency energy while subjected to a strong magnetic field
• Protons on different parts of a molecule (in different molecular environments) absorb and emit (resonate) different radio frequencies
• The magnetic field strengths of protons in organic compounds are measured and recorded on a spectrum
• The resonance energy is unique to specific H atoms in molecules that are located in the same chemical environment
• Information from the spectrum tells us the number of different H environments
• A reminder about low resolution NMR:

A low resolution ¹H NMR for ethanol showing the key features of a spectrum

Tetramethyl silane

• The horizontal scale on an NMR spectrum represents chemical shift (δ)
• Chemical shift is measured in parts per million (ppm) of the magnetic field strength needed for resonance in a reference chemical called tetramethylsilane, abbreviated to TMS

The displayed formula of tetramethylsilane

• TMS is used universally as the reference compound for NMR as its methyl groups are particularly well shielded and so it produces a strong, single peak at the far right of an NMR spectrum
• The signal from the carbon atoms in TMS is defined as having a chemical shift of 0 ppm

The NMR reference peak for TMS

Chemical Shift

• The chemical shift values of peaks on an ¹H NMR spectrum give information about the likely types of proton environment in a compound
  
  

The chemical shift values can be used to identify specific proton environments

Peak Splitting

• High resolution NMR gives more complex signals giving more structural details
• The signals sometimes appear to be split into a number of sub-peaks called doublets, triplets and quartets
  • This is known as multiplicity

• The splitting pattern of each peak is determined by the number of protons in neighbouring environments
• The complex signal produced indicates the number of protons on adjacent carbon atoms
• Neighbouring protons produce weak magnetic fields that can interact with each other
• Depending on how that interaction takes place, it allows you to determine the number of neighbouring protons
• Suppose you have a particular view point on an issue
  • You ask your neighbour’s opinion
  • Your neighbour could reinforce your argument and make your belief stronger
  • Alternatively, your neighbour could contradict your argument and make it weaker
    
    

Aligned and opposite spins on neighbouring protons

• If the spin of a neighbouring proton is aligned with the spin of the proton in question, the magnetic field from this spin strengthens the magnetic field
  • The resonance is stronger and results in a slightly higher chemical shift

• The magnetic field from the spin on a neighbouring proton that spins against the first proton weakens the magnetic field
  • The resonance is weaker and results in a slightly lower chemical shift

• The resulting high resolution NMR peak shows a split into a doublet - two equal peaks
• This pattern can only be obtained when there is one neighbouring proton so it gives us useful information about the structure of the molecule
• When there are two neighbouring protons, there are four possible combinations, but two of them have the same outcome on field strength, so three separate peaks are obtained

Table showing the effect of two neighbouring protons on peak splitting

• The resulting peak is split as a triplet
• This is what is seen when a proton is next to a -CH₂- group.

• When there are three neighbouring protons, there are eight possible combinations, four with the same outcome, so four separate peaks are seen, called a quartet
• This is what is seen when a proton is next to a –CH₃ group, in other words, a proton that is next to the end of a chain
• The number of split peaks is related to the neighbours following what is termed the n+1 rule
  • Where there are n neighbours there are n+1 split peaks

¹H NMR peak splitting patterns table

• In summary, an NMR spectrum provides several types of information
  • number of signal groups...    ...the number of different proton environments
  • chemical shift...     ...the general environment of the protons
  • peak area...     ...the relative number of protons in each environment
  • multiplicity...    ...how many protons are on adjacent atoms

• In many cases, this information is sufficient to deduce the structure of an organic molecule but other forms of spectroscopy are used in conjunction with NMR to confirm structural information