3.3.15 - Nuclear magnetic resonance spectroscopy

What is the typical sequence to identify a compound?

Typical sequence to identify a compound: elemental analysis, mass spectrometry, infrared spectrometry NMR spectrometry.

What kind of compounds is NMR used to analyse?

NMR is used for analysing organic compounds.

Which atom show signals on NMR? why?

Only atoms with odd mass numbers show signals on NMR spectra because they have a nuclear spin.

What is the organic compound dissolved in before being introduced to the magnetic field?

TMS (Tetramethylsilane)

Why is TMS used?

TMS is used as a reference compound, because it is non toxic, inert, volatile (easily separated from the sample), produces one strong, sharp absorption peak on the spectrum.

Why does TMS only produce one absorption peak?

TMS only produces one absorption peak as all the protons are in the same molecular environment.

What value does the reference peak have in HNMR? How are the H peaks measured?

This reference peak has a value of 0. When a sample is analysed, its H peaks are measured and recorded by their shift away from the sharp TMS peak.

What value does the reference peak have in CNMR? Why? How are the C peaks measured?

TMS also produces one peak on the carbon NMR as all the carbons are in the same molecular environment. This peak is high intensity as it accounts for the absorption of energy from 4 C13 nuclei. When a sample is analysed, the C13 peaks are measured and recorded by their shift away from the sharp TMS peak.

What are the features of a H NMR spectra?

NMR spectra show the intensity of each peak against its chemical shift. The area under each peak gives information about the number of protons in a particular environment. The height of each peak shows the intensity/absorption from protons. A single sharp peak is seen to the far right of the spectrum. This is the reference peak from TMS, usually at 0 ppm.

Why are different hydrogen atoms in differenc chemical shifts?

Because they are in different environments.

What must happen to a sample before it is anaylsed through NMR?

Samples must be dissolved in a solvent before it is analysed through NMR.

Why is TMS sometimes not useful?

Even though TMS shows one sharp peak, the proton atoms can still interfere with peaks of a sample compound

What can be done to prevent this interference of TMS?

to avoid this interference, solvents with deuterium can be used (e.g. CDCl3 (deuterated chloroform))

What is deuterium?

Deuterium is an isotope of H. Deuterium nuclei absorb radio waves in a different region to the protons analysed so the reference solvent peak will not interfere with the sample.

What can high resolution H NMR show?

High resolution H NMR can show you the structure of the molecule but also the peak can be split into sub-peaks or splitting patterns.

What is the splitting pattern caused by?

These are caused by a proton's spin interacting with the spin states of nearby protons that are in different environments.

What can the splitting pattern show?

This can provide information about the number of protons bonded to adjacent carbon atoms.

What can he peaks be classified into?

The peaks can be classified into singlet, doublet, triplet and quartet using the (n+1) rule.

Overall, what information does H NMR provide?

H NMR provides several types of information based on- number of peaks, chemical shift, peak area, splitting patterns.

Does C NMR spectra have splitting?

C NMR spectra usually displays sharp single signals- no splitting.

On the C NMR spectra, is the height of each signal proportional to the number of C atoms?

The height of each signal is not proportional to the number of carbon atoms. C atoms in different environments will give different heights at different chemical shifts.

What can you do to calculate the number of C environments in aromatic compounds?

For aromatic compounds, you can use symmetry to calculate the number of C environments.