F324: Module 3: Proton NMR Flashcards
C) What is nuclear spin and why is it important?
Nuclear spin is a property of nuclei with odd mass numbers such as 1H and 13C.
It is important because it allows the nuceli to behave as magnets in a magnetic field when subjected to a pulse of radio frequency radiation so therefore is useful as the positions in a compound can be determined.
C) How does NMR/MRI work?
- Substance is put in a large magnetic field
- Subjected to a pulse of radio frequency radiation
- Some nuceli absorb energy
- A spectrum is produced from the different energies absorbed by different nuclei
- Allowing the position of protons in the compound to be determined
C) Which solvent is used in NMR spectroscopy and why?
CCl4 or CDCl3. The former because it contains no protons so will not produce a peak, the latter because one of the chlorines has been replaced with deuterium which has no nuclear spin so will also not produce a peak (this is called a deuterated solvent).
C) Describe low resolution and high resolution NMR.
- Low resolution only produces one peak per proton environment, the RELATIVE numbers of each type of proton are shown by the relative peak areas (shown by an integration trace), the chemical shift shows what environment that/those protons are in
- High resolution- Similar to low res but the peaks are split and the splitting pattern depends on the surrounding protons. If there is one proton adjacent, there will be a doublet peak (peak split into 2) because the splitting pattern is n+1. –> This is due to the spins of the protons interacting with eachother (PROTONS ON THE SAME CARBON ARE EQUIVALENT TO EACHOTHERN AND DO NOT CAUSES A SPLITTING PATTERN)
C) What is used to produce a reference peak?
Tetramethylsilane (TMS)
C) Why do OH and NH groups not produce a splitting pattern in neighbouring CH groups?
The OH proton rapidly exchanges protons with water (or acid) very rapidly- so rapid that the signal for the OH proton becomes a single peak. This proton exchange also occurs in some organic compounds containing nitrogen so therefore can allow these groups to be identified (usually peptides)
C) How can the single peaks from OH or NH be identified and removed?
- Add deuterated water to the sample and the deuterium will replace the protons and the peaks will disappear.
- Comparing the spectra before and after deuteration allows the OH and NH peaks to be identified.
C) What is the name for the proton in OH and NH
Labile Proton
C) How does Carbon 13 spectroscopy work? What produces the reference peak
An NMR spectrum shows chemical shift for non equivalent carbon atoms in a molecule, with the reference as tetramethylsilane. Also there are no splitting patterns with C spectroscopy.
C) What does the area of the peak represent in C13 spectroscopy?
Nothing. Unlike HNMR, the peak area is not proportional to the number of carbons in that environment.
C) How does an MRI scanner work?
…….
C) How does mass spectrometry contribute to our knowledge of a compound?
- Accurate molecular mass from the molecular ion peak
- Possible structure from fragmentation pattern
- Identify the compound by ‘fingerprinting’ using a computer database
C) How does infra-red spectroscopy contribute to our knowledge of a compound?
- Presence of functional groups from the wave numbers of the absorption bands between 1500cm^-1 to 3500cm^-1
- Identity of the compound using the fingerprint region 700-1500cm^-1 by comparison of spectra of known compounds.
C) How does NMR contribute to our knowledge of a compound?
- Identity of chemical groups containing protons from chemical shift in proton NMR spectra
- Arrangement of proton containing groups in the molecule from the spin-spin splitting patterns
- Identity of non equivalent carbon atoms from chemical shift in C13 NMR spectra
C) In which situations in infrared spectroscopy will the absorption band caused by OH be sharp peaks rather than broad peaks?
They will appear as sharp peaks if in a dilute solution or in the vapour phase. (This is because the broad peak is caused by hydrogen bonding
