NMR Spectroscopy Flashcards
(21 cards)
What does NMR spectroscopy stand for? And what does it determine?
Nuclear magnetic resonance spectroscopy
It determines the structure of a molecule
What property do some atoms have, that allows an NMR spectrum to be created?
Atoms with an ODD mass number have a ‘nuclear spin’ and can behave like tiny magnets in a magnetic field
Describe how an NMR spectrum is created.
The sample of atoms with an odd mass number are dissolved in a solvent, then placed in a magnetic field, the atoms align with or against the field. A reference standard is added
Radioactive waves are passed through the sample, cause it the nuclei to absorb the energy and flip between two nuclear states. This is called resonance
Nuclei in different chemical environments will require different amounts of energy to flip between states, creating different peaks on the spectrum
Once the spectrum is created the solvent is evaporated off
What is resonance?
Where atoms with odd mass numbers flip between two nuclear spin states when radioactive waves are passed through them in a magnetic field. This is because the nuclei absorb the energy
How are different peaks on an NMR spectrum created?
Nuclei in different chemical environments (bonded to different atoms/ not symmetrical) cause peaks in different positions on an NMR spectrum
Describe what solvents are used in NMR spectroscopy
Deuterated solvents such as CDCl3 or tetrachloro are used to dissolve the samples of atoms. Once the spectrum is crated, the solvent can be evaporated and so must be volatile
In NMR spectroscopy, what is a reference standard?
A reference standard is added to the sample under study, and acts as a reference fro the ppm values
Describe what will be seen on an NMR spectrum and why
It will contain peaks that occur in different positions
A scale called the chemical shift is used to compare the peaks positions (units are PPm)
The chemical shift of a nucleus depend on its molecular environment
Why can carbon 13 be analysed using NMR spectroscopy?
1.1% of carbon atoms in molecules are carbon 13
They have an odd mass number, so have a nuclear spin
Therefore they can absorb radio wave radiation and flip between two nuclear spin states
On a carbon NMR spectrum, what does the number of peaks and the chemical shift indicate?
The number of peaks indicates the number of different carbon environments
The chemical shift indicates the type of environment that the carbon atom is in
What does deuterated mean?
A deuterated compound is a compound where the normal hydrogen isotope is replaced by deuterium
What does the x axis on an NMR spectrum?
Chemical shift (PPM)
What are the units for chemical shift?
PPM
In proton NMR, what does the number of peaks and chemical shifts indicate?
Number of peaks- the number of different hydrogen environments
Chemical shift- the type of environment of the protons/ hydrogens
In proton NMR, what does integration indicate?
The number of hydrogen ions/ protons in that specific environment
It also tells us the area under the peak
What is the splitting pattern in proton NMR? What does it indicate?
Where the number of hydrogen ions on the adjacent carbons causes the chemical shift peak to split into n+1 smaller peaks.
The splitting pattern indicates the number of adjacent hydrogens, by taking a number away from the value
State the first four splitting patterns and the number of adjacent hydrogens they indicate
(Use the n+1 rule)
Singlet- 0
Doublet- 1
Triplet- 2
Quadret- 3
In splitting patterns, how do O-H and N-H environments differ?
They always only have one peak, because the do not take part in splitting
When answering a proton NMR question, where you need to deduce the shape of a molecule, how do you answer it?
State the chemical shift
State the integration number and how many hydrogens it therefore has
Use the (n+1) rule and the splitting patterns to find the number of adjacent hydrogen ions.
Draw all of the environments, and police them together so the end molecule matches the molecular formula
Give an example of a molecule that can be used as a reference standard in NMR, why is it a good reference value?
Tetramethylsilane Si(CH3)4
- contains four identical carbon and hydrogen environments, so its ppm is seen as 0
- its peaks are far away from other typical peaks
- it is volatile, so can easily be removed
- inert so doesn’t react with the samples
What are the features that make a good solvent for nmr?
Inert
Deuterated, so they do not have hydrogen ions and will not cause peaks on the ppm spectrum
If a polar organic compound is being dissolved, the solvent should be a polar covalent molecule
If a non-polar molecule is being dissolved, a non-polar solvent should be used
