NMR Flashcards
What does the splitting pattern tell you?
What does the integration factor tell you?
How many hydorgens are in that same environment e.g. how many hydrogens are bonded to the same carbon
What does the chemical shift data tell you?
What group e.g. ester, alcohol is the one giving the peaks in that region of the spectrum
What three factors must you double check when forming a molecule from its hydrogen NMR chemical shift data?
Integration number
Splitting pattern
Chemical shift data
What are the two types of NMR?
Carbon-13 NMR and H-1 NMR
If an atomic nucleus has an ODD number of nucleons then it has what?
A nuclear spin
What are nucleons?
Protons and neutrons
What does the nuclear spin of the nucleus create?
A weak magnetic field
What does NMR detect?
It detects how the magnetic field created by the nuclear spin are affected by a larger external magnetic field.
Why does hydrogen have a nuclear spin?
Because it has one proton
WHEN does carbon have a nuclear spin? Why does it not normally have one?
Carbon usually has 6 protons and 6 neutrons however approx 1% of carbons are 13C which has 7 neutrons and thus gives carbon a nuclear spin
What direction do nuclei spin in when there is no magnetic field?
Random
What direction does nuclei spin in when there is an external magnetic field applied?
They align in two directions.
Either, they spin in the same direction of the magnetic field or they spin against it.
In terms of energy, what does NMR measure?
The amount of energy absorbed
How does NMR actually work?
NMR fires out radio waves and at a specific frequency the nuclei that are aligned with the magnetic field absorb the energy and flip to a higher energy level. Those with higher energy can also drop to lower energy and emit radio waves. Initially there are more nuclei aligned with the magnetic field so overall more energy is absorbed than emitted. AND NMR measures the amount of energy absorbed.
Does the nuclei spinning in the same direction as the magnetic field have the lower energy or the nuclei spinning in the opposite direction have the lower energy?
The nuclei spinning in the same direction as the magnetic field has the lower energy
What can shield a nucleus from an external magnetic field?
electrons surrounding the nucleus
What can affect the level of electron shielding? Explain how by giving an example.
Atoms and groups of atoms adjacent to the nucleus affect the level of electron shielding. e.g. an electronegative element such as oxygen near a carbon atom will reduce the electron shielding on the carbon atom.
What determines the environment of the carbon/hydrogen?
The groups of atoms that exist near the nuclei that is being examined. You look along the FULL CHAIN NOT just the atoms that are immediately bonded.
How can the NMR spec determine between hydrogens in different environments?
the magnetic field will be felt by the nuclei differently depending on the environment it is in as they absorb different amounts of energies and various frequencies. This is the difference that the NMR spec picks up.
Give an example of a chemical used as a standard when looking at chemical shift in NMR spectra:
TMS (tetramethylsilane)
What is the difference between the TMS peak and the peaks produced by the substance under test called?
The chemical shift
Why do we need to use a standard in NMR spectra?
because nuclei absorb different amounts of energy at different frequencies it is difficult to measure the magnitude of these without a standard chemical or reference to measure against.
What is the chemical shift?
the difference between the TMS peak and the peaks produced by the substance under test
What is the structure of TMS?
(CH3)4Si
A tetrahedral molecules with Si as the central atom
What is TMS used for in terms of the NMR machines?
it is used to calibrate NMR machines when analysing samples
Where is the peak of TMS on NMR spectra?
chemical shift = 0
Why is TMS a good standard to use?
It is inert, non-toxic and volatile so easy to remove from the sample
Why does TMS produce a large single peak well away from sample peaks?
Because it has 12 hydrogens all in identical environments
What does the number of peaks represent on a carbon NMR spec?
The number of peaks = the number of carbon environments
What is the relationship between chemical shift and electron shielding?
High electron shielding = low chemical shift
Lower electron shielding = higher chemical shift
Draw out 1,3-dichlorocyclohexane and determine how many carbon environments are:
4 carbon environments
What group does a peak at 190 suggest in carbon NMR?
A carbonyl group (aldehyde/ketone)
What do the numbers above the peak suggest in hydrogen NMR?
The ratio of the hydrogens, allows us to work out the relative number of hydrogens in each environment and overall.
What is the splitting pattern?
Peaks that split into smaller peaks. The number of smaller peaks corresponds to the number of hydrogen atoms on the adjacent carbon +1 – the (n+1 rule)
What is a singlet peak?
0 hydrogens on the neighbouring carbon
What is a duet peak?
1 hydrogen on the neighbouring carbon
What is a triplet peak?
2 hydrogens on the neighbouring carbon
What is a quartet peak?
3 hydrogens on the neighbouring carbon
Give an example of when a singlet peak would be produced:
When the hydrogen is bonded to an oxygen, because this is an oxygen not a carbon the n+1 rule applies here.
What solvents have to be used to dissolve the substances that we are testing in order to prevent confusing the spectra?
Non-hydrogen based solvents otherwise the solvent would show peaks in the spectrum
What solvents can we use in hydrogen NMR?
Deuterated solvents and alternatives such as CCl4
Why do we use deuterated substances as solvents?
Because they do not have hydrogen. Deuterium is an isotope of a hydrogen atom with 2 nucleons instead of 1. As deuterium has an even number of nucleons, it is not detected by the NMR machine. The only peaks will be from the test sample.
Why does deuterium not produce a peak on the NMR spectrum?
Because it has an even number of nucleons, so it does not have a nuclear spin and therefore is not detected by the NMR machine.
