Principles of NMR spectroscopy

Question 1

Nuclear magnetic resonance spectroscopy is

Answer 1

a technique used to find the structures of organic compounds. it depends on the ability of nuclei to resonate in a magnetic field

Question 2

the chemical shift if a proton (or a group of protons) is

Answer 2

a number (in the units ppm) that indicates its behaviour in a  magnetic field relative to tetramethylsilane.
it can be used to identify the chemical environment of the carbon atom or of the hydrogen atoms (protons) attached to it

Question 3

in hospitals, NMR is called

Answer 3

MRI (magnetic resonance imaging) so s not to confuse with radioactive substances due to the word ‘nuclear’

Question 4

the key points and explanation for how NMR works are:

Answer 4

• Nucleons (protons and neutrons), have spin, with protons in the nucleus having opposite spins just like electrons in orbitals
• Nuclei either have odd or even numbers of nucleons (an even number means n protons= n neutrons, and an odd number means n protons : n+1 neutrons)
• where n protons=n neutrons, the spins of the cancel each other out
• Nuclei with an odd number of nucleons means there is a residual spin on the nucleus
• residual spin cause tiny magnetic field to be created
• these nuclei are therefore affected by external magnetic fields
• nuclei can be wither line up with or oppose the external magnetic field
• this different positions can be made to change by switching the external magnetic field, and as the nuclei change position depending on the external magnetic field, they absorb electromagnetic radiation
• the nuclei of two adjacent (neighbouring) atoms in a molecule influence each other and electrons also ave an effect
• this means that the electromagnetic radiation is absorbed different by different atoms depending on neighbouring atoms in a molecule, and these different levels of absorbance are detected as different signals at different frequencies

Question 5

the vertical and horizontal on an NMR spectrum are:

Answer 5

vertical: absorption for absorption of radio frequency energy (no units)
horizontal: chemical shift (δ) with units ppm (parts per million)

Question 6

for an atom to show up in an NMR spectrum,

Answer 6

it must have an odd number of nucleons

Question 7

as C-12 has an equal number of nucleons, …………can be detected

Answer 7

C-13 can be detected as 1.1% of naturally occurring carbon is the C-13 isotope

Question 8

the two atoms most commonly used for NMR are:

Answer 8

C-13
H-1

N-15 and P-31 are also used less commonly

Question 9

the organic compound being analysed has to be dissolved in ………………….

Answer 9

has to be dissolved in a solvent, but most solvent molecules contain carbon and hydrogen atoms

Question 10

the issue with most solvents is that

Answer 10

they contain C and H atoms which will interfere with those produced by the compound being analysed

Question 11

the commonest solvent used has the formula:

Answer 11

CDCl3

where D is deuterium, which is the H-2 isotope

Question 12

deuterium contain…………..which means

Answer 12

H-2 contains a proton and a neutron, so their spins cancel out and the solvent CDCl3 produces no signals could interfere with the signals from H-1 atoms
it only produces one system that can interfere with the signals from carbon atoms, and this can be removed from the spectrum

Question 13

TMS is

Answer 13

tetramethylsilane

Question 14

a reference standard is necessary because:

Answer 14

because carbon and hydrogen atoms in different chemical environments will produce different signals, so a reference standard to produces a prominent signal is necessary

Question 15

the structure of TMS is

Answer 15

(CH3)4-Si

in a tetrahedral shape

Question 16

TMS is a suitable standard because: (3 reasons):

Answer 16

• chemically unreactive
• contains 12 hydrogen atoms all joined in the same way joined in a symmetrical arrangement
• this means it produces a strong signal that is easy to identify

Question 17

splitting occurs because:

Answer 17

-the magnetic environment of a proton in one group is affected by the magnetic field of protons on a neighbouring carbon atom

Question 18

explain how splitting occurs in the following structure:

CH - CH2

Answer 18

-the CH hydrogen atom can have its spin aligned, ↑, or opposed, ↓
-this results in two different field affecting the CH2 hydrogen atoms, so their peak split into two or equal height, 1:1
-the CH2 hydrogen atoms affect the field on the CH hydrogen atom. here, because of multiple spins, the situation is more complex:
there are four following ways in which the spins can be aligned or opposed as: ↑↑, ↑↓ , ↓↑ or ↓↓
the effect of ↑↓ is the same as ↓↑, so they produce one peak with the value of 2, which results in the CH peak splitting into three sub-peaks in the ratio of 1:2:1
-this is called spin-spin coupling (splitting)
-therefore, if the proton of a hydrogen atom has n hydrogen atoms on neighbouring carbon atoms, its peak will be split into n+1 sub-peaks
-you can see this as the proton of the hydrogen atom in the CH group is neighboured by a carbon atom that has 2n hydrogen atoms, so its peak splits into 2n+1 sub-peaks, or a triplet, int he ratio 1:2:1