NMR

Question 1

What does NMR Stand For?

Answer 1

Nuclear Magnetic Resonance

Question 2

Explain The Principles of How NMR Works (5)

Answer 2

Nuclei with Spin produce weak magnetic fields

NMR fires out EM (radio) waves at a specific frequencies

The Nuclei with spin that are parallel with the larger magnetic Field absorb the energy

and flip to the higher energy level and are now anti-parallel with the larger magnetic field

This Energy Change can be monitored and recorded

Question 3

An Nucleus has spin when ……

Answer 3

The atomic nucleus has a odd number of nucleons

-NMR only works with nucleons with uneven nucleons

Question 4

How would you carry out NMR spectroscopy? (7)

Answer 4

Dissolve liquid sample in a suitable solvent

place dissolved sample along with a small amount of TMS inside a tube

put the tube into the NMR machine

zero the spectrometer against the value read for TMS

The sample is spun to even out ant imperfections in the magnetic field

Radiation with different radio frequencies but a constant magnetic field is applied to the sample

any absorbtions due to resonance are detected and plotted

Question 5

Name a use of NMR

Answer 5

MRI scans

Question 6

What kind of Nuclei does NMR work with?

Provide Examples

Answer 6

(protium) H-1 & C-13

Those Nuclei with an uneven number of nucleons

Question 7

What defines the resonant frequency of a C-13 atom

(What affects the chemical shift of an NMR)

Answer 7

• The chemical environment it is in

-The amount of electron shielding it has

Question 8

What type of graph is produced by NMR

Answer 8

Energy absorbed against Chemical shift

Question 9

What is Chemical Shift?

What are its units?

Answer 9

The Resonant frequency of the nuclei compared to that of a H-1 atom in TMS

Parts per million (ppm)

Question 10

What is the range of Chemical shift for C-13 NMR?

Answer 10

0-200 ppm

Question 11

What does it mean when C-13 atoms show a different chemical shift value?

Answer 11

Having different chemical environments

Question 12

What kind of environment leads to a greater chemical shift? (C-13)

Answer 12

When a carbon atom is adjacent to a more electronegative atom/portion (such as C=C, or -Br)

Question 13

Why is it easier to get a spectrum of H-1 NMR then C-13 NMR

Answer 13

Most atoms that are Hydrogen are protium (H-1) so most have Nuclear spin that can be recognised by NMR

Much more abundant then C-13 relative to C-12

Question 14

What is the Range of Chemical Shift for H-1 NMR

Answer 14

0-10 ppm

Question 15

What leads to a smaller chemical shift value for H-1 NMR

Answer 15

H-1 with a greater amount of electrons surrounding them

• Further away from electronegative groups/atoms

Hydrogens resonate more like a hydrogen atom in TMS

Question 16

On a low resolution spectrum

What peaks you would expect to see for H-1 NMR

Answer 16

One peak for each set of inequivalent H-1 Atoms

(each chemical shift environment shows 1 peak)

Question 17

What does the area below a peak representation in H-1 NMR

Answer 17

The area below the peak is proportional to the number of H-1 atoms causing the peak

Integration Trace

Question 18

What is the Integration Trace

Answer 18

A Line located above a graph of H-1 NMR

That is equal to the area below the graph at every value of Chemical shift

Easier to measure the relative abundance of H-1 atoms within each environment

Question 19

What is TMS

Name?

Structure?

Answer 19

Question 20

What state is TMS at room Temperature

Answer 20

Liquid

Question 21

Why is TMS Used?

Answer 21

Inert, non toxic and easy to remove form sample

Provides a peak at 0 ppm

Used as a reference point so that every chemical shift caused by an environment is measured comparatively to it

Question 22

When does Splitting coupling occur?

Answer 22

Neighbouring hydrogen atoms (3 or fewer bonds away/ on an adjacent carbon atom) affect the magnetic field of H-1 atoms.

Causing their Peaks to split

Question 23

What is the N+1 rule?

Answer 23

If there are n inequivalent H-1 Atoms on the adjacent carbon

then the peaks will be split into n+1 peaks of varying sizes

Question 24

Answer 24

Question 25

What is a requirement for the solvent used to dissolve samples for H-1 Nmr

Answer 25

No Hydrogen atoms/ protium

Would cause major interference with results, so that they become unreadable

Question 26

Which solvents are used for NMR?

Answer 26

Deuterated Solvents - CDCl3 , D2O ,C6D6
CCl4 - Tetrachloromethane

Question 27

Deuuterated solvents contain Hydrogen atoms

Why does this have no effect on NMR values

Answer 27

Has even number of nucleons

No Nuclear Spin

Isn’t affect by the applied external magnetic field in an NMR machine , so isn’t detected

Question 28

Explain 1 problem with using D2O as a solvent

Answer 28

• If molecules contain at O-H bond

O-D bond will form in its place

due to labile protons that move from 1 molecule to another

so the chemical shift peak caused by O-H will disappear

Question 29

Explain why CCl4 and CDCl3 are used in Proton NMR

Why are their Properties suitable for this?

Answer 29

CCl4 & CDCl3 are inert solvents

ClCl4:
- Non-polar solvent that is ideal for preforming proton NMR with non-polar molecules
-Dipoles exhibit symmetry making it non-polar
-Doesn’t interfere with proton NMR spectra as there are no hydrogen atoms present

CDCl3:
-Polar solvent ideal for proton NMR of polar organic molecules
-Doesn’t interfere with proton NMR spectra as a deuterated hydrogen as no nuclear spin due to an even number of nucleons —> hence no weak magnetic field that is detected by the NMR machine

Question 30

Explain why TMS is used in NMR

Why is it suitible?

Answer 30

Tetramethylsilicone is used as a standard that all values of chemical shift are measured against

Produces single Peak at 0 PPM

TMS has 12 equivalent hydrogen environments due to symmetry of the molecule —> only produces a single peak that is easily identifiable.

TMS is volatile , low BP so easily Removed from sample once spectra is completed

Question 31

(just the first one)

Answer 31

H attached to both C-Cl and adjacent to C=O which isn’t on the Table

Question 32

Answer 32

R has 4 Carbons next to a C=O
S has 2 Carbons next to a C=O

R has 2 peaks in the range of 20-50 ppm while S will only have 1 peak in this range

S has 1 peak in the range 5-40 ppm

S has the peak with the lowest chemical shift

Question 33

Answer 33

Both contain 2 hydrogen environments each producing 2 peaks

R produces 2 singlet peaks, As no hydrogens attached to a carbon, are adjacent to another molecule with hydrogens attached in a different chemical environment

S produces a triplet peak and a Quartet peak