3.3.15 NMR Spectroscopy

Question 1

Give three reasons why T.M.S. is a good standard in proton NMR spectroscopy

Answer 1

Volatile
Inert
Cheap
Vibes one signal far right
Low boiling point so can easily be removed after
Non toxic
Provides peak at 0ppm - reference

Question 2

Principles of NMR

Answer 2

Nuclear Magnetic Resonance
Gives information about 13C or 1H atoms in a molecule
Structures in magnetic field have electromagnetic waves of radio frequency applied to them
If radio waves of the right frequency are absorbed, the nuclei flip from parallel to applied magnetic to field to anti parallel
Energy change monitored and recorded
Uses resonance of nuclei with spin

Question 3

Chemical shift is affected by

Answer 3

Molecular environment

Question 4

Solvent used in 1H NMR spectra to dissolve sample

Answer 4

CCl4 - tetrachloromethane

Question 5

Explain what the integration trace shows

Answer 5

Relative areas under peak
Integrator trace crosses peak/group of peaks + gains height
Height gained proportional to area under peak/group of peaks

Question 6

What causes splitting and spin-spin coupling

Answer 6

Magnetic spins of resonating nuclei interact + affect the magnetic field of 1H atoms
Cause peaks to split

Question 7

How would you carry out NMR spectroscopy

Answer 7

Dissolve liquid sample in suitable solvent
Put in tube with small amount TMS
Put tube in NMR machine
Spin sample to even out any imperfections in magnetic field and spectrometer zeroed against TMS or
Radiation with different radio frequencies + constant magnetic field is applied to sample
Any absorptions detected

Question 8

Use of NMR

Answer 8

MRI scanners

Question 9

What defines the resonant frequency of a 13C atom

Answer 9

Chemical environment
Amount of electron shielding

Question 10

What graph is produced by NMR spectroscopy

Answer 10

Energy absorbed against chemical shift

Question 11

What is chemical shift + units

Answer 11

Resonant frequency of nuclei compared to that of 1H atom in TMS
Parts per million

Question 12

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

Answer 12

Most H atoms are 1H
Much more abundant than 13C
So almost all H atoms have spin to show up

Question 13

What does the area under the peak represent for H NMR

Answer 13

Proportional to number of 1H atoms represented by the peak

Question 14

What is the integration trace

Answer 14

A stepped line that makes it easier to measure the area under the curve

Question 15

TMS structures + state

Answer 15

Si(CH3)4 - tetramethylsilane
Liquid

Question 16

Why does the peak from O-H bond disappear if D2O is used as a solvent

Answer 16

O-D bond formed in preference to O-H due to labile protons that move/swap from one molecule to another