3.3.15 Nuclear magnetic resonance spectroscopy (A2)

Question 1

what are the basic principles of NMR?

Answer 1

you can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them
if radio waves of the right frequency are absorbed, the nuclei flips from parallel to applied magnetic to field to anti-parallel
this energy change can be monitored and recorded
uses the resonance of nuclei with spin

Question 2

how would you carry out NMR spectroscopy?

Answer 2

dissolve the liquid sample in a suitable solvent, put in a tube along with a small amount of TMS and put the tube into an NMR machine
sample is spun to even out any imperfections in the magnetic field and the spectrometer is zeroed against the TMS
radiation with different radio frequencies but a constant magnetic field is applied to the sample and any absorptions (due to resonance) are detected

Question 3

give one use of NMR?

Answer 3

MRI scans

Question 4

what kind of nuclei does NMR work with (and examples)?

Answer 4

those with an uneven number of nucleons, meaning they will spin eg. 1H, 13C

Question 5

what percentage of carbon atoms are 13C?

Answer 5

1% - but modern instruments are sensitive enough to detect this?

Question 6

what defines the resonant frequency of a 13C atom?

Answer 6

the chemical environment that is in; the amount of electron shielding it has.

Question 7

what graph is produces by NMR spectroscopy?

Answer 7

energy absorbed against chemical shift

Question 8

what is chemical shift? what is its symbol? what are its units?

Answer 8

the resonant frequency of the nuclei, compared to that of a 1H atom in TMS
symbol: δ
units: parts per million (ppm)

Question 9

what is the range of the chemical shift for 13C NMR?

Answer 9

0-200ppm

Question 10

what means 13C atoms show a different chemical shift value?

Answer 10

having different chemical environments (but equivalent atoms show the same peak)

Question 11

what kind of environment leads to a greater chemical shift?

Answer 11

a C atom next to more electronegative atom has a greater chemical shift

Question 12

summarise what these mean for 13C NMR?
number of signals, chemical shift, area under peak, splitting

Answer 12

number of signals: one signal for each carbon environment (each set of inequivalent 13C atoms)
chemical shift: greater δ from atoms closer to electronegative atoms or C=C
area under peak: no meaning
splitting: there is no splitting for 13C NMR

Question 13

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

Answer 13

most H atoms are 1H- it is much more abundant than 13C
this means almost all H atoms have spin so show up

Question 14

what is the range of chemical shift for 1H NMR?

Answer 14

0-10ppm

Question 15

what leads to a lower chemical shift value for H NMR?

Answer 15

1H with more electrons around them i.e. further from electronegative groups/atoms

Question 16

on a low resolution spectrum, what peaks would you expect to see for H HMR?

Answer 16

One peak for each set of inequivalent H atoms (each chemical environment shows 1 peak)

Question 17

what does the area under the peak represent (for H NMR)?

Answer 17

the area under the peak is proportional to the number of 1H atoms represented by the peak

Question 18

what is the integration trace?

Answer 18

a stepped line that makes it easier to measure the area under the curve (height of line = area under that peak)

Question 19

what is TMS (name and structure)?

Answer 19

tetramethylsilane

Question 20

what state is TMS at room temperature?

Answer 20

liquid

Question 21

why is TMS used?

Answer 21

can be added to sample to calibrate NMR equipment
provides a peak at exactly δ = 0ppm
it is the reference point against which all δ are measured

Question 22

what are the advantages of using TMS?

Answer 22

inert, non-toxic, easy to remove from the sample (as relatively volatile)

Question 23

when does splitting/spin-spin coupling occur?

Answer 23

neighbouring hydrogen atoms (3 or fewer bonds away, or on the adjacent carbon) affect the magnetic field of 1H atoms and causes their peaks to split

Question 24

what is the n+1 rule?

Answer 24

if there are n inequivalent 1H atoms on the neighbouring carbon then the peak will split into (n+1) smaller peaks

Question 25

why must solvents used for 1H NMR not contain any hydrogen atoms?

Answer 25

signals from the solvent would swamp signals from the sample, as there is much more solvent than sample

Question 26

which solvents are used for NMR of 1H?

Answer 26

deuterated solvents: CDCl3, D2O, C6D6, CCl4 - tetrachloromethane

Question 27

summarise what these indicate for 1H NMR?
number of signals, chemical shift, splitting, area under peak

Answer 27

number of signals: one main signal for each set of inequivalent 1H atoms (for each hydrogen environment)
chemical shift: larger for 1H atoms closer to electronegative atoms or C=C
splitting: number of smaller peaks = 1 + number of inequivalent hydrogen atoms 3 bonds away
area under peak: proportional to the number of atoms represented by that peak

Question 28

why does the peak from O-H bonds disappear if D2O is used as a solvent?

Answer 28

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