nmr spectroscopy

Question 1

nmr stands for

Answer 1

nuclear magnetic resonance

Question 2

basic principles of nmr

Answer 2

find structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them

if radiowaves of the right frequency are absorbed, the nuclei flips from parallel to applied magnetic field to anti-parallel

this energy change can be monitored and recorded

uses resonance of nuclei with spin

Question 2

basic principles of nmr

Answer 2

find structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them

if radiowaves of the right frequency are absorbed, the nuclei flips from parallel to applied magnetic field to anti-parallel

this energy change can be monitored and recorded

uses resonance of nuclei with spin

Question 3

how would you carry out nmr spectroscopy

Answer 3

dissolve the liquid in suitable solvent, put in a tube along with a small amount of TMS and put the tube into a NMR machine

sample 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 4

give one use of nmr

Answer 4

mri scans

Question 5

what kind of nuclei does nmr work with

Answer 5

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

Question 6

what percentage of carbon atoms 13C

Answer 6

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

Question 6

what percentage of carbon atoms 13C

Answer 6

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

Question 7

what defines the resonant frequency of a 13C atom

Answer 7

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

Question 8

what defines the resonant frequency of a 13C atom

Answer 8

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

Question 9

what graph is produced by nmr spectroscopy

Answer 9

energy absorbed against chemical shift

Question 10

what is chemical shift? what are its units?

Answer 10

resonant frequency of nuclei, compared to that of a 1H atom in TMS

parts per million

Question 11

what is the range of chemical shift 13C nmr

Answer 11

0-200ppm

Question 12

what means 13C atoms show a different chemical shift value

Answer 12

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

Question 13

what kind of environment leads to a greater chemical shift

Answer 13

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

Question 14

what do the number of signals mean for 13C nmr

Answer 14

one signal for each carbon environment

Question 15

what do chemical shift mean for 13C nmr

Answer 15

greater chemical shift from atoms closer to electronegative atoms or C=C

Question 16

what does the area under each peak mean for 13C nmr

Answer 16

no meaning

Question 17

is there any splitting in 13C nmr

Answer 17

there is no splitting

Question 18

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

Answer 18

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

Question 19

what is the range of chemical shift for 1H nmr

Answer 19

0-10 ppm

Question 20

what leads to a lower chemical shift value for H nmr

Answer 20

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

Question 21

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

Answer 21

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

Question 22

what does the area under the peak represent for H nmr

Answer 22

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

Question 22

what does the area under the peak represent for H nmr

Answer 22

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

Question 23

what is the integration trace

Answer 23

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

Question 23

what is the integration trace

Answer 23

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

Question 24

what is TMS (name and structure)

Answer 24

tetramethylsilane
CH3
H3C–Si–CH3
CH3

Question 25

what state is tms at room temperature

Answer 25

liquid

Question 26

why is tms used

Answer 26

can be added to sample to calibrate the nmr equipment

it provides a peak at exactly chemical shift = 0 ppm

reference point against which all chemical shift are measured

Question 27

what are other advantages of using TMS

Answer 27

inert
non-toxic
easy to remove from the sample

Question 28

when does splitting/spin-spin coupling occur

Answer 28

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 29

what is the n+1 rule

Answer 29

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

Question 30

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

Answer 30

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

Question 31

which solvents are used in 1H nmr

Answer 31

• deuterated solvents such as CDCl3 , D2O and C6D6
• CCl4 (tetrachloromethane)

Question 32

what does the number of signals indicate in 1H nmr

Answer 32

one main signal for each set of inequivalent 1H atoms (for each hydrogen environment)

Question 33

what does the chemical shift indicate in 1H nmr

Answer 33

larger chemical shift for 1H atoms closer to electronegative atoms or C=C

Question 34

what does splitting mean for 1H nmr

Answer 34

number of smaller peaks = 1 + number of inequivalent hydrogen atoms 3 bonds away

Question 35

what does the area under peak mean in 1H nmr

Answer 35

proportional to the number of atoms represented by that peak