spectrometry

Question 1

what does NMR stand for

Answer 1

nuclear magentic resonance

Question 2

what are the basic principles

Answer 2

• 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 frquency are absorbed
• the nuclei flips from parellel to applied magentic to field to anti-paraellel
• the energy change can be monitored and recorded
• uses ther esonance of nuclei with spin

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 e.g 1H, 13C

Question 5

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

Answer 5

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

Question 6

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

Answer 6

• one peak for each set of equivalent H atoms
• each chemical environment shows 1 peak

Question 7

what does the area under the peak represent for H NMR

Answer 7

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

Question 8

what is the integration trace

Answer 8

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

Question 9

what is TMS (name and structure)

Answer 9

tetramethylsilane

H3C—Si(CH3)2—CH3

Question 10

why is TMS used

Answer 10

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

Question 11

what are the other advantages of using TMS

Answer 11

• inert
• non-toxic
• easy to remove from the sample as relatively volatile

Question 12

when does splitting/spin-spin coupling occur

Answer 12

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

Question 13

what is the n+1 rule

Answer 13

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

Question 14

what are the splitting patterns for 0, 1, 2, and 3 inequivalent H atoms 3 bonds or less away

Answer 14

singlet = no coupled protons

doublet = 1coupled protons

triplet = 2 coupled protons

quartet = 3 coupled protons

Question 15

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

Answer 15

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

Question 16

which solvents are used

Answer 16

deuterated solvents:
1. CDCl3
2. D2O
3. C6D6
4. CCl4

Question 17

how do you analyse a carbon-13 NMR spectrum of an organic molecule

Answer 17

• number of peaks = number of carbon environments
• chemical shifts = type of environment
• carbon atoms bonded to different atoms/groups differ in environments and absorb at different chemical shifts
• 2 carbon atoms positioned symmetrically are equivalent
• this means the same chemical environment, absorption at a chemical shift and contribution to the peak

Question 18

how do you analyse a proton NMR spectrum of an organic molecule?

Answer 18

• number of peaks = number of proton environments
• chemical shift = type of environment
• relative peak areas = relative number of each type of H+
• splitting pattern = number of non-equivalent H+ on adjacent carbon atom (n+1 rule) to give singlets, doublets, triplets, quartets or multiples

Question 19

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

Answer 19

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