3.3.15 Nuclear magnetic resonance spectroscopy

Question 1

what types of NMR are there

Answer 1

1) C13 NMR
2) H ( proton) NMR

Question 2

what is the sample called which is used to calibrate the spectrum

Answer 2

Tetramethylsilane ( TMS)

Question 3

what’s the formula for TMS

Answer 3

Si in the middle
4 methyl groups attached

Question 4

why is TMS used

Answer 4

• its signals is away from all the others
• it only gives one signal
• it’s non toxic
• it is inert
• it has a low boiling point and so can be removed from sample easily

Question 5

what is the delta sign

Answer 5

is a measure in parts per million (ppm) is a relative scale of how far the frequency of the proton signal has shifted away from that for TMS

Question 6

what is the chemical shift

Answer 6

how much the field has shifted away from the field for TMS

Question 7

what is in a C13 NMR spectrum

Answer 7

there is one signal ( peak) for each set of equivalent C atoms

Question 8

EQ : A compound is usually mixed with Si( CH3)4 and either CCl4 or CDCl3 before recording the compounds H NMR spectrum
State why Si(CH3)4 , CCl4 and CDCl3 are used in H NMR spectroscopy
Explain how their properties make them suitable for use in H NMR spectroscopy (6)

Answer 8

Stage 1:
• CDCl3 or CCl solvent
• TMS as a reference / standard
• Inert => unlikely to react with sample
Stage 2 :
- CCl4 and CDCl3 have no H atoms
- CCl4 is non polar so good solvent for non polar organic molecules
- CDCl3 is a polar covalent molecule so good solvent for polar organic compounds
Stage 3:
• TMS gives one signal
• Signal in an area away from other typical H signals
• Easy to remove / low bp

Question 9

what solvent is used in HNMR spectrum

Answer 9

CCl4 or CDCl3

Question 10

EQ : suggest why CDCl3 is a more effective solvent than CCl4 for polar molecules such as atenolol (1)

Answer 10

• CDCl3 is polar