NMR

Question 1

What is an NMR wave

Answer 1

• Radio wave that has the longest wavelengeth

Question 2

Characteristics of elemental isotopes

Answer 2

• Have a spin
• Some are integral (1,2,3), some are fractional (1/2, 3/2, 5/2), some have no spin l = 0 (12C, 16O)
• The most useful in organic chemistry are 1H, 13C, 19F and 31P, all l = ½

Question 3

What does spinning nuclei show

Answer 3

• Create a magnetic moment
• Can spin in all directions until an external field is introduced

Question 4

What is magnetic field

Answer 4

• Spinning nuclei can now be aligned with or against the field

Question 5

What is resonance?

Answer 5

• Right combination of magnetic field and electromagnetic radiation the spin can flip back and forth
• The nucleus is in resonance and this is the energy detected by NMR

Question 6

What is Shielding?

Answer 6

• Magnetic force exterted by the electrons small induced and it is opposed by the external field

Question 7

What happens when theres a decrease in magnetic field in the nucleus?

Answer 7

a stronger field being needed for resonance to occur

Question 8

Oxygen desheilding

Answer 8

• Pull electrons away from the neighbouring atoms

Question 9

What is a Chemical Shift

Answer 9

• Difference in magnetic field strength between the resonance field of the proton being observed and that of tetramethylsilane (ppm)
• Distance of the shift and how much it resonates
• Si is less electronegative than C, so CH3 groups are relatively electron rich and so H well shielded
• NMR signal is too the left

Question 10

Electronegativity and chemical shifts

Answer 10

Increasing electronegativity, there is more deshielding of neighbouring atoms as the electrons are drawn towards the more electronegative atom

Question 11

Distance from Electron-Withdrawing Group and chemical shift

Answer 11

• The further away an atom is from the electronegative group, the less deshielding effect is felt by the protons of that atom
• More electron withdrawing the deshielding effects increase

Question 12

Aromatic Protons effect on chemical shift

Answer 12

• Electron circulation causes deshielding so lower fields observed (5-6)

Question 13

Alderhydes and chemical shift

Answer 13

• Carbonyl group in alderhydes cause the desheilding effect (9-10)

Question 14

OH and NH groups effect on chemical shift

Answer 14

• Concentrated solutions, deshielding caused by H-bonding, NH (δ 3.5) and OH (δ 4.5)
• Diluted solutions, no H-Bonding, NH and OH (δ 2.0)
• Broad peaks are observed due to H-bonding and proton exchange

Question 15

COOH chemical shift

Answer 15

C=O and O present so even greater deshielding

Question 16

Number of signal

Answer 16

• Dependant on the proton environment

Question 17

Integration line

Answer 17

• Measure both peaks and compare them
• The ratio between them shows the difference of protons in the environment

Question 18

Splitting of peaks

Answer 18

• Number of hydrogens on the bond next to the one observed N+1 rule
• Chemically equivalent protons do not split each other

Question 19

Complex splitting patterns

Answer 19

• Ayral groups have a complex splitting pattern
• 1 strongly EWG or EDG
• Symmetrical ortho distribution
• Unsymmetrical para distribution

Question 20

A coupling constant

Answer 20

• The distance between the peaks of multiplets measured in Hz
• They do not vary with the field strength of the spectrometer

Question 21

Homotopic

Answer 21

• If any H are replaced by deuterium (D) nothing changes as the other two groups are equivalents

Question 22

Enantiotopic

Answer 22

If any of the H are replaced by D the molecule becomes chiral

Question 23

Diastereotopic

Answer 23

• If either of the prochiral H are replaced by D we create a second chiral centre and the two resulting molecules will be diastereomers