NMR Flashcards
What is an NMR wave
- Radio wave that has the longest wavelengeth
Characteristics of elemental isotopes
- 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
What does spinning nuclei show
- Create a magnetic moment
- Can spin in all directions until an external field is introduced
What is magnetic field
- Spinning nuclei can now be aligned with or against the field
What is resonance?
- 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
What is Shielding?
- Magnetic force exterted by the electrons small induced and it is opposed by the external field
What happens when theres a decrease in magnetic field in the nucleus?
a stronger field being needed for resonance to occur
Oxygen desheilding
- Pull electrons away from the neighbouring atoms
What is a Chemical Shift
- 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
Electronegativity and chemical shifts
Increasing electronegativity, there is more deshielding of neighbouring atoms as the electrons are drawn towards the more electronegative atom
Distance from Electron-Withdrawing Group and chemical shift
- 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
Aromatic Protons effect on chemical shift
- Electron circulation causes deshielding so lower fields observed (5-6)
Alderhydes and chemical shift
- Carbonyl group in alderhydes cause the desheilding effect (9-10)
OH and NH groups effect on chemical shift
- 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
COOH chemical shift
C=O and O present so even greater deshielding
Number of signal
- Dependant on the proton environment
Integration line
- Measure both peaks and compare them
- The ratio between them shows the difference of protons in the environment
Splitting of peaks
- Number of hydrogens on the bond next to the one observed N+1 rule
- Chemically equivalent protons do not split each other
Complex splitting patterns
- Ayral groups have a complex splitting pattern
- 1 strongly EWG or EDG
- Symmetrical ortho distribution
- Unsymmetrical para distribution
A coupling constant
- The distance between the peaks of multiplets measured in Hz
- They do not vary with the field strength of the spectrometer
Homotopic
- If any H are replaced by deuterium (D) nothing changes as the other two groups are equivalents
Enantiotopic
If any of the H are replaced by D the molecule becomes chiral
Diastereotopic
- If either of the prochiral H are replaced by D we create a second chiral centre and the two resulting molecules will be diastereomers