NMR

Question 1

Strategy to determine the structure of a molecule via NMR:

Answer 1

1. H-NMR
2. COSY
3. C-NMR
4. DEPT-135
5. HSQC

Question 2

DEPT types?

Answer 2

DEPT-45 = All peaks positive
DEPT-90 = Only signals of CH groups will show
DEPT-135 = CH and CH3 positive, CH2 negative

Question 3

Simple description of what NMR maps.

Answer 3

NMR maps the carbon-hydrogen framework of molecules.

Question 4

How does NMR work?

Answer 4

Measures the resonance transition between magnetic energy levels. (Measures the relaxation resonance back to a stable state, resonance causes nuclei to flip into an anti-parallel state)

Question 5

Commonly used reference sample in NMR:

Answer 5

Tetramethylsilane (TMS), used to calibrate the chemical shift

Question 6

What do H-NMR and COSY show?

Answer 6

Assigns protons based on chemical shift, splitting, and coupling.
COSY correlates H atoms through bonds with other H atoms.

Question 7

What do C-NMR and DEPT show?

Answer 7

Determines the number of protons attached to each carbon.

Question 8

What does HSQC show?

Answer 8

Assigns carbons with attached protons.

Question 9

What is the coupling constant (J) ?

Answer 9

The distance between the peaks in a given multiplet, a measure of the magnitude of splitting effect.
Measured in Hz
In simple cases, J is just the difference (in Hz, not ppm) between the peaks.

Question 10

Multiplicity

Answer 10

The n+1 rule. Due to splitting.
The splitting is into 1 more peak than the number of H’s on the adjacent carbon, thus n+1 rule.
e.g. CH3 – CH2 – O – CH3
A quartet means that these hydrogens have three neighbouring hydrogens on adjacent carbons. In the above case this quartet would represent the hydrogens in the CH2 region.

Question 11

Integration

Answer 11

Integration is the measurement of peak areas on the NMR spectrum. It corresponds to the amount of energy absorbed or released by all nuclei participating in chemical shift during the nuclear spin flip process. It is used to determine the ratio of hydrogens that correspond to the signal.
Simple ratio to calculate the number of H’s.
e.g. ~7 ~14 ~7 ~21 ~14, divide all by 7
7 = 1H, 14 = 2H’s, 21 = 3H’s