Chapter 15: NMR Spectroscopy

Question 1

What determines the energy gap between spin states

NMR Spectroscopy

Answer 1

The magnitude of the energy gap depends on the strength of the imposed external magnetic field

Question 2

Chemical equivalence categories

NMR Spectroscopy

Answer 2

Homotopic- protons are chemically equivalent and are interchangeable by rotational symetry

Enantiotopic- protons are chemically equivalent and are interchangeable only by reflectional symmetry

Diasterotopic- protons are not chemically equivalent and cannot be interchanged by symmetry

Question 3

Number of expected signals

NMR Spectroscopy

Answer 3

A signal is expected for each chemically equivalent hydrogen atom

Question 4

Chemical shifts

NMR Spectroscopy

Answer 4

A LEFT shift is described as DOWNFIELD and results from deshielded protons that absorb a higher frequency of rf radiation

A RIGHT shift is described as UPFIELD and results from shielded protons that absorb a lower frequency of rf radiation− a lower frequency wave can penetrate further

Question 5

Integration

NMR Spectroscopy

Answer 5

1. Divide each individual integration value by the smallest integration value
2. This gives the ratio of protons relative to each other
3. Compare the ratio of protons to the molecular formula to determine the number of protons in the compound

Question 6

Multiplicity

NMR Spectroscopy

Answer 6

1. Identify all chemically equivalent protons
2. Identify number of immediate neighbors for each equivalent proton; either geminal or vicinal
3. Apply the n+1 rule determine splitting

Question 7

Ethyl splitting patterns

NMR Spectroscopy

Answer 7

2H quartet and 3H triplet

Question 8

Isopropyl splitting pattern

NMR Spectroscopy

Answer 8

1H septet and 6H doublet

Question 9

tert-Butyl splitting pattern

NMR Spectroscopy

Answer 9

9H singlet

Question 10

Alcohol splitting pattern

NMR Spectroscopy

Answer 10

Generally no splitting is observed across an alcohol oxygen atom

Chemical shift ~2-5 ppm