spectroscopy

Question 1

What are equivalent carbons? How do they show up in 13C NMR?

Answer 1

Carbons which are in the same environment.

There is one signal peak for each set of equivalent carbons.

Question 2

How do equivalent hydrogens differ from equivalent carbons?

Answer 2

The intensity (peak integration value) ∝ number of equivalent H’s it represents. Eg,

Question 3

How and why must samples be dissolved in 1H NMR? (with examples)

Answer 3

Dissolved in solvents without any 1H’s (eg, CCl4 and CDCl3) meaning no interference.

Question 4

How are both types of NMR calibrated

Answer 4

By adding a small amount of TMS (tetramethylsilane).

Question 5

Why is TMS used in calibrating both types of NMR?

Answer 5

1. Non-toxic.
2. Inert.
3. Low b.p. so easily removed afterwards. 4. Signal is far away from others.

Question 6

What does chemical shift / δ represent in NMR?

Answer 6

It is how far the frequency of signal is shifted from TMS measured in parts per million (ppm).

Question 7

How does electronegativity affect 1H NMR?

Answer 7

If a 1H is closer to more electronegative group, greater shift (further left).

Question 8

What is the issue with identifying -OH and -NH groups in 1H NMR? How is this solved?

Answer 8

● They’re very variable and don’t split.
1. Run two spectra of the molecule - one with D2O added. 2. If -OH, -NH present, it’ll swap proton as shown below:
Eg, CH3CH2OH + D2O → CH3CH2OD + HOD • This is called PROTON EXCHANGE.
This works since deuterium doesn’t absorb radio since even number of nucleons.

Question 9

What is spin-spin coupling in high-resolution NMR?

Answer 9

● Each signal can be split based on how many neighbouring NON-EQUIVALENT 1H’s (neighbouring means within 3 bonds).
● Yet, hydrogens bonded to nitrogen or oxygen don’t split or are themselves split.
● Split number of peaks = number of nonequivalent 1H’s within 3 bonds + 1