A-level Chemistry: 3.3.15 Nuclear Magnetic Resonance Spectroscopy
This flashcard set introduces the fundamentals of NMR spectroscopy, covering the main types (^1H and ^13C NMR), the concept of nuclear spin, and why only certain nuclei (like ^1H and ^13C) exhibit it. It explains how isotopes influence spin behavior and the role of magnetic fields in NMR analysis.
Name 2 types of NMR
1H (or proton) NMR
13C NMR
Key Terms
Name 2 types of NMR
1H (or proton) NMR
13C NMR
What type of atomic nuclei spin?
Atomic nuclei with an odd number of nucleons (protons and neutrons)
What does nuclear spin cause an atom to have?
Why do hydrogen nuclei have spin?
Hydrogen nuclei are single protons
Why doesn’t carbon have nuclear spin?
C usually has 6 protons and 6 neutrons
1% of carbon atoms are isotope 13C (___ protons and ____ neutrons) so does have ____
But about 1% of carbon atoms are isotope 13C (6 protons and 7 neutrons) so does have spin
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| Term | Definition |
|---|---|
Name 2 types of NMR |
|
What type of atomic nuclei spin? | Atomic nuclei with an odd number of nucleons (protons and neutrons) |
What does nuclear spin cause an atom to have? | |
Why do hydrogen nuclei have spin? | Hydrogen nuclei are single protons |
Why doesn’t carbon have nuclear spin? | C usually has 6 protons and 6 neutrons |
1% of carbon atoms are isotope 13C (___ protons and ____ neutrons) so does have ____ | But about 1% of carbon atoms are isotope 13C (6 protons and 7 neutrons) so does have spin |
What partly shields nuclei from the effects of external magnetic fields? |
|
State what will happen to the amount of electron shielding around its nucleus, if carbon atoms bond to a more electronegative atom (e.g. oxygen)? | Will decrease |
Why do nuclei in different environments absorb different amounts of energy at different frequencies? | ∵ in different environments, nuclei will feel different magnetic fields |
What does NMR spectroscopy essentially look for? | Differences in energy absorption between environments |
For an atom to be in same environment, what must happen? | 2 atoms must be joined to exactly the same things |
What substance is used as a standard in NMR spectroscopy? | Tetramethylsilane (TMS), Si(CH3)4 |
What is chemical shift (δ)? | The differences (in energy absorbed) NMR spectroscopy measures relative to the standard substance (TMS) |
Tetramethylsilane has 12 H atoms in ____ ________ | identical environments |
Name 5 reasons why TMS is used as a standard |
|
What is chemical shift measured in? | parts per million (ppm) relative to TMS |
State the chemical shift of the single peak produced by TMS | 0 |
| ∵ TMS is added to test compound for calibration purposes |
Draw tetramethylsilane (TMS) | |
What does the number of peaks in 13C NMR tell you? | no. of different carbon environments in molecule |
State the number of carbon environments in cyclohexane-1,3-diol | 3 |
What does 1H NMR tell you? | Tells you how many environments and how many hydrogens are in each (1H NMR is about how hydrogen nuclei react to magnetic field) |
What is each peak in 1H NMR spectrum due to? | one or more hydrogen nuclei (protons) in a particular environment |
1H NMR What does the numbers above the peaks on a spectrum tell you? |
|
1H NMR: Splitting Patterns What is meant by spin-spin coupling? | When peaks are split into smaller peaks |
1H NMR What is the n+1 rule? | Peaks always split into number of hydrogens on the neighbouring carbon, plus one |
1H NMR What are used instead of peak ratios to show the area more clearly? | Integration Traces |
Explain how integration traces work | The increases in height are proportional to areas under each peak |
Describe how you can calculate a integration ratio from integration traces | Can use ruler to measure height of each vertical bit of trace and then use heights to work out ratio of peak areas |
State the integration ratio for this spectrum and what it means |
|
1H NMR What solvents are used to dissolve samples? | Deuterated solvents |
What are the hydrogen atoms in deuterated solvents replaced by? | D or 2H |
What is deuterium? | Isotope of H that has 2 nucleons (proton and neutron) |
Why does deuterium not have a spin (and doesn't create a magnetic field)? | ∵ deuterium has even no. of nucleons |
1H NMR Name a solvent that is used to dissolve samples | CCl4 |
Explain why CCl4 can be used as a solvent to dissolve a sample | ∵ doesn't contain any 1H atoms |
Two isomers of C6H12O2 are both esters. Each isomer has only two peaks in their 1H NMR spectrum. The integration ratio for both esters is 3:1. Draw the two esters. (2) | |
Another isomer of C6H12O2 is a carboxylic acid with a chiral centre. This isomer has five peaks in its 13C NMR spectrum. Draw the structure of this isomer. | |
What solvent should you use when obtaininng the 1H NMR spectrum of an amino acid? | D2O |