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chemistry > A - NMRS > Flashcards

A - NMRS Flashcards

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1
Q

What does NMR spectroscopy help to determine?

A

The structure of a molecule.

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2
Q

What are the two types of NMR spectroscopy I need to know? What do each give information about?

A
  1. 13C NMR - gives information about how many different carbon environments a molecule has. There will be one peak for each carbon environment.
  2. 1H NMR - gives information about how the hydrogen atoms in a molecule are arranged.
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3
Q

What does NMR stand for?

A

Nuclear magnetic resonance

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4
Q

What are nucleons?

A

Protons and neutrons in a nucleus.

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5
Q

What property does any atomic nucleus with an odd number of nucleons have?

A

A nuclear spin.

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6
Q

What does an atomic nucleus with a nuclear spin have?

A

A weak magnetic field.

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7
Q

Briefly explain what NMR spectroscopy looks into.

A

How the tiny magnetic field of an atomic nucleus with an odd number of nucleons reacts when you put it in a much larger external magnetic field.

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8
Q

Why do atoms’ magnetic fields usually cancel out?

A

Because the nuclei are spinning in random directions.

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9
Q

What happens when a strong external magnetic field is applied to nuclei with different magnetic fields?

A

The nuclei will all align either with the field or loosed to it.

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10
Q

Comment on the difference in energy levels of nuclei aligned with the external field than the opposed nuclei.

A

The nuclei aligned with the external field are at a slightly lower energy level than the opposed nuclei.

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11
Q

How do radio waves affect nuclei which have an applied external magnetic field?

A

Radio waves of the right frequency can give the nuclei that are aligned with the external magnetic field enough energy to flip up to the higher energy level.

The nuclei opposed to the external field can emit radio waves and down to the lower energy level.

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12
Q

Why is there an overall absorption of energy when an external magnetic field is initially applied to atoms?

A

Because to start with, there are more nuclei aligned with the external field so there is an overall absorption of energy.

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13
Q

What exactly does NMR spectroscopy measure?

A

The absorption of energy when a magnetic field is applied to nuclei.

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14
Q

How can a nucleus be shielded from the effects of external magnetic fields?

A
  1. Partly by its surrounding electrons.
  2. Will also be affected by any other atoms and groups of atoms that are around the nucleus (for example, if a carbon atom bonds to a more electronegative atom like oxygen, the amount of electron shielding around the nucleus will decrease).
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15
Q

What does an atom’s environment depend on?

A

All the groups that it’s connected to, going right along the molecule. Not just the atoms it’s actually bonded to.

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16
Q

How could two atoms be in the same environment?

A

They must be joined to exactly the same atoms.

17
Q

How does absorption of energy differ with nuclei in different environments?

A

Nuclei in different environments will absorb different amounts of energy at different frequencies.

18
Q

What is the chemical shift?

A

Nuclei in different environments absorb energy of different frequencies. NMR spectroscopy measures these differences relative to a standard substance- the difference is called the chemical shift.

19
Q

What is the standard substance used in NMR spectroscopy? Give the formula.

A

Tetramethylsilane (TMS)

Si(CH3)4

20
Q

Describe and explain why TMS is used as the standard substance in NMR spectroscopy.

A

It has 12 hydrogen atoms all in identical environments so produces a single absorption peak, well away from most other absorption peaks.

It’s also inert (so won’t react with the sample), non-toxic, and volatile (so it’s easy to remove from the sample).

21
Q

What is chemical shift measured in? What is the chemical shift value for TMS?

A

In parts per million (ppm) relative to TMS. Single peak produced by TMS is given a chemical shift value of 0.

22
Q

Why is TMS added to the test compound?

A

For calibration purposes.

23
Q

How can you look up chemical shift values?

A

In a data table.

24
Q

Which gives simpler spectra out of 13C NMR and 1H NMR?

A

13C NMR.

25
Q

What does chemical shift depend on?

A

The molecular environment.

26
Q

What is 1H NMR also known as and why?

A

Proton NMR because the nucleus of a hydrogen atom is a single proton.

27
Q

What is each peak on a 1H NMR spectrum due to?

A

One or more hydrogen nuclei in a particular environment.

28
Q

What do the numbers above the peaks on a 1H spectrum tell you?

A

The ratio of the areas under the peaks. This relative area under each peak tells you the relative number of hydrogen atoms in each environment.

29
Q

Why do different tables of 1H NMR chemical shifts have different values?

A

They depends on the solvent, temperature and concentration used.

30
Q

What is the n + 1 rule?

A

Peaks on 1H spectrum may be split into smaller peaks. The peaks always split into the number of hydrogens on the neighbouring carbon atom, plus one. So if there’s two hydrogens on the neighbouring carbon, the peak will be split into three.

31
Q

What are the steps to interpreting a proton NMR spectrum?

A
  1. Look at the chemical shift of the peaks
  2. Look at the ratio of the areas under the peaks
  3. Look at the splitting patterns
32
Q

What are integration traces and how and why are they used?

A

A line on the spectrum that indicates the relative numbers of 1H atoms in different environments.

They’re used as sometimes it’s not easy to see the ratio of the areas under the peaks when the peaks are split. The increases in height of the integration trace are proportional to the areas under each peak.

You can use a ruler to measure the height of each vertical bit of the trace and then use the heights to work out the ratio of the peak areas.

33
Q

If a sample has to be dissolved in proton NMR, what solvents can be used and why?

A

A solvent is needed that doesn’t contain any 1H atoms because these would show up on the spectrum.

Deuterated solvents are often used (their H atoms have been replaced by deuterium (D or 2H). Because deuterium had an even number of nucleons, it doesn’t have a spin and so doesn’t create a magnetic field.

CCl4 can also be used as a solvent as it doesn’t contain any 1H atoms either.

34
Q

What is deuterium?

A

An isotope of hydrogen that has two nucleons.

chemistry (34 decks)

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