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Chemistry Year 2 > NMR & Chromatography > Flashcards

Flashcards in NMR & Chromatography Deck (28)
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1
Q

How are the components of a mixture detected at then end of GLC by flame ionisation?

A

The outlet gas is mixed with hydrogen and air and burned to form ions. This allows a current to be transmitted, which is then converted to a signal on a chart recorder. The relative sizes of the peaks are related to how much of a compound is present in a mixture.

2
Q

Why is the coupling effect not observed in C-13 NMR spectroscopy?

A

There is a low natural abundance of the C-13 isotope, meaning the probability of two C-13 atoms being bonded to one another is very low. Therefore C-13 spectra consist only of a series of singlet resonances

3
Q

Chemical environment

A

Neighbouring atoms surrounding a particular atom producing differing NMR absorptions

4
Q

Why are samples for NMR spectroscopy investigated in dilute solution?

A

This separates the sample molecules from each other, preventing them from interacting and causing very complex absorptions

5
Q

Chemical shift

A

The position, relative to the proton absorption of tetramethylsilane, where a nucleus absorbs in an NMR spectrum

6
Q

Why is tetramethylsilane (TMS) used as a reference point for NMR spectroscopy?

A
  • Silicon has a very low electronegativity, meaning that the hydrogen nuclei in TMS resonate at a field strength well above that of any H nuclei in common organic molecules
  • it gives one strong, sharp, easily detected absorption because it is caused by the combined effects of 12 equivalent H atoms
  • TMS is non-toxic and cheap
  • it doesn’t react with the sample
  • TMS is easily separated from the sample molecule due to its low boiling point
7
Q

In TLC, each component has an Rf value, what is this?

A

The distance moved by that component ÷ the distance moved by the solvent. This can. This can be compared to known data to figure out what was in a mixture

8
Q

N + 1 rule

A

The NMR absorption of a proton that has n equivalent neighbouring protons is split into n + 1 peaks

9
Q

Low resolution proton NMR

A

shows a peak for each chemical environment of a H in the molecule. The strengths of the absorptions are proportional to the number of H atoms in that environment, which is proportional to the area under each peak

10
Q

Gas-liquid chromatography

A

The stationary phase is usually diatomaceous earth coated with a non-volatile liquid. This is packed into a long coiled tube and put into a heated he mobile phases is an inert has, e.g. dry nitrogen or helium. The sample must either be a gas or a volatile liquid at the temperature of the oven. Sample is injected into the column through self-sealing disc and the vapour formed is carried through the stationary phase by the mobile phase. The temperature of the stationary phase can be varied to increase separation of the mixture in a sample. The components of the mixture leaving the column can be detected by thermal conductivity or flame ionisation.

11
Q

Thin-layer chromatography (TLC)

A

The stationary phase is later if silva on glass, aluminium or plastic. A small spot of a mixture is added near the bottom of the TLC plate, which is then dipped into a solvent (mobile phase). As the solvent moves up the plate, the components of a mixture interact with the stationary phase differently and move up the plate at varying rates. TLC plate removed when the solvent nearly reaches the top of the plate. Compounds visualised under UV radiation or by a chemical staining technique.

12
Q

Mobile phase

A

The fluid in chromatography that moves through the solid medium and carries a sample

13
Q

Coupled

A

Two atoms in a molecule are couple when the magnetic field experienced by one is altered by the effect of the magnetic field created by the other

14
Q

What happens if a molecule in a strong magnetic field is irradiated with radio frequency electromagnetic waves?

A

The nuclei of some of the atoms can absorb the radio waves and move from the low energy to the high energy state. He’s absorptions, called resonances, occur at different energies depending upon the surrounding environment of the atom

15
Q

What solvents are used when preparing samples for NMR spectroscopy? Why?

A

Tertrachloromethane (CCl4) or deuterated trichloromethane (CDCl3) because these are very powerful solvents for organic compounds and do not contain hydrogen atoms. This means they do not resonate and therefore don’t interfere with the H NMR spectrum of the sample

16
Q

Chromatography

A

A technique that enables the separation of mixtures due to the principle that components in a mixture when dissolved in a fluid will flow through another material (stationary phase) at varying rates.

17
Q

What does separation in chromatography depend on?

A

How the components interact with the stationary phase (their retention) and how soluble they are in the fluid

18
Q

The nucleus been of an atom can align with an external magnetic field or against it, which has a higher energy?

A

When the atom is aligned against the magnetic field

19
Q

Retention time

A

The time taken to pass through a GLC column; depends on the nature of and attraction between the solute and the stationary and mobile phases and the volatility of the solute.

20
Q

Similarities between column chromatography and TLC

A

They both use a solid stationary phase and a liquid mobile phase

21
Q

Magnetic field

A

The region surrounding a magnetic north or south pole in which attractive and repulsive forces are felt

22
Q

Examples of different types of chromatography

A
  • paper chromatography
  • thin-layer chromatography (TLC)
  • column chromatography
  • gas-liquid chromatography (GLC)
  • high-performance liquid chromatography
23
Q

Coupling effect

A

The interaction between the nuclear spins of neighbouring atoms in a molecule

24
Q

Nuclear spin

A

A property of the nuclei with odd atomic numbers or mass numbers, behaving as if they were bar magnets

25
Q

Resonance

A

The natural vibrational frequency of a bond when subjected to IR radiation or the protons when subjected to a radio-frequency radiation

26
Q

Integrated spectrum

A

A series of leaks that represent the areas under NMR peaks. These are equivalent to the number of protons in various groups

27
Q

High resolution proton NMR spectra

A

Each peak is split into groups of signals, due to the influence H atoms bonded to neighbouring atoms (n + 1 rule)

28
Q

Column chromatography

A

The stationary phase is an inert solid (silica gel or alumina), this is placed in a column with the mobile phase which is a liquid solvent phase. The sample mixture is dissolved in the solvent and introduced at the top of the column. The sample flows down the column via gravity l, the component with the greatest attraction to the stationary phase takes the longest time to flow through the column.