Chapter 29: Chromotography and NMR Spectroscopy Flashcards

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1
Q

how does NMR spectroscopy work?

A

Nuclear Magnetic Resonance (NMR) spectroscopy uses a combination of strong magnetic field and radio frequency radiation. The nuclei of some atoms absorb and release the radiation repeatedly in a process called nuclear magnetic resonance

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2
Q

what is used as the standard for chemical shift values?

A

Tetramethylsilane (TMS)

(CH3)4Si

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3
Q

what effects something’s chemical shift?

A

chemical shift depends on the chemical environment of atoms and is greatly influenced by the presence of nearby electronegative atoms or pi-bonds

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4
Q

what does the analysis of a C-13 NMR give us?

A

number of carbon environments (number of peaks)

types of carbon environments present (chemical shift)

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5
Q

equivalent carbon atoms

carbon atoms that are positioned symmetrically within a molecule

A

have the same chemical environment and therefore the same chemical shift (contribute to the same peak)

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6
Q

non-equivalent carbon atoms

A

have different environments and therefore different chemical shifts

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7
Q

what does analysis of a proton NMR spectrum give us?

A
  • numer of proton environments (number of peaks)
  • types of proton environment (chemical shift)
  • relative numbers of each type of molecule (relative peak areas)
  • number of adjacent non-equivalent protons (spin-spin splitting pattern)
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8
Q

splitting pattern rule

A

n + 1
i.e. for n protons (H+) on an adjacent carbon atom, the peak will be split into n + 1

-OH and -NH are always singlets

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9
Q

issues with identifying O-H and N-H peaks

A
the NMR peaks are:
often broad (from hydrogen bonding) and do not have any splitting
of variable chemical shift
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10
Q

proton exchange using D2O

A

this is for identifying O-H and N-H peaks

  1. a H+ NMR spectrum is ran as normal
  2. a few drops of D2O are added, the mixture is shaken, and a second spectrum is run
  3. deuterium (2H) exchanges with any OH or NH protons (1H) , removing their peaks from the second NMR so that they can be identified
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11
Q

spectroscopy table headings

A
peak number
chemical shift
environment
splitting 
no. of adjacent hydrogens
relative peak areas
no. of atoms in environment 
conclusion
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12
Q

What is the difference between a mobile phase and a stationary phase?

A

The mobile phase does move and is normally a liquid or a gas whereas, the stationary face does not move and is normally a solid or liquid supported on a solid

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13
Q

What is the stationary phase in TLC?

A

The adsorbent, usually silica.

Adsorption is the process by which the silica holds different substances in the mixture to its surface

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14
Q

What is the mobile phase in TLC?

A

The separation of the substance achieved by the relative adsorptions of substances with the stationary phase.

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15
Q

How do you carry out TLC?

A

1) Pencil line 1cm from bottom - base line
2) Capillary tube used to spot solution
3) Prepare a tank and pour some solvent up to 0.5cm
4) Place TLC plate in beaker and make sure it does not cover spot
5) Allow solvent to rise up the plate until it is about 1cm below the top of the plate, remove plate and immediately mark point with pencil

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16
Q

What is gas chromatography used for?

A

Separating and identifying volatile organic compounds present in mixtures

17
Q

What is the stationary phase in gas chromatography?

A

A high boiling liquid adsorbed onto an inert solid support

18
Q

What is the mobile phase in gas chromatography?

A

An inert carrier gas e.g neon or helium

19
Q

How does gas chromatography separate the components?

A

Mobile carrier gas carries the components through the capillary column which contains the liquid stationary phase.

It is the relative solubility of the compounds with the stationary phase. More soluble, the slower it moves through the column

20
Q

How do we know which components are less soluble in the stationary phase?

A

The compound retained for the shortest time has the lower retention time and is detected first.

21
Q

How do you interpret a gas chromatography?

A

1) Retention times can be used to identify the components present in the sample by comparing these to known retention times
2) Area under each peak can be used to determine concentration

22
Q

How can you work out the concentration of a component?

A

1) Plot a calibration curve of concentration against relative peak area with known values
2) Run the sample through the gas chromatograph and compare the compounds peak area with the calibration curve

23
Q

why do we use deuterated solvents to dissolve substances for NM?

A

Deuterium is the 2H isotope of hydrogen, and therefore D2O (heavy water) doesn’t produce either H+ or C-13 peaks

24
Q

How do you calculate a components retention factor?

A

Distance moved by the component / Distance moved by the solvent front

24
Q

What wave on the electromagnetic spectrum does NMR use?

A

Radio waves

25
Q

Medical use of NMR?

A
  • Diagnostic information

- Magnetic resonance scanners

26
Q

what is the solvent for NMR

A

CDCl3