Chapter 29: Chromotography and NMR Spectroscopy

Question 1

how does NMR spectroscopy work?

Answer 1

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

Question 2

what is used as the standard for chemical shift values?

Answer 2

Tetramethylsilane (TMS)

(CH3)4Si

Question 3

what effects something’s chemical shift?

Answer 3

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

Question 4

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

Answer 4

number of carbon environments (number of peaks)

types of carbon environments present (chemical shift)

Question 5

equivalent carbon atoms

carbon atoms that are positioned symmetrically within a molecule

Answer 5

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

Question 6

non-equivalent carbon atoms

Answer 6

have different environments and therefore different chemical shifts

Question 7

what does analysis of a proton NMR spectrum give us?

Answer 7

• 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)

Question 8

splitting pattern rule

Answer 8

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

Question 9

issues with identifying O-H and N-H peaks

Answer 9

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

Question 10

proton exchange using D2O

Answer 10

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

Question 11

spectroscopy table headings

Answer 11

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

Question 12

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

Answer 12

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

Question 13

What is the stationary phase in TLC?

Answer 13

The adsorbent, usually silica.

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

Question 14

What is the mobile phase in TLC?

Answer 14

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

Question 15

How do you carry out TLC?

Answer 15

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

Question 16

What is gas chromatography used for?

Answer 16

Separating and identifying volatile organic compounds present in mixtures

Question 17

What is the stationary phase in gas chromatography?

Answer 17

A high boiling liquid adsorbed onto an inert solid support

Question 18

What is the mobile phase in gas chromatography?

Answer 18

An inert carrier gas e.g neon or helium

Question 19

How does gas chromatography separate the components?

Answer 19

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

Question 20

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

Answer 20

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

Question 21

How do you interpret a gas chromatography?

Answer 21

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

Question 22

How can you work out the concentration of a component?

Answer 22

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

Question 23

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

Answer 23

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

Question 24

What wave on the electromagnetic spectrum does NMR use?

Answer 24

Radio waves

Question 25

Medical use of NMR?

Answer 25

• Diagnostic information

- Magnetic resonance scanners

Question 26

what is the solvent for NMR

Answer 26

CDCl3

Question 27

How do you calculate a components retention factor?

Answer 27

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