NMR & Chromatography Flashcards
How are the components of a mixture detected at then end of GLC by flame ionisation?
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.
Why is the coupling effect not observed in C-13 NMR spectroscopy?
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
Chemical environment
Neighbouring atoms surrounding a particular atom producing differing NMR absorptions
Why are samples for NMR spectroscopy investigated in dilute solution?
This separates the sample molecules from each other, preventing them from interacting and causing very complex absorptions
Chemical shift
The position, relative to the proton absorption of tetramethylsilane, where a nucleus absorbs in an NMR spectrum
Why is tetramethylsilane (TMS) used as a reference point for NMR spectroscopy?
- 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
In TLC, each component has an Rf value, what is this?
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
N + 1 rule
The NMR absorption of a proton that has n equivalent neighbouring protons is split into n + 1 peaks
Low resolution proton NMR
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
Gas-liquid chromatography
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.
Thin-layer chromatography (TLC)
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.
Mobile phase
The fluid in chromatography that moves through the solid medium and carries a sample
Coupled
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
What happens if a molecule in a strong magnetic field is irradiated with radio frequency electromagnetic waves?
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
What solvents are used when preparing samples for NMR spectroscopy? Why?
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
