NMR and chromatography Flashcards
Why does NMR only work with nuclei with odd mass number?
4
Odd vs even?
Magnetic field?
- Nuclei with odd mass numbers (1H, C13) have spin
- Nuclei with even mass numbers do not have magnetic properties
- Nuclear spin induces weak magnetic fields which can be influenced by an external magnetic field
- Hydrogens nucleus can align w/ (stable) or against (unstable) an applied field
How is it possible to make H flip from more to less stable alignment?
2
- Supplying an exact amount of energy
- the amount of energy depends on the strength of the magnetic field
What is the resonance condition?
Energy peak when (H) flipped form more to less stable alignment on a graph
What solvents are used for NMR?
2
CCl4
CDCl3 (sometimes gives a clearer split of peaks so used for complex spectra
What is TMS and why is it the standard?
(1H nuclei of TMS given chemical shift value of 0)
7 Environment? Resonance? Cost? Reactivity?
TMS: tetramthylsilane SiC4H12
- all H and C are in the same environment so one intense peak
- H nuclei are highly shielded due to Si’s low electronegativity so it has high resonance and strong sharp easily detected absoptions
- non-toxic
- inert
- cheap
- easy to remove from sample (low b.p)
What are spectra?
1
Resonance conditions
What are spectra affected by?
3
- Environment of H: causes different peak shapes and chemical shifts
- T4 affected by no. H in molecule
- T4 affected by electronegativity of surrounding atoms
What is the position of the peak influenced by?
1
Electronegativity of surrounding atoms
What is the size of the peak influenced by?
1
Relative to number of H’s in that environment
What does the amount of absorptions in NMR spectra represent?
1
Amount of non-equivalent protons
What does the intensity of absorptions in NMR spectra represent?
1
Quantity of protons with each peak
What does the position of absorptions in NMR spectra represent?
1
Environment of each proton (electronegativity of surrounding atoms)
What does the splitting of absorptions in NMR spectra represent?
1
Amount of neighbouring inequivalent protons on C (or other atom) directly adjacent
What is the split with 0 inequivalent H’s?
Singlet
What is the split with 1 inequivalent H’s?
Doublet
What is the split with 2 inequivalent H’s?
Triplet
What is the split with 3 inequivalent H’s?
Quartet
What is the split with 4+ inequivalent H’s?
Multiplet
What stationary phase and mobile phase is used in column chromatography?
Stationary: powder
Mobile: solvent
What stationary phase and mobile phase is used in paper chromatography?
Stationary: absorbent paper
Mobile: solvent
What stationary phase and mobile phase is used in TLC (thin layer) chromatography?
Stationary: powder on glass/plastic plate
Mobile: solvent
What stationary phase and mobile phase is used in gas chromatography?
Stationary: powder (sometimes) coated with liquid
Mobile: inert gas
What is chromatography?
1
Process of separating substances by their relative affinity to a stationary and mobile phase
How fast do substances with a stronger affinity for the mobile phase move?
More quickly
How fast do substances with a stronger affinity for the stationary phase move?
More slowly
What (type of) powders are used for stationary phase?
SiO2 (silica) (or Al2O3 (alumina))
-Polar powders
What are some examples of solvents used as mobile phases?
Non-polar: alkanes
Polar: water, alcohols
What type of solvent is used with a polar stationary phase and why?
3/4
Non-polar
For separation
-Polar molecules will move slowly as higher affinity for the polar stationary phase
-Non-polar molecules will move quickly as higher affinity for the non-polar mobile phase
What is the Rf value?
Calculation?
-always less than 1
Distance moved by sample / distance moved by solvent
How do you view results of colourless samples?
UV light
What is column chromatography?
Large scale TLC
How does column chromatography work?
3
- Apply solvent and pass it through column
- Sample separates into coloured bands
- If keep passing through solvent the bottom band will reach the bottom of the column and can be collected for further testing
How does gas chromatography work?
4
- Long coiled tube packed with powder
- Inert gas passes through tube (H2, N2, He)
- Retention time: time to travel through tube
- Samples are often analysed by mass spec as they leave
What is the likely group with triplet:quartet peaks at a 3:2 ratio?
Ethyl group
What is the group with singlet with intensity of 3?
Methyl group
What is the group with a singlet at intensity of 6?
Two methyl groups on same carbon with two other non-H molecules bonded
What is the group with triplet:triplet at 2:2 ratio of relative intensity?
Middle of carbon chain
CH2-CH2
What is group with doublet:quartet at 3:1 ratio?
CH3-CHR2
What is group with singlet at 9 intensity?
3 methyl groups on one carbon
What is group with singlet with intention of 1?
Likely alcohol group
Other wise CHR3
