analytical 2 p2 Flashcards
what does NMR stand for?
nuclear magnetic resonance
what are the basic principles of NMR?
you can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them
if radio waves of the right frequency are absorbed, the nuclei flips from parallel to applied magnetic field to anti-parallel
this energy change can be monitored and recorded
uses the resonance of nuclei with spin
how would you carry out NMR spec?
dissolve liquid sample is suitable solvent
put in tube with small amount of TMS and place in machine
the sample is spun to even out any imperfections in the magnetic field and the spectrometer is zeroed against the TMS
radiation with different radio frequencies but a constant magnetic field is applied to the sample and any absorptions are detected
use of NMR?
MRI scans
what kind of nuclei does NMR work with?
those with an uneven number of nucleons meaning they will spin e.g. 1H, 13C
what % of carbon atoms are 13C?
1% - but modern instruments are sensitive enough to detect this
what defines the resonant frequency of a 13C atom?
the chemical environment that it is in; the amount of electron shielding it has
what graph is produced by NMR spec?
energy absorbed against chemical shift
what is chemical shift? what is its symbol? what are its units?
the resonant frequency of the nuclei, compared to that of a 1H atom in TMS
𝛿
parts per million (ppm)
what is the range of chemical shift for 13CNMR?
0-200 ppm
what means 13C atoms show a different chemical shift value?
having different chemical environments (but equivalent atoms show the same peak)
what kind of environment leads to a greater chemical shift?
a C atom next to more electronegative atom has a greater chemical shift
summarise for 13C NMR
number of signals, chemical shift, area under peak, splitting
number of signals - one signal for each carbon environment
chemical shift - greater shift from atoms closer to electronegative atoms or C=C
area under peak - no meaning
splitting - no splitting
why is it easier to get a spectrum of 1H NMR than 13C NMR?
most H atoms are 1H - it is much more abundant than 13C
this means almost all H atoms have spin therefore show up
what is the range of chemical shift for 1H NMR?
0-10 ppm
what leads to a lower chemical shift value for H NMR?
1H with more electrons around them i.e. further from electronegative groups/atoms
on a low resolution spec, what peaks would you expect to see for H NMR?
one peak for each set of individual H atoms (each chemical environment shows 1 peak)
what does the area under the peak represent for H NMR?
the area under the peak is proportional to the number of H atoms represented by the peak
integration trace
a stepped line that makes it easier to measure the area under the curve (height of line - area under that peak)
what is TMS? what state at room temp?
tetramethylsilane
liquid
why is TMS used?
can be added to sample to calibrate the NMR equipment
it provides a peak at exactly 𝛿 = 0ppm
it is the reference point against which all 𝛿 are measured
what are advantages of using TMS?
inert, non-toxic, easy to remove from the sample as relatively volatile
when does splitting/spin-spin coupling occur?
neighbouring hydrogen atoms (3 or fewer bonds away or on the adjacent carbon) affect the magnetic field of 1H atoms and causes their peaks to split
what is the n+1 rule?
the peak will split into n+1 smaller peaks
n being the number of H atoms on the neighbouring carbon
what are the splitting pattern names?
singlet, doublet, triplet, quartet
why must solvents used for 1H NMR not contain any hydrogen atoms?
signals from the solvent would swamp signals from the sample, as there is much more solvent than sample
which solvents are used for H NMR?
deuterated solvents: CDCl3, D2O, C6D6
CCl4
summarise 1H NMR
number of signals, chemical shift, splitting, area under peak
number of signals - one main signal for each set of inequivalent H atoms (for each hydrogen environment)
chemical shift - larger 𝛿 for H atoms closer to electronegative atoms or C=C
splitting - number of smaller peaks = 1 + number of inequivalent hydrogen atoms 3 bonds away
area under peak - proportional to the number of atoms represented by that peak
why does the peak from O-H bonds disappear if D2O is used as a solvent?
O-D bond is formed in preference to O-H due to labile protons that move/swap from one molecule to another
what are the basic principles of all kinds of chromatography?
a family of separation techniques that depend on the principle that a mixture is separated if it is dissolved in a solvent and this mobile phase is passed over a solid (the stationary phase)
what is the mobile phase?
carries the soluble components of the mixture
what relationship between a sample and the mobile phase makes the sample move faster?
more soluble components/components with more affinity to the solvent move faster
what does the stationary phase do?
holds back components of the mixture that are attracted to it
what is the relationship between a sample and the stationary phase that make the sample move slower? what kind of bonding does this often involve?
more affinity for the stationary phase means that a component moves slower; often attracted by hydrogen bonding
how are substances separated by chromatography?
if suitable stationary/mobile phases are chosen, the balance between affinity for each is different for each component of the mixture
thus, they move at different rates and are separated over time
why will different substances show different Rf values?
they are bonded differently and have different polarities - more polar bonds mean longer retention time or smaller Rf value, since hydrogen bonding/dipoles are attracted more strongly to the stationary phase
what does TLC stand for?
Thin Layer Chromatography
what is the stationary phase in TLC?
plastic/glass/metal sheet or ‘plate’ coated in silica (SiO2) or alumina (Al2O3)
what are the advantages of TLC over paper chromatography?
runs faster, smaller amounts of mixture can be separated, TLC plates are more robust than paper
how can you observe colourless spots?
shine UV light on them
or
spray with a developing agent e.g. ninhydrin turns amino acid spots from colourless to purple (needs to be heated though)
how do you calculate the Rf value?
measure the distance from the initial line to the solvent front and the distance from the initial line to the spot
Rf = distance moved by spot / distance moved by solvent front
what does Rf value stand for?
retention factor; a measure of the rate of movement of a component through the chromatography apparatus; a ratio between the rate of movement of the solvent and that component
how could you confirm the identity of a substance from its Rf value?
compare your Rf value to accepted values Rf for that substance in the same solvent and set-up
if they match, identity is confirmed
what is column chromatography?
column packed with silica, alumina or resin has solvent run through it downwards
what is the stationary phase in column chromatography?
silica, alumina or resin packed into a column
what is the mobile phase in column chromatography? what is it also known as?
solvent added at the top and runs down the column; called eluent
advantages of column chromatography
more than one eluent can be used which leads to better separation
fairly large amounts can be separated and collected after separation
what is the stationary phase in gas-liquid chromatography?
powder, coated with oil
packed into a long, thin, capillary tube (100m long, 0.5mm diameter)
coiled and placed in an oven, the temperature of which can be varied
what is the mobile phase in gas-liquid chromatography?
carrier gas, inert e.g. N2 or He
what do you measure in gas-liquid chromatography?
retention time; different components of the mixture take different amounts of time to move through
what are the advantages of GLC?
very sensitive; GC can detect minute traces of substances in foodstuffs, and link oil pollution on beaches to the specific tanker the oil came from
what are GLC’s uses?
test athletes’ and horses’ blood and urine for drugs
how can you use GC or GCMS to identify substances?
match gas chromatograph to that of a known substance under the same conditions; retention time should exactly match
substance’s identity can be confirmed by mass spectrometry, NMR or infrared spec
how does GCMS work?
gas chromatography is run, retention time recorded, then mixture is run through a mass spec
fragmentation pattern/molecular ion peak confirms identity
will an alcohol or an aldehyde have a shorter retention time by column chromatography?
aldehyde has shorted retention time since it has less polar bond than an alcohol
therefore adsorbs less strongly to the stationary phase, so moves down the column at a quicker rate
force of attraction between stationary phase and aldehyde is less
