nmr spectroscopy Flashcards
nmr stands for
nuclear magnetic resonance
basic principles of nmr
find structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them
if radiowaves 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 resonance of nuclei with spin
basic principles of nmr
find structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them
if radiowaves 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 resonance of nuclei with spin
how would you carry out nmr spectroscopy
dissolve the liquid in suitable solvent, put in a tube along with a small amount of TMS and put the tube into a NMR machine
sample 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 (due to resonance) are detected
give one 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 and 13C
what percentage of carbon atoms 13C
1% - but modern instruments are sensitive enough to detect this
what percentage of carbon atoms 13C
1% - but modern instruments are sensitive enough to detect this
what defines the resonant frequency of a 13C atom
chemical environment that it is in; the amount of electron shielding it has
what defines the resonant frequency of a 13C atom
chemical environment that it is in; the amount of electron shielding it has
what graph is produced by nmr spectroscopy
energy absorbed against chemical shift
what is chemical shift? what are its units?
resonant frequency of nuclei, compared to that of a 1H atom in TMS
parts per million
what is the range of chemical shift 13C nmr
0-200ppm
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
what do the number of signals mean for 13C nmr
one signal for each carbon environment
what do chemical shift mean for 13C nmr
greater chemical shift from atoms closer to electronegative atoms or C=C
what does the area under each peak mean for 13C nmr
no meaning
is there any splitting in 13C nmr
there is 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 meaning almost all H atoms have spin so 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 spectrum, what peaks would you expect to see for H nmr
one peak for each set of inequivalent H atoms (each chemical environment shows 1 peak)
what does the area under the peak represent for H nmr
area under the peak is proportional to the number of 1H atoms represented by the peak
what does the area under the peak represent for H nmr
area under the peak is proportional to the number of 1H atoms represented by the peak
what is the 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 the 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 (name and structure)
tetramethylsilane
CH3
H3C–Si–CH3
CH3
what state is tms at room temperature
liquid
why is tms used
can be added to sample to calibrate the nmr equipment
it provides a peak at exactly chemical shift = 0 ppm
reference point against which all chemical shift are measured
what are other advantages of using TMS
inert
non-toxic
easy to remove from the sample
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
if there are n inequivalent 1H atoms on the neighbouring carbon then the peak will split into (n+1) peaks
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 in 1H nmr
- deuterated solvents such as CDCl3 , D2O and C6D6
- CCl4 (tetrachloromethane)
what does the number of signals indicate in 1H nmr
one main signal for each set of inequivalent 1H atoms (for each hydrogen environment)
what does the chemical shift indicate in 1H nmr
larger chemical shift for 1H atoms closer to electronegative atoms or C=C
what does splitting mean for 1H nmr
number of smaller peaks = 1 + number of inequivalent hydrogen atoms 3 bonds away
what does the area under peak mean in 1H nmr
proportional to the number of atoms represented by that peak
