6.3.2 Spectroscopy
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NMR spectroscopy
involves interaction of material with low energy radiowave region of the electromagnetic spectrum uses the same technology as MRI to obtain diagnostic information about internal structure in body scanners
2 types of NMR
Carbon-23 NMR Proton NMR
in a carbon 13 spectra what is there one single peak for
each set of equivalent carbon atoms
in carbon 13 nmr when do you have different peaks `
when each carbon is bonded to a different group label them C a,b,c,d
in a H NMR spectrum what is there one signal for
each set of equivalent H atoms
what else is shown on the H peaks in proton NMR
the intensity of the peaks (integration values) proportional to the number of equivalent H atoms it represents
solvents
samples are dissolved in solvents with no H atoms so that the H NMR there will be no peaks from the solvent same solvent used in C 13 NMR however it is a known peak so can be ignored
examples of solvents used
CCl4 CDCl3
good solvent for non-polar organic molecules
CCl4 as it is a non-polar compound
good solvent for polar organic molecules
CDCl3 as it is a polar covalent compound
which calibration compound is used in H and C13 spectra
TMS tetramethylsilane
why is TMS used to calibrate the spectrum
signal is away from all of the others only gives one signal strong signal so a small amount is needed non-toxic inert low boiling point so can be easily removed
what is the scale called that the spectra are recorded on?
chemical shift how much the field has shifted from the field for TMS measured in ppm relative scale of how far the frequency of the proton signal has shifted from that for TMS
what does the chemical shift of a H NMR depend on
which other atoms or groups are near the H more electronegative give a greater shift
proton exchange using D2O
then a process of proton exchange happens with the H in any OH and NH bonds has the effect of removing the peaks from the H NMR spectra can help with the identification of OH and NH peaks on the spectra
if the carbons are in the same range what is it not possible to identify in C13 NMR
the exact carbon corresponding to each peak
in high resolution H NMR what can happen to each signal
can be split into further lines due to equivalent H’s on neighbouring C atoms
splitting of peak is equal to
the number of inequivalent H’s on neighbouring C atoms +1
singlet: appearance split number of peaks number of neighbouring inequivalent H atoms relative size
1 peak drawn straight up 1 peak 0 inequivalent H
doublet: appearance split number of peaks number of neighbouring inequivalent H atoms relative size
2 peaks straight up 2 split peaks 1 neighbouring inequivalent H 1:1
triplet: appearance split number of peaks number of neighbouring inequivalent H atoms relative size
3 peaks, 1st and 3rd same size and 2nd is taller 3 split peaks 2 neighbouring inequivalent H atoms 1:2:1
quartet: appearance split number of peaks number of neighbouring inequivalent H atoms relative size `
4 peaks, 1st and 4th the same size and are smaller, 2nd and 3rd are taller and the same size 4 split peaks 3 neighbouring inequivalent H atoms 1:3:3:1
quintet: appearance split number of peaks number of neighbouring inequivalent H atoms relative size
1st and last small and same size, 2nd and 4th taller and same size, 3rd is tallest 5 split peaks 4 neighbouring inequivalent H atoms 1:4:6:4:1
what do H bonded to N or O do on an NMR spectra
do not couple with other protons and appear as singlets