[29.3-4] carbon-13 and proton NMR spectroscopy
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what information can be found from a carbon-13 spectrum?
• number of different carbon environments - from the number of peaks • types of carbon environments - from the chemical shifts
what is the chemical shift referenced against?
TMS at δ = 0 ppm
how can you find the chemical environment of a carbon atom?
• carbon atoms that are bonded to different atoms or groups of atoms have different environments that will absorb at different chemical shifts • if two carbon atoms are positioned symmetrically within a molecule, they are equivalent and have the same chemical environment. they will absorb radiation at the same chemical shift and contribute to the same peak
what information can be found from a proton NMR spectrum?
• number of different proton environments - from the number of peaks • types of proton environments present - from the chemical shift • relative numbers of each type of proton - from integration traces or ratio numbers of the relative peak areas • number of non-equivalent protons adjacent to a given proton - from the spin-spin splitting pattern
how can integration (peak intensity) be found?
• area under a signal is proportional to the number of hydrogen atoms present • an integration device scans the area under the peaks • lines on the spectrum show the relative abundance of each hydrogen type • computers can now display the information as numerical data
what can you work out by measuring the integration lines?
the simple ratio between the various types of hydrogen (not actual number of H’s)
describe the difference between low and high resolution NMR
• low resolution NMR gives 1 peark for each environmentally different group of protons • high resolution gives more complex signals eg. doublets, triplets
what does the signal produced in proton NMR indicate?
• the number of protons on adjacent carbon atoms • number of peaks = number of chemically different H’s on adjacent atoms + 1
describe the spin-spin splitting patterns when there are no H’s on adjacent atoms
• splitting pattern = singlet • relative peak areas within splitting = 1
describe the spin-spin splitting patterns when there is an adjacent CH
• splitting pattern = doublet • relative peak areas within splitting = 1:1
describe the spin-spin splitting patterns when there is an adjacent CH₂
• splitting pattern = triplet • relative peak areas within splitting = 1:2:1
describe the spin-spin splitting patterns when there is an adjacent CH₃
• splitting pattern = quartet • relative peak areas within splitting = 1:3:3:1
when is splitting seen?
if adjacent H’s are no more than 3 bonds apart
how do OH and NH signals appear in proton NMR?
• -OH and -NH signals are singlets • no splitting • peaks can appear over a wide range of chemical shifts and may be broad
why do OH and NH signals appear this way?
• H on OH or NH rapidly exchanges with protons on other molecules (eg. water, acids) • not attached to any particular oxygen long enough to register a splitting signal
what is the technique proton exchange used for?
identifying -OH and -NH protons
describe the technique of proton exchange
• run proton NMR as normal • add small volume of deuterium oxide, D₂O • shake mixture • run second spectrum
what happens after a D₂O shake?
• any signal due to an exchangeable proton disappears • H in group changes place with a deuterium atom from D₂O • deuterium doesn’t exbhibit NMR under the conditions used for proton NMR so the signal is removed to another part of the spectrum
how do you interpret NMR spectra?
• analyse types of proton and how many of each type (number of peaks, relative peak areas) • analyse splitting patterns to find information about adjacent protons • analyse chemical shifts • combine information to suggest a structure