Proton NMR • Carbon-13 NMR and proton NMR both depend on the ability of an odd nucleon to spin and also flip in an applied magnetic field. • The energy to bring a proton to resonance is in a different part of the spectrum from that required for carbon-13 NMR but the same machine can be used after recalibration. • As before different chemical environments affect the fields required to flip the proton and these energy differences can be used diagnostically.
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Proton NMR Carbon-13 NMR and proton NMR both depend on the ability of an odd nucleon to spin and also flip in an applied magnetic field. The energy to.
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Transcript
Proton NMR
• Carbon-13 NMR and proton NMR both depend on the ability of an odd nucleon to spin and also flip in an applied magnetic field.
• The energy to bring a proton to resonance is in a different part of the spectrum from that required for carbon-13 NMR but the same machine can be used after recalibration.
• As before different chemical environments affect the fields required to flip the proton and these energy differences can be used diagnostically.
Differences from C-13 NMR• In proton NMR the area under a peak is
proportional to the number of protons in that chemical environment. This is not true of C-13 NMR peaks.
• The machine calibrates this information as an INTEGRATION TRACE – a step superimposed on the spectrum.
• By measuring the height of each step on the trace the ratio of protons in each chemical environment can be worked out.
• Analyse a proton NMR spectrum to make predictions about the different types of proton present, the relative numbers of each type and possible structures for the molecule.
• Predict the chemical shifts of the protons in a given molecule.
• Analyse a proton NMR spectrum to make predictions about the number of non-equivalent protons and possible structures for the molecule.
• Predict the splitting patterns of the protons in a given molecule.
Low resolution NMR spectrum ethanol
Spin – spin splitting• The previous slide showed a simple, low
resolution NMR spectrum of ethanol with peak identification and integration interpretation given.
• If the analysis is carried out in a stronger magnetic field the resulting ‘high resolution’ spectrum gives far more detail.
• Two of the peaks are split.
• The CH3 peak is split into a triplet and the CH2 peak is split into a quartet.
• This is called ‘spin-spin splitting’.
High Resolution NMR Spectrum Ethanol
Cause of spin – spin splitting
• 1. Protons on the same carbon are equivalent and DO NOT affect each other.
• Splitting is caused by the spins of protons on ADJACENT carbon atoms affecting (coupling with) each other.
• Since protons, when spinning, will produce their own magnetic field protons on adjacent carbon atoms will make a small difference in the magnetic field experienced by a proton.
• The difference depends on whether the spin of the adjacent proton is aligned with or against the external field.
• Each proton can be with or against the external field.
• In the next slide the left hand diagram shows ONE PROTON WITH or AGAINST the external field.
• This generates 2 identical fields which will split an adjacent peak into 2 peaks of identical height.
• The ‘doublet’ so formed in any peak in the spectrum indicates that the group in question in NEXT to a SINGLE hydrogen atom
• The second diagram in from the left shows the possible ways of aligning 2 protons in the external field.
• Both protons can be with the field, 1 can be with the field and 1 against which is equivalent to 1 against and 1 with, and both can be against the field.
• This leads to 3 different fields with intensity 1:2:1 – TRIPLET diagnostic that the group in question is NEXT TO a CH2 group.
• The 3rd diagram shows the possible felds generated by a CH3 group.
• As before all protons can be with or against the field ( the left and right sets of arrows) but each proton can also be independently with or against leading to a 1:3:3:1 quartet diagnostic of an ADJACENT CH3 group.
• Once this is grasped it is easier to remember the n+1 rule:
• For n protons on an adjacent carbon the number of peaks in the splitting pattern = n+1.
• Analyse a proton NMR spectrum to make predictions about the number of non-equivalent protons adjacent to a given proton, and possible structures for the molecule.
• Predict the splitting patterns of the protons in a given molecule.