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.

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• 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.

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Proton chemical shifts

The actual shifts vary slightly depending on the environment of the protons.

O-H and N-H positions vary considerably depending on concentration and solvent.

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Proton NMR spectrum of ethyl methanoate with integration data. What’s wrong with this heading?

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• 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.

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Different combinations of spin states for protons in adjacent H atoms

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Proton NMR spectrum of methyl propanoate showing splitting patterns

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• Describe the identification of O–H and N–H protons by proton exchange using D2O.

-NH and -OH• NH and OH groups can be difficult to

identify because they are prone to be all over the place in NMR spectra.

• This leads to confusion with other groups.• The signals are broad and usually have no

splitting pattern.• The signal can, however be removed

completely from the spectrum by the addition of non-NMR active D2O.

• The D atom exchanges with the OH H atom making OD, effectively removing the signal from the spectrum.

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Water, H2O, and heavy water, D2O

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Proton NMR spectrum of ethanol, C2H5OH: (a) without D2O; (b) with D2O

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Compound with molecular formula C3H7NO2

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• 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.

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Proton NMR spectra of ClCH2CH2COOH and CH3CHClCOOH, both in D2O

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Proton NMR spectra of two esters of C4H8O2

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Proton NMR spectrum of 2-chloropropane, CH3CHClCH3

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Proton NMR spectrum of 1,2-dichloroethane, ClCH2CH2Cl

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• Explain that NMR spectroscopy is the same technology as that used in magnetic resonance imaging (MRI).