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• State that NMR spectroscopy involves interaction of materials with low-energy radio frequency radiation.

• Describe the use of tetramethylsilane, TMS, as the standard for chemical shift measurement.

• State the need for deuterated solvents, e.g. CDCl3, when running an NMR spectrum.

NMR TECHNIQUE VIDEO

• http://www.youtube.com/watch?v=uNM801B9Y84

General Principles (Not Tested)• Protons and neutrons have a property

called spin.• Generally nucleons spin pair like electrons

so in a nucleus with an even number of nucleons the spins cancel each other out.

• Where the number of nucleons is odd the spinning unpaired nucleon generates a small magnetic field.

• Just like bar magnets these nuclear magnets can align themselves with or against an external magnetic field.

• When the sample is put into a large magnetic field in the NMR spectrometer the nuclear minimagnets can align themselves in the field so that they are parallel with the field direction.

• This is a low energy option.• They can however, align themselves

against the field (antiparallel) which is a higher energy state.

• The difference between the 2 energy levels is small and equivalent to radiowave energy.

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Different nuclear spins in an external magnetic field

Resonance

• By subjecting the sample to a burst of radio-frequency radiation some of the nuclei will absorb energy and ‘flip’ to the higher energy anti-parallel state – excitation.

• When it drops back to the ground state it will re-emit the energy it absorbed in the first place –relaxation.

• Together these 2 processes are called resonance which continues as long as the correct frequency radiation is supplied.

Nuclear Shielding and Chemical Shift

• The size of the energy gap between the 2 energy states for a particular nucleus depends on the atoms in the rest of the molecule.

• Nearby electrons will shield the nucleus from some of the applied magnetic field- nuclear shielding.

• The higher the electron density nearby the greater the shielding and the greater the energy gap between the energy states.

• This in turn increases the frequency of the radiation needed to cause resonance.

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Nuclear shielding – electrons modify the magnetic field experienced by the nucleus

Identification of Different Groups

• Because different chemical environments affect the nuclear response to the external field the signals at different frequencies in the NMR spectrum can be used diagnostically to identify particular chemical groups.

• Chemical shift is the place in a spectrum where a nucleus absorbs energy. This is given the δ scale, measured in ppm.

• Shift is determined using TMS as a reference peak which has a single sharp absorption and is given a shift value of 0.

Week 11

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Tetramethylsilane (TMS), (CH3)4Si

Solvents

• Organic solvents will produce an absorption in the same frequency range as the comounds being analysed.

• Deuterated solvents (where all hydrogen atoms (with spin) are replaced by deuterium with 2 nucleons and therefore no residual spin) are always used for NMR purposes.

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Isotopes of hydrogen, 1H and 2H; 2H is commonly called ‘deuterium’

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• Analyse carbon-13 NMR spectra to make predictions about the different types of carbon atoms present.

• Predict the chemical shifts of carbons in a given molecule.

Carbon-13 NMR

• 1.1% of carbon atoms are carbon 13 with an odd number of nucleons which can be used to generate NMR spectra where carbon 12 atoms cannot.

• The chemical shift values in carbon – 13 spectra range up to about 230 ppm because carbon 13 is sensitive to nuclear shielding.

• Actual shift values quoted in data sheets are APPROXIMATE and can vary slightly depending on solvent concentration and substituents.

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Carbon-13 chemical shifts

Shift values

• Notice that the more polar the group the higher the resonance frequency – the further left in the spectrum the absorption appears.

• From the absorptions and peaks the NUMBER and TYPES of different chemical environments in an organic compound can be determined BUT NO information about the number of carbon atoms responsible for the peaks observed.

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Carbon-13 NMR spectra of propan-1-ol and propan-2-ol

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Carbon-13 NMR spectra of propan-1-ol and propan-2-ol showing peak assignments

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Carbon-13 NMR spectra of (a) CH3CHBrCH3 and (b) CH3COOCH3

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• Analyse carbon-13 NMR spectra to make predictions about possible structures for an unknown compound.

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Carbon-13 NMR spectra of isomer of C3H6O

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Structural isomers of the carbonyl isomers of C3H6O showing different carbon environments

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Carbon-13 NMR spectra of an aromatic compound with molecular formula C8 H8

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Structural isomers of the aromatic isomers of C8H8