W HAT IS NUCLEAR MAGNETIC RESONANCE ? 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. CDCl 3, when running an NMR spectrum.

N UCLEAR M AGNETIC RESONANCE Analytical technique for examining molecular structure Most commonly studies the isotopes 1 H, 13 C, 19 F and 31 P We will be most interested in carbon-13 and proton ( 1 H) NMR spectroscopy The requirements for NMR spectroscopy are: A strong magnetic field applied using an electromagnet Low-energy radio frequency radiation

N UCLEAR SPIN chemistry/collections/spectroscopy/Content/FileRepositor y/NMR/NMR%20002.swf Protons and neutrons have a property called ‘spin’ – this can be in one of two directions In the nucleus opposite spins pair up. Fine until there are an uneven number of nucleons (protons and neutrons) Those isotopes with an unpaired nucleon have a small residual nuclear spin which generates a magnetic field

N UCLEAR SPIN Nuclei with a magnetic spin can be thought of as tiny magnets and so in a magnetic field can line up either: With the field Opposite to the field Nuclei that oppose the field have higher energy than those aligned The stronger the field, the larger the energy gap, ΔE, between these energies

RESONANCE A nucleus in its low-energy spin state can be promoted to its high energy state by providing the energy that is equal to ΔE (excitation) It is supplied by low-energy radio-frequency radiation. ΔE is proportional to frequency so the larger the energy gap, the higher the frequency required The excited nucleus will return to its low-energy ground state (relaxation) This cycle of excitation and relaxation is called resonance. It continues as long as the frequency matches the energy gap between spin states

NMR chemistry/collections/spectroscopy/Content/FileRepositor y/NMR/NMR%20003.swf olc/ch13nmr.html Quite a useful site for background reading

N UCLEAR SHIELDING Nuclear shielding is caused by an atom’s electrons – the amount of shielding depends on the electron density in groups or atoms near by It alters the environment of the nucleus and this the energy gap NMR identifies these differences and can be interpreted.

C HEMICAL S HIFT Key definition – chemical shift, δ, is a scale that compares the frequency of an NMR absorption with the frequency of the reference peak of tetramethylsilane, Si(CH 3 ) 4 (TMS) at δ = 0 i.e. The differences in energy levels are measured relative to a standard compound (TMS). Why might we use TMS as a standard? All other compounds are compared to TMS by quoting the shift away from the standard TMS line. The units for chemical shift are parts per million (ppm)

S OLVENTS FOR NMR SPECTROSCOPY NMR – usually carried out in solution so a solvent is required. What could the problem be with using an organic solvent? Deuterated solvents are used What is deuterium, D? Why does the use of deuterium solve the problem?

Deuterium – isotope of hydrogen with 1 proton and one neutron Even number of nucleons so produces no signal in the NMR spectrometer Solvents like CDCl 3 are commonly used when running proton and carbon-13 NMR spectra The sample can be recovered by evaporating the solvent Complete questions 1 and 2 on page 85