NMR Lectures - Organic chemists rely on two main techniques...

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Organic chemists rely on two main techniques for determining the structure of organic molecules. Spectroscopy and Mass Spectrometry Mass spectrometry: measurement of the mass of molecules (topic for Chem 3BL). Spectroscopy: measurement of the absorption of electromagnetic radiation in discrete “packets” or quanta.
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Spectroscopy • When absorption occurs, something changes from a lower energy state to a higher energy state. E = h ν E = change in energy h = Plank’s Constant (6.62608 * 10 -34 J·s) ν = frequency of electromagnetic radiation
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The Electromagnetic Spectrum > 300 (kcal) 300 – 40 40 – 2 2 – 0.1 10 -4 10 -6 X-rays Electronic Transitions of core electrons in atoms UV Vis. Electronic Transitions of valence electrons Near IR Far IR Micro- waves Vibrational Transitions Rotational Transitions Radio- waves Nuclear Spin Transi- tions λ 10 nm 50 400 800 20 µ m 1mm 100mm 1m E = h ν ν= c λ λ = wavelength
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Nuclear Magnetic Resonance (NMR) The name is quite appropriate: Nuclear (spins) resonating (when electromagnetic radiation or EMR is applied) in a magnetic field. As with electrons, nuclei can have spin numbers. 1 H, 13 C, 15 N, 19 F, 31 P all have spins of “1/2.” • Any nuclei with a non-zero spin is, in principle, NMR active. That is, it can be “seen” in the NMR experiment.
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“Spinning Nuclei” An NMR active nucleus spins about its nuclear axis (i.e. a charged particle). This circulating charge creates a small magnetic field. Thus we can think of such nuclei as “little” randomly oriented bar magnets. + + +
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Nuclei will orient in the presence of an external magnetic field + + + + + + + + + External Magnetic Field α Spin State β Spin State E +
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α and β Spin States • Since the α state is lower in energy than the β state, it is more “populated.” The energy difference between these two states is proportional to the applied external magnetic field (H 0 ). β
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NMR Lectures - Organic chemists rely on two main techniques...

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