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l15a - CH 203 O R G A N I C C H E M I S T R Y I NMR...

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NMR spectroscopy: Physical basis and the chemical shift © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I NMR spectroscopy: Physical basis and the chemical shift In a previous lecture we pointed out that infrared (IR) spectroscopy is used to demonstrate which functional groups are present in a molecule. In this lecture we turn to nuclear magnetic resonance (NMR) spectroscopy, the preemi- nent technique for establishing the structure of organic molecules. The physical basis of NMR spectroscopy You are already familiar with the quantum mechanical concept of spin: elec- trons are either spin-up or spin-down when they occupy atomic orbitals. The proton, that is, the 1 H nucleus, also has spin: protons are either spin-up or spin-down, just like electrons. A variety of symbols are used to represent the proton’s spin. Spin-up protons are denoted by an upward pointing ar- row ( ! ), or are said to be in the " spin state, or are said to be in the +1/2 spin state. Spin-down protons are denoted by a downward pointing ar- row ( # ), or are said to be in the $ spin state, or are said to be in the –1/2 spin state. The presence of a magnetic field influences which spin state the proton adopts. Magnetic fields are vector fields: every point in a magnetic field is associated with a magnetic vector that possesses magnitude (high or low mag- netic field strength) and direction (the magnetic field points somewhere). By convention, the field direction is taken to be outward from the north pole and inward to the south pole of the magnet. When a collection of protons is immersed in the magnetic field furnished by a strong laboratory magnet, the protons partition themselves between their two spin states: slightly more than 50% of the protons assume the lower energy +1/2 spin state whereas slightly less than 50% of the protons assume the higher energy –1/2 spin state. Protons in the lower energy +1/2 spin state are imagined as tiny tops spinning about an axis parallel to the direction of the magnetic field, whereas protons in the higher energy –1/2 spin state are imagined as tiny tops spinning about an axis antiparallel to the direction of the magnetic field:
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NMR spectroscopy: Physical basis and the chemical shift © Bruno I. Rubio 2 S N +1/2 spin state –1/2 spin state Energy An important quantity to which we refer repeatedly is the magnetic field strength B p experienced by the proton. For an isolated proton, that is, for a proton that is not in a molecule, B p = B lab where B lab is the strength of the magnetic field in units of megahertz (MHz, 1 megahertz = 10 6 hertz = 10 6 s –1 ) furnished by the laboratory magnet. The quantity B p is important because the energy difference % E between the pro- ton’s two spin states is given by % E = hB p where h is Planck’s constant (6.626 10 –34 J·s). Thus, for an isolated pro- ton, % E = hB p = hB lab If the collection of isolated protons immersed in the magnetic field are ir- radiated with electromagnetic radiation of energy
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This note was uploaded on 02/27/2012 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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l15a - CH 203 O R G A N I C C H E M I S T R Y I NMR...

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