notes_14C_nmr01 - Proton Nuclear Magnetic Resonance...

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Proton Nuclear Magnetic Resonance Spectroscopy Introduction: The NMR Spectrum serves as a great resource in determining the structure of an organic compound by revealing the hydrogen and carbon skeleton. Historically, NMR was initially used to study the nuclei of Hydrogen atoms; however, any atom with an odd mass or atomic number has a nuclear spin that can be studied by NMR. Without the application of an applied magnetic field, protons are spinning in a randomly oriented manner and are generating a magnetic field (called the magnetic moment ) 1 . However, once an external (applied) magnetic field is present the protons either align with (parallel) or against (anti parallel) it. The parallel orientation, called the alpha spin, has a lower energy than the anti parallel (beta) spin. The stronger the applied magnetic field the greater the energy difference ( E) between the parallel and anti parallel states (Diagram 1) 2 . Therefore, the strength of the magnetic field determines the energy required to cause a nuclear spin flip . The energy difference ( E) between the ground and excited states is approximately 0.02 cal/mol which correlates to radio wave photons. An NMR signal is created once the radio wave photons supplied match the ( E) of the nucleus. Structural Information from NMR Spectrum: 3 1. Number of Signals: In order to use the number of signals to gain structural information one needs to understand the concept of chemically equivalent protons. Equivalent Protons: Identical protons, which experience the same magnetic force, will create identical and overlapping signals on the NMR spectrum. Such protons which are in the same magnetic environment are termed chemically equivalent . 4 The number of signals on the NMR spectrum represents the number of equivalent sets of protons. Protons with 1 . P.539, L.G.Wade, Organic Chemistry (5 th ) 2 . (08/27/06) 3 . p.55, Steven Hardinger, PhD. The Think Book (chem14C). 4 . P.552, L.G.Wade, Organic Chemistry (5 th )
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any one difference between them are no longer equivalent. For finding these differences one must begin by first looking at the atom directly attached to the proton and continue down the bonded atoms to find one difference. Protons aligned across a plane of symmetry, such as in Benzene, are equivalent. Another useful method for testing equivalency is replacing each proton with another atom, such as deuterium, to form two molecules. If the two molecules obtained are identical then the protons are equivalent. Example 1: Indicate the expected number of signals in the H NMR spectrum for each of the following molecules. 5 OCH 3 H 3 C (A) C C H Cl CH 3 H (B) *Part A has 4 signals, one from the methyl group, one from the methoxy group, and two from the benzene ring due to the plane of symmetry. Part B has 3 different signals due to the methyl group, and one from each H due to their attachment to different carbons. 2.
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This note was uploaded on 07/27/2009 for the course CHEM 14C-14D taught by Professor Hardinger during the Fall '08 term at UCLA.

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notes_14C_nmr01 - Proton Nuclear Magnetic Resonance...

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