HandoutNMRSummary - NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY...

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- 1 - NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY BASIC INFORMATION FROM 1 H NMR SPECTRA I. NUMBER OF SIGNALS Number of different types of protons. II. CHEMICAL SHIFTS Type of environment for protons. III. INTEGRATION Ratios of numbers of protons. IV. SIGNAL SHAPE Dynamics of proton environments. V. SIGNAL SPLITTING Spin-Spin coupling between nuclei with non-zero nuclear spins. Typically, the number and geometry of nearby protons. I. NUMBER OF SIGNALS The number of signals indicates the number of different types of protons. Be careful about overlapping signals. Stereotopism defines types of protons or groups. There are three categories as defined by Mislow and Raban in Topics in Stereochemistry 1967 , 1 , 1. Type of Group Symmetry Criterion Substitution Criterion Excluded Point Groups Examples Homotopic Interchangeable by C n ( >n>1) No isomers are generated with achiral or chiral test groups. Conical (C v ) and non- axial (C 1 , C s , C i ) protons of methyl groups, axial protons of cyclohexane. Enantiotopic Interchangeable only by S n Enantiomers are generated with achiral test groups. Diaster- eomers are generated with chiral test gorups. Linear (C v , D h ) and chiral (C n , D n , T, O I) protons of CH 2 Cl 2 , equatorial protons at C2 and C6 of cyclohexanone. Diastereotopic Not interchangeable by any symmetry operation Diastereomers are generated with all test groups. Linear (C v , D h ) many
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- 2 - HOMOTOPIC Homotopic nuclei or groups are interchangeable by an axis of rotation. Homotopic protons are equivalent and will not couple to each other. H H Cl Cl Br H H H C 2 Axis C 3 Axis Substitution Test: Two atoms or groups are homotopic if substitution of an atom or group by something else generates a structure that is identical to that generated by replacing the other atom or group. Me Me H H Me Me D H Me Me H D OR ENANTIOTOPIC Enantiotopic nuclei or groups are interchangeable by a rotation and a reflection. Enantiotopic protons are equivalent by most NMR techniques and do not couple to each other. H H ' Me Cl H ' H Cl Me H ' H Me Cl Rotation C 2 Reflect through plane of H-C-H Substitution Test: Two atoms or groups are enantiotopic if substitution of an atom or group by something else generates the enantiomer of the compound generated by replacing the other atom or group.
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  • Spring '10
  • MiguelGarciaGaribay
  • Protons, Nuclear magnetic resonance, Proton NMR, protons Allylic protons, protons Aldehyde protons

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