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Published byAubrey Sparks Modified over 9 years ago
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Proton NMR Spectroscopy
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The NMR Phenomenon Most nuclei possess an intrinsic angular momentum, P. Any spinning charged particle generates a magnetic field. P = [I(I+1)] 1/2 h/2 where spin quantum # I = 0, 1/2, 1, 3/2, 2, …
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Which nuclei have a “spin”? If mass # and atomic # are both even, I = 0 and the nucleus has no spin. e.g. Carbon-12, Oxygen-16 For each nucleus with a spin, the # of allowed spin states can be quantized: For a nucleus with I, there are 2I + 1 allowed spin states. 1 H, 13 C, 19 F, 31 P all have I = 1/2 E = h/2 )Bo
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Spin states split in the presence of B 0
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When a nucleus aligned with a magnetic field, B 0, absorbs radiation frequency (Rf), it can change spin orientation to a higher energy spin state. By relaxing back to the parallel (+1/2) spin state, the nucleus is said to be in resonance. Hence, NMR
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Presence of Magnetic Field
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NMR instruments typically have a constant Rf and a variable B 0. A proton should absorb Rf of 60 MHz in a field of 14,093 Gauss (1.4093 T). Each unique probe nucleus ( 1 H perhaps) will come into resonance at a slightly different - and a very small percentage of - the Rf. All protons come into resonance between 0 and 12/1,000,000 (0 – 12 ppm) of the B 0.
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Nuclei aligned with the magnetic field are lower in energy than those aligned against the field The nuclei aligned with the magnetic field can be flipped to align against it if the right amount of energy is added (DE) The amount of energy required depends on the strength of the external magnetic field
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Energy Difference ( E) Between Two Different Spin States of a Nucleus With I=1/2
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What Does an NMR Spectrum Tell You? # of chemically unique H’s in the molecule # of signals The types of H’s that are present e.g. aromatic, vinyl, aldehyde … chemical shift The number of each chemically unique H integration The H’s proximity to eachother spin-spin splitting
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Chemical Equivalence How many signals in 1 H NMR spectrum?
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Number of Equivalent Protons
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Homotopic H’s –Homotopic Hydrogens Hydrogens are chemically equivalent or homotopic if replacing each one in turn by the same group would lead to an identical compound
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Enantiotopic H’s If replacement of each of two hydrogens by some group leads to enantiomers, those hydrogens are enantiotopic
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Diastereotopic H’s If replacement of each of two hydrogens by some group leads to diastereomers, the hydrogens are diastereotopic –Diastereotopic hydrogens have different chemical shifts and will give different signals
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Vinyl Protons
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Typical 1H NMR Scale is 0-10 ppm
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The Scale
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Tetramethylsilane (TMS)
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Chemical Shift Ranges, ppm
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Diamagnetic Anisotropy Shielding and Deshielding
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Deshielding in Alkenes
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Shielding in Alkynes
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Methyl t-butyl ether (MTBE)
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Toluene at Higher Field Splitting patterns in aromatic groups can be confusing A monosubstituted aromatic ring can appear as an apparent singlet or a complex pattern of peaks
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Integral Trace
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Spin-Spin Splitting
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The Doublet in 1 H NMR
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H b in 1,1,2-Tribromoethane
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The Triplet in 1 H NMR
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H a in 1,1,2-Tribromoethane
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1,1,2-Tribromoethane
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The Quartet in 1 HMR
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1,1-Dichloroethane
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Ethyl benzene
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CH 3 CH 2 OCH 3
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Equivalent Protons do not Couple
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Pascal’s Triangle
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Methyl Isopropyl Ketone
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1-Nitropropane
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Differentiate using 1 H NMR
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Coupling Constants (J values)
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Para Nitrotoluene
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Bromoethane
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para-Methoxypropiophenone
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Styrene
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H a splitting in Styrene “Tree” Diagram
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In the system below, Hb is split by two different sets of hydrogens : Ha and Hc –Theortically Hb could be split into a triplet of quartets (12 peaks) but this complexity is rarely seen in aliphatic systems
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C-13 NMR Spectroscopy
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C-13 chemical shifts
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Methyl Propanoate C-13 proton decoupled
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Ethyl Acrylate; C-13 prediction
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Ethyl Acrylate; C-13 NMR
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Coupling in C-13 NMR
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Butanone - Coupled and Decoupled
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1,2,2-Trichloropropane 1 H and 13 C NMR Spectra
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Coupled C-13 NMR Spectrum
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