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Lecture 5

CHEM 2OA3 Lecture 5: Podcast 5 Proton NMR Spectroscopy

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Paul Harrison

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Characteristics of a H NMR Spectrum
- Measure: number of signals, signal location shift, signal area integration,
signal shape splitting pattern
- Number of signals: number of unique H environments
- Range is smaller: 0 to 10. Amount of shielding and deshielding is less dramatic
- Signal location: tells what the environments are. Left = e withdrawing group,
right = no e withdrawing group
- Area under the peaks = number of protons in the environment.
- Sp3 on the right
- Splitting patterns: coupling, number of neighbours
- Single line: no neighbouring H
Number of Signals
- Homotopic: perfectly overlapping signals, axis of rotational symmetry that allows
one proton to be rotated onto the other without changing the molecule
- Emanitiopic: perfectly overlapping signals, molecule has a plane of reflection that
makes one proton the mirror image of the other
- Replacement test
- If one is out of the page and the other is into the page, they cannot be
- 3 Protons on a methyl group with always be equivalent to each other
- Resonate at same frequency: chemically equivalents, same environment, one
Chemical Shifts
- Tetramethylsilane (TMS) is the standard
- TMS has only one type of H environment
- Chemical shift = (observed shift from TMS in Hz * 10^6) / operating frequency of
the instrument in Hz
- Chemical shift is constant, it is independent of the instrument
- Upfield: shielded, right, require stronger magnetic field to be excited, lower
- Downfield: deshielded (lower than normal e density), left, higher energy
- As number of halogens increases, so does chemical shift to the left.
- Chemical shift tells us which functional groups that are attached to C that have
the H
- Diamagnetic anisotropy: e interact with the magnetic field in a pi bonded
system. Protons get extra magnetic field in the same direction as the applied
magnetic field.
- Aldehyde H: cannot be distinguished from ketone by C-13 NMR or IR, but can use
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