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Chemical Shift (1)

Chemical Shift (1). ν is measured in Hz and varies with spectrometer frequency The position of a signal from a particular proton is therefore measured relative to TMS This is known as chemical shift ( δ ) in parts per million (ppm). δ = Distance from TMS signal (Hz) in ppm

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Chemical Shift (1)

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  1. Chemical Shift (1) • ν is measured in Hz and varies with spectrometer frequency • The position of a signal from a particular proton is therefore measured relative to TMS • This is known as chemical shift (δ) in parts per million (ppm). δ = Distance from TMS signal (Hz) in ppm Spectrometer frequency (MHz) • 1H NMR spectra are normally of the range 0-10ppm (gives a 900 Hz range on a 90MHz machine).

  2. Neighbouring electronegative atoms Deshielded nucleus Increased ν needed for resonance Downfield shift in signal (increased ppm) Chemical Shift (2) Electronegative Neighbour Atoms: e.g. O, F 1H δ for RCH3 (1-2ppm)< RCH2Br (3-4ppm)< RCH2OH (3.5-4.5ppm) ALSO: Because hydrogen is more electropositive than carbon, increasing substitution (branching) also gives a downfield shift. i.e 1H d for RCH3 < RR’CH2 < RR’R”CH Both effects decrease rapidly with distance.

  3. π -bonds have high electron density Asymmetric magnetic fields Anisotropy Downfield shift for adjacent protons Chemical Shift (3) Unsaturated systems: i.e. Alkenes, alkynes Typical Benzene C-H is at 7-8.5ppm Aldehyde (RCHO) protons are observed the furthest downfield (>9.5ppm) as they have an electronegative atom and a double bond. Amines and Alcohols: RNH2, ROH Protons attached directly to nitrogen or oxygen give broad, variable position signals, because they become involved in hydrogen-bonding which affects their electron density.

  4. Chemical Shift (4) Chemical Equivalence: Nuclei in identical environments have the same chemical shifts and therefore give only one signal. • Atoms are said to be chemically equivalent if mentally substituting one of them would give identical results as substituting another. • Equivalence can be identified using symmetry: • Plane of symmetry: Atoms that are reflections of each other through a plane of symmetry are equivalent. • Rotational symmetry: Atoms that can be interchanged by rotation about a chemical bond are equivalent (e.g. methyl protons) provided that bond is able to rotate freely.

  5. 11 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (5) Typical Chemical Shifts (1H) CH3C=C CH2Cl CH=0 CONH, NH2 HC=C CCH2C CH3Si CH2Br COOH, Phenol OH Aromatic CH3N- CH3C CH02 CH3O, CH2O CH3C=O Low field High field δ/ppm

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