NMR Spectroscopy
Made using OCR Chemistry Textbook. Pages 84-93. Gent and Richie.
- Created by: Former Member
- Created on: 19-12-12 20:24
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- NMR Spectroscopy
- Background
- Nuclear spin
- In nucleus, nucleons (protons and neutrons) with opposite spins pair up
- If uneven no. of nucleons, small residual nuclear spin= magnetic field. Nuclei with magnetic spin can line up with or against a magnetic field
- Those that oppose MF have a higher energy than those aligned, stronger MF= larger gap btn these energies
- If uneven no. of nucleons, small residual nuclear spin= magnetic field. Nuclei with magnetic spin can line up with or against a magnetic field
- In nucleus, nucleons (protons and neutrons) with opposite spins pair up
- Resonance
- Cycle of excitation and relaxation
- Nucleus in low-energy spin state can be promoted to upper- energy. = excitation. Emits energy as radiation= relaxation.
- In NMR energy supplied by low-energy radio-frequency radiation
- Nucleus in low-energy spin state can be promoted to upper- energy. = excitation. Emits energy as radiation= relaxation.
- Cycle of excitation and relaxation
- Nuclear Shielding
- An atoms e- shield the nucleus from some of the applied magnetic field.
- Extent of shielding depends on e- density of near atoms so atoms in diff enviroments have different resonance frequencies= diff signals on NMR spectrum
- Chemical Shift
- is a scale that compares the frequency of an NMR absorption with the frequency of a reference peak of TMS
- Tetramethylsilane, 12 equivalent protons so single signal @ 0 ppm.
- When NMR spectra is recorded small amount of TMS is added to calibrate.
- TMS can easily be removed from sample later.
- Tetramethylsilane, 12 equivalent protons so single signal @ 0 ppm.
- is a scale that compares the frequency of an NMR absorption with the frequency of a reference peak of TMS
- Nuclear spin
- Solvents for NMR
- Usually carried out in solution but can't use solvent that will produce a signal.
- So, deuterated solvents are used ie. CDCl3
- Sample recovered by evaporating off solvent
- Deuterium = even nucleons (1P, 1N) = no signal
- Usually carried out in solution but can't use solvent that will produce a signal.
- Carbon-13 NMR
- Carbon-13 = sensitive to nuc. shielding= chemical shift 0-230ppm
- Interpreting
- No. of peaks= No. of different C. environments
- Work out type of environment from chemical shift
- Presence of electronegative group= significant c.s.
- Proton NMR
- Narrow range of c.s.= overlaps. More abundant than C-13 so smaller sample needed.
- Interpreting
- No. of peaks= no. of H environments
- Chemical shifts= types of environm.
- Relative peaks areas= proportions of H per environ.
- = Integration trace
- Spin-spin coupling= information about adjacent protons
- Arises from interactions of spin states of protons on adjacent c's
- Adjacent Hs may align their spins states in different ways
- N+1 rule.
- Only happens btn non-equivalent Hs
- Arises from interactions of spin states of protons on adjacent c's
- Background
- Only happens btn non-equivalent Hs
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