What is nuclear magnetic resonance? 84-85 Flashcards
What are common isotopes studied using NMR spectroscopy?
1H, 13C, 19F and 31P
In organic chemistry, proton(1H) NMR and carbon-1 (13C) NMR spectroscopy and are the most useful general-purpose techniques, but the basic principles are the same for NMR spectroscopy of other isotopes.
What are the basic requirements for NMR spectroscopy?( 2 points to be made)
- A strong magnetic field applied using an electromagnet.
- Low-energy radio-frequency radiation
What is a nucleon?
Atom or neutron
Explain the property spin found in nucleons.
And how spin in isotopes can create a magnetic field.
Protons and neutrons have a property called spin, which can be in one of two opposite directions - rather like electrons can have opposite spins. In the nucleus, opposite spins pair up. However, the nuclei of some isotopes have an uneven number of nucleons. These isotopes have an unpaired nucleon, which produces a small residual nuclear spin. This generates a magnetic field.
Explain what happens to nuclei with magnetic spin in a strong magnetic field.
In a strong magnetic field, these nuclei can line up in one of two directions:
- With the field
- Opposed to the field
Whats the diffirence in energy between nuclei that oppose or line up with the magnetic field.
How is the energy diffirence increased?
Nuclei that oppose the magnetic field have a higher energy than those aligned with the magnetic field. The stronger the magnetic field, the larger the energy.
What happens in the process of excitation?
A nucleus in its low-energy spin state can be promoted to its upper-energy spin state by providing energy that is exactly equal to the energy gap, E.
In an NMR spectrometer, this energy supplied by low-energy radio-frequency radiation. The change in energy (E) is proportional to frequency so the larger the energy gap, the higher the frequency required.
The excited nucleus will later drop back to its low-energy ground state, emitting exactly the same amount of energy in the form of radiation- this process is called ‘relaxation’.
What is resonance?
The cycle of excitatio and relaxation of the nucleus is called resonance. Resonance continues so long as the frequency of the radiation supplied has an energy that exactly matches the energy gap between the spin states. The name nuclear magnetic resonance describes this process aptly.
Define the term chemical shift.
Chemical shift, is a scale that compares the frequency of an NMR absorption with the frequency of the reference peak of TMS at 0 ppm.
What are the two factors that the magnetic field felt by a nucleus depends on?
- The applied strong magnetic field.
- A weaker magnetic field generated from electrons surrounding the nucleus and in nearby atoms.
Explain why an atom’s electrons change the magnetic field felt by the nucleus.
An atom’s electrons shield the nucleus from some of the applied magnetic field - this is called nuclear shielding. The extent of nuclear shielding depends on the electron density in nearby atoms or groups. This alters the environment of a nucleus, changing the energy gap. The result is that atom in diffirent environments have diffirent resonance frequencies.
In an NMR spectrum any diffirences in environment will show up as diffirent signals and these can be identified.
What is chemical shift?
Chemical shift,δ, is the place in an NMR spectrum at which a nucleus absorbs energy. NMR spectra are calibrated using the δ (delta) scale, measured in units of parts per million, or ppm. The δ scale is based on the frequencies used by the NMR spectrometer.
What is chemical shift measured relative to?
Why is chemical shift recorded relative to the resonance frequency of this compound?
Chemical shift is measured relative to a reference signal from a standard compound called tetramethylsilane (TMS) , (CH3)4Si.
Each molecule of TMS has 12 equivalent protons and these gives rise to single, sharp NMR signal which can be identified easily. The chemical shift of TMS is defined as δ = 0ppm.
When NMR spectra are recorded, a small amount of TMS is usually added so that the spectrometer can be calibrated against the TMS reference peak. TMS is chemically unreactive and volatile so it can easily be removed from a sample after running an NMR spectrum.
Why can’t normal organic solvents be used to dissolve the sample in NMR solutions?
Organic solvents contain carbon and hydrogen and these atoms themselves produce a signal.
Explain how the use of deuterated solvents allows to produce solutions that can be used in NMR.
Deuterium, D, is an isotope of hydrogen with one proton and one neutron in the nucleus . Deuterium has an even number of nucleons and produces no signal in an NMR spectrum.
