5: Magnetic Resonance Spectroscopies
- Page ID
- 22737
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- 5.1: Nuclear Magnetic Resonance (NMR) - Intrinsic Spins
- Proton Nuclear Magnetic Resonance Spectroscopy is a powerful method used in the determination of the structure of unknown compounds. Many useful properties can be extracted from NMR technique.
- 5.2: Nuclear Magnetic Resonance (NMR) - Turning on the Field
- The absence of external fields, there is no preferred orientation for a magnetic moment. That is, the different m values of the orientation of the spin are degenerate. However, since a nucleus is a charged particle in motion, it will develop a magnetic field. Randomly oriented nuclear spins are aligned when a magnetic field applied on it. Hence, in the presence of a magnetic field, the energy of a magnetic moment depends on its orientation relative to the applied field lines.
- 5.3: Spin 1/2 Spectra
- for 1H, 13C, 15N and other nuclei, we only have to deal with two orientations. The energy difference between the two spin states at a given magnetic field strength will be proportional to their magnetic moments.
- 5.4: Chemical Shifts
- The chemical shift in absolute terms is defined by the frequency of the resonance expressed with reference to a standard compound which is defined to be at 0 ppm. The standard reference that was chosen is tetramethylsilane (TMS). This compound has four -CH3 methyl groups single bonded to a silicon atom. All of the protons on the methyl groups are in the same electronic environment. Therefore only one NMR signal will be generated.
- 5.5: Boltzmann Statistics
- The energy separation between these states is relatively small and the energy from thermal collisions is sufficient to place many nuclei into higher energy spin states. The number of nuclei in each spin state can be described by the Boltzmann distribution. The Boltzmann equation expresses the relationship between temperature and the related energy.
- 5.6: Larmour Frequency
- When placed in a magnetic field, charged particles will precess about the magnetic field. In NMR, the charged nucleus, will then exhibit precessional motion at a characteristic frequency known as the Larmor Frequency. The Larmor frequency is specific to each nucleus. The Larmor frequency is measured during the NMR experiment, as it is dependent on the magnetic field that the nucleus experiences.
- 5.7: Ensemble Effects
- The net magnetization for a sample is the sum of the individual magnetic moments in the sample.