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Principles (1)

Principles (1). Nuclear Magnetic Resonance Spectrometry Developed in the 1960’s The study of the absorption of radio waves by a sample in a magnetic field. Measures transitions in nuclear spin. Samples (1-30mg) are dissolved in a solvent not containing the type of nucleus of interest.

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Principles (1)

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  1. Principles (1) Nuclear Magnetic Resonance Spectrometry • Developed in the 1960’s • The study of the absorption of radio waves by a sample in a magnetic field. • Measures transitions in nuclear spin. • Samples (1-30mg) are dissolved in a solvent not containing the type of nucleus of interest

  2. Principles (2) • Atomic nuclei with an odd number of either protons or neutrons (e.g. 1H, 13C, 31P) have a property called nuclear spin. • Spin induces a local magnetic field around the nucleus to arise -the nucleus behaves like a small magnet. • In an external magnetic field, the nucleus may align with or against the magnetic field. Alignment against the field is a higher energy state than alignment with the field.

  3. Principles (3) • Irradiation with the correct frequency (ν) of radio waves causes the nucleus to “spin-flip” from ground-state (aligned with the field, called α) to the higher energy β spin state (against the applied field). • This absorbs energy (ΔE) and the nucleus is said to be in resonance. • The energy is then released in a variety of complex processes, and the nucleus can relax back to the ground state.

  4. Principles (4) ν for any particular nucleus is proportional to ΔE ΔE depends on atomic identity and local chemical environment ΔE is also proportional to the strength of the applied field (H0) • H0 ranges from 14,100 to 176,250Gauss for modern spectrometers - operating frequencies (ν) normally 60 to 750MHz (based on the resonance of protons in (CH3)4Si or TMS) • Increasing field strength gives increased resolution & better spectra. • Modern high-field (>270MHz) machines use a superconducting magnet and complex hardware and computer software to collate and process the data. • Powerful computers allow more complex data analysis and programming of “pulse” sequences

  5. Principles (5) • All nuclei are surrounded by electrons, which produce their own magnetic field • Electrons shield the nucleus from the applied field • Shielding varies according to the local chemical environment, because this affects gives rise to variation in electron density and distribution. • Increased electron density gives increased shielding and vice versa For any particular nucleus ν field strength felt by nucleus Beff Beff= Applied field (B0) - local electronic field (Blocal) .

  6. Principles (6) • ν of chemically different nuclei are different, but not as different as νof different atomic types • Can do NMR spectrometry of one type of atomic nuclei at a time. • by using a defined range of frequencies suitable for the type of nuclei to be observed (e.g. the protons in a molecule) a spectrum can be recorded. • Spectrum consists of distinct signals for each type of chemical environment

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