Rules of the Day 1st midterm

Rules of the Day 1st midterm - Rules of the Day 1-16-08 1....

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Rules of the Day 1-16-08 1. Physics: Moving charge generates a magnetic field , and a magnetic field causes charges to move 2. NMR, nuclear magnetic resonance , is used to assign structures of organic molecules. 3. Atomic nuclei, like electrons, have a quantum mechanical property of "spin" . Spin can be thought of as a small magnetic field around the nucleus created as if the positive charge of the nucleas were circulating. 4. We care about the nuclei 1H and 13C since these are commonly found in organic molecules and they have spin quantum numbers of 1/2 . 5. Nuclei with spin quantum number 1/2 are quantized in one of two orientations , "+1/2" (lower energy) or "-1/2"(higher energy) in the presence of an external magnetic field, that is, with and against the external field, respectively. 6. The difference in energy between the +1/2 and -1/2 nuclear spin states is proportional to the strength of the magnetic field felt by the nucleus. 7. NMR spectra record the energy (plotted as frequency) necessary for the nuclei to be excited from the lower energy spin state to the higher energy spin state in the presence of a strong external magnetic field. Different atoms in a molecule take different amounts of energy to accomplish this, and the different energies can be correlated to structure of the molecule . 8. Electron density is induced to circulate in a strong external magnetic field, which in turn produces a magnetic field that opposes the external magnetic field . This shields nuclei from the external magnetic field. The greater the electron density around a nucleus, the more shielded it is, and the lower the energy (frequency) of electromagnetic radiation required to flip its nuclear spin . 9 . Electronegative atoms, pi bonds and hybridization state of carbon atoms attached to an H atom influence shielding in predictable ways by removing differing amounts of electron density around adjacent nuclei. Rules of the Day 1-18-08 1 . Electronegative atoms, pi bonds and hybridization state of carbon atoms attached to an H atom influence shielding in predictable ways by removing differing amounts of electron density around adjacent nuclei. 2. NMR spectra record the energy (plotted as frequency) necessary for the nuclei to be excited from the lower energy spin state to the higher energy spin state in the presence of a strong external magnetic field. Different atoms in a molecule take different
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amounts of energy to accomplish this, and the different energies can be correlated to structure of the molecule . 3. The location of a given signal with respect to a standard, TMS, is called chemical shift ( δ ) and this has the units ppm (parts per million). The more shielded the nucleus, the smaller the chemical shift. Different functional groups have characteristic chemical shifts . 4.
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Rules of the Day 1st midterm - Rules of the Day 1-16-08 1....

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