Rules of the Day 3 - chemical shift. Different functional...

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Rules of the Day 1-26-09 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 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
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Unformatted text preview: chemical shift. Different functional groups have characteristic chemical shifts . 4. Equivalent hydrogen atoms in a molecule give the same NMR signal. Equivalent hydrogen atoms in a molecule have an identical relationship to all the other atoms in the molecule, and are found on the same sp3 atom (bond rotation makes them equivalent) or entire groups are equivalent due to symmetry in the molecule (i.e. the six equivalent hydrogens on the two methyls of an isopropyl group). Determining how many equivalent hydrogens are in a molecule can be very tricky (Skull and Crossbones!) so PRACTICE. 6. The size (integration ) of the signal in an 1H NMR spectrum is proportional to the number of equivalent hydrogens in each signal....
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