Question

1. Would you expect phosphorus to exhibit an NMR spectrum? Why or why not?
2. When radio-frequency energy is absorbed by a nucleus with a spin of 1/2 that is in a magnetic field, what happens?
3. How does the irradiation of nuclei in FT-NMR instruments differ from irradiation in other types of NMR instruments? What are the advantages of FT-NMR over the other NMR techniques?
4. In many respects, the spectrum of CH3CH2I (iodoethane) appears to be identical to that of ethanol. What are the differences in the two spectra and why do these differences arise?

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2. When radio-frequency energy is absorbed by a nucleus with a spin of 1/2 that is in a magnetic field, what happens?
Ans: According to the quantum mechanics a nucleus of spin quantum number I will have 2I + 1 possible orientation. So, nuclei with spin quantum number of 1/2 will be having two spin states or orientation i.e. +1/2 and –1/2. In the absence of external magnetic field, nuclei in either a +1/2 or –1/2 spin state are of the same energy. When an external magnetic field is applied, radio-frequency energy is absorbed by a nucleus and the two spin states will be degenerated having different energies. The difference in energy is directly proportional to the strength of the external magnetic field. The +1/2 spin state is lower in energy, and the –1/2 spin state is higher in energy. This causes a small excess in the population of the lower energy spin state, which results in a small bulk magnetization of the sample in the direction of the magnetic field. When irradiated at a 90 degree angle to the large magnetic field, the nuclei resonate with the magnetic component of the radio frequency pulse at the respective Larmor frequency causing the bulk magnetization to rotate and oppose to the magnetic field. This net rotating magnetic moment induces a current in the detector coil resulting in a free induction decay curve that is converted into a spectrum....

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