Q1P - Organic Chemistry (Mcmurry)

Question

The amount of energy required to spin-flip a nucleus depends both on the strength of the external magnetic field and on the nucleus. At a field strength

of 4.7 T, rf energy of 200 MHz is required to bring a nucleus into resonance,

but energy of only 187 MHz will bring a nucleus into resonance. Calculate

the amount of energy required to spin-flip a nucleus. Is this amount greater

or less than that required to spin-flip a nucleus?

Step-by-Step Solution

Answer

The energy needed to spin flip anucleus is . This is lower in energy than the energy needed to spin flip a hydrogen nucleus. 

Step 1: Spin-flip a nucleus

The application of a very strong magnetic field makes the energy difference between the two spin states to be larger and high energy is required for spin-flip. The application of a weaker magnetic field leads to a lesser energy between the spin states.

Step 2: Amount of energy needed to spin flip the fluorine-19 nucleus

To determine the energy needed to spin flip of ¹H  and ¹⁹F nucleus, we will need to multiply the frequency of electromagnetic energy required to initiate the transition by the Planck’s constant and Avogadro number. This gives the energy in kilojoules per mole.

 

For ¹H

For  ¹⁹F

The electromagnetic energy needed to spin flip the ¹H   nucleus is 200 MHz , which is higher in frequency than the radiation needed for ¹⁹F  nucleus. Because the energy is directly proportional to the frequency of the radiation, the hydrogen nucleus will require higher energy.

The energy needed to spin flip a ¹⁹F  nucleus is 1.24x10^-25  J . This is lower in energy than the energy needed to spin flip a hydrogen nucleus.