Set 1 – Basic Concepts

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Page ID: 79415

Thomas Wenzel
Bates College

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I usually start by pointing out what each of the terms in “nuclear magnetic resonance spectroscopy” implies about this instrumental analysis method. Then they are asked the following question.

What makes up the nucleus of a hydrogen atom?

Groups always are able to remember that there is a single proton in the nucleus. I then remind them of what they have already learned about electron configurations and point out that there are nuclear configurations for the protons and neutrons, and that these particles also have quantum numbers analogous to the electron quantum numbers. They are then asked the following two questions.

What are the allowable spin quantum numbers for an electron?

What do you think are the allowable spin quantum numbers for a proton?

They usually have no problem answering these two questions. After describing what is meant by I, the total spin quantum number, they are asked the following.

What is the magnitude of I, the total nuclear spin, for a hydrogen nucleus?

Groups typically have no problem answering this.

What do you think is produced by the spinning, charged proton that is the hydrogen nucleus?

Students’ ability to answer this may depend on whether they have recently taken a physics course where magnetism has been discussed. Others can often guess at an answer given that we’re talking about nuclear magnetic resonance spectroscopy.

I then identify for them that we will use of B_p to express the magnetic field of the hydrogen nucleus. I also point out that magnetic fields have a direction and that we use the right-hand rule to determine the orientation of the magnetic field produced by a spinning proton. I then indicate that a sample is placed into an applied magnetic field when recording an NMR spectrum. They are then asked the following question.

What happens when two magnetic fields (B_p and B_APPL) are in contact with each other?

Students realize that these will interact with each other. I give them the formula to determine how many orientations the proton’s magnetic field will have relative to the applied magnetic field and we determine that there are two possible orientations.

What do you think are the two allowable orientations of B_p relative to B_APPL?

Students intuitively propose the “with” and “against” orientations. I then draw a picture similar to that shown in Figure 2 in the text.

Do you think the two allowable orientations have the same or different energy?

Students intuitively thing they have different energy, in part because they realize it must be necessary if we are to actually be able to perform NMR spectroscopy.

Which of the two do you think is lower in energy?

Again, students intuitively propose that the “with” state is lower in energy. After this, I draw a picture similar to that shown in Figure 3 in the text. We also look at the nature of the “spin flip” that will occur for an individual proton as it is excited. Finally, we consider how there is a specific frequency that will lead to the excitation.