What causes the tilt angle of a precessing proton to change in a magnetic field?

[dangus]

What determines the tilt angle of a precessing proton in a magnetic field? Is it a constant the assuming no additional magnetic forces?

In an NMR a perpendicular magnetic field is applied which rotates the precessing magnetic moment into the X-Y plane. After this perpendicular magnetic pulse is removed the magnetic moment returns back to its original tilt angle. This runs counter to the spinning top analogy that is often used for the precessing proton. My understanding is that the tilt angle of a precessing top will remain fixed assuming no friction. So If I took a spinning top precessing at angle θ1 and then applied a force to increase the tilt angle to θ2, when that force is removed the top will continue to precess at this new tilt angle θ2. Is this all correct? Why is the proton different from the top in this case?

 

[Jano L.]

Hello dangus,

one way to understand the gradual alignment of the magnetic moment with the external field is in terms of suspended eternally-spinning top in uniform gravitational field, whose precession is damped by friction forces in the suspension or by the air.

In the presence of friction, things tend to lose as much energy as possible. In this case the state of lowest energy is such that magnetic moment loses all its " precession energy " (I do not know whether there really is such thing, but seems to work in this case).

With protons, because friction forces are present too, in the course of time, their average magnetic moment leaves the lateral plane and tends to be aligned with the field (individual moments can fluctuate around).