A spectral density function (three of them depicted below) is 'power' vs frequency. These functions apparently inform us on the dynamics of spins in a protein somehow but I'm having the hardest time grasping how they do this. In the highest function, the correlation time is long, indicating the motion is slow and 'predictable'... or has a long memory function. In the second, there is more motion with less power at each frequency. In the third... even faster motion with even less power at each frequency.
My question is... what frequency are we talking about here? Are we talking about the frequency of the physical motion of a magnetic moment vector? Or a Larmor precession frequency?