text: In 1948, Nicolaas Bloembergen, Edward Mills Purcell, and Robert Pound proposed the so-called Bloembergen-Purcell-Pound theory (BPP theory) to explain the relaxation constant of a pure substance in correspondence with its state, taking into account the effect of tumbling motion of molecules on the local magnetic field disturbance. The theory agrees well with experiments on pure substances, but not for complicated environments such as the human body.
  This theory makes the assumption that the autocorrelation function of the microscopic fluctuations causing the relaxation is proportional to , where  is called the correlation time. From this theory, one can get T > T for magnetic dipolar relaxation:
  where  is the Larmor frequency in correspondence with the strength of the main magnetic field .  is the correlation time of the molecular tumbling motion.  is defined for spin-1/2 nuclei and is a constant with  being the magnetic permeability of free space of the  the reduced Planck constant, γ the gyromagnetic ratio of such species of nuclei, and r the distance between the two nuclei carrying magnetic dipole moment.
  Taking for example the HO molecules in liquid phase without the contamination of oxygen-17, the value of K is 1.02×10 s and the correlation time  is on the order of picoseconds =  s, while hydrogen nuclei H (protons) at 1.5 tesla precess at a Larmor frequency of approximately 64 MHz (Simplified. BPP theory uses angular frequency indeed). We can then estimate using τ = 5×10 s:
  :(dimensionless)
  := 3.92 s
  := 3.92 s,
  which is close to the experimental value, 3.6 s. Meanwhile, we can see that at this extreme case, T equals T.
  As follows from the BPP theory, measuring the T times leads to internuclear distances r. One of the examples is accurate determinations of the metal – hydride (M-H) bond lengths in solutions by measurements of H selective and non-selective T times in variable-temperature relaxation experiments via the equation:
  :, with 
  where r, frequency and T are measured in Å, MHz and s, respectively, and I is the spin of M.