surrounding charges. Thermal motion of a nucleus can result in fluctuating electrostatic interaction energies. These fluctuations produce transitions between the nuclear spin states in a similar manner to the magnetic dipole-dipole interaction. BPP theory 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 T1 > T2 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 H2O molecules in liquid phase without the contamination of oxygen-17, the value of K is 1.02×1010 s−2 and the correlation time is on the order of picoseconds = s, while hydrogen nuclei 1H (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 τc = 5×10−12 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, T1 equals T2. As follows from the BPP theory, measuring the T1 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 1H selective and non-selective T1 times in variable-temperature relaxation experiments via the equation: , with where r, frequency and T1 are measured in Å, MHz and s, respectively, and IM is the spin of M. See also Nuclear magnetic resonance Nuclear magnetic resonance spectroscopy Nuclear magnetic resonance spectroscopy of carbohydrates Nuclear magnetic resonance spectroscopy of nucleic acids Nuclear magnetic resonance spectroscopy of