The 131Xe isotope (32.8 MHz resonance frequency at 9.4 T, 21.2% natural abundance) has a spin I = 3/2 and thus possesses a nuclear electric quadrupole moment (Q = −11.4 fm2) [16]. The electric quadrupole moment of the 131Xe nucleus is susceptible to interactions with electric field gradients (EFGs) and therefore serves as a sensitive probe for environmentally induced distortions of its large surrounding electron cloud [14]. Unless high concentrations of paramagnetic substances are present, these quadrupolar interactions are the dominant cause of 131Xe nuclear spin relaxation in all phases. Further, 131Xe coherent quadrupolar interactions can be induced when the Sunitinib xenon atoms are contained within an anisotropic environment.

In solid, natural abundance xenon, Warren

and Norberg [17] and [18] found that 131Xe had a very short longitudinal relaxation time of T1 ≈ 200 ms at temperatures close to the melting point (161 K). However, the T1 increased monotonically by more than three orders of magnitude with decreasing temperature and reached T1 = 390 s at 9 K. The relaxation times in liquid xenon show the opposite trend compared to the solid and increase from T1 ≈ 40 ms at 161 K to T1 ≈ 80 ms at 250 K and 3 MPa. Later work [19] determined T1 = 110 ms at conditions just below the critical point, i.e. 298 K and 5.8 MPa. The 131Xe relaxation behavior of xenon dissolved Y27632 in various solvents was subject to experimental and computational studies in the past (see [20] for a review). Longitudinal relaxation in polar solvents is quite fast (T1 < 10 ms) due to the electric field gradient fluctuations induced by the solvent molecule dipoles. Even in non-polar solvents, the 131Xe T1 relaxation times are typically below 50 ms. In gas phase, it was theoretically predicted by Staub and later confirmed experimentally by Brinkmann

et al. [21] that the 131Xe longitudinal relaxation time (T  1) is inversely proportional to the gas density, ρ  , with equation(1) 1/T1131Xe=ρ·3.96×10-2amagat-1s-1.1 amagat is the density of the specific gas at standard pressure and temperature of 101.325 kPa and 273.15 K. For xenon the atomic number density of one amagat is reported with 2.7048 × 1025 m−3 [22]. (Note that in literature the Celastrol amagat is often alternatively defined as the density of an ideal gas at standard pressure and temperature resulting to the slightly different value of 2.6868 × 1025 m−3.) Brinkmann’s result was obtained at a temperature of 298 K and 0.76 T magnetic field strength. In later theoretical work, Adrian [23], considered separately the relaxation dependence on van der Waals and exchange contributions during binary collisions. He obtained 1/T1131Xe=ρ·4.61×10-2amagat-1s-1 for the gas at room temperature but also noted a temperature dependence of the 131Xe relaxation. From these equations, a 131Xe gas-phase relaxation time of T1 ≈ 22–25 s would be expected at ambient pressure (∼1 amagat).