9Be NMR relaxation measurements of bis(acetylacetonato)beryllium(II) in liquid and supercritical carbon dioxide:: A clear evidence of near-critical solvation effect on rotational correlation time

Be-9 NMR longitudinal relaxation times (T-1) of bis(acetylacetonato)beryllium(II) in liquid and supercritical carbon dioxide were precisely measured at four different temperatures of 293.4, 313.2, 332.9, and 351.4 K over a wide pressure range from 8.0 to 25.0 MPa. Because the quadrupolar Be-9 relaxes only through intramolecular interaction with the electric field gradient, the rotational correlation time (tau(r)) of the complex was purely determined from T-1 with a known value of the quadrupole coupling constant. It was observed that tau(r) increased with increasing density Of CO2 at each temperature. The plot of tau(r) vs eta/T, where eta is the viscosity Of CO2 and T is the thermodynamic temperature, gave a straight line at each temperature except for in the near-critical regime. This fact indicates that the viscosity of solution is the predominant factor in the change in tau(r) of the complex in CO2. At near-critical temperatures of 313.2 and 332.9 K, moreover, it was clearly found that tau(r) deviates upward beyond the experimental errors at intermediate eta/T. The upward deviation was discussed in terms of solute-solvent interactions.