Experimental Determination of Reorientational Correlation Time of CO₂ over a Wide Range of Density and Temperature

We have precisely measured 17 O longitudinal relaxation times, T 1 ( 17 O), of CO 2 under gaseous, liquid, and supercritical conditions at four different temperatures of 293.4, 313.0, 331.8, and 351.4 K over a wide pressure range between 1 and 20 MPa, corresponding to a density range between 0.03 and 0.94 g cm -3 . Assuming that the quadrupolar interaction is a dominant mechanism in the 17 O relaxation, the reorientational correlation time, τ 2r, of CO 2 was obtained from T 1 ( 17 O) with a known value of the quadrupole coupling constant. The τ 2r in CO 2 thus determined showed a unique density dependence; τ 2r decreased rapidly with increasing density at lower densities up to ∼0.2 g cm -3, τ 2r seemed to be insensitive at intermediate densities between 0.2 and 0.6 g cm -3, and τ 2r moderately increased at higher densities more than 0.6 g cm -3 . It was demonstrated that the density dependence of τ 2r in CO 2 has a clear minimum near the critical density. The present experimental results of τ 2r perfectly agreed with the calculation ones recently reported by Adams and Siavosh-Haghighi ( J. Phys. Chem. B 2002, 106, 7973). The rotational dynamics of CO 2 was discussed over a wide range of density and temperature in terms of τ 2r together with the angular momentum correlation time.