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

We have precisely measured 17O longitudinal relaxation times, T 1(17O), of CO2 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 17O relaxation, the reorientational correlation time, τ2r, of CO2 was obtained from T 1(17O) with a known value of the quadrupole coupling constant. The τ2r in CO2 thus determined showed a unique density dependence; τ2r decreased rapidly with increasing density at lower densities up to ∼0.2 g cm-3, τ2r seemed to be insensitive at intermediate densities between 0.2 and 0.6 g cm-3, and τ2r moderately increased at higher densities more than 0.6 g cm-3. It was demonstrated that the density dependence of τ2r in CO2 has a clear minimum near the critical density. The present experimental results of τ2r perfectly agreed with the calculation ones recently reported by Adams and Siavosh-Haghighi (J. Phys. Chem. B 2002, 106, 7973). The rotational dynamics of CO2 was discussed over a wide range of density and temperature in terms of τ2r together with the angular momentum correlation time.