Relaxation and Linewidth Studies in SCFs

Studies on T values for protons in SC H2O [16], CHCI3 [27], H2S and HCl [28], CH4 [29], C2H4 [30] and toluene [31], and also of dissolved CeHe [10] and hexadecane [32] in SC CO2 show the increased importance of relaxation by spin-rotation in the supercritical phase, at the expense of dipole-dipole relaxation. This is not surprising given the equation governing these relaxation mechanisms for intramolecular dipole-dipole relaxation [33], 

Thus the rate of dipole-dipole relaxation is greater for longer Xc values which corresponds to high viscosity, t), and low temperature, T, and of course the converse is true for spin-rotation relaxation. The low viscosity of SCFs (and, frequently, use of higher temperatures) should clearly lead to enhanced spin-rotation and reduced dipole-dipole relaxation. In the studies of SCFs mentioned above and for in SC CO2 the decrease in relaxation time as temperature was increased showed that spin-rotation had become the dominant relaxation pathway (for all other relaxation mechanisms increasing temperature results in a lengthening of T whereas in the case of the much larger hexadecane, dipole-dipole relaxation remained dominant. In the latter study the authors point out that spin-rotation relaxation dominates for the C resonance in CO2. In the study of SC H2O by Lamb and Jonas [16] it was noted that under supercritical conditions the proton T values were not affected by addition of O2, unlike in normal solvents. 

In the complex Mn(q-C5H5)(CO)2H2 [38,39], in which the H2 ligand is bound, T for the H2 ligand is surprisingly long [40], 500 ms in SC Xe at 298 K. This makes it difficult to apply the T criterion to test for coordinated H2 in SCFs, but we were able to confirm the presence of an H-H bond using HD /hd was 33 Hz, compared with 43 Hz in free HD. 

The same system was studied in the presence of a small amount of Cr(acac)3 (acac = acetylacetonato) (with a little CeD6 in the case of xenon to help 

---