NMR of Quadrupolar Nuclei in Supercritical CO

Supercritical media have very promising features for NMR spectroscopy. Because of their low bulk viscosity and the associated small molecular rotational correlation times of solutes in these media [68], relaxation times of quadrupolar nuclei increase significantly and, hence, their line widths (HHW) will be considerably reduced compared to those in common solvents [30,69,70]. This feature may be advantageously employed for the detection of NMR resonances of quadrupolar nuclei, because measurements in the liquid state are often troubled due to extremely broad line widths. An illustrative example involving Co was described in Section 3.2-3. 

For measurements of solutes in supercritical media, toe phase behavior is always complicated by the presence of two or more components. Although the critical point can be estimated from the equation of state, this estimate is not always accurate. An ideal and fast probe for toe experimental determination of toe state of CO2 is to monitor the chemical shift and intensity of its resonance at natural abundance. Once the fluid reaches its supercritical state, the intensity of the resonance decreases concomitant with a change 

copper wire G, Viton 0-ring H, Vespel 0-ring I, Teflon O-ring J, Viton O-ring K, gas inlet. 

The appropriateness of the method has been demonstrated by measuring the line widths of a series of tertiary amines and a-diimines in supercritical CO2 [75]. The line widths are reduced 3-5 times upon going from benzene to supercritical CO2 as the solvent. For instance, for (/-Bu-N=CH)2 the 

Jonas and co-workers have pointed out that often a compromise must be found between sensitivity and resolution in NMR spectroscopy [76]. Line narrowing is optimum in regions of low supercritical fluid density (where the viscosity is low), but then the solubility of compounds is also low. Sometimes, admixtures with small amounts of low-viscosity solvents such as acetone may be tried to obtain a reasonable concentration of the compounds studied, i.e. coordination compounds such as (R-N=CH)2Mo(CO)4. However, line widths for this compound decrease by a factor of about four to six when comparing benzene-dg solutions to supercritical CO2 (with 8% acetone-de). The dispersion of the nitrogen chemical shift ensures identification of coordinated ligands by using NMR, in the above molybdenum complex, where A5( n) = -36 ppm) [75]. 

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