5028 ppm at 13.8 MHz) with an intensity ratio of ca. 4:l. The poor resolution vitiated accurate, simultaneous determination of the individual relaxation times. Addition of small quantities of Eu(fod)3 to the extent of p = 0.01 ( p being the molar ratio of lanthanide reagent to acetone-d6) resulted in complete separation of the two resonances (Av = 0.4 ppm) and allowed simultaneous determination of the individual relaxation times. The relaxation times determined in this manner were found to agree, within experimental error (&lo%), with those determined on separate chloroform solutions of the individual components not containing the shift reagent. Subsequent additions of Eu(fod)3 up to and including p = 0.02 (Av = 0.8 ppm) led to the same result. At p > 0.02 significant shortening of the relaxations times of the components was observed.
To quantify the effect of Eu(fod)3 on the relaxation time of acetone-d6 and to probe the nature of the interactions contributing to this effect, the relaxation time of aCetOne-dh in a chloroform solution of acetonpd6 (0.81 M ) and CDC13 (1 .O M ) (for internal referencing) containing varying amounts of Eu(fod)3, and in separate experiments the diamagnetic La(f0d)3,~was determined (six samples of each reagent in the range 0.0 5 p 5 0 . 1 4 ) . At least five separate T i measurements were performed on each sample. In this range of p the chemical shift of the acetone resonance with respect to internal CDC13 is linearly correlated with the concentration of Eu(fod)3 (slope of the best least-squares line = 10.2 ppm per unit p ) . Similarly, 1/T1 is linearly correlated with respect to p . In the case of Eu(fod)3 the slope is 7.03 X lo-' sec-l per unit p (correlation coefficient 0.934) and the intercept ( p = 0.0) is 0.223 sec-I; for La(fod)3 the slope is 6.64X lo-' sec-' per unit p (correlation coefficient 0.927) and the intercept ( p = 0.0) is 0.223 sec-'. These findings suggest that the pseudocontact paramagnetic interaction effect (ca. 6%), if experimentally significant, is at best very small in the entire range of p studied. Thus, the relaxation of acetone-d6 in the presence of the lanthanide shift reagent is still primarily dominated by the quadrupolar relaxation mechanism, suggesting that the molecular reorientation time of the acetone molecule is significantly affected by complexation with the bulkier shift reagent molecule. For small p (
