Uranocene isotropic shift

Uranocenes. Edelstein and co-workers (5) proposed that the XH isotropic shift in uranocene can be approximated by... 

Since the calculated pseudocontact shifts are smaller in magnitude than the observed isotropic shift, Edelstein, et.al., concluded that an upfield contact component contributes to the total isotropic shift, indicative of covalency in the ligand metal bonds of uranocene. 

B. The Temperature Dependent H NMR of Uranocene and Octamethyluranocene. Our initial interest was in verifying the temperature dependence of the isotropic shift in uranocene and the reported non-zero intercept at T =0. Recent laser Raman studies by Spiro and co-workers (41) have established that the first excited state in uranocene is 466 cm- above the ground state. Thus, the isotropic shift may not vary linearly with the inverse of the temperature from -100°C to 100°C. Indeed, below 100°K some controversy exists concerning the temperature dependence of the magnetic moment in uranocene (42,43). 

The temperature dependence of the isotropic shift in uranocene was measured on two independent samples from -80°C to 100°C. At the same nominal temperature slight differences in the shift between the two samples are undoubtedly due to slight differences in the true temperature of the samples and provide an estimate of the error in temperature measurement or measurement of the resonance frequency in this study. 

Octamethyluranocene, 35, has effective 4-fold symmetry and Xx and Xy are constrained to be equal on the nmr time scale. The temperature dependence of the ring protons of this compound is compared with uranocene in Fig. 6 and Table V. The non-zero intercept is probably due to referencing the isotropic shift to the tetramethylCOT dianion note in Table IV that the ring protons of dimethylthorocene differ from methylCOT dianion by almost 1 ppm. 

Figure 6. Isotropic shift vs. T 1 for uranocene and the ring protons in 5,5, 7,7 -octamethyluranocene, 3 5...

The discussions above have shown that the pseudocontact component of the isotropic shift in 1,1 -dialkyluranocenes is accurately given by the axially symmetric form of eq. 3 and thus, these systems can be used in evaluating both the assumptions employed in deriving, and the value of the anisotropy term (xn Xj[ ) used, by previous workers in factoring isotropic shifts in uranocenes. 

Fischer has proposed useful and important methods for factoring the isotropic shifts of uranocenes into contact and pseudocontact components (15) values were reported for uranocene, 1,-1, 3,3, 5,5, 7,7 -octamethyluranocene, and 1 1 -bis(trimethyl-si lyl) uranocene using a non-zero value of Xj Fischer arrived at values of yjj2 and y 2 at several temperatures from the ratio of the geometry factor and the isotropic shift for methyl protons in bis(trimethylsilyl)-uranocene, and bulk magnetic susceptibility data, assuming no contact contributions to the isotropic shift of the methyl protons. From the published data of Fischer, the value of y( - y2 at 30°C is 8.78 BM2. 

Using Fischer s value of y j2 - yj 2=8.78 BM2 the calculated pseudocontact shifts for the t-butyl groups in 1,1 -di-t-butyl-and 1,1 -dineopentyl uranocene are -12.1 ppm and 7.28 ppm, respectively, for coplanar substituents, and -14.6 ppm and 3.22 ppm, respectively, for tipped substituents. Agreement between the calculated pseudocontact shifts and the observed isotropic shifts is rather good. Calculation of the pseudocontact shifts for the eye-... 

Previous attempts at factoring the isotropic NMR shifts in uranocene and substituted uranocenes have assumed that these systems can be viewed as having effective axial symmetry. The temperature dependent 1h NMR spectra of uranocene and a variety of substituted uranocenes clearly verify this assumption and show that eq. 9 can be used to evaluate the pseudocontact contribution to the total isotropic shift in uranocenes. In this equation xx = Xy f°r substituted uranocenes and are replaced by Xj. ... 

Early attempts to factor the isotropic shifts in alkyl-uranocenes using eq. 9 were not completely successful because of failure to correctly assess the conformation of the substituent in solution and overestimation of the value of the anisotropy term jj-Xj (5,6,14). ... 

Another important result of this study is the confirmation of Fischer s demonstration that Xj is not equal to zero in uran-ocene. Early attempts to factor isotropic shifts in uranocene have generally assumed that Xj[=0, and leads to overestimation of the anisotropy term. A precise value of Xj is difficult to determine rigorously from analysis of available NMR data. We have found that y 2 - yj 2 = 12.5 BM2 leads to the best internal consistency factored isotropic shifts for a wide variety of 1,1 -dialkyluranocenes. Assuming yav2 = 5.76 BM2 and yjj - yj 2 = 12.5 BM2, at 30°C/ the corresponding values of y. 2 and yj2 are 14.09 and 1.59 BM2, respectively. This implies that Xj j/X L 8 uran ocene, a value substantilly larger than Fischer s ratio of X /Xj = 2.8 (15). 

As a result of Xjj 0, early work on factoring the isotropic shift of the ring protons in uranocene underestimated the magnitude of the contact shift. Using our value of Uj2 yj = 12.5 BM2, the pseudocontact and contact shifts for uranocene ring protons are -8.30 ppm and -34.2 ppm, (G = -2.34 x 1021 cm-3), respectively. Thus, this study confirms that both contact and pseudocontact interactions contribute to the observed isotropic shifts in uranocenes. The contact component is dominant for ring protons, but rapidly attenuates with increasing number of Q-bonds between the observed nucleus and the uranium such that the contact shift is effectively zero for g-protons. 

Expressions have been derived for the contact shift s but for uranocenes this term is normally obtained by subtracting a calculated pseudocontact shift from the experimental isotropic shift. The pseudocontact shift for uranocene is generally represented by the axially symmetric expression may be expressed as 76,78... 

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