Uranocene temperature-dependent

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. 

The temperature dependence of the unsubstituted ring proton resonances are linear functions of T 1 and the slopes of shift vs. T-- - are identical within experimental error to that of uranocene (fig. 7, Table V). The slight difference in intercepts at T-1=0 undoubtedly result from using the proton resonance of cycloocta-tetraene dianion as a diamagnetic reference for all the compounds. 

Recently, Fischer (15/45) has independently arrived at the same conclusion based on the temperature dependence behavior of the H NMR resonances of the two monosubstituted uranocenes,... 

The high degree of linearity in the temperature dependence of the ring proton shifts is evident from the correlation coefficients of the least squares regression lines (Table IX). The slopes of the lines are all negative and similar in magnitude to that of uranocene. However, the standard deviations of the extrapolated intercepts at T-1=0 indicate that a number of the intercepts are non-zero. Ideally, eq. 3 predicts that all of the intercepts should be zero at T 1=0. 

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. ... 

There have been several studies of shift anisotropy in paramagnetic solids, obtained from the orientation analysis of powder lineshapes. Bloembergen [76], Rundle [77], and Poulis and Hardeman [78] analyzed the orientation dependent shifts in single crystals of hydrated Cu salts at low temperatures. McGarvey and Nagy [79, 80] have investigated the temperature dependence of static powder spectra of uranocene U(CgHg)2, a system where all the protons are chemically equivalent. 

The magnetic susceptibility of uranocene has been measured as a function of temperature and is approximately linear in 1/T except at rather low temperatures.QO oi Over this temperature range, the proton chemical shift is also expected to be linear in 1/T many such linear dependences have been documented.77 Consequently, the anisotropy term,... 

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