Proton uranocenes, temperature

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. 

E. H NMR of Substituted Uranocenes. Table VIII summarizes the chemical shifts relative to TMS for a number of uranocenes at a common temperature (30°C). The results are summarized for ring and substituent protons for convenience. 

Note in these results that the total difference between the highest and lowest field resonance of the non-equivalent ring protons in all of the uranocenes increases as the temperature decreases. Moreover, the relative pattern of the ring proton resonances in each uranocene remains constant as a function of temperature except for the two phenyl-substituted uranocenes and 1,1 -... 

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. 

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. 

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,... 

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. 

---