Proton uranocene ring

Least Squares Linear Regression Lines For Alkyl Uranocene Ring Proton Data... 

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. 

Subsequently, the H NMR of 1,1, 3,3, 5,5, 7,7 -octamethyl-uranocene was analyzed in a similar manner (6). The contact shifts for the ring and ot-protons were found to be similar in magnitude, but opposite in sign, implying spin density in a 7T-MO, and transfer of spin density via a spin polarization type mechanism (entries 3 and 4 in Table II). In this paper, a new, significantly smaller, value for the pseudocontact shift in uranocene... 

Least Squares Linear Regression Analysis of VT H NMR Data for Ring Protons in Uranocene, Octamethyluranocene and the Unsubstituted Ring in Monosubstituted Uranocenes. 

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 importance of these compounds for nmr interpretations is that we can look at the unsubstituted ring in systems where Xx and Xy are not constrained by symmetry to be equal. In both of the monosubstituted uranocenes investigated, the proton resonance of the unsubstituted ring is a singlet. 

At 30°C, the protons of the unsubstituted ring in mono-t-butyluranocene resonate at 0.51 ppm lower field and those in the mono-ester resonate at 0.43 ppm higher field than the ring protons in uranocene. These differences are small but real and were established independently by observing the spectrum of mixtures of these compounds. 

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. 

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

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. 

For purposes of convenient identification, the ring proton resonances in the NMR of substituted uranocenes will be labeled alphabetically starting with the lowest field resonance. This does not imply that the "A" resonancees in two different uranocenes correspond to the same ring position. We shall discuss below the assignment of the individual ring proton resonances. 

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. 

Ill. Identification of Ring Proton Resonances in Substituted Uranocenes. 

We can now return to our conclusion in the last section where we deduced from the pattern of ring proton resonances and from steric considerations that t-butyl substituents in uranocenes must be tilted away from uranium. This argument does not apply to the neopentyl group which is a normal primary alkyl substituent for which the ring-CH2 bond can be tilted towards uranium without difficulty. 

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