Uranocene proton resonances

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. 

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. 

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 -... 

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. 

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. 

In all of the mono- and 1,1-disubstituted uranocenes prepared to date, the NMR resonances of the non-equivalent protons in... 

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