Uranocene resonances

Uranocene, U( -C8H8) provides an excellent example. The f ionization band, the first of the spectrum, shows two characteristic features, a delayed maximum and a giant resonance. Figure 13(a) shows the cross section of this band. The next two bands are primarily ring C 2p r bands of 62 symmetry. They show the characteristic decay associated with nodeless 2p orbitals (Figure 13(b)). The second band, however, has, superimposed on this decay, a maximum around 40 eV and a double resonance between 95 and 125 eV. This indicates partial 5f character and assigns the orbital to the e2 orbital. The third band is consequently associated with the C2g orbital and its higher IE is indicative of the fact that the U 6d orbitals form more effective bonds that the 5f in this instance. 

Figure 13 PE cross sections of uranocene (a) f band showing a delayed maximum and a giant resonance (b) e2u and e2g bands, the 62u band shows features characteristic of f character...

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. 

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. 

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,... 

CqH8)(CqH7R)U, R = P(t-C4H9)2 and Sn(t-C4Hg)3. In both of these compounds the unsubstituted ring resonances is reported to be identical with that in uranocene. 

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. 

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

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. 

The ionization cross section of the first band of uranocene (the/-band) shows a dramatic rise to a delayed maximum at 40 eV and subsequently resonance structure between 95 and 120eV (Figure 23). Assignment of the second and third bands is readily made from their cross sections. Both decay with an increase in photon energy, as expected for C... 

The mechanism of the reaction to form the bis([8]annulene) complexes is not known. It is clear, however, that there are differences in the mechanism in the formation of uranocene compared to thorocene. When the reaction of two equivalents of n-butyl[8]annulene dianion with thorium tetrachloride was carried out in an NMR tube and monitored by iH-NMR, a resonance was observed which cannot be attributed to the bis([8]annulene) complex.22 Further experiments showed that this apparent intermediate in the reaction is the mono-ring complex, [8]annulenethorium(IV) dichloride (vide infra).22 The mechanism for the formation of bis([8]annulene)thoriuro evidently involves the stepwise substitution of [8]annulene dianion for two chloride ions. 

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