Isotropic ring proton

Table V displays the XH-NMR data of [Cp2Yb(ketim)]2 at various temperatures. The rather different isotropic shifts(opposite signs ) of the two methyl groups suggest their location in fairly different "magnetic environments". It is, however, not possible to decide if there are N- or O-bridging links. As only one Cp ring proton resonance appears, some fluxional behaviour of the bridging ketim ligand is not unlikely.

The temperature dependence behavior of the ring proton isotropic shifts also reflects the effects of lower symmetry. While the ring proton shift in Cp U shows a linear dependence on T from -106°C to 133°C, the ring proton shifts of Cp3U-X compounds 2-5, 8-10, 12, 17-18, and 23-25 all show marked deviations from linearity. The alkyl-substituted systems show linear behavior from ca. -150°C to room temperature but deviate from linearity above room temperature. The alkoxy compounds show apparent linearity from ca. 200°C to 400°C but deviations from linearity below 200°C. All of the halides except for the fluoride display a slight curvature from 200°C to 400°C. The variable temperature behavior of the fluoride is solvent dependent and reflects the formation of dimers. 

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. 

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

Figure 8. Isotropic shift vs. T1 for the ring protons in l,r-dimethyluranocene, 27...

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. 

Numerical results for the shielding field of the benzene molecule are collected in Table 1 for the center of the molecule (labelled COM), and for points along a quarter circle of radius 2.47 A from the -ajcis to the x-axis, see Figure 3 for specification of axes. The radius of the circle corresponds to the distance from the ring center to a proton but, as defined, the points lie in the entirely nucleus-free xz-plane. Except for COM, the entries in the table are labelled by the angle between the z-axes and the direction to the field point. The table includes the isotropic part of the shielding, and the principal... 

The spin densities deduced from the formula aftft, = ( A1 + 2 A )/3 are presumed to be positive because of direct delocalization of the unpaired electron from the (a3da + b4s) hybrid orbital. It should be noted, however, that Aft1 and A , measured for rotating rings at T = 150 K, are not principal values of the proton hfs tensor, i.e. a , determined in this way does not exactly correspond to the isotropic coupling obtained by the common formula aftft, = (A" + A" + A )/3. 

It is known that five-membered Cr(V) chelates are favored over six-membered ones.19,50,61 For Cr(V)-diolato complexes formed with linear diols, it was observed that all of the protons are equivalent in the isotropic EPR spectra,62 although the strain of a six-membered ring imparts inequivalence to the magnetic environment of the protons in the second coordination sphere.63 This observation at room temperature again points to rotational flexibility in the chelate ring (namely, puckering in the 8 or X configuration),20 similar to that observed for 1. 

As well as providing a means of measuring 1 H/2H-exchange in proteins, NMR is a most powerful technique for studying the mobility of individual amino acids. For example, the rotational freedom of the aromatic side chains of tyrosine and phenylalanine about the C 3—Cy bond is readily studied by various NMR methods. ]H NMR can detect whether or not the aromatic ring is constrained in an anisotropic environment. In an isotropic environment or where there is rapid rotation on the NMR time scale, the 3 and 5 protons of phenylalanine and tyrosine are symmetrically related, as are the 2 and 6 (structures 1.12). The resultant spectrum is of the AA BB type, containing two pairs of closely separated doublets. But if there is slow rotation in an anisotropic environment, the symmetry breaks down to give four separate resonances (an ABCD spectrum), since the 5 and 6 protons are in different states from the 2 and 3. At an intermediate time... 

In hemes and hemoproteins contact shifts arise if finite amounts of unpaired electron spin density are delocalized from the iron orbitals into the jr-orbital systems of the porphyrin and the axial ligands, as indicated by the arrows in Fig. 25. Electron density is then further transferred from the aromatic ring carbon atoms to the protons (Fig. 2), thus giving rise to contact interactions. The measured isotropic contact coupling constants for the protons, A in Eq. (4), can be related to the integrated spin density on the neighboring ring carbon atom by (McConnell (73)] Bersohn (5) Weissman (107). 

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