Pseudocontact shift component

Eq. (2.20), or its simplified version in the axial case, Eq. (2.18), are of general validity. However, the principal directions and components of the molecular X tensor are seldom available. Pseudocontact shifts can be still evaluated by expressing the principal molecular magnetic susceptibility values as a function of the principal g values, in analogy with Eq. (1.38) ... 

The coupling of the unpaired electrons with the nucleus being observed generally results in a shift in resonance frequency that is referred to as a hyperfine isotropic or simply isotropic shift. This shift is usually dissected into two principal components. One, the hyperfine contact, Fermi contact or contact shift derives from a transfer of spin density from the unpaired electrons to the nucleus being observed. The other, the dipolar or pseudocontact shift, derives from a classical dipole-dipole interaction between the electron magnetic moment and the nuclear magnetic moment and is geometry dependent. 

B. NMR of Uranium(IV) Organometallic Compounds. Current interest in the NMR of U(IV) organometallic compounds has been concerned with the relative contributions of contact and pseudocontact shifts to the observed isotropic shifts. Much of this interest arises from the possible presence, and relative role of covalency in ligand metal bonds in organoactinide compounds. Ideally, if the isotropic shifts in U(IV) compounds can be factored into contact and pseudocontact components, the contact shift can be correlated with electron delocalization and bond covalency. 

Since the calculated pseudocontact shifts are smaller in magnitude than the observed isotropic shift, Edelstein, et.al., concluded that an upfield contact component contributes to the total isotropic shift, indicative of covalency in the ligand metal bonds of uranocene. 

The interactions of LSR with various organophosphorus substrates have been reported (460-463). Yb(fod)3 and Pr(fod), are considered to be the best LSR for organophosphorus compounds. Proton shifts are, as usual, dominated by pseudocontact interactions. shifts are predominantly pseudocontact in nature but have sizeable contact contributions for phosphine and phosphoryl compounds. In contrast P shifts have large contact components where direct phos-phoryl-oxygen or phosphorus-lanthanide interactions occur. Large pseudocontact P shifts for triethyl phosphite indicate little or no direct phosphorus-lanthanide interaction. 

A. Theory. A detailed derivation of the theory behind paramagnetic shifts in the NMR of paramagnetic compounds, or a complete review of the literature concerning separation of observed isotropic shifts into contact and pseudocontact components is well beyond the scope of this paper. Several books and reviews of these subjects are available (16-21). ... 

The discussions above have shown that the pseudocontact component of the isotropic shift in 1,1 -dialkyluranocenes is accurately given by the axially symmetric form of eq. 3 and thus, these systems can be used in evaluating both the assumptions employed in deriving, and the value of the anisotropy term (xn Xj[ ) used, by previous workers in factoring isotropic shifts in uranocenes. 

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. 

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. 

These complexes, which either are salts, or at least contain coordinated anionic ligands in which case a large Ln-ligand bond moment should result from the rather electrostatic nature of the bond, tend to be insoluble in solvents other than polar solvents in which they dissociate. Hence physicochemical studies in solution are limited. However, solid state fluorescence spectra of phenanthroline and bipyridyl complexes of Eu have been obtained and correlated with the molecular site symmetry (for a discussion of this tyrc of fluorescence, see Section 39.2.7.2). Magnetic susceptibility values have been measured for the series of complexes M(N03)3(phen)2 at 293 K and are (BM) as foUows La, 0 Ce, 2.46 Pr, 3.48 Nd, 3.44 Sm, 1.64 Eu, 3.36 Gd, 7.97 Tb, 9.81 Dy, 10.6 Ho, 10.7 Er, 9.46 Tm, 7.51 Yb, 4.47 Lu, 0. These values correspond closely with the calculated values. The complexes Ln(N03)3(4,4-di- -butyl-2,2 -bipyridyl)2 and Ln(N03)3(5,5 -di-n-butyl-2,2 -bipyridyl)2 are soluble in organic solvents and give NMR spectra which show paramagnetic shifts. These were at first believed to be of a covalently induced, or contact, nature but it later became evident that they incorporate a substantial dipolar, or pseudocontact, component also. Electronic spectra of these complexes were also obtained in solution. They showed only a small nephelauxetic effect with =0.96, which indicated very little covalent character in the Ln—bond. 

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