Pseudocontact interaction

The observed hyperfine shifts could come from contact coupling or pseudocontact interactions between the electrons and the protons. Contact shifts arise when a finite amount of unpaired electron density is transferred to the observed protons. The contact shifts of the proton resonances for isotropic systems are given by Bloembergen s (9) expression... 

The pseudocontact interaction (perhaps more appropriately called a dipolar interaction) arises from the magnetic dipolar fields experienced by a nucleus near a paramagnetic ion. The effect is entirely analogous to the magnetic anisotropy discussed in Section 4.5. It arises only when the g tensor of the electron is anisotropic that is, for an axially symmetric case, j> g . The g value for an electron is defined as... 

Pseudocontact interactions are thought to be important for V(acac)3 and Mn(acac)3. Spin-lattice relaxation times have also been reported (94,36) using the PRFT method (95). The Tj values are dominated by DD interactions which may be both metal- and ligand-centred. Assuming that the former are dominant, it follows that the ratio of the CH3 and -CH= carbon Ti values is related to the sixth power of the ratio of their distances from the metal, namely ... 

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. 

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. 

The observed large values of the carbon contact shifts (rel to H) indicate that the pseudocontact interactions are probably of secondary importance (data for these complexes is in Table LVI). A comparison of the Ni(2+) (acac)2 complexes with the Co(2-l-) derivatives shows that the mode of delocalization is probably different. The shift at the y carbon (shielded carbon for both Ni and Co, deshielded proton Ni, shielded Co) was interpreted in terms of a dominant n delocalization mechanism due to the close agreement of the aicjffiu fatio with that predicted by theory. Both the pC and are deshielded (/ C > fJH). The observed ratio is too large (relative to theory)... 

The conditions necessary for observation of proton magnetic resonance spectra in paramagnetic systems are well established (1). Either the electronic spin-lattice relaxation time, T, or a characteristic electronic exchange time, Te, must be short compared with the isotropic hyperfine contact interaction constant, in order for resonances to be observed. Proton resonances in paramagnetic systems are often shifted hundreds of cps from their values in the diamagnetic substances. These isotropic resonance shifts may arise from two causes, the hyperfine contact and pseudocontact interactions. The contact shift arises from the existence of unpaired spin-density at the resonating nucleus and is described by 1 (2) for systems obeying the Curie law. 

We are able to explain the difference in isotropic shift ratios between the cobalt and nickel compounds on the basis of a pseudocontact interaction... 

Chemists did not follow the caution voiced by Weissman [30] that both contact and pseudocontact (dipolar) shifts are to be expected in molecules coordinated to Europium shift reagents. Numerous workers applied a simplified form of the McConnell Robertston relationship (Equation (3)) for pseudocontact interaction as the sole interpreter of the lanthanide induced shifts in rigid organic molecules. It has been shown that especially when C data are compared to H data [31], that both the dipolar and contact effects operate simultaneously ... 

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