Time-averaged shifts

This conclusion is at variance with current interpretations of observed dimerization shifts in pu-py dimers (19-24) and prompts us to propose new (but tentative) interpretations for some of the stacking shifts 6 - d, remendbering that represents a time-average shift of micro-... 

Proton resonance shifts for salts in water have been studied for many years [25,26], but have proven difficult to interpret. Apart from the other problems associated with time-averaged shifts, this arises because of the problem of assigning separate shifts to cations and anions in a non-empirical fashion. Because of its relative simplicity, we have studied methanol rather than water [27] and endeavoured to extrapolate to aqueous solutions [28]. 

Equation (2.3) describes line positions correctly for spectra with small hyperfine coupling to two or more nuclei provided that the nuclei are not magnetically equivalent. When two or more nuclei are completely equivalent, i.e., both instantaneously equivalent and equivalent over a time average, then the nuclear spins should be described in terms of the total nuclear spin quantum numbers I and mT rather than the individual /, and mn. In this coupled representation , the degeneracies of some multiplet lines are lifted when second-order shifts are included. This can lead to extra lines and/or asymmetric line shapes. The effect was first observed in the spectrum of the methyl radical, CH3, produced by... 

Nitrogen donors usually form weak complexes with the lanthanides in aqueous solution. Sinha and Green 26) have measured the NMR spectra of complexes formed between 1,10-phenanthrohne and Ce(III), Pr(III), Nd(HI) and Eu(IH) in D2O. As the spectra were measured at room temperature, only time-average signals for the phenanthrohne protons were obtained. However, a plot of the shift of the protons as a function of the mole ratios of the hgand and metal ion (Fig. 2)... 

There is some evidence that while mono adducts of the j8-diketonate shift reagents may show time-averaged uniaxial symmetry, bis adducts often do not. Thus the spectra of [M(dpm)3L2], where M = Pr, Sm, Dy, Ho, Er or Yb and L = 3-picolinc or 3,5-lutidine, have been interpreted in terms of biaxial magnetic anisotropy. One magnetic axis is defined by the molecular C2 axis present in the X-ray crystal structure and the other two axes lie in similar orientations in all the adducts. A variable-temperature study of this system shows restricted rotation of the aromatic rings with a very solvent-dependent conformational equilibrium resulting from this.558-560... 

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