Contact ion pair, formation

Avoid broadening through contact ion-pair formation, either by using a good ionizing solvent or a cation with a non-magnetic nucleus. 

In UV-visible studies of metal-14 anions showed bathochromic shifts of their absorption maxima in comparison with those of the precursor organogermanes. This absorption band (Table 5) can be explained in terms of a transition from the non-bonding orbital of the metal (HOMO) to the lowest anti-bonding orbital (LUMO) of the metal-carbon bonds for alkyl metal-14 anions or of the phenyl groups in the aryl series . Moreover, the bathochromic shifts on going from hthium to potassium are indicative of CIP (Contact Ion Pair) formation for the aryl group-14 anions with an electron localization at the metal. 

The enantiomerization mechanism of racemic a-phenylselanylalkyllithiums has been studied. It seems that the rate limiting step is a reorganization with a contact ion pair formation [22,23] (Scheme 21). 

Equation (65) represents the contact ion-pair formation between complex cyanide and alkali metal ions and Eq. (66) the bridging of the lone pair on iSOs with the cation of the contact ion pair. Isomer I is the dominant form of SOa in aqueous medium (79)... 

From the geometrical model described above, an average coordination number of 6 around a lithium ion was resulted for both solutions. In the LiNCS solution the cation is fiilly solvated, while in the LiCl solution 5 DMF molecules and 1 chloride ion are located in the neighborhood of a cation. The Li-O distance values agree within the limits of errors. The contact ion-pair formation seems to be plausible because the solution of LiCl was already near to saturation and thus the probability of the contact ion-pair formation is mote enhanced. 

Infrared spectroscopy in the CO stretching region provides a sensitive probe for detecting contact ion pair formation between cations and carbonylates (12,13). Good examples are provided by the early work on the interaction of cations with tetracarbonylcobaltate (12) and by more recent work on a variety of metal carbonyl anions (13,14). In solvents such as THF, alkali metal carbonylates appear to be present in two or three different forms tight (contact) ion pairs, looser ion pairs, and ion triplets. For example, Fig. 5 shows the IR spectrum of Na[Mn(CO)s] in THF and its deconvolution into two different sets of bands, which are associated with two different species in solution (14). For one of these the evidence points to a contact ion pair of structure 6. 

Takeuchi, M. Matubayasi, N. Kameda, Y. Minofar, B. Ishiguro, S. Umebayashi, Y, Free-Energy and Structural Analysis of Ion Solvation and Contact Ion-Pair Formation of li with BF4 and PF in Water and Carbonate Solvents. J. Phys. Chem. B 2012,116,6476-6487. 

This observation is in accordance with expectation, that is, enhanced ion-ion interactions in concentrated solutions. Furthermore, Hr(LiPF6-PC) > Hr(LiBF4-PC) is valid for the whole concentration range, indicating that the contact ion pair formation ability of PFg" is weaker than that of Bp4. This conclusion is in accordance with results from conductivity measurements for dilute solutions, see Table 17.7. Hr ratios were also derived from molecular dynamics (MD) trajectories and qualitatively agree with experimental values. 

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