The structures of the hydrated zinc(I1) and nitrate ions in 3.72 mo! dm-3 aqueous ZniN03)z have been determined bv X-ray diffraction. Octahedral coordination in Zn;; is confirmed and the average Zn-Owater distance is 2.17 (4) A; there is no evidence for coordination by NO;. Approximately three water molecules are hydrogen bonded to each nitrate 0-atom.The use of X-ray diffraction for determining the molecular geometry of metal complexes in solution has been very limited compared with the solid-state applications, and such studies as have been reported have largely involved the heavier metals and simple monodentate ligands. A major reason for our interest in it is the possibility of directly determining the coordination of zinc in its complexes in solution. While it is generally agreed that the important role played by Zn" in enzyme chemistry is linked with its ability to adopt a coordination number (CN) of 4, 5 or 6 and to change its coordination geometry comparatively easily according to the circumstances, its d lo configuration has so far precluded the direct determination of its CN by spectroscopic techniques. We have developed a simple but accurate photographic technique which uses Cu K, radiation and an enclosed cylindrical sample and have applied it initially to a study of aqueous Z I I ( N O~)~. Its use to determine the coordination geometry of the 2,2',2"-triaminotriethylamine (tren) complex of zinc (chloride) will be reported elsewhere.A full single-crystal structural analysis of Zn(N03j2.6H20 has been reported by Ferrari et al.,' who showed that the the coordination at the metal ion is octahedral. The only X-ray report on solutions of this salt has been by Bol et L Z~. ,~ who compared the diffraction pattern of a unimolar aqueous solution with that of the same concentration of Mg(NO3)2. Assuming both metals to be octahedrally hydrated, Bol et al. argued that the anion-solvent, solvent-solvent and anion-anion interactions would be same in the two cases and would therefore be eliminated by subtracting one radial distribution function (r.d.f.) from the other. They determined the inner-sphere zinc-oxygen distance but were unable to comment on any other characteristic distances other than to suggest that there might be 12k2 water molecules in the second hydration shell, with rZn-o of ca. 4.2 A.The only other reports on X-ray diffraction studies of solutions of aqua-zinc have been by Shapovalov3 on the sulphate and Ohtaki et al.4 on the sulphate and perchlorate. In the former case, three concentrations were studied (corresponding to mole ratios xH20:xZn of 31, 50 and 100) and the inner-sphere rZn-0 was determined for all three. The CN of zinc was assumed to be six throughout, and no evidence was found to indicate any deviation from this, or of sulphate coordination; no other distances were reported. Ohtaki et al. found the CN of zinc to be close to 6 for both the sulphate (xHz0 : x Z n = 19.2) and the perchlorate (15.4), and also found no evidence of sulphate coordination. They reported the zinc-oxygen distan...
The acid dissociation constants have been determined by potentiometric titration in H,O and D,O for the zinc(I1) complexes of 2,2'-ethylenedi-iminobis(ethylamine) (trien), 2,2'trimethylenedi-iminobis(ethy1amine) (2,3,2-tet) and 2,2',2"-nitrilotris(ethylamine) (tren) and by '"C n.m.r. spectroscopy in D,O for ethylenediamine (en;2), glycine (gly;2), L-alanine (ala), acetic acid (ace) and N-(2-aminoethyl)propane-1,3-diamine (2,3-tri ;3). The equilibrium deuterium isotope effects are similar, pKa(D,O) -pKa(H,O) = 0.63 & 0.07. This result supports the treatment of the protonated forms of polyamines as 'hybrid' ions in respect of their contribution to ionic strength and indicates that the coordination number of Zn" in the low-pH forms of the three complexes is at least five.
The 13C n.m.r. spectra are reported as a function of pH for 1,2-diaminoethane [en], N-(2aminoethyl)propane-l ,3-diamine [2,3-tri 3, 1,4,7,1O-tetra-azadecane [trien], and 1,4,8,11 -tetraazaundecane [2,3,2-tet]. The previously reported amine shift parameters ~t and ~t + are used to determine the protonation sites in the charged forms of the polyamines and the associated microscopic pK, values. AnalysisThe atoms and non-equivalent C atoms in the polyamines are identified as in the Scheme. The measured chemical shifts 6 of the C atoms in 2,3-tri, trien, 2,3,2-tet, and en at different pH are represented by the points in Figure 1. The titration curves in
The reactions of phenols with the free radical galvinoxyl (which is stable to air and solvents and deeply coloured) constitute a series of H-atom-transfer reactions where the solvent and substituents can conveniently be varied and accurate kinetic measurements can be made. The kinetics for a series of substituted phenols in toluene solution have been studied; we report rate constants (which vary over a range of more than lo4 at 25 "C), activation parameters and some deuterium isotope effects (rate ratio ca. 6 at 25 "C). The results can be interpreted in terms of the electronic effects of substituents in stabilising the incipient phenoxy radical in the transition state and the steric effects of o-substituents in reducing the interaction of the phenol with the solvent. The reaction of one of these phenols, 2,4,6-tri-t-butylphenol, has been studied in a series of aprotic solvents of varying polarity and hydrogen-bond-acceptor properties. The rate constant decreases markedly with increasing polarity of the solvent; the isotopic rate ratio also varies appreciably from solvent to solvent. Desolvation of the phenol is apparently involved. Interpretation of the isotope effects may be sought in terms of tunnelling or of transition-state asymmetry.
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