ION PAIRING IN H2O AND D2O SOLUTIONS OF LEAD NITRATE, AS DETERMINED WITH 207Pb NMR SPECTROSCOPY (300,00 руб.)

The results are analyzed in terms of exchange between a solvated lead ion and the Pb(NO3)+ contact-ion pair. <...> Predictions of the chemical shift difference between the aquated ion and contact-ion pair are carried out for the gas-phase entities and for the solvated species with a DFT calculation. <...> Previously reported data on 207Pb NMR chemical shifts of Pb(NO3)2 in H2O are reevaluated. <...> From the analysis, the enthalpy of dissociation of the contact-ion pair is found to be –42.31.0 kJ/mol. <...> INTRODUCTION Electrochemical and spectroscopic measurements have demonstrated that dissolution of certain salts in water leads to the formation of contact-ion pairs in equilibrium with fully aquated ions [ 1— 11 ]. <...> In aqueous solutions of the nitrates, it is found that Pb2+ has the strongest proclivity for the formation of contact ion pairs [ 11 ]. <...> The strong temperature variation of the 207Pb NMR chemical shift of solid Pb(NO3)2 is consistent with a significant sensitivity of the electronic state of the lead ion to small structural changes such as variations of interionic distances induced by thermal expansion [12—16 ]. <...> Studies of the variation of the 207Pb NMR chemical shift of lead-containing solids with applied pressure also point to the significant variation of electronic state with structural change [ 15, 16 ]. <...> The electrochemical and spectroscopic properties of ion pairs in solution are more complex than those of the solid, in part because of the dynamics of exchange in solution, as well as the interaction of the ions with the solvent [ 1—10]. <...> For example, Nancollas interpreted changes in conductivity of aqueous lead nitrate solutions as evidence for rapid exchange between contact-ion pairs and solvated ions by the mechanism of Eq. (1) [ 2 ]: Pb(NO ) Pb NO3(aq). 3 (aq)  (aq) 2 (1) From the temperature and concentration dependences of the conductivity of lead nitrate solutions, he determined an equilibrium constant, Kdiss, of 0.065 at 298.15 K, and Hdiss = 2.38 kJ/mol and Sdiss = = –14.6 JK–1mol–1 for the process of Eq. (1). <...> The NMR chemical shifts of nuclei such as 207Pb in solution, which are rapidly interchanging among sites by processes such as those described in Eq. (1), depend strongly on temperature and concentration [ 4—6, 10 ]. <...> Harrison et al. analyzed room-temperature concentration-dependent <...>