CALCULATIONS OF THE STRUCTURES, STABILITIES, AND RAMAN AND ZN NMR-SPECTRA OF ZNCLN(OH2)A2-N SPECIES IN AQUEOUS-SOLUTION

Ab initio self-consistent-field molecular orbital (MO) methods have been used to calculate minimum-energy geometries, total energies, and the totally symmetric Raman mode and Zn NMR spectra for Zn-containing species which may exist in chloride-containing aqueous solutions, such as ZnCl+, ZnCl2, ZnCl3-, ZnCl4(2-), ZnCl(OH2)5+, ZnCl2(OH2)4, ZnCl3(OH2)2-, and Zn(OH2)6(2+). Calculated bond distances, Raman frequency, and absolute Zn NMR shielding for Zn(OH2)6(2+) are in good agreement with experiment. The calculated structure, Raman frequency, and Zn NMR shielding are also in good agreement with experiment for ZnCl4(2-), indicating that involvement of water in the first coordination sphere is weak for this ion. For ZnCl2 and ZnCl+, on the other hand, calculated Raman frequencies for Zn-Cl stretching are too large and Zn NMR shieldings much too diamagnetic, indicating substantial H2O participation in the first coordination sphere. Calculations on the aquated complexes Zncl(OH2)5+ and ZnCl2(OH2)4 confirm this interpretation, yielding equilibrium Zn-Cl distances which are larger by approximately 0.20 angstrom and Zn NMR shieldings which are lower by hundreds of parts per million in the aquated species (compared to anhydrous Zncl(n)2-n). ZnCl(OH2)5+ is found to have a long Zn-Cl bond and a short Zn-O bond and to give a Zn NMR shielding very similar to that of Zn(OH2)6(2+) while ZnCl2(OH2)4 has a much longer Zn-O distance and is deshielded by about 250 ppm with respect to Zn(OH2)6(2+). ZnCl3- interacts rather weakly with H2O to give a ZnCl3(OH2)2- complex with a slightly lengthened Zn-Cl bond, a long Zn-O bond, and intermediate Zn NMR shielding, and a reduced Zn-Cl stretching frequency. From calculated hydration energies for ZnCl2 and ZnCl3- and literature values of the Cl- hydration energy, we have estimated the energy change for the ZnCl2 (+CVl-) --> ZnCl3- equilibrium. The Zn NMR shieldings can be obtained semiquantitatively for all the species by calculating the diamagnetic contribution from an atom superposition model and semiquantitatively for all the species by calculating the diamagnetic contribution from an atom superposition model and correlating the magnitude of the paramagnetic term with the Zn 4p Mulliken orbital population. Calculated Zn NMR shieldings show the same trend as experiment if energy-optimized geometries are employed. Our results indicate in general that ab initio SCF MO techniques are capable of giving quantitative information about the speciation of Zn in aqueous solutions.