Other
Scientific paper
May 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.1935t&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 57, Issue 9, pp.1935-1945
Other
3
Scientific paper
Ab initio self-consistent-field molecular orbital (SCF-MO) calculations have been employed to determine the structures and properties of Zn and Cd bisulfides and their hydrates. Calculated M-S bond lengths, R(M-S), for M ( SH ) n ( OH 2 ) 2- n a species are consistently about 0.05 Å longer than the experimental ones but show the right trends with n and a . Calculated Zn-S symmetric and average stretching frequencies scale linearly with R(Zn-S) -1 , so that Zn-S distances can be estimated from vibrational spectra and vice versa. The total NMR shieldings of Zn and Cd in M ( SH ) 2- n n species show a shallow minimum for n = 3, and the shift compared to free M 2+ can be well fitted to a function increasing linearly with n and decreasing exponentially with R(M-S). The calculated shielding anisotropy for Cd(SH) - 3 is in good agreement with observed values for the C 3 h symmetry species Cd(SR) - 3 in solids. Complexation of water to Zn(SH) 2 produces slightly larger Zn-S distances and smaller Zn shieldings due to the deshielding effect of the tightly bound water. For Zn(SH) - 3 , on the other hand, hydration is less exothermic since it requires greater Zn-S bond elongation and results in a longer Zn-O bond. Reduction of the magnitude of deshielding from S, coupled with only a small deshielding from O, causes the Zn NMR shielding of Zn(SH) 3 OH - 2 to be larger than that of Zn(SH) - 3 . Such effects of hydration on structure and shielding are much like those previously seen in studies of Zn and Cd chlorides. Comparison of the isomeric species Zn(SH) 3 OH -2 and ZnS(SH) 2 (OH 2 ) -2 shows the former to be more stable by more than 50 kJ/mol. The two species also differ spectroscopically, with ZnS(SH) 2 (OH 2 ) -2 having a higher stretching frequency and a lower Zn NMR shielding. Equivalently, for the species Zn(SH) 3 (OH 2 ) - , the H 2 O is calculated to be more acidic than the SH - . Both H 2 O and SH - are more acidic in the complexes studied herein than when occurring as the free molecules. The quantitative change is, in fact, greater for SH - , but not enough to make it more acidic than H 2 O. Comparisons of the energies of intermediate members in some relevant chemical series indicate that Zn(SH) - 3 is enhanced in stability compared to Zn(SH) 2 and Zn(SH) -2 4 , and that Zn(SH) 3 (OH) -2 is enhanced in stability compared to Zn(SH) -2 4 and Zn(OH) -2 4 ; whereas Zn(SH) 2 Cl -2 2 is unstable energetically compared to Zn(SH) -2 4 and ZnCl -2 4 . This supports the occurrence of Zn(SH) - 3 and Zn(SH) 3 (OH) -2 species and argues against that of Zn(SH) 2 Cl -2 2 . Zn(SH) -3 also provides a model for Zn sites on the surface of solid ZnS, showing the geometric relaxation expected compared to the bulk. Hydration of the ZnS surface can be modeled by the species Zn(SH) 3 (OH 2 ) -1 . Such Zn and Cd species have large NMR shielding anisotropies, and the Cd species may be observable using modern NMR techniques.
Tossell John A.
Vaughan David J.
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