The effects of hydrolysis and oligomerization upon the NMR shieldings of Be+2 and Al+3 species in aqueous solution

The Be-9 and Al-27 NMR shieldings have been calculated for the species Be(OH2)(4)(+2), Be(OH2)(3)OH+1, Be-3(OH)(3)(OH2)(6)(+3), Be-2(OH)(2)(OH2)(4)(+2), Be-2(OH)(OH2)(6)(+3), Al(OH2)(6)(+3), Al(OH2)(5)OH+2, and Al-2(OH)(2)(OH2)(8)(+4) using a 6-31G* basis set and the gauge-including atomic orbital (GIAO) method. Our results indicate that although hydrolysis deshields the Be and Al nuclei, oligomerization shields them. Since experiment indicates that the most stable Be+2 and Al+3 species in aqueous solution at moderate concentration and pH are Be-3(OH)(3)(OH2)(6)(+3) and Al-2(OH)(2)(OH2)(8)(+4), respectively, which are both hydrolyzed and oligomerized, their shieldings are little different from those of their unhydrolyzed, unpolymerized parent species Be(OH2)(4)(+2) and Al(OH2)(6)(+3). The calculated deshielding of Be-3(OH)(3)(OH2)(6)(+3) with respect to Be(OH2)(4)(+2) is only 0.8 ppm (compared to 0.61 ppm observed experimentally) while the calculated deshielding of Al-2(OH)(2)(OH2)(8)(+4) compared to Al(OH2)(6)(+3) is 2.1 ppm (compared to 3.5 ppm observed experimentally). The corner sharing Be dimer, Be-2(OH)(OH2)(6)(+3) is about 0.1 ppm more strongly shielded than Be(OH2)(4)(+2). The hydrolyzed monomeric species Be(OH2)(3)OH+1 and Al(OH2)(5)OH+2 are calculated to be deshielded by 1.97 and 8.8 ppm, respectively, vs their parent ions. Such species may be observable at very low concentration, but their quadrupole coupling constants are large and they may have large linewidths. Calculated changes in average H-1 NMR shieldings for the hydrolyzed, oligomerized species derived from aqueous Be+2 are also in accord with experimental NMR data. Calculated energetics for the formation of oligomeric species from the hydrolyzed ions are consistent with the greater relative stability of oligomerized species in the Be case compared to Al. (C) 1998 Academic Press.