Tossell J A
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland, 20742, USA.
J Magn Reson. 1998 Nov;135(1):203-7. doi: 10.1006/jmre.1998.1565.
The 9Be and 27Al NMR shieldings have been calculated for the species Be(OH2)+24, Be(OH2)3OH+1, Be3(OH)3(OH2)+36, Be2(OH)2(OH2)+24, Be2(OH)(OH2)+36, Al(OH2)+36, Al(OH2)5OH+2, and Al2(OH)2(OH2)+48 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 Be3(OH)3(OH2)+36 and Al2(OH)2(OH2)+48, respectively, which are both hydrolyzed and oligomerized, their shieldings are little different from those of their unhydrolyzed, unpolymerized parent species Be(OH2)+24 and Al(OH2)+36. The calculated deshielding of Be3(OH)3(OH2)+36 with respect to Be(OH2)+24 is only 0.8 ppm (compared to 0.61 ppm observed experimentally) while the calculated deshielding of Al2(OH)2(OH2)+48 compared to Al(OH2)+36 is 2.1 ppm (compared to 3.5 ppm observed experimentally). The corner sharing Be dimer, Be2(OH)(OH2)+36 is about 0.1 ppm more strongly shielded than Be(OH2)+24. The hydrolyzed monomeric species Be(OH2)3OH+1 and Al(OH2)5OH+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 1H 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.
使用6-31G*基组和含规范原子轨道（GIAO）方法，计算了Be(OH₂)₂⁺⁴、Be(OH₂)₃OH⁺¹、Be₃(OH)₃(OH₂)₃⁺⁶、Be₂(OH)₂(OH₂)₂⁺⁴、Be₂(OH)(OH₂)₃⁺⁶、Al(OH₂)₃⁺⁶、Al(OH₂)₅OH⁺²和Al₂(OH)₂(OH₂)₄⁺⁸等物种的⁹Be和²⁷Al核磁共振屏蔽常数。我们的结果表明，虽然水解会使Be和Al原子核去屏蔽，但齐聚作用会使其屏蔽。由于实验表明，在中等浓度和pH值的水溶液中，最稳定的Be²⁺和Al³⁺物种分别是Be₃(OH)₃(OH₂)₃⁺⁶和Al₂(OH)₂(OH₂)₄⁺⁸，它们既发生了水解又发生了齐聚，因此它们的屏蔽与未水解、未聚合的母体物种Be(OH₂)₂⁺⁴和Al(OH₂)₃⁺⁶的屏蔽几乎没有差异。相对于Be(OH₂)₂⁺⁴，计算得到的Be₃(OH)₃(OH₂)₃⁺⁶的去屏蔽仅为0.8 ppm（实验观察值为0.61 ppm），而相对于Al(OH₂)₃⁺⁶，计算得到的Al₂(OH)₂(OH₂)₄⁺⁸的去屏蔽为2.1 ppm（实验观察值为3.5 ppm）。角共享Be二聚体Be₂(OH)(OH₂)₃⁺⁶的屏蔽比Be(OH₂)₂⁺⁴强约0.1 ppm。水解的单体物种Be(OH₂)₃OH⁺¹和Al(OH₂)₅OH⁺²相对于它们的母体离子，计算得出的去屏蔽分别为1.97和8.8 ppm。这些物种在非常低的浓度下可能可以观察到，但它们的四极耦合常数很大，可能具有很大的线宽。计算得到的源自水溶液中Be²⁺的水解、齐聚物种的平均¹H核磁共振屏蔽的变化也与实验核磁共振数据一致。计算得到的从水解离子形成齐聚物种的能量学与Be情况下齐聚物种相对于Al具有更高的相对稳定性相一致。
