Computational Studies of Polyelectrolytes Containing Zeolitic Fragments

The structures and sodium affinities of a series of zeolitic fragments [H₃Al(OCH₃)_x(OSiH₃)^- _1-x, 2T, H₂Al(OCH₃)_x(OSiH₃)^- _2-x, 3T, Al(OCH₃)_x(OSiH₃)^-_4-x, 5T] that mimic the charge sites in polyelectrolytes are calculated by ab initio molecular orbital methods at different levels of theory. At the HF/6-31G * level, the decrease in the sodium affinity due to the substitution of an OCH₃ group by an OSiH₃ group is about 8 kcal/mol in the 2T and 3T systems. In the 5T systems, the replacement of a sodium-coordinated OCH₃ group by an OSiH₃ group causes a decrease of 7 kcal/mol in the sodium affinity, while the substitution for a non-sodium-coordinated OCH3 group results in a 2.7 kcal/mol decrease. The lower sodium affinity indicates a weaker Coulombic interaction, suggesting an enhanced ionic conductivity with the substitution of carbon by silicon, consistent with experimental results. Natural bond orbital (NBO) analyses show that silicon-bonded oxygen atoms have smaller lone-pair dipole moments, resulting in a lower sodium affinity. The substitution of aluminum by boron leads to a higher sodium affinity, although the effect of replacing an OCH₃ group by an OSiH₃ group still reduces the sodium affinity. The effect of the sodium cation on the bond angles in these systems is also investigated.