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.