Computational studies of polyelectrolytes containing zeolitic fragments

The structures and sodium affinities of a series of zeolitic fragments [H3A l(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 c alculated by ab initio molecular orbital methods at different levels of the ory. 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-coordina ted OCH3 group by an OSiH3 group causes a decrease of 7 kcal/mol in the sod ium affinity, while the substitution for a non-sodium-coordinated OCH3 grou p results in a 2.7 kcal/mol decrease. The lower sodium affinity indicates a weaker Coulombic interaction, suggesting an enhanced ionic conductivity wi th the substitution of carbon by silicon, consistent with experimental resu lts. Natural bond orbital (NBO) analyses show that silicon-bonded oxygen at oms have smaller lone-pair dipole moments, resulting in a lower sodium affi nity. The substitution of aluminum by boron leads to a higher sodium affini ty, 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 an gles in these systems is also investigated.