(281g) Density Functional Theory Investigation of the Hydrolysis and Condensation of Polysilicic Acid: a Mechanistic and Energetic Analysis

The discovery of new materials is
often a heuristic pursuit.  Today, many nano-porous materials with functionality
beyond their traditional roles of catalysis, ion exchange, and separations are
discovered quite serendipitously.  The need for a systematic synthesis approach
has kept this subject an area of active research.  With the advent of modern
computational and experimental techniques and the increasing speed of
computers, research has focused on obtaining a detailed understanding of the
synthesis mechanism.  To this end, we conducted a reaction path analysis of the
hydrolysis and condensation reaction of various polysilicic acid species using
density functional theory (DFT).  We considered clusters containing up to eight
silicons: the dimer Q₂¹, the linear trimer Q₂¹Q₁²,
the cyclic-trimer Q₃², the branched cyclic-trimer Q₂²Q₁³Q₁¹,
the cyclic-tetramer Q₄², the six silicon prism Q₆³,
and the octamer cage Q₈³.  In particular, we elucidate
the reaction mechanism as well as report the energies and configurations of the
reactive intermediates and transition states along the reaction pathway for
each cluster under neutral, acidic, and basic conditions ? both in vacuo and
incorporating hydration effects. 

Previous studies of these systems
have lead to a variety of proposed mechanism with two being accepted as most
probable: the first involving an S_N2-type attack and the second
occurring through a ?flank' or lateral attack.  Our results show the S_N2-type
or ?backside' attack with inversion of the molecular geometry to be the lowest
energy reaction pathway for every species studied.  In addition, we found that
in all cases the reaction proceeds through the formation of a stable
pentacoordinated intermediate.  The results obtained from thermochemical
analysis of each reaction under varying ionic conditions provide valuable
insight into the chemical behavior of these clusters, such as the preferential
formation of linear trimer species prior to trimer rings and the marked absence
of stable hydrolysis products for neutral clusters larger then six silicons.  With
the ultimate goal of modeling these clusters in solution, we present first the
results of our calculations on clusters in vacuo followed by a discussion of
our techniques and a comparison of conformational and energetic changes that
occur when hydration effects are incorporated into the model.  In particular,
we are interested in the effects the presence of excess water has on the
hydrolysis and condensation rate-limiting activation barriers.