title: Mechanisms of silicon alkoxide hydrolysis-oligomerization reactions: a DFT investigation.
abstract: Silica aerogels possess a variety of unique and remarkable properties, but the mechanisms of silicon alkoxide, Si(OR)(4), hydrolyses and oligomerization in the initial stage of sol-gel processes are still not well understood. On the basis of density functional theory calculations at the B3LYP/6-31G(d,p)//B3LYP/6-311++G(d,p) basis set level, the hydrolysis and oligomerization reactions of Si(OR)(4) in neutral, acidic, and alkaline solutions were systematically investigated and we found that in acidic solutions the precursor Si(OCH(3))(4) was inclined to hydrolyze rather than to condense and the hydrolysis processes were energetically more favorable than the neutral ones. In alkaline solutions, the hydrolysis products oligomerize through an S(N)1 dimerization mechanism and the condensation rates are fast to form denser colloidal aerogels. Our calculations also testify that the subsequent cyclization reactions are energetically unfavorable.
authors.affiliation: Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, China.
authors.forename: Xueli
authors.initials: X
authors.lastname: Cheng
authors.forename: Dairong
authors.initials: D
authors.lastname: Chen
authors.forename: Yongjun
authors.initials: Y
authors.lastname: Liu