silicon for the chemical and solar industry XIII

performed on the substances taken from the site of the accident. The results of the

investigation and the proposals were made public on June 12, 2014 and the plant

operation was resumed soon after that. The cause of the accident identified by the

committee was the explosive chemical reaction of chlorosilane polymers remained in

the heat exchanger and hydrolyzed in situ at low ambient temperatures. Some basic

chemistry of explosive substance, however, remained unidentified or unquantified. It

included the mechanism of the chemical reaction, the thermodynamic and the kinetic

properties related to the explosion, though hypothetical explosion processes have been

proposed elsewhere [1, 2].

Ab initio molecular orbital calculations

For preliminary calculations, a tool of molecular mechanical (MM) calculations,

Avogadro (Version 1.0.3) [3], which includes empirical parameters was used to get

approximate molecular structures. The set of the obtained atomic coordinates was

served as the initial condition for the further structural optimization.

Based on the geometries obtained by MM, more sophisticated ab initio molecular

orbital calculations were carried out by Gaussian 09 [4]. The geometries of the models

and some stationary points on the potential energy surface of the reaction path were

fully optimized at the restricted Hartree-Fock (RHF) level of theory using the 6-31G*

basis set [5] which is one of the split-valence type basis functions with a set of d-type

polarization functions on the heavy atoms. In addition, the geometries were refined

using second-order Møller-Plesset perturbation theory (MP2) [6].

Then, all optimized molecules were characterized as minima or transition states by

normal mode (vibrational) analysis.

Structural modeling

Possible molecular skeletons

The chemical composition and the infrared spectra obtained in the investigation of the

cause of the accident were used for structural modeling of the chlorosilane polymers

hydrolyzed at low temperatures. Several pieces of information found in the literature

were also utilized in the modeling. One is the existence of Si-SiOH group [7] in the

substance. Another one is decrease in the stability of the compound as increase in the

number of directly linked Si atoms in it [8, 9]. And the other is that the so-called

“silicooxalic acid” which can be synthesized by hydrolysis of Si

cooled by ice [10]

has two directly linked Si atoms [11].

On the basis of the above observed and literature information the molecular formula of

with four Si-Si bonds was chosen as a basis for the modeling. Also the same

molecular formula with smaller number of Si-Si bonds was considered.

There are six possible molecular skeletons or possible stereoisomers with four Si-Si

bonds. The schematic diagrams of those models are shown in Figure 1. They are

divided into two groups. Models A, B and C are all featured by partial hexahedral cage

structure with one open ring. Models D, E and F have twisted cage with one diagonal

Si-O-Si bond. All four Si-Si bonds are arranged almost in parallel for Models A and

D. A set of two parallel Si-Si bonds is in orthogonal arrangement to the other two

parallel Si-Si bonds for the other models. It should be noted that those concept of

“parallel” or “orthogonal” is applicable only for idealized schematic diagram in Figure

1 and not for actual structure.

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