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Tetramethylsilane contains no chromophores that would absorb visible or UV radiation so direct photolysis is not likely to be significant.  Indirect photolysis resulting from gas-phase reaction with photochemically-produced hydroxyl radicals may occur. 

The SRC AOPWIN program has been used to obtain values of the rate constant kOH for reaction of tetramethylsilane with hydroxyl radicals. This prediction method has not been validated to assess applicability to siloxanes/silanes/silanols; therefore, there is uncertainty associated with the calculated values obtained.

The overall half-life in air under default conditions of hydroxyl radical concentration was calculated using the following expressions:

kdegair(d-1) = kOH(cm3/molecule.sec) x OHConcair(molecules/cm3) x 24 x 3600

DT50 (d) = ln 2/ kdegair(d-1)

Where:

kdegair= total rate constant for degradation in air

kOH= rate constant for reaction with hydroxyl radicals

OH Concair= concentration of hydroxyl radicals in air =5 x 105 OH molecules / cm3

DT50 = half-life

The results are given in Table 4.1.

Table 4.1: Results of photodegradation in air calculations

Parameter

Result, tetramethylsilane

kOH(cm3/ molecule.sec)

0.6 x 10-12

kdegair(d-1)

0.03

DT50 (days)

23