Dimethoxydimethylsilane

Phototransformation in air

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

phototransformation in air

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

Please refer to the justification for grouping of substances provided in IUCLID Section 13.

Reason / purpose for cross-reference:

read-across source

Preliminary study:

Prior to the product study, an exploratory photolysis of an air mixutre of demethylsilanediol, CH3ONO and NO with initial concentration of 1.8 E+14, 2.4 E+14 and 1.8 E+14 molecules/cm³ was carried out.

Reaction with:

OH radicals

Rate constant:

0 cm³ molecule-1 s-1

Transformation products:

yes

KINETIC STUDY (IR Spectra and Dark Decay in the 5870 L chamber)

Vapor samples of dimethylsilanediol ((CH3)2Si(OH)2; 1.3 E+14 molecules/cm³) showed the initial presence and growth of another component with time. From the known amount of the sample and the differences between the spectra with time, the spectrum of the decay products at t = 61 min and of dimethylsilanediol at t = 0 were derived. The disappearance of (CH3)2Si(OH)2 was faster in the evacuated chamber than in the presence of 1 atm of air. In both cases, however, the decays could be described as first order in (CH3)2Si(OH)2, with observed decay rates of 0.015 min^-¹ at reduced pressure (< 5 Torr N2 diluent) and 0.0022 min^-¹ at 1 atm.

The observed decay rates of (CH3)2Si(OH)2 inits mixtures with (CH3)2O and O3 prior to injection of N2H4 were in the range of 0.6 – 1.3 E^-²/min and these decay losses (due to self-reaction and/or wall losses) were significant fractions (48 – 62%) of the total losses after introduction of N2H4. Hence, only one injection of N2H4 per experiment was made, with the last spectral record of the “pre-reaction” decay measurement being used to establish the initial concentrations of (CH3)2Si(OH)2 and (CH3)2O and the final concentrations being determined from the next spectrum recorded after N2H4 injection and the mixing time of ca. 3 min duration. The elapsed time between these 2 spectra was 6 – 8 min. No measurable decays of the reference compound (CH3)2O in the reaction chamber prior to N2H4 injection were observed. The data was plotted and resulted in a slope for (k1/k2) of 0.285 ± 0.02 (where k1 and k2 are the rate constants for reactions 1 and 2 respectively, see below).

(1)   Organosilicon + OH -> products

(2)   (CH3)2O + OH -> products

 

The k1/k2 value can be converted to an absolute value by plugging in 2.84 E-12 cm³/molecule* s for k2, the rate constant for the (CH3)2O + OH reaction (reaction 2), resulting in a rate constant of 0.81 ± 0.10 E+12 x k1 (cm³ molecule^-1s^-1), corresponding to 8.1 ± 1.0 E-13 cm3/molecule*s.

TROPOSPHERIC LIFETIME

Based on the measured OH radical rate constant and a 24 h average OH radical concentration of 1.0 E+06 molecules/cm³, a tropospheric lifetime of 15 d was calculated for dimethylsilanediol.

PRODUCT STUDY

Product studies for dimethylsilanediol could not be carried out on pure samples due to its inherent tendency to form condensation products in the bulk sample or in the gaseous reaction mixtures. Thus, the investigation of the fates of the alkoxy radicals, formed after the initial OH radical and/or Cl atom abstraction of an H atom from a methyl group was complicated by formation and subsequent reaction of these impurities. For diemthylsilanediol, the possible interferences from a number of these products limited the analysis by FT-IR spectroscopy, although the product spectra provided evidence for the formation of formate ester intermediates. API-MS analysis indicated the formation of methylsilanetriol from dimethylsilanediol.

The findings provide support for the following tropospheric degradation scheme:

(CH3)2Si(OH)2 --> CH3Si(OH)2C•H2 (reaction with OH)

CH3Si(OH)2C•H2 --> CH3Si(OH)2OCH2O• (reaction with O2)

CH3Si(OH)2OCH2O --> CH3Si(OH)2OCHO (reaction with O2)

CH3Si(OH)2OCHO --> CH3Si(OH)3 (reaction with H2O)

Since dimethylsilanediol will be released into the atmosphere at concentrations where autocondensation will be negligible, incorporation into aerosols (and subsequent washout) could be significant removal pathways for dimethylsilanediol in the atmosphere in addition to reaction with OH radicals and wet deposition from the gas phase.

Conclusions:

An experimental rate study with the source substance Dimethylsilanediol (DMSD) found a reaction with the OH radical rate constant of 8.1E-13 cm³/molecule and second. DMSD is the hydrolysis product of the target substance. Therefore comparable results are expected with the target substance Dimethoxydimethylsilane.

Reference 2

Endpoint:

phototransformation in air

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

The study was well documented and meets generally accepted scientific principles, but was not conducted in compliance with GLP.

Principles of method if other than guideline:

A kinetic and product study were conducted in two different reaction chambers to measure the rate constant for reaction of the OH radical with dimethylsilanediol (CH3)2Si(OH2) and to investigate the products of the atmospheric reactions formed after H-atom abstraction by OH radicals and/or Cl atoms from dimethylsilanediol.

KINETIC STUDY
In the kinetic study, the fast reaction of hydrazine (N2H4) with ozone (O3) was used to generate OH radicals. During the experiment, aliquots of hydrazine were introduced to an O3-organosilicon-dimethyl ether air mixutre for 2 - 3 min (the N2H4 + O3 reaction was essentially complete at each stage). Dimethyl ether ((CH3)2O) was used as reference compound. The decay rate of the organosilicon compounds (e.g. dimethylsilanediol) were analysed by FT-IR absorption spectroscopy prior to and after the injection of N2H4. Significant decay losses of dimethylsilanediol (CH3)2Si(OH2) in its mixtures with (CH3)2O and O3 were observed prior to the injection of N2H4 due to self-reaction and/or wall losses (48 - 62%). The observed decay rates were significant fractions of the total losses after the injection of N2H4. Therefore, only one N2H4 injection was made per experiment.

PRODUCT STUDY
In the product study, Chlorine atom-initiated photooxidations of dimethylsilanediol were conducted in both reaction chambers by photolyzing mixtures of the organosilicon compound (e.g. dimethylsilanediol) and Cl2 in dry synthetic air at 740 Torr total pressure and ca. 5% relative humidity. Irradiations were then carried out continuously for 20 - 40 min and spectra were recorded every 5 min. Prior to this experimet, an exploratory photolysis of an air mixture of dimethylsilanediol, CH3ONO and NO was carried out. The chamber contents were sampled directly into an API mass spectrometer source and MS (scanning) as well as MS/MS analyses of the mixtures were performed before and after each illumination period.

GLP compliance:

no

Estimation method (if used):

PHOTOCHEMICAL REACTION WITH OH RADICALS (KINETIC STUDY)
- Concentration of OH radicals: 4.8 E+14 molecules O3/cm³; 1.0 - 2.4 E+14 molecules N2H4/cm³
- Degradation rate constant: Calculated for reaction 1 (organosilicon + OH -> products) and reaction 2 ((CH3)2O + OH -> products)
- Temperature for which rate constant was calculated: 298 ± 2 K
- Pressure in reaction chamber: 740 Torr of dry synthetic air
- Other: The applicability of the N2H4 + O3 reaction as a non-photolytic source of OH radicals for OH rate constant measurements has previously been validated elsewhere (Tuazon, E .C.; Carter W. P. L.; Atkinson R.; Pitts, J . N., Ir. Int. J. Chem. Kinet. 1983, 15, 619. and Tuazon, E .C.; Carter W. P. L; Atkinson R.; Winer A.M; Pitts J. N., Ir. Environ. Sci. Teclnol. 1984, 18, 49.)

Light source:

Xenon lamp

Light spectrum: wavelength in nm:

> 300

Details on light source:

- Emission wavelength spectrum: 650 - 1850 cm-¹ / 700 - 1900 cm-¹ (wavenumber)
- Light source: 24 kW xenon arc lamp
- Filters used and their purpose: 6.4 mm thick Pyrex pane, used to remove wavelengths < 300 nm
- Duration of light/darkness: Irradiations were carried out continuously for 20 - 40 min and spectra were recorded every 5 min (product study).

Details on test conditions:

KINETIC STUDY (REACTION WITH HYDROXYL RADICAL & MEASUREMENT OF THE CORRESPONDING RATE CONSTANT)
- Reaction chamber: Evacuable, teflon-coated 5870 L chamber with an in-situ multiple-reflection optical system interfaced to a Nicolet 7199 Fourier transform infrared (FT-IR) absorption spectrometer
- Irradiation: 24 kW xenon arc lamp, filtered through a 6.4 mm thick Pyrex pane to remove wavelengths < 300 nm
- Temperature: 298 ± 2 K
- Pressure: 740 Torr of dry synthetic air (80% N2 + 20% O2)
- Analysis: Before (20 - 60 min) and after N2H4 injection by FT-IR absorption spectrometry

PRODUCT STUDY (INVESTIGATION OF REACTION PRODUCTS: CHLORINE ATOM INITIATED PHOTOOXIDATION)
- Reaction chamber: 5870 L chamber (see above) and a teflon-coated 7500 L Teflon chamber equipped with 2 parallel banks of blacklamps and interfaced to a PE SCIEX API III MS/MS direct air sampling, atmospheric pressure ionization tandem mass spectrometer (API-MS)
- Temperature: 298 ± 2 K
- Pressure: 740 Torr
- Relative humidity: 5%
- Initial organosilicon concentration: 1.2 - 1.8 E+14 molecules/cm³
- Initial Cl2 concentrations: 2.4 - 5.1 E+14 molecules/cm³
- Irradiation: Continuous for 20 - 40 min
- Analysis: Spectra were recorded every 5 min by API-MS analysis

Duration:

61 min

Temp.:

298 K

Reference substance:

yes

Remarks:

Dimethyl ether (for measurement of OH radical rate constants by the relative rate method in the kinetic study)

Preliminary study:

Prior to the product study, an exploratory photolysis of an air mixutre of demethylsilanediol, CH3ONO and NO with initial concentration of 1.8 E+14, 2.4 E+14 and 1.8 E+14 molecules/cm³ was carried out.

Reaction with:

OH radicals

Rate constant:

0 cm³ molecule-1 s-1

Transformation products:

yes

KINETIC STUDY (IR Spectra and Dark Decay in the 5870 L chamber)

Vapor samples of dimethylsilanediol ((CH3)2Si(OH)2; 1.3 E+14 molecules/cm³) showed the initial presence and growth of another component with time. From the known amount of the sample and the differences between the spectra with time, the spectrum of the decay products at t = 61 min and of dimethylsilanediol at t = 0 were derived. The disappearance of (CH3)2Si(OH)2 was faster in the evacuated chamber than in the presence of 1 atm of air. In both cases, however, the decays could be described as first order in (CH3)2Si(OH)2, with observed decay rates of 0.015 min^-¹ at reduced pressure (< 5 Torr N2 diluent) and 0.0022 min^-¹ at 1 atm.

The observed decay rates of (CH3)2Si(OH)2 inits mixtures with (CH3)2O and O3 prior to injection of N2H4 were in the range of 0.6 – 1.3 E^-²/min and these decay losses (due to self-reaction and/or wall losses) were significant fractions (48 – 62%) of the total losses after introduction of N2H4. Hence, only one injection of N2H4 per experiment was made, with the last spectral record of the “pre-reaction” decay measurement being used to establish the initial concentrations of (CH3)2Si(OH)2 and (CH3)2O and the final concentrations being determined from the next spectrum recorded after N2H4 injection and the mixing time of ca. 3 min duration. The elapsed time between these 2 spectra was 6 – 8 min. No measurable decays of the reference compound (CH3)2O in the reaction chamber prior to N2H4 injection were observed. The data was plotted and resulted in a slope for (k1/k2) of 0.285 ± 0.02 (where k1 and k2 are the rate constants for reactions 1 and 2 respectively, see below).

(1)   Organosilicon + OH -> products

(2)   (CH3)2O + OH -> products

 

The k1/k2 value can be converted to an absolute value by plugging in 2.84 E-12 cm³/molecule* s for k2, the rate constant for the (CH3)2O + OH reaction (reaction 2), resulting in a rate constant of 0.81 ± 0.10 E+12 x k1 (cm³ molecule^-1s^-1), corresponding to 8.1 ± 1.0 E-13 cm3/molecule*s.

TROPOSPHERIC LIFETIME

Based on the measured OH radical rate constant and a 24 h average OH radical concentration of 1.0 E+06 molecules/cm³, a tropospheric lifetime of 15 d was calculated for dimethylsilanediol.

PRODUCT STUDY

Product studies for dimethylsilanediol could not be carried out on pure samples due to its inherent tendency to form condensation products in the bulk sample or in the gaseous reaction mixtures. Thus, the investigation of the fates of the alkoxy radicals, formed after the initial OH radical and/or Cl atom abstraction of an H atom from a methyl group was complicated by formation and subsequent reaction of these impurities. For diemthylsilanediol, the possible interferences from a number of these products limited the analysis by FT-IR spectroscopy, although the product spectra provided evidence for the formation of formate ester intermediates. API-MS analysis indicated the formation of methylsilanetriol from dimethylsilanediol.

The findings provide support for the following tropospheric degradation scheme:

(CH3)2Si(OH)2 --> CH3Si(OH)2C•H2 (reaction with OH)

CH3Si(OH)2C•H2 --> CH3Si(OH)2OCH2O• (reaction with O2)

CH3Si(OH)2OCH2O --> CH3Si(OH)2OCHO (reaction with O2)

CH3Si(OH)2OCHO --> CH3Si(OH)3 (reaction with H2O)

Since dimethylsilanediol will be released into the atmosphere at concentrations where autocondensation will be negligible, incorporation into aerosols (and subsequent washout) could be significant removal pathways for dimethylsilanediol in the atmosphere in addition to reaction with OH radicals and wet deposition from the gas phase.

Validity criteria fulfilled:

not applicable

Conclusions:

An experimental rate study found a reaction with the OH radical rate constant of 8.1E-13 cm³ molecule-1 second-1.

Additional information

Dimethoxydimethylsilane (CAS 1112-39-6) hydrolyses rapidly to dimethylsilanediol and methanol under environmental conditions (DT50 < 0.6 h at pH 7, pH 9 and 25 °C). Thus, the hydrolysis products rather than the parent substance are expected to be present in the environment after environmental release.

Direct photolysis is unlikely to be relevant for both hydrolysis products as they contain no chromophores that would absorb visible or UV radiation. However, indirect photolysis with hydroxyl radicals may occur.

Measured photodegradation rate constant data for indirect photodegradation with hydroxyl radicals are available for both dimethylsilanediol and methanol. A rate constant value for reactions with hydroxyl radicals of 8.1E-13 cm3/molecule*sec was determined for DMSD using a relevant test method (Tuazon et al. 2000). Using an average concentration of OH-radicals of 5E+5 mol cm-3, a half-life of 20 days can thus be estimated.

Methanol is degraded in the atmosphere by photochemical, hydroxyl-radical dependent reactions, with a rate constant of 0.932E-12 cm3/molecule*sec (Atkinson, 1989). A half-life in the troposphere of about 17 days can be estimated.

In addition, the AOPWIN program (v1.92, EPA 2010) has been used to obtain a value of the rate constant kOH for reaction of dimethoxydimethylsilane with hydroxyl radicals. The rate constant of 1.96E-12 cm3/molecule*sec, results in an estimated half-life of about 8 days for the parent substance. 

Thus both hydrolysis products and the parent substance dimethoxydimethylsilane are slowly degraded by photochemical processes in air.

Reference:

Atkinson R (1989). Kinetic and Mechanisms of the Gas-Phase Reactions of the Hydroxyl Radical with Organic Compounds. Journal of Physical and Chemical Reference Data. Monograph No. 1: 160