A systematic experimental study of the kinetics and mechanisms of the chemical reactions in the gas phase between ground-state Si(+)2p and a variety of astrophysical molecules. The aim of this study is to identify the reactions which trigger the formation of chemical bonds between silicon and carbon, oxygen and sulphur, and the chemical pathways which lead to further molecular growth. Such knowledge is valuable in the identification of new extraterrestrial silicon-bearing molecules and for an assessment of the gas-phase transition from atomic silicon to silicon carbide and silicate grain particles in carbon-rich and oxygen-rich astrophysical environments

Bohme, Diethard K.

Fox, Arnold

Wlodek, Stanislaw

English

NASA Technical Reports Server

N91-15014
GAS-PHASE FORMATION OF SILICON CARBIDES, OXIDES AND SULPHIDES FROM
ATOMIC SILICON IONS
Diethard K. Bohme, Stanislaw Wlodek and Arnold Fox
Department of Chemistry and Centre for Research in
Experimental Space Science, York University, North
York, Ontario, Canada M3J IP3.
We have recently embarked on a systematic experimental study of the
kinetics and mechanisms of the chemical reactions in the gas phase
between ground-state Si+(2P) and a variety of astrophysical molecules.
The aim of this study is to identify the reactions which trigger the
formation of chemical bonds between silicon and carbon, oxygen and
sulphur and the chemical pathways which lead to further molecular
growth. Such knowledge is valuable in the identification of new
extraterrestrial silicon-bearing molecules and for an assessment of the
gas-phase transition from atomic silicon to silicon carbide and
silicate grain particles in carbon-rich and oxygen-rich astrophysical
environments
Ground-state Si+(2P) ions are generated in our laboratory experiments
in a Selected-lon Flow Tube (SIFT) apparatus and reacted in helium buffer
gas (at ca 0.35 Tort and 296 K) with added neutral molecules. Reactant
and product ions are monitored as a function of the concentration of the
added gas. These data provide rate constants and product distributions
for the primary reactions. Secondary and higher-order reactions which
lead to further ionic growth are also monitored, including the sequential
growth of ions in which the silicon becomes increasingly co-ordinated.
In natural environments which are partially ionized, such as diffuse or
dense interstellar clouds or the atmospheres of certain stars, molecular
ions formed in this manner may ultimately be neutralized by recombination
with electrons without significantly disrupting their molecular
composition and structure, and so provide sources for complex
silicon-bearing molecules in these environments.
Ground-state silicon ions havebeen found to be unreactJve toward H2
and so should be avai}nble for reactions with other constituents in
astrophysical environments in which hydrogen gas predominates (Wlodek et
al, 1987).
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FORMA rio :OFSILICONCARBIDES
Si-C bond formation has been found to be efficient with acetylene and
cyanoacetylene which react rapidly with Si+(2P) to form SiC2 H+ by H-atom
and CN elimination, respectively:
Si + + C2H 2
Si + + HC3N
0.7
..... > SiC2 H+ + H (la)
0.3
......> SiC2H2 + (Ib)
0.7
..... > SiC2H + + CN (2a)
0.3
...... > _ ,."C,,HN+ (2b)
A secondary reaction of SiC2 H+ with acetylene was observed to build up
the carbon content further to produce SiC4H + by H2 elimination:
0.9
SiC2 H+ + C2H2 ..... > SiC4H3 + (3a)
0.1
..... > SiC4 H+ + H2 (3b)
SiC2 H+ and SiC4 H+ may neutralize by recombination with electrons or
proton transfer to produce the carbide molecules SiC 2 and SiC 4.
Ground-state silicon ions will also extract carbon from amines to form
directly the neutral molecules SiCH and SiCH 3 as well as the ion SJCH2 +
from acetonitrile to form SiCII2+, from acetone to form SiCB 3, from
ethylene to form SJC2H3 +, and from methylacetylene to form SiCH2 +.
SiC2 H+, and SiC3H3 + (Wlodek and Bohme, 1988: Bohme et al, 1988).
Scheme 1 provides a summary of the observed ion chemistry which can
be directed to the formation of silicon-carbide molecules.
It is interesting to note that Si + reacts only slowly with methane
under our experimental conditions, k = 5 x 10 -13 cm 3 molecule -1 s -l, to
form adduct ions, probably in a termolecular fashion:
Si+ + CH4 ..... > SICH4 + (4)
Also diacetylene does not entrain silicon, reacting instead to form SiH:
Si + + C4H 2 ..... > Sill + C4 H+ (5)
FORMATION OF SILICON OXIDES
A rich chemistry was observed to be initiated and propagated with
hydroxyl-containing molecules (Wlodek et al, 1987). Ground-state silicon
ions were found to react with the molecules H20, CH30H , C2H50H , HCOOH,
and CH3COOH to produce the silene cation SiOH ÷ which may neutralize to
form silicon monoxide:
474
Si+ XOH ...... > SiOH+ + X (6)
SiOH" _ e (B) ..... > SiO + H (BH *) [7}
SiH302 +, which may neutralize to produce silanoic acid, is tile predominat
product in the reactions of SiOH + with H20, C2HsOH and HCOOH, while
direct formation of sJlanoic acid is likely in the reaction with CH3COOH.
Further chemistry can be propagated by SiH302 + It associates with H20
and C2H50H and produces SiCH303 + and SiHs03 + with formic acid. Reaction
sequences identified with CH3OH, C2HsOH and HCOOH are postulated to lead
to the complete saturation of Si + forming ions of the type HSi(OCH3)3 +,
HSJ(OC2Hs)3 H+, and HSi(OH)3 H+ which may neutralize to generate
trimethoxysilane, triethoxysilane and trihydroxysilane, respectively.
Analogous reactions can be proposed which lead to the formation of
tetrahydroxysilane which is a known building block for condensational
synthesis of hydrated silica networks.
Scheme 1. Limited reaction scheme for the synthesis of silicon-carbide
molecules initiate bu atomic silicon ions. The double arrows represent
neutralization reactions such as proton transfer and electron/ion
recombination.
..................................................................
CH2NH 2, (CH 3)2NH
(CH3)2CO, (CH3)2N
•- SiCH/HSiC
SiCH3/HSiCH 2
CH3CN, (CH3)2N, CH3C2H
SiCH2 _ .... >> SiCH/HSiC
C2H 2. HC3N, CH3C2H
• SiC2 H+ .... >>
C2H2 I
SIC4 H_ .... >>
C2H 4
SiC 2
CH3C2H
SiC 4
SiC2H3 _ .... >> SiC2H 2
SJC3H3 + .... >> SiC3H 2
Schemes 2 and 3 provide a summary of the observed ion chemistry which
can be directed to the formation of silicon-oxide molecules.
475
Scheme2. Limited schemefor the synthesis of silicon-oxide molecules
with ion chemistry initiates by Si + in ethanol and formic acid. The
double arrows represent neutralization reactions such as proton transfer
and electron/ion recombination.
Si ÷
C2H5OH, HCOOH
SiOH +
C2H5OH, HCOOH
HSi(OH)2 +
..... >> SiO
..... >> HSJOOH/HSi(OH) 2
HCOOH(-CO)
C2H5OH(-C2H 4)
HCOOH(-H20)
HSi(OH)3 H+
HSi(OCHO)OH +
HCOOH(-CO)
HSi(OCHO)(OH)2 H_
..... >> HSi(OH) 3
..... >> HSJ(O)(OCHO)
..... >> HSi(OCHO)(OH) 2
C2H5OH(-H20)
HSi(OC2H5)OH + ..... >>
C2H50H
HSi(OC2H5)(OH)2 + ..... >>
HSi(O)(OC2H 5)
HSi(OC2H5)(OH) 2
The oxidation of ground-state atomic silicon ions with diatomic and
triatomic molecules not containing hydrogen was explored with 02, NO, CO,
CO 2, NO 2, SO 2, and N20. Bimolecular oxidation reactions which may
proceed with these molecules include oxygen-atom transfer as indicated in
reaction (8) and oxide-anion transfer as indicated in reaction (9). Both
Si _ _ XO ..... > SiO + + X (8)
Si + + XO ......> SiO + X + (9)
reactions (8) and (9) were observed to occur rapidly when exothermic.
Oxygen-atom transfer was observed with N20 and NO2, while oxide-anion
transfer was observed to occur with NO 2 and SO 2. Slow adduct formation
(with 02 , NO and C02), or no reaction {with CO) was observed when
formation of SiO + or SiO was endothermic.
476
With N20 oxidation was observed to proceed sequentially to produce
polyoxide ions of the type SiOn ÷ as indicated in reaction (10):
N20 N20 N20 N20
Si * ..... > SiO + ..... > Si02 + > Si03 + ..... > Si04 ÷ (10)
The production of these polyatomlc ions was observed to compete with
adduct formation and elimination of 02 + and SiO 2 which becomes important
in the last two steps in the reaction sequence. The polyoxide ions pose
interesting questions regarding their structure and the molecules which
they may spawn upon neutralization.
Scheme 3. Limited reaction scheme for the synthesis of silicon-oxide
molecules initiated by SlOB + in methanol and ethanol. The double arrows
represent neutralization reactions such as proton transfer and
electron/ion recombination.
...........................................................................
SiOCH3 _-[
CH30H (-H20)
SiOH +
I
=====_
CH30H
c-SiOH 2
c-SiOC(H)CH 3
Si(OCH3) 2
Si(0C2H5) 2
HSi(OCH3) 2
HSI(OC2H5) 3
HSi(OCH3)2 ÷ ..... >
CH30 H
HSi(OCH3)3 H+ ..... >
C2H50H (-H20)
< ..... SiOC2H5 e
C2H50H l
< ..... HSJ(OC2H5)2 _
C2H50H 1
<..... HSi(OC2H5)3 B+
FORMATION OF SILICON SULPHIDES
Si-S bond formation was observed to proceed with H2S and with COS
in a reaction analogous to reaction (8):
Si _ _ H2S ..... > SiSH _ + H (II)
Si _ + COS ..... > SiS + ÷ CO (12)
477
The polysulphide ions SiS2 + and SiS3 + were observed to be formed
sequentially with COS as reactant.
COS COS COS
Si + ..... > SiS + ..... > SiS2 _ ..... > SiS3 _ (13)
Charge-transfer reactions with molecules which have ionization energies
lower than the recombination energies of these ions can lead to formation
of the polysulphide molecules.
SUMMARY
The experimental studies reported here are providing clues to the
formation of a large number of silicon-containing molecules in partially
ionized astrophysical environments. For example, the gas-phase
ion/mo]ecule reactions which have been identified in our studies so far
represent possible contributors to the formation of the following
silicon-carbide, silicon-oxide and silicon sulphide molecules:
SiCH, SiC 2, SiCH 3, SIC 4, SiO, SiO 2, SiS, SiS 2, HSiOOH. SiOCH 2,
HSiC(H)CH 3, HSi(OH) 3, HSi(O)(OCHO), HSi(O)(OC2H5), HSi(OCHO)(OH) 2,
Si(OCH3) 2, HSi(OCH3) 3, Si(OC2H5) 2, HSi(OC2H5) 3.
Current investigations are being directed toward ion/molecule reactions
of Si+(2P) which can lead to bonding with still other heavy atoms, and
toward a further elucidation of the kinetics and mechanisms of the
higher-order chemistry which can lead to still further growth and perhaps
ultimately to the formation of small silicon-carbide and silicon-oxide
particles.
Acknowledgment
We thank the Natural Sciences and Engineering Research Council of
Canada for financial support of this research.
References
Bohme, D.K., Wlodek, S. and Fox, A.: 1988, in "Rate Coefficients in
Astrochemistry", T.J. MJllar and D.A. Williams (Eds.), Rluwer Academic
Press, 193 (1988).
Wlodek, S., Bohme, D.K.: 1988, J. Am. Chem. Soc. 110, 2396.
Wlodek, S., Fox, A. and Bohme, D.K.: 1987, J. Am. Chem. Soc. 109, 6663.
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Gas-phase formation of silicon carbides, oxides, and sulphides from atomic silicon ions

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