Mechanism of thermal decomposition of allyltrichlorosilane with formation of three labile intermediates: dichlorosilylene, allyl radical, and atomic chlorine

Article abstract of DOI:10.1007/s11172-016-1438-4

It is experimentally found that allyltrichlorosilane dissociates under vacuum pyrolysis (~10^–2 Torr) at temperatures above 1100 K to form three labile intermediates: allyl radical, dichlorosilylene, and monoatomic chlorine. On the basis of experimental and theoretical data obtained, it is shown that the decomposition reaction proceeds in two steps. The first step is a typical reaction of homolytic decomposition to two radicals (C₃H₅ and SiCl₃) at the weakest Si—C bond. Due to weakness of the Si—Cl bond in the SiCl₃ radical, the energy of which is even somewhat lower than the dissociation energy of the Si—C bond in starting AllSiCl₃, this radical undergoes further dissociation to SiCl₂ and Cl, thus resulting in three intermediates of different classes of highly reactive species formed from AllSiCl₃.

Full text of DOI:10.1007/s11172-016-1438-4

Russian Chemical Bulletin, International Edition, Vol. 65, No. 5, pp. 1216—1224, May, 2016
1216
Mechanism of thermal decomposition of allyltrichlorosilane
with formation of three labile intermediates:
dichlorosilylene, allyl radical, and atomic chlorine*
S. E. Boganov,a V. M. Promyslov,a I. V. Krylova,а G. S. Zaitseva,a,b and M. P. Egorova
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 8941. Eꢀmail: bog@ioc.ac.ru
bDepartment of Chemistry, M. V. Lomonosov Moscow State University,
1/3 Leninskie Gory, 119991 Moscow, Russian Federation.
Fax: +7 (495) 932 0067.
It is experimentally found that allyltrichlorosilane dissociates under vacuum pyrolysis
(~10–2 Torr) at temperatures above 1100 K to form three labile intermediates: allyl radical,
dichlorosilylene, and monoatomic chlorine. On the basis of experimental and theoretical data
obtained, it is shown that the decomposition reaction proceeds in two steps. The first step is
a typical reaction of homolytic decomposition to two radicals (C3H5 and SiCl3) at the weakest
Si—C bond. Due to weakness of the Si—Cl bond in the SiCl3 radical, the energy of which is
even somewhat lower than the dissociation energy of the Si—C bond in starting AllSiCl3, this
radical undergoes further dissociation to SiCl2 and Cl, thus resulting in three intermediates of
different classes of highly reactive species formed from AllSiCl3.
Key words: allyltrichlorosilane, allyl radical, dichlorosilylene, trichlorosilyl radical, matrix
IR spectroscopy, quantum chemical calculations, G4(MP2) level.
Matrix isolation1 is one of the most widely used techꢀ
niques for studying highly reactive intermediates of chemꢀ
ical reactions. The decomposition of compoundꢀprecurꢀ
sors under thermal action, various types of radiation, and
a flow of charged or excited particles is most frequently
used for the generation of labile intermediates. When orꢀ
ganic radicals (i.e., species with unpaired electrons) are
generated from organic compounds characterized by only
closed electron shells, two radical particles are formed,2
although a stable accompanying product is often formed
along with the radicals.2 When such labile intermediates
as nitrenes,3—5 carbenes,4,6 and their analogs (silylenes,7
germylenes, and stannylenes8) bearing an even number of
electrons on the outer shell are generated, a labile interꢀ
mediate is eliminated from the precursor molecule to form
one or several molecules of stable products. The examples
(Eqs (1)—(5)) of the generation of allyl radical9—15 (C3H5)
and dichlorosilylene16 (SiCl2), used in matrix isolation
studies and illustrating the above described general schemes
of decomposition of precursor molecules are given below.
(2)
(3)
(4)
(5)
Undoubtedly, the formation of minimally possible
number of labile species upon stable precursor decompoꢀ
sition is not a law but it has a clear physical sense. In the
general case, the formation of labile molecules implies
a significant increase in the energy of the molecular system.
The more labile molecules are formed by the decomposiꢀ
tion, the stronger is the increase in the energy of the sysꢀ
tem and the more energy should be transferred to the preꢀ
cursor molecule, which, in turn, increases the probability
of secondary processes to occur. Therefore, a good precurꢀ
sor mainly decomposing to a target intermediate should
(1)
* Dedicated to Academician of the Russian Academy of Sciences
O. G. Sinyashin on the occasion of his 60th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1216—1224, May, 2016.
1066ꢀ5285/16/6505ꢀ1216 © 2016 Springer Science+Business Media, Inc.

Thermal decomposition of allyltrichlorosilane
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
1217
calculation of the eigenvalues of the matrix of the second derivaꢀ
tives of the energy. The structures related to the transition states
were established using IRC.19,20 The thermodynamic functions
were calculated in the harmonic oscillator—rigid rotor approxiꢀ
mation.
be a compound generating a minimum number of labile
molecules for its decomposition to require minimum enꢀ
ergy. In this respect, the thermal decomposition of allylꢀ
trichlorosilane (AllSiCl3) turned out to be quite untypical.
The formation of almost only allyl radical and SiCl2, two
labile species belonging to different classes of highly reacꢀ
tive intermediates, was detected by matrix IR spectroꢀ
scopy upon vacuum pyrolysis of AllSiCl3
Results and Discussion
The matrix IR spectrum of allyltrichlorosilane
(AllSiCl3) earlier nonꢀdescribed in literature is presented
in Fig. 1. The detected vibration frequencies of AllSiCl3
and their most probable assignment to normal modes,
which is based on the calculation of fundamental vibraꢀ
tions by the B3LYP/6ꢀ31G(2df,p) method (a part of the
G4(MP2) calculations) are presented in Table 1.
(6)
This work deals with the study of the mechanism of
this unusual process.
Experimental
The pyrolytic decomposition of AllSiCl3 started at
~850 °C. As the temperature increased, the conversion of
AllSiCl3 increased and reached ~50% at ~1000 °C (see
Fig. 1, b). The bands of the pyrolysis products observed in
the IR spectra are presented in Table 2. The most part of
them is easily identified on the basis of literature data and
the matrix IR spectra of individual compounds and labile
intermediates detected previously at our laboratory. The
bands of SiCl2 (see Table 2) near 500 cm–1 having a charꢀ
acteristic splitting on the Cl isotopes are most intense.16,21
The band of radical C3H5 at 801 cm–1 is almost of the
same intensity. In addition to this band, 15 bands belongꢀ
ing to radical C3H5, whose matrix IR spectrum is well
known,9—15 were revealed in the spectra of the products.
The bands of such stable unsaturated hydrocarbons as
allene,22,23 acetylene,24—29 methylacetylene,22,30,31 and
especially ethylene32,33 are noticeably less intense. The
formation of the listed hydrocarbons is characteristic of
the processes of pyrolytic generation of allyl radical from
various precursors9,11—14 and, most likely, it is predomiꢀ
nantly a result of secondary transformations of C3H5.14
According to published data,14 the weak band at
3040 cm–1 belongs to cyclopropyl radical (cꢀC3H5), an
isomer allyl radical. This assignment is confirmed by the
disappearance of this band under the irradiation of the
matrices with the full light of a mercury lamp (230—1000
nm) due to the isomerization of this radical to C3H5.34
The formation of cyclopropyl radical was observed upon
the thermal generation of C3H5 from various precurꢀ
sors.11,14 Two very weak bands at 3308 and 687 cm–1 were
assigned to propargyl radical,22,30,35,36 since the first of
them is characteristic of this radical.
A setup for matrix isolation was constructed on the basis of
a Displex CSWꢀ208R closedꢀcycle refrigeration system (APD
Cryogenics). The matrices were formed on a cooled mirror
copper plate located inside a standard vacuum cryostat (APD
Cryogenics) with optical KBr windows (for IR measuremens)
and CaF2 windows (for UV and visible irradiation of matrices).
The temperature of the copper plate during matrix deposition
and IR spectra recording was 10—12 K, and the matrices were
annealed at the temperatures specified in a range of 24—41 K.
The temperature was measured using a DTꢀ470ꢀSD12 silicon
diode fixed on the plate with a Lake Shore Model 330ꢀ11 conꢀ
troller (Lake Shore Cryotronics). The cryostat was evacuated
with a CVLꢀ100 diffusion pump through a liquid nitrogen trap to
a residual pressure of ~10–6 Torr.
Argon (99.998%, OAO Moskoks) served as a matrix gas. An
Ar flow was passed over P2O5 to remove water traces, supplied to the
cryostat along the nonꢀheated line with a rate of 7—9 mmol h–1
,
and condensed on the copper plate together with pyrolysis prodꢀ
ucts. The pyrolysis of AllSiCl3 (97%, ABCR GmbH) or allyl
chloride (AllCl, Acros Organics, 99%) was carried out in a quartz
reactor connected to the cryostat. The reactor was a quartz tube
with the external and internal diameters equal to 6 and 4 mm,
respectively, and the heated zone length equal to 100 mm. The
outlet hole of the reactor was remote at a distance of ~50 mm
from the copper plate. The reactor was heated with a nichrome
coil to 900—1000 °C. The temperature was measured with
a chromel—alumel thermocouple connected to an IRT 5301
digital meter (Elemer Company). The pressure of the reactant in
the flow through the reactor was monitored using a PMTꢀ4M
manometric converter and maintained not higher than 10–2 Torr
to provide the dilution of the reactant with argon (in mol) in
a ratio of at least 1 : 600. The typical time of matrix deposition
was 1.5—2 h.
A DRShꢀ500 highꢀpressure mercury lamp (500 W) equipped
with a water filter to absorb IR radiation served as a light source
for the photolysis of matrices.
IR spectra were recorded in a range of 4000—400 cm–1 with
a resolution of 0.5 cm–1 via the reflection scheme on an IFS
113v FTꢀIR spectrometer (Bruker) with a DTGS/KBr detector.
Quantum chemical calculations were performed at the
G4(MP2) level of theory17 using the Gaussian 09, revision D.01
program package.18 The character of the found stationary points
(a minimum or a first order saddle point) was monitored by the
Minor amounts of HCl were also detected in the matrꢀ
ices.37—40 However, the intensity of its bands was comparable
with that of the bands of water, which is always present in
the matrices because of microleakages in the vacuum sysꢀ
tem and desorption from its internal surfaces, rather than
with the band intensity of other products, indicating the
formation of HCl due to the side reaction of hydrolysis of
the Si—Cl bonds that are very sensitive to water traces.

1218
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
Boganov et al.
a
w
w
CO2
CO2
2
b
c
1
1
2
1
1
w
6
6
5
1
4
1
1
6
1
7
7
7
7
1
?
5
1
1
1
8
3
7
7
1
w
1
5
1
4
1
CO2
1
1
1
6
5
5
1
1
3
1
1
6
?
d
w
CO2
w
3200
3000 2000
1800
1600
1400
1200
1000
800
600
400 ν/cm–1
Fig. 1. Matrix IR spectra of AllSiCl3 (a), products of pyrolysis of AllSiCl3 (b) and AllCl (c) at 1000 °C, and unpyrolyzed AllCl (d). The
bands of the following compounds are marked: 1, C3H5; 2, SiCl2; 3, cꢀC3H5; 4, propargyl radical HC≡CCH2; 5, HC≡CMe;
6, HC≡CH; 7, H2C=C=CH2; 8, H2C=CH2; ?, nonꢀidentified products; w, water; and CO2, carbon dioxide (H2O and CO2 are always
present in the matrices because of desorption of these compounds from the internal surface of the vacuum systems and the presence of
microleakages in the vacuum setups).
Scheme 1
In addition to the bands of the listed products, a numꢀ
ber of weak bands was observed in the IR spectra, and the
most noticeable of these bands are included in Table 2.
The low intensity of these bands impeding their assignꢀ
ment implies their belonging to products of secondary
reactions of transformation of primary products or minor
directions of the decomposition of AllSiCl3.
Annealing of the matrices resulted in the weakening
of bands of the labile species (C3H5, cꢀC3H5, and
HC≡CCH2, SiCl2) because of the decay of their decay in
intermolecular reactions upon diffusion in the softened
matrices. However, even for matrix annealing at 41 K for
10 min, the band intensities of radical C3H5 decreased by
not more than twice, indicating a relatively low efficiency
of these reactions. For the photolysis of the matrices with
the full light of a mercury lamp, in addition to the disꢀ
appearance of the band belonging to cꢀC3H5, the bands of
radical C3H5 appreciably decreased and a series of new
weak bands of nonꢀidentified products appeared.
Thus, the observed pyrolytic transformations of AllSiCl3
can be presented by Scheme 1.
The main direction of the thermal decomposition of
AllSiCl3 is the decomposition to SiCl2, C3H5, and, as a conꢀ
sequence of the material balance, to atomic Cl. This deꢀ
composition can include the primary detachment of SiCl2
which is a reaction reverse to one of the characteristic
reactions of silylenes: insertion in the σꢀC—Cl bond7 folꢀ
lowed by C—Cl bond cleavage in the formed AllCl, the more
so AllCl itself is known as a thermal precursor of C3H5.9,11
To check this assumption, we carried out experiments
on the pyrolysis of AllCl in the same temperature range
(the matrix IR spectrum of this compound is shown in
Fig. 1, d, also see Refs 41 and 42). Like AllSiCl3, AllCl
began to decompose at ~850 °C. The degree of decompoꢀ
sition increased with temperature, and the formation of
the same hydrocarbon products as in experiments on the
pyrolysis of AllSiCl3 was observed (see Fig. 1, c), although
(7)
,

Thermal decomposition of allyltrichlorosilane
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
1219
Table 1. Experimental (Ar matrix, 12 K) and calculated (B3LYP/6ꢀ31G(2df,p)) vibration frequencies (ν/cm–1) of AllSiCl3, their
intensities (I/km mol–1), and their assignment to normal modes based on the calculations
ν
ν
calcd (I)
Assignment
ν
ν
calcd (I)
Assignment
exp
exp
gaucheꢀconformer cisꢀconformer
gaucheꢀconformer cisꢀconformer
—
3233 (10)
3157 (5)
3146 (6)
3081 (2)
3235 (14)
3130 (17)
3157 (2)
3034 (2)
ν
ас(=CH2)
ν(=CH)
988 m
987 sh
923 w
919 m
916 sh
915 s
909 m
778 sh
775 sh
773 s
772 sh
767 sh
765 sh
763 s
760 sh
623 s
619 s
590 vs
589 sh
583 vs
582 sh
580 sh
—
—
—
—
—
—
—
—
—
3093 w
3016 w
2989 w
—
ν (=CH2)
ν
—
с
ас(CH2)
941 (13)
—
952 (31)
—
—
—
793 (41)
—
—
—
759 (32)
—
—
624 (82)
—
580 (172)
—
569 (134)
—
441 (25)
395 (0)
297 (1)
220 (7)
207 (5)
171 (2)
149 (0)
120 (0)
65 (0)
48 (0)
936 (11) ν(C—C) + ρ(=CH2)
3025 (5)
3004 (7)
ν (CH2)
—
956 (35)
—
—
—
812 (28)
—
—
—
711 (59)
—
—
517 (0)
—
586 (170)
—
577 (168)
—
411 (10)
534 (32)
240 (7)
213 (5)
208 (2)
164 (1)
153 (1)
57 (0)
101 (0)
67 (0)
—
с
1841 w
1836 sh
1834 sh
1832 w
1637 m
1423 sh
1422 sh
1420 m
1419 sh
1397 w
1388 m
1304 w
1300 w
1212 sh
1210 w
1207 sh
1190 m
—
—
—
—
—
—
—
—
—
—
ω(=CH2)
—
—
—
—
—
1708 (9)
1718 (19)
ν(C=C)
ρ(CH2)
—
—
—
—
—
1459 (5)
—
—
1427 (6)
—
1332 (1)
—
—
—
1462 (5)
—
—
1421 (5)
—
1347 (2)
—
—
1274 (7)
—
—
δ (=CH2)
с
—
—
ν(Si—C)
—
δ (CH2)
—
ν(Si—C) + δ(=CH)
—
с
—
δ(=CH)
—
ν(Si—Cl)
—
—
—
—
—
ω(CH2)
ν(Si—Cl)
—
ν(Si—Cl)
δ(C=C—C)
—
—
1182 w, br 1213 (3)
1170 (12)
τ(CH2)
—
1173 m
1168 m
1166 sh
1044 sh
1030 m
1024 sh
998 sh
—
1201 (6)
—
—
—
—
—
—
—
—
—
—
—
—
ω(CH2)
—
—
—
—
1060 (21)
1059 (0) ν(C—C) + ρ(=CH2)
—
—
—
—
—
—
—
—
—
—
—
995 sh
992 sh
—
—
989 sh
1031 (6)
1034 (9) τ(=CH2) + δ(=CH)
Note: br is broad, s is strong, vs is very strong, w is weak, m is medium, and sh is shoulder; for the most part of bands, matrix splitting is
observed resulting in the appearance of shoulders at the major absorption bands; matrix splitting is partially eliminated by matrix
annealing.
the relative amount of allene was a little higher and those
of acetylene and ethylene were noticeably lower than for
the pyrolysis of AllSiCl3 at the same temperatures. Of the
most intense nonꢀassigned bands presented in Table 2,
only the band at 674 cm–1 was observed in the spectra,
whereas the bands at 976, 974, and 788 cm–1 were absent,
indicating their belonging to the minor products specific
for the decomposition of AllSiCl3. Even at 1000 °C (see
Fig. 1, c) the intensity of the strongest bands of nonꢀ
decomposed AllCl exceeded the intensity of the strongest
band of allyl radical at 801 cm–1 (see Fig. 1, c); i.e.,
a considerable portion of AllCl underwent no decomposiꢀ
tion, whereas no formation of AllCl was observed for the
pyrolysis of AllSiCl3. Therefore, the pyrolysis of AllSiCl3
proceeds via a different route.
In the theoretical analysis of AllSiCl3 decomposition,
we considered three possible routes of transformations of
this compound (Scheme 2).
Scheme 2

1220
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
Boganov et al.
Table 2. IR bands of the pyrolysis products of AllSiCl3
According to the calculations of six listed reactions,
only reaction c1 proceeds with the formation of the transiꢀ
tion state(s) (TS(s)). The absence of TSs in the reactions
of simple bond cleavage, such as reactions a1, b1, and c2,
is a fairly known fact related to the absence of barriers in
recombination of radicals (i.e., in reactions inverse to the
presented ones) and the principle of microscopic reversꢀ
ibility. The absence of TSs in reactions a2 and b2 is conꢀ
sistent with the data of calculations for similar systems.43
The calculated structures of molecules involved in the
transformations presented in Scheme 2 and transition
states for reaction c1 (TS1—TS3) are shown in Fig. 2. The
structures of more stable gaucheꢀconformers (gꢀAllSiCl3
and gꢀAllCl) are presented for AllSiCl3 and AllCl. Acꢀ
cording to the calculations, the energies of less stable
cisꢀconformers of these molecules are higher by 2.0 and
0.4 kcal mol–1, respectively. Note that the calculated difꢀ
ference in enthalpies under normal conditions for cisꢀ and
gaucheꢀconformers of AllCl is 0.35 kcal mol–1, which alꢀ
ν/cm–1
Product
ν/cm–1
Product
3776 w
3756 w
3711 w
3323 w
3308 vw
3303 vw
3289 w
3109 vw, br
3053 vw
3040 vw
3027 vw
3020 vw
2888 w
2886 sh
2345 w
2339 w
2137 vw
1998 vw
1956 m
1680 vw
1624 w
H2O
H2O
H2O
1331 vw
1317 vw
1284 vw
1247 vw
1183 vw
984 w
976 vw
974 sh
973 sh
948 vw
844 sh
837 w
HC≡CH
C3H5
C3H5
HC≡CMe
C3H5
C3H5
HC≡CMe
HC≡CCH2
HC≡CH
HC≡CH
C3H5
C3H5
cꢀC3H5
C3H5
Was not assigned*
Was not assigned*
C3H5
H2C=CH2
H2C=C=CH2
H2C=C=CH2
C3H5
C3H5
H35Cl
801 s
H37Cl
788 w, br Was not assigned*
CO2
CO2
737 m
734 sh
731 sh
687 vw
HC≡CH
HC≡CH
HC≡CH
HC≡CMe
H2C=C=CH2
H2C=C=CH2 674 w, br
H2C=C=CH2
H2O
H2O
C3H5
H2O
C3H5
C3H5
H2C=CH2
C3H5
C3H5
HC≡CH
C3H5
most coincides with the value of 0.42+0.6 kcal mol–1
,
HC≡CCH2
Was not assigned
CO2
which was experimentally obtained in solutions in liquid
Xe in a temperature range of 176—216 K.41 Such a small
difference in energies of the conformers is insubstantial for
the analyzed processes, and the transformations involving
only gꢀAllSiCl3 and gꢀAllCl will be considered further, the
more so all found TSs of reaction c1 are related to these
conformers.
664 w
662 w
630 sh
512 s
510 m
507 vw
506 vw
501 s
CO2
1608 w
HC≡CMe
28Si35Cl2
28Si35Cl37Cl
29Si35Cl2
28Si37Cl2
28Si35Cl2
28Si35Cl37Cl
1603 vw
1589 vw
1477 w
1463 vw
1440 vw
1389 w
1386 sh
1335 vw
1242 vw
Many conformers can exist in the case of radical
AllSiCl2. By analogy to AllSiCl3 and AllCl, it is reasonable
to assume that the conformers of AllSiCl2 formed by the
rotation of the CH2SiCl2 group relative to the simple
C—C bond do not strongly differ in energy. To check the
influence of the turning angle of the SiCl2 group relative to
the Si—C bond, we calculated conformers AllSiCl2ꢀ1,
AllSiCl2ꢀ2, and AllSiCl2ꢀ3 (see Fig. 2). As expected, their
498 s
496 m 28Si37Cl2 29Si35Cl2
+
493 w
491 w
488 vw
29Si35Cl37Cl
30Si35Cl2
30Si35Cl37Cl
* The band belongs to a nonꢀestablished minor product of
AllSiCl3 decomposition (was not observed for the thermal deꢀ
composition of AllCl).
energies differ insignificantly (less than by 1 kcal mol–1
)
and, therefore, only conformer AllSiCl2ꢀ1 was considered
in an analysis of the thermochemical characteristics.
The total energies, enthalpies, and free energies at difꢀ
ferent temperatures for the structures shown in Fig. 2 are
given in Tables 3 and 4. The changes in the energy, enꢀ
thaly, and free energy at different temperatures for the
reactions shown in Scheme 2 are presented in Tables 5 and 6.
Unfortunately, the calculated and experimentally obꢀ
tained47,48 values of ΔH° (see Table 5) can be compared
only for two reactions (a2 and c2). As can be seen, the
G4(MP2) calculations reproduce well the experimental
data. The agreement is also observed between ΔH and ΔG
calculated at the G4(MP2) level for different temperaꢀ
tures for reaction a2 and the corresponding values obꢀ
tained earlier by the CCSD(T)/augꢀccꢀpVTZ//MP2/augꢀ
ccꢀpVDZ method.49
As follows from the set of the major final products
(SiCl2, C3H5, and Cl), the thermal decomposition of
AllSiCl3 comes to the cleavage of the Si—C and Si—Cl
bonds. The simultaneous cleavage of two bonds is enerꢀ
getically rather expensive and, hence, the decomposition
should undoubtedly consist of at least two steps. In the
case of the decomposition via route a (see Scheme 2), the
initial cleavage of the Si—C bond occurring in AllSiCl3
(reaction a1) is followed by the Si—Cl bond cleavage (reꢀ
action a2) in trichlorosilyl radical (SiCl3) formed in the
first step. Route b consists of the initial cleavage of the
Si—Cl bond (reaction b1) and subsequent Si—C bond
cleavage (reaction b2) in allyldichlorosilyl radical (AllSiCl2).
Route c is also included into the consideration for comꢀ
parison as a route that certainly does not occur. Route c
starts from the detachment of SiCl2 from the AllSiCl2
molecule (reaction c1) and completes with the C—Cl bond
cleavage in the AllCl molecule (reaction c2).
The typical values of energies (D) of the Si—C and
Si—Cl bonds are about 90 and 110 kcal mol–1, respectively
(see Ref. 48); i.e., the Si—Cl bonds are appreciably stronꢀ

Thermal decomposition of allyltrichlorosilane
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
1221
Fig. 2. Structures of molecules involved in the transformations (see Scheme 2) according to the B3LYP/6ꢀ31G(2df,p) calculations.
46
Bond lengths are given in Å, and angles are given in deg. The experimental values for AllCl,44 C3H5,45 and SiCl2 are given in
parentheses.
ger than the Si—C bonds. The presence of alkyl groups at
the Si atom results in some increase in the strength of
other bonds at this atom, such as Si—H, Si—C, Si—Si,
and Si—Cl.48 The more alkyl groups are added to the Si
atom, the higher is the strengthening of these bonds. As
shown by a comparison of the bond energies in SiH3Cl
The effect of presence of Cl atoms and the dependence on
their number are less pronounced: the D(Si—Cl) values in
SiH3Cl and SiCl4 are 109 and 110 kcal mol–1, respectiveꢀ
ly,48 i.e., almost coincide. Unfortunately, as far as we
know, literature data on the influence of the allyl group on
the strength of the bonds at the Si atom and on the D(Si—C)
value for AllSiCl3 are lacking. Nevertheless, it can be asꢀ
and SiMe3Cl,48 their difference can reach ~10 kcal mol–1
.
Table 3. Total energies including ZPE (Е0) and enthalpies (H) at different temperatures and p = 1 atm for the compounds involved in
the transformations (see Scheme 2) according to the G4(MP2) calculations
Comꢀ
pound
–Е0/au
–H/au at T/K
298.15
600
900
1100
1300
1500
1700
a
gꢀAllSiCl3 1785.663971
1785.653559 1785.634201 1785.610361 1785.592949 1785.574693 1785.555804 1785.536435
117.044338 117.034593 117.021310 117.011178 117.000306 116.988863 116.976981
1668.487047 1668.478213 1668.468978 1668.462744 1668.456480 1668.450197 1668.443902
1325.777947 1325.761054 1325.739891 1325.724313 1325.707911 1325.690888 1325.673394
C3H5
SiCl3
117.049137
1668.493239
AllSiCl2ꢀ1a 1325.786993
Cl
459.703691
576.862832
1208.683756
1785.507271
1785.533392
1785.530444
459.701330
576.857049
459.698940
576.845666
459.696565
576.830220
459.694982
576.818470
459.693398
576.805898
459.691815 459.690232
576.792703 576.779033
gꢀAllCla
SiCl2
TS1
1208.678941 1208.672666 1208.6666167 1208.661789 1208.657395 1208.652990 1208.648579
1785.496363 1785.477200 1785.454000 1785.437131 1785.419465 1785.401192 1785.382452
1785.522926 1785.504064 1785.481005 1785.464186 1785.446552 1785.428301 1785.409577
1785.519419 1785.500184 1785.476943 1785.460057 1785.442380 1785.424100 1785.405355
TS3
TS2
a The values of Е0, H(298.15 K), and G(298.15 K) for cisꢀAllSiCl3: –1785.660752, –1785.650256, and –1785.697395 au; for cisꢀAllCl:
–576.862221, –576.856500, and –576.890137 au; for AllSiCl2ꢀ2: –1325.785828, –1325.776739, and –1325. 821179 au; and for
AllSiCl2ꢀ3: –1325.785421, –1325.776321, and –1325.820807 au.

1222
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
Boganov et al.
Table 4. Total free Gibbs energies (G) calculated at the G4(MP2) level at different temperatures and p = 1 atm for the
compounds involved in the transformations (see Scheme 2)
Comꢀ
pound
–G/au at T/K
298.15
600
900
1100
1300
1500
1700
gꢀAllSiCl3 1785.700605 1785.755272 1785.820830 1785.869388 1785.921206 1785.975893 1785.033137
C3H5
SiCl3
117.073638
1668.523563 1668.563907 1668.608744 1668.640449 1668.673297 1668.707113 1668.741771
1326.075934 1326.128387
117.106720
117.145546
117.174189
117.204751
117.237036
117.270880
AllSiCl2ꢀ1 1325.822200 1325.873109 1325.933585 1325.978226 1326.02579
Cl
459.719368
576.890882
459.738562
576.929140
459.758886
576.974091
459.772899
577.007263
459.787202
577.042658
459.801750
577.080042
459.816509
577.119223
gꢀAllCl
SiCl2
TS1
TS3
TS2
1208.711042 1208.745947 1208.783989 1208.810625 1208.838063 1208.866181 1208.894888
1785.545220 1785.601687 1785.668826 1785.718298 1785.770925 1785.826328 1785.884205
1785.570255 1785.625051 1785.690405 1785.738665 1785.790074 1785.844253 1785.900905
1785.569598 1785.627444 1785.695981 1785.746390 1785.799958 1785.856302 1785.915122
Table 5. Relative energies including ZPE (ΔЕ0) and enthalpies (ΔH) obtained at the G4(MP2) level at different
temperatures and p = 1 atm for the reactions presented in Scheme 2
Reaction
ΔЕ0
ΔH/kcal mol–1 at T/K
/kcal mol–1
298.15
600
900
1100
1300
1500
1700
a1
b1
TS1
TS2
TS3
с1
a2
b2
176.3
108.7
198.3
183.8
181.9
173.7
166.4
133.2
169.0
142.7
176.7
109.4
198.6
184.2
182.0
176.2
109.3
198.5
184.1
181.7
172.7
166.9
133.8
170.4
143.1
175.3
109.1
198.1
183.7
181.2
171.5
166.7
132.9
170.5
142.0
174.7
109.0
197.8
183.4
180.8
170.7
166.5
132.2
170.5
141.2
174.0
108.8
197.4
183.0
180.4
169.9
166.3
131.5
170.4
140.3
173.3
108.6
197.0
182.6
180.0
169.1
166.1
130.8
170.3
139.4
172.5
108.4
196.6
182.3
179.6
168.3
165.9
130.0
170.2
138.5
173.8
67.0 (66 3)*
134.3
69.9 (69)*
143.7
с2
gꢀAllSiCl3 →
→ C3H5 + SiCl2 + Cl
* The values of ΔH° obtained on the basis of the available experimental data47,48 on ΔHf° are given in parentheses.
Table 6. Free Gibbs energies (ΔG) calculated at the G4(MP2) level at different temperatures and
p = 1 atm for the reactions presented in Scheme 2
Reaction
ΔG/kcal mol–1 at T/K
298.15
600
900
1100
1300
1500
1700
a1
b1
TS1
TS2
TS3
с1
a2
b2
64.9
99.8
97.5
82.2
81.8
61.9
58.5
23.5
61.4
123.3
53.1
90.1
96.4
80.2
81.7
50.3
49.8
12.8
52.6
102.9
41.8
80.5
95.4
78.3
81.8
39.4
41.3
2.5
34.4
74.2
94.8
77.2
82.0
32.3
35.7
–4.1
37.8
70.1
27.1
67.9
94.3
76.1
82.3
25.4
30.1
–10.7
31.8
57.2
19.9
61.6
93.9
75.0
82.6
18.6
24.6
–17.1
25.9
44.5
12.9
55.4
93.5
74.1
83.0
11.9
19.1
–23.5
20.0
31.9
с2
43.7
83.1
gꢀAllSiCl3 →
→ C3H5 + SiCl2 + Cl

Thermal decomposition of allyltrichlorosilane
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 5, May, 2016
1223
sumed that the effects of the All and Me groups are simiꢀ
lar, and by analogy with the data on D(С—С) in hydrocarꢀ
bons and chlorinated hydrocarbons,47 the bond between
the Si atom and allyl group should be much weaker (in the
of detectable amounts of SiCl3, although, taking into acꢀ
count that the most intense band of SiCl3 at 582 cm–1
exists50 in the same range as the intense band of Si—Cl
vibrations of nonꢀdecomposed AllSiCl3, we cannot comꢀ
pletely exclude its presence in the matrix in minor amounts.
The obtained data also indicate that if reaction b1 would
occur under the considered conditions, we could not deꢀ
tect the formation of radical AllSiCl2, since its existence
at T > 1100 K is thermodynamically unfavorable.
Thus, the decomposition of AllSiCl3 represents a typical
decomposition of a precursor molecule to two radicals:
C3H5 and SiCl3. However, since the Si—Cl bond in radiꢀ
cal SiCl3 is weak and its energy is somewhat lower than
the energy of the Si—C bond in the initial AllSiCl3, this
radical undergoes the further decomposition, resulting in
the formation of three intermediates from AllSiCl3, viz.,
C3H5, SiCl2, and Cl, which belong to different classes of
labile compounds. This determines that the whole process
is very untypical of the processes of generation of labile
intermediates. It is most likely that a similar stepped deꢀ
composition is inherent in other allylsilanes. For example,
the formation of C3H5 was observed for the pyrolysis of
AllSiMe3, whereas no SiMe3 radical was detected.51 Unꢀ
doubtedly, a deeper fragmentation of the trimethylsilyl
radical occurred in this case, which is indicated, in particꢀ
ular, by the band of CH3 in the detected matrix IR spectra
of the pyrolysis products.51
case of hydrocarbons, by ~30 kcal mol–1 47
than the typiꢀ
)
cal Si—C bond. In fact, the value of D(Si—C) in AllSiCl3
(77 kcal mol–1) obtained by us in the calculations is
~13 kcal mol–1 lower than the typical value and the D(Si—Cl)
value (109 kcal mol–1) coincides with the typical one.
A change in the enthalpy for all the six reactions and
the activation enthalpy for reaction c1 depend weakly on
the temperature (see Table 5) and insignificantly decreasꢀ
es with temperature. The least endothermic reaction is the
detachment of SiCl2 from AllSiCl2 (reaction b2), the ΔH°
value of this reaction is 34 kcal mol–1; i.e., the Si—C bond
in radical AllSiCl2 is substantially weakened.
Of the primary reactions (a1, b1, c1), the decomposiꢀ
tion of AllSiCl3 to AllCl and SiCl2 (reaction c1) is the least
endothermic at all temperatures. However, this reaction
to occur needs to surmount the activation barrier (TS3)
with the activation enthalpy exceeding the enthalpy of
AllSiCl3 decomposition to C3H5 and SiCl3 (reaction a1).
Reaction b1 is the most endothermic process at all temꢀ
peratures. Thus, starting from the thermochemical data
only, it can be expected that the main primary process of
AllSiCl3 decomposition at the temperatures 1120—1270 K
is the cleavage of the Si—C bond, whereas the detachment
of SiCl2 to form AllCl is the next in accessibility.
The values of changes in the free Gibbs energies should
be compared for a more reliable establishment of the prefꢀ
erential process. In all cases, except for the formation of
TS1—TS3, the values of ΔG decrease noticeably with temꢀ
perature, which is determined by the contribution of the
entropy factor. It should be mentioned that for TS1 and
TS2 the ΔG values also somewhat decrease, whereas ΔG of
the formation of TS3 slightly increases (see Table 6). Alꢀ
though reactions a1, a2, b1, b2, and c2 are decomposition
reactions with the formation of two species, a decrease in
ΔG with temperature for these reactions differs noticeably
(from 39 kcal mol–1 for reaction a2 to 52 kcal mol–1 for
reaction a1). Nevertheless, the data on both ΔG and ΔH
indicate that it is the decomposition of AllSiCl3 to C3H5
and SiCl3 (reaction a1) which is the main direction of the
primary transformations of this compound. The reaction of
AllSiCl3 decomposition to AllSiCl2 and Cl (reaction b1) is
thermodynamically significantly less favorable at elevated
temperatures, whereas reaction c1, which is thermoꢀ
dynamically more favorable and kinetically accessible at
room temperature compared to reaction b1, becomes
kinetically least accessible beginning from 900 K.
This work was financially supported by the Russian
Science Foundation (Project No. 14ꢀ13ꢀ01456).
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Received December 30, 2015;
in revised form February 18, 2016