Si is a metalloid that readily donates or shares its four valence electrons to form chemical bonds, combining with many other element to form various compounds.
5
Unlike carbon it sometimes accept additional electrons to form hypervalent products with five or six bonds.
6
Recent matrix spectroscopic studies have shown that silicon, like transitionmetals, readily undergoes C-X (X = H, halogen) insertion and following X migration from C to Si in reactions with methane and halomethanes, generating small methyl silylenes, silaethenes, and silyl carbenes without a trace of hypervalent products.
7
Maier
et al
. have identified CH
3
-SiH and CH
2
=SiH
2
in reactions of Si + CH
4
from the matrix spectra and also proposed a reaction path (initial C-H insertion by Si and following H migration),
2
which is in fact similar to that for reaction of a transition-metal with methane.
7,8
They have later reported production of halogenated derivatives (CH
3
-SiX and CH
2
=SiHX) in reactions of Si with methyl halides.
3
Schreiner
et al
. have observed HC-SiH
3
from matrix ESR and infrared spectra in reaction of C(
3
P) with SiH
4
and the ESR hyperfine structures indicate that the silyl carbene owns a triplet ground state like other carbenes (CX
2
, X = H, halogen).
4
Cho and Andrews have shown that the silaethene is the primary product in reactions of dihalomethanes and the silyl carbenes in reactions of tri- and tetrahalomethanes.
7
These Si bearing products from its s
2
p
2
electron configuration are in fact similar to those from Ti (s
2
d
2
) reactions, but the Ti methylidynes (XC÷TiX
3
) own a much stronger C-Ti bond by effective electron donation from C to Ti’s low lying d-orbitals.
2−4,7,8
The previous studies have shown that Ti also generates CH
3
-MH and CH
2
=MH
2
in reactions with CH
4
, but they produce CH
2
=MX
2
with CH
2
X
2
and triplet methylidynes (HC÷CX
3
and XC÷MX
3
) with tri- and tetrahalomethanes.
7,8
Substitution of H with halogen increases the stability of the higher oxidation-state compounds due to the stability of M-X bond.
Density functional theory (DFT) calculations
9
were carried out for the Si bearing products and transition states using the Gaussian 09 package,
10
the B3LYP hybrid density functional,
11
and the 6-311++G(3df, 3pd) basis sets for H, C, F, and Si.
12
Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis, all real frequencies for a stable conformation and a single imaginary frequency for a transition state. Each transition state was linked to its reactant and product by intrinsic reaction coordinate (IRC) calculation.
1
Natural bond orbital (NBO) calculations
13
were also done to examine the bond properties of the Si compounds and electron-donations between the MO’s. The vibrational frequencies were calculated analytically. In the calculation of binding energy for a Si product, the zero-point energy is included.
Due to the law of spin conservation and the ground states of C and Si (
3
P), these Si reactions most probably occur in a triplet potential surface.
. 1
illustrates the energies of the products and transition states relative to the reactants [Si(
3
P) + CH
4
and Si(
3
P) + CH
3
F]. These results reveal that both the C-H and C-F insertion reactions by atomic Si have to go over a considerable barrier, requiring extra photon energy to provide enough kinetic energy to the reactants in a cold matrix. It is consistent with the observed results that the Si products were generated during photolysis after original deposition.
2−4
Energies of the products and transition states relative to Si(3P) + CH4 (a) and those of the monofluorided analogues to Si(3P) + CH3F (b) calculated at the the B3LYP/6-311++G (3df, 3pd) level of theory. Each transition state is confirmed by linking it to its reactant and product by intrinsic reaction coordinate (IRC) calculations as shown in Figs. 2-4.
TS1, TS2, and TS3 in the methane reaction are considerably higher than the reactants [107, 126, and 177 kJ/mol higher than Si(
3
P) + CH
4
as shown in
. 1(a)
], indicating that the reaction becomes increasingly more difficult as moving toward its end [SiH
4
+ C(
3
P)]. The previous studies have also shown that while the insertion compound (CH
3
- SiH) is the primary reaction product on UV photolysis (λ = 185 or 254 nm), and the silaethane is detected after additional UV irradiation (λ = 254 nm), but HC-SiH
3
is not observed.
2
The silyl carbene is provided later in reaction of SiH
4
+ C(
3
P),
4
which is 372 kJ/mol higher in energy than Si(
3
P) + CH
4
. This reaction [SiH
4
+ C(
3
P) → HC-SiH
3
(T)] is evidently barrierless as shown in
. 1(a)
[and
. 4(a)
] while the next reaction step from HC-SiH
3
(T) to CH
2
-SiH
2
(T) requires a substantial activation energy (151 kJ/mol), leading to provision of the silyl carbene (HC-SiH
3
).
4
Inter-system crossing is supposed to take place once CH
3
-SiH and CH
2
=SiH are generated; the singlet methyl silylene and silaethene are 153 and 171 kJ/mol more stable than the reactants. Maier
et al
. have also reported that the observed vibrational characteristics of the methyl silylene and silaethene correlate with the predicted values for their singlet states.
2
On the other hand, all our attempts to optimize the geometry of singlet HC-SiH
3
ended up with the structure of CH
2
=SiH
2
, suggesting that singlet HC-SiH
3
is not a meaningful energy minimum. The observed vibrational characteristics and ESR hyperfine structures also correlate with those predicted for triplet HC-SiH
3
.
4
In reaction of Si(
3
P) + CH
3
F, TS1, TS2, and TS3 are considerably lower than those in the Si(
3
P) + CH
4
system, 56, -65, -37 kJ/mol higher than the reactants [
. 1(b)
]. Therefore, C-F insertion by Si to form CH
3
-SiF is expected to be easier, but the following conversions to CH
2
-SiHF and to HC-SiH
2
F to be more difficult. The F bearing Si products are relatively more stable than the corresponding analogues in the Si(
3
P) + CH
4
system due to the strong Si-F bond (540 kJ/mol).
14
Maier
et al
. have observed a Si adduct (CH
3
F···Si) in the original deposition spectrum, which easily converts to CH
3
-SiF by short UV or visible photolysis.
3
CH
2
=SiHF was observed later on much longer irradiation with λ = 366 nm.
The fluorosilyl carbene (HC-SiH
2
F), which is expected to be more difficult to provide as shown in
. 1(b)
, has not been reported. However, the present results indicate that provision of HC-SiH
2
F is probably possible via reaction of C(
3
P) + SiH
3
F, parallel to the case of HC-SiH
3
, where the silyl carbene is produced in reaction of laser-ablated graphite plume with a SiH
4
/Ar gas mixture on the surface of a cold window (1-12 K) by Schreiner
et al
.
4
As shown in
. 1(b)
and
4(b)
, the Si-H insertion reaction by C is apparently barrierless and continuous downhill to HC-SiH
2
F, while the reaction from HC-SiH
2
F to CH
2
-SiHF has to go over a considerable barrier (161 kJ/mol), making the fluorosilyl carbene a stable conformation.
CH
3
-SiF and CH
2
=SiHF are also expected to undergo system crossing to their singlet ground states, which are 182 and 134 kJ/mol more stable than the triplet counter parts. Maier
et al
. have also reported that the observed vibrational characteristics of the fluorided products correlate with the computed values for the singlet species.
3
The silyl carbene, on the other hand, has a triplet ground state. Our attempts for geometry optimization of singlet HC-SiH
2
F all lead to the structure of singlet CH
2
=SiHF, indicating that this singlet fluorosilyl carbene is also not a meaningful energy minimum.
. 2
and
3
illustrate our IRC computation results for the reaction steps in the course of Si(
3
P) + CH
4
→HC-SiH
3
(T) and those for the fluorided counterparts, showing that each step is a smooth conversion over its barrier.
4
shows the energy variations in conversions of HC-SiH
3
to C(
3
P) + SiH
4
(a) and HC-SiH
2
F to C(
3
P) + SiH
3
F (b), both of which are barrierless and continuous uphill.
. 2
-
4
also show that the product of a previous reaction step is the reactant of the next reaction step, the entire reaction path being a series of smooth conversions from one to another product. This suggests that the generally accepted reaction mechanism (initial C-H or C-F insertion and subsequent H migrations) is most probably the true reaction course, which connects the saddle points in the triplet potential surface.
Energy variations in the reaction steps form Si(3P) + CH4→HC-SiH3(T). Intrinsic reaction coordinate (IRC) calculations are done for the transition states at the level of B3LYP/6-311++G(3df, 3pd), showing that the reported silicon bearing products are the energy minima in the triplet potential surface.
Energy variations in the reaction steps form Si(3P) + CH3F→HC-SiH2F(T). Intrinsic reaction coordinate (IRC) calculations are done for the transition states at the level of B3LYP/6-311++G(3df,3pd), showing that the reported silicon bearing products are the energy minima in the triplet potential surface.
Energy variations in reactions from HC-SiH3 to C(3P) + SiH4 (a) and from HC-SiH2F to C(3P) + SiH3F (b). These IRC calculation results show that they are barrierless, uphill reactions to the energetically high products.
The structures of triplet CH
3
-SiH and CH
3
-SiF in
. S1
and
S2
are similar to those of the singlet counterparts,
2,3
whereas triplet CH
2
-SiH
2
and CH
2
-SiHF own non-planar structures due to lack of C-Si π-bond (natural C-Si bond orders
13
0.99 and 0.98), which contrasts the more usual planar or near planar geometry in the singlet states.
2,3,7,8
The structure of triplet HC-SiH
3
is almost identical to the reported structure,
4
carrying a bent HCSi moiety (160.5°) due to the unpaired electrons staying on C. Other carbenes also have a bent structure (e.g. 135.2° for CH
2
at the same level of theory). The non-bonding electrons reportedly have considerable contribution to the C-Si bond (π characters), leading to a shorter C-Si bond (
Figs
. S1 and S2).
7
In conclusion, the Si bearing products and transition states in the triplet reaction paths from Si(
3
P) + CH
4
to C(
3
P) + SiH
4
and from Si(
3
P) + CH
3
F to C(
3
P) + SiH
3
F have been examined. Each transition state is linked by IRC computation to its reactant and product, confirming that the reaction path is a series of smooth conversions from one to another reported product.
2−4
Unlike other reaction steps, conversion of silyl carbene to C(
3
P) + SiH
3
X (X = H or F) is a continuous, barrierless uphill. Therefore, the unreported fluorosilyl carbene may be prepared in reaction of atomic C with SiH
3
F, similar to the case of HC-SiH
3
.
4
The silyl carbenes have a triplet ground state, leading to a bent HCSi moiety. The triplet methyl silylene and silaethenes with unusual non-planar structures are supposed to undergo a system crossing to the singlet ground state, whose vibrational characters correlate with the experimentally observed values.
2−4
Supporting Information. Figs. S1 and S2 showing the molecular structures of methyl silylene, silaethene, and silyl carbene and the involved transition states in reaction paths of Si(3P) + CH4 → C(3P) + SiH4 and Si(3P) + CH3F → C(3P) + SiH3F.
Acknowledgements
This work is supported by Incheon National University Research Grant in 2013.
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