Unusual isotope effect in the reaction of chlorosilylene with trimethylsilane-1-d. Absolute rate studies and quantum chemical and Rice-Ramsperger-Kassel-Marcus calculations provide strong evidence for the involvement of an intermediate complex

Article abstract of DOI:10.1021/ja301924b

Time-resolved studies of chlorosilylene, ClSiH, generated by the 193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reaction with trimethylsilane-1-d, Me₃SiD, in the gas phase. The reaction was studied at total pressures up to 100 Torr (with and without added SF₆) over the temperature range of 295-407 K. The rate constants were found to be pressure independent and gave the following Arrhenius equation: log[(k/(cm³ molecule ^-1 s^-1)] = (-13.22 ± 0.15) + [(13.20 ± 1.00) kJ mol^-1]/(RT ln 10). When compared with previously published kinetic data for the reaction of ClSiH with Me₃SiH, kinetic isotope effects, k_D/k_H, in the range from 7.4 (297 K) to 6.4 (407 K) were obtained. These far exceed values of 0.4-0.5 estimated for a single-step insertion process. Quantum chemical calculations (G3MP2B3 level) confirm not only the involvement of an intermediate complex, but also the existence of a low-energy internal isomerization pathway which can scramble the D and H atom labels. By means of Rice-Ramsperger-Kassel-Marcus modeling and a necessary (but small) refinement of the energy surface, we have shown that this mechanism can reproduce closely the experimental isotope effects. These findings provide the first experimental evidence for the isomerization pathway and thereby offer the most concrete evidence to date for the existence of intermediate complexes in the insertion reactions of silylenes.

Full text of DOI:10.1021/ja301924b

Article
pubs.acs.org/JACS
Unusual Isotope Effect in the Reaction of Chlorosilylene with
Trimethylsilane-1-d. Absolute Rate Studies and Quantum Chemical
and Rice−Ramsperger−Kassel−Marcus Calculations Provide Strong
Evidence for the Involvement of an Intermediate Complex
Rosa Becerra,*,† Sergey E. Boganov,‡ Mikhail P. Egorov,‡ Irina V. Krylova,‡ Vladimir M. Promyslov,‡
and Robin Walsh*,§
†Instituto de Quimica-Fisica “Rocasolano”, Consejo Superior de Investigaciones Científicas (CSIC), C/Serrano 119, 28006 Madrid,
Spain
‡N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospekt 47, 119991 Moscow, Russian
Federation
§Department of Chemistry, University of Reading, Whiteknights, P.O. Box 224, Reading RG6 6AD, United Kingdom
S
* Supporting Information
ABSTRACT: Time-resolved studies of chlorosilylene, ClSiH,
generated by the 193 nm laser flash photolysis of 1-chloro-1-
silacyclopent-3-ene, have been carried out to obtain rate
constants for its bimolecular reaction with trimethylsilane-1-d,
Me3SiD, in the gas phase. The reaction was studied at total
pressures up to 100 Torr (with and without added SF6) over
the temperature range of 295−407 K. The rate constants were
found to be pressure independent and gave the following
Arrhenius equation: log[(k/(cm3 molecule−1 s−1)] = (−13.22
0.15) + [(13.20
1.00) kJ mol−1]/(RT ln 10). When
compared with previously published kinetic data for the
reaction of ClSiH with Me3SiH, kinetic isotope effects, kD/kH, in the range from 7.4 (297 K) to 6.4 (407 K) were obtained. These
far exceed values of 0.4−0.5 estimated for a single-step insertion process. Quantum chemical calculations (G3MP2B3 level)
confirm not only the involvement of an intermediate complex, but also the existence of a low-energy internal isomerization
pathway which can scramble the D and H atom labels. By means of Rice−Ramsperger−Kassel−Marcus modeling and a necessary
(but small) refinement of the energy surface, we have shown that this mechanism can reproduce closely the experimental isotope
effects. These findings provide the first experimental evidence for the isomerization pathway and thereby offer the most concrete
evidence to date for the existence of intermediate complexes in the insertion reactions of silylenes.
the tunneling rate.8 Until the present, the insertion reactions of
INTRODUCTION
■
silylenes into Si−H bonds have not been considered likely to
produce unusual KIE values. Indeed, Conlin et al.9 have
obtained a value for kH/kD of 1.11 0.12 in a study of SiMes2
with Et3SiD and Et3SiH in solution using direct laser
monitoring of the transient silylene, and Weber and
colleagues10 measured a value of 1.32 0.19 for the related
reaction of SiMe2 + nBuMe2SiH(D) in cyclohexane solution
using end-product analysis.
Silylenes are important because they are implicated in the
thermal and photochemical breakdown mechanisms of silanes
and organosilanes11 as well as being key intermediates in
chemical vapor deposition (CVD).12 The insertion process of
silylenes into Si−H bonds leading to the formation of disilanes
is one of the most fundamental and widely studied reactions of
Kinetic isotope effects (KIEs) are a well-established diagnostic
for chemical mechanisms, the most widely exploited variety
being that of D-for-H substitution.1 Values of kH/kD are
normally in excess of unity,1 and values as high as 8 can be
accommodated by classical transition-state theory.2 Even higher
values can be found at low temperatures,3 where tunnel effects
become important.4 Values less than unity (inverse isotope
effects) are much less common and indicate something unusual
about a mechanism. Pressure-dependent, gas-phase, uni-
molecular reactions can show inverse isotope effects,5 and we
have found examples in reverse association reactions.6 Some
recent examples of inverse isotope effects in the range of 0.4−
0.8 in the field of transition-metal oxidative elimination
reactions have caused some excitement,1d,7 as well as a
predicted value of 0.37 for the cyclopropylcarbinyl radical
rearrangement at 20 K, where D-for-H substitution at the
exomethylene carbon appears to increase, rather than reduce,
Received: February 27, 2012
Published: May 25, 2012
© 2012 American Chemical Society
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silylenes.11 The mechanism of this process is thought to involve
a weakly bound intermediate complex,13−15 viz.
Our preliminary findings,25 limited to room temperature
(295 K) and a pressure of 5 Torr, gave a value for kD/kH of 7.4
0.426 (an inverse isotope effect of 0.135
0.007). An
R3SiH + SiXY → R3Si···H···SiXY → R3SiSiHXY
approximate Rice−Ramsperger−Kassel−Marcus (RRKM)
model showed that this was consistent with the quantum
chemical energy surface. The questions that remain, however,
include whether the kD/kH value is mainly entropic or enthalpic
in origin and to what extent any intermediate complex is
collisionally stabilized, thus opening up the possibility of a
longer lived species. To these ends we have extended our
measurements to cover the temperature range of 295−407 K
and the pressure range of 5−100 Torr. We have also extended
the range of our RRKM model to explore the effects of different
barrier heights and well depths on the KIE predictions for this
process and to see whether experiment can help refine further
its energy surface.
The first step consists of an association of the silylene with the
substrate silane largely through interaction with the substrate H
atom. The second step involves a rearrangement of the complex
in which the silylene moiety straddles the Si−H bond in a
relatively tight transition state before formally inserting into the
bond. The evidence for this comes for the most part from the
interpretation of gas-phase kinetic studies, allied with
theoretical (ab initio) calculations. The theoretical argument
in favor of such complexes is the existence of low-energy
minima on the calculated potential energy surfaces16−20 for
these reactions, and the experimental evidence is the finding of
negative activation energies,16a−c,e,21 indicating the presence of
low-energy secondary barriers which affect reaction rates.
Despite these arguments, the experimental evidence may still be
regarded as indirect. The complexes themselves have not been
observed by any spectroscopic technique,22−24 nor has any
pathway unique to such a complex itself yet been identified. We
report here an experiment, employing KIE measurements,
which provides much stronger evidence of such a process.
Our choice for this experiment is based on findings from an
earlier kinetic study of the reaction ClSiH + Me3SiH.16e ClSiH
reacts more slowly and selectively with silanes than does
SiH2,16c and the kinetic study of this reaction16e revealed a tight
transition state (A = 10−13.97 cm3 molecule−1 s−1) and
significantly negative activation energy (Ea = −12.6 kJ
mol−1), indicating the strong likelihood of a complex. However,
the significant finding was the prediction from quantum
chemical calculations16e that not only does the reaction
proceed via a complex, but the complex has a degenerate
rearrangement process, indicated by the exchange of H and H*
via a low-energy transition state in which both H atoms are
coordinated to each Si atom as shown in Scheme 1. If correct,
EXPERIMENTAL SECTION
■
Rate Measurements. The apparatus and equipment for these
studies have been described in detail previously.16b,27 Only essential
and brief details are therefore included here. ClSiH was produced by
flash photolysis of 1-chlorosilacyclopent-3-ene (CSCP) using a
Coherent Compex 100 exciplex laser operating at 193 nm (ArF fill).
Photolysis laser pulses (energies ca. 50−70 mJ) were fired into a
variable-temperature quartz reaction vessel with demountable windows
at right angles to its main axis. ClSiH concentrations were monitored
in real time by means of a Coherent Innova 90-5 argon ion laser. The
monitoring laser beam was multipassed 36 times along the vessel axis,
through the reaction zone, to give an effective absorption path length
of ca. 1.5 m. A portion of the monitoring beam was split off before
entering the vessel for reference purposes. The monitoring laser was
tuned to the 457.9 nm line, coincident with the known transition
A A″(0,2,0) ← X A′(0,0,0) in the ClSiH vibronic band.28 Light signals
were measured by a dual photodiode/differential amplifier combina-
tion, and signal decays were stored in a transient recorder (Datalab DL
910) interfaced to a BBC microcomputer. This was used to average
the decays of typically five photolysis laser shots (at a repetition rate of
1 Hz or less). The averaged decay traces were processed by fitting the
data to an exponential form using a nonlinear least-squares package.
This analysis provided the values for the first-order rate coefficients,
kobsd, for removal of ClSiH in the presence of a known partial pressure
of Me3SiD.
1
1
̃
̃
Scheme 1
Gas mixtures for photolysis were made up containing 13−30 mTorr
of the transient precursor, CSCP, and variable pressures of Me3SiD.
An inert diluent, SF6, was added to give a total pressure of 5 Torr in
most experiments. Pressure was measured with a capacitance
manometer (MKS Baratron).
1-Chlorosilacyclopent-3-ene was prepared as described previously.29
Me3SiD was prepared by standard means.30 Its purity was checked by
IR (<1.2% Me3SiH). Sulfur hexafluoride, SF6 (no GC-detectable
impurities), was from Cambrian Gases. All gases used in this work
were deoxygenated thoroughly prior to use.
this indicates that the introduction of a D label would provide a
means of lifting the degeneracy. Although the process cannot
be observed directly, its consequences should be detectable
since formation of the complex from its reactants is reversible,
viz.
Quantum Chemical Calculations. The electronic structure
calculations were performed with the Gaussian 03, revision C.02,
software package.31 The DFT calculations, including the finding and
verification of stationary points, were done with the B3LYP
functional32 using the 6-31G(d) basis.33 All the structures obtained
were verified by examination of their Hessian matrix as minima (all
frequencies real) or transition states (one imaginary frequency). The
identities of transition states for particular reactions were established
by B3LYP calculations along the minimum-energy paths (intrinsic
reaction coordinate, IRC) connecting them with local minima. The
standard G3(MP2)//B3LYP/6-31G(d)34 method (denoted
G3MP2B3 for short) was employed to determine final energies for
all minimum-energy species and transition states of interest. For all
stationary points, enthalpies, entropies, and Gibbs free energies at 298
K were also calculated using harmonic oscillator and rigid rotor
models.
ClSiH + Me3SiH ⇌ Me3SiH···SiHCl
This fact is a necessary consequence of the low A factor and
negative activation energy referred to above.16e Thus, the
reaction of ClSiH + Me3SiD should lead to H/D exchange, and
the result of this, allied with the reversibility of the first step,
should provide a rapid sink pathway leading to formation of
Me3SiH + ClSiD. Provided the barrier to the complex
rearrangement process is low enough, this extra pathway will
lead to an enhanced rate for reaction of ClSiH with Me3SiD
compared with Me3SiH. The calculated quantum chemical
barrier16e suggests that this should be so, and the present study
was undertaken to obtain experimental evidence for this.
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deviations. The total pressure in these experiments was 5 Torr,
maintained by addition of SF6. A few experiments with
pressures up to 100 Torr (SF6) gave no change in value of
RESULTS
■
Kinetic Measurements. It was independently verified
during preliminary experiments that, in a given reaction
mixture, kobsd values were not dependent on the exciplex laser
energy (50−70 mJ/pulse routine variation) or number of
photolysis shots. Because static gas mixtures were used, tests
with up to 10 shots were carried out. The constancy of kobsd
(five shot averages) showed no effective depletion of reactants
in any of the systems. The sensitivity of detection of ClSiH was
high but decreased with increasing temperature. Precise
concentrations of ClSiH are not known, but are not required
since the decay kinetics were pseudo-first-order; a rough
estimate would be somewhere within the range of 0.1−1.0
mTorr (ca. 1013 molecules cm−3). Increasing quantities of
precursor were required at higher temperatures. However, at
any given temperature precursor pressures were kept fixed to
ensure a constant (but fairly small) contribution to kobsd values
of decay by reaction of ClSiH with the precursor. A series of
experiments were carried out at five temperatures in the range
of 295−407 K. The temperatures and the temperature range
were selected to match those of our earlier study of ClSiH with
Me3SiH.16e At each temperature, a number of runs (at least
eight) at different Me3SiD partial pressures were carried out.
The results of these experiments are shown in Figure 1, which
k
obsd, thus showing that the rate constants were not pressure
dependent.
Figure 2 shows an Arrhenius plot of the rate constants. The
resulting equation is
log[kD/(cm3 molecule−1 s−1)] =
(−13.22 0.15) + [(13.20 1.00) kJ mol−1]/(RT ln 10)
Uncertainties are again quoted as single standard deviations.
Figure 2. Arrhenius plots for ClSiH + Me3SiD and ClSiH + Me3SiH.
In addition to these experiments, the reaction between ClSiH
and Me3SiH was also reinvestigated at 344 K because the
previous rate constant16e (7.03 × 10−13 cm3 molecule−1 s−1)
was in poor agreement with the temperature trend of other
values. The new second-order plot value (1.13 × 10−12 cm3
molecule−1 s−1) is shown in Table 1, and the revised Arrhenius
plot is included in Figure 2. The revised Arrhenius parameters
are given in Table 2. Also shown, for comparison purposes, are
those obtained previously16e together with those for ClSiH +
Me3SiD and SiH2 + Me3SiH.16c
Table 2. Arrhenius Parameters for Chlorosilylene and
Silylene Insertion Reactions with Me3SiD/H
Figure 1. Second-order plots for the reaction of ClSiH + Me3SiD at
different temperatures (indicated).
log[A/(cm3
reaction
molecule−1 s−1)]
Ea/(kJ mol−1
)
ref
demonstrates the linear dependence of kobsd on [Me3SiD], as
expected for second-order kinetics. The second-order rate
constants, kD, obtained by least-squares fitting to these plots,
are given in Table 1. The error limits are single standard
ClSiH + Me3SiD
ClSiH + Me3SiH
ClSiH + Me3SiH
SiH2 + Me3SiH
−13.22 0.15
−13.90 0.21
−13.97 0.25
−10.37 0.10
−13.20 1.00 this work
−12.43 1.36 this work
−12.57 1.64 16e
−4.65 0.76 16c
26
Table 1. Experimental Second-Order Rate Constants for
ClSiH + Me3SiD (kD) and ClSiH + Me3SiH (kH)
The kinetic isotope effects were calculated as kD/kH from
the individual rate constants and are also shown in Table 1.
Where temperatures of each study differed slightly, values were
averaged (for modeling purposes (see later)). An Arrhenius-
type plot of these gives
a
kD/(10−12
kH/(10−12
T/K cm3 molecule−1 s−1
)
T/K cm3 molecule−1 s−1
)
kD/kH
295
318
341
363
407
13.90 0.53
8.52 0.21
7.33 0.31
4.45 0.17
297
316
344
368
407
1.876 0.065
1.304 0.041
1.128 0.062
0.657 0.029
0.480 0.013
7.41 0.38
6.53 0.26
6.49 0.45
6.77 0.40
6.35 0.25
log(AD/AH) = 0.67 0.09 and
b
Ea(D) − Ea(H) = −1.02 0.56 kJ mol−1
3.05 0.09
Quantum Chemical Calculations. The results of these
calculations have been reported previously for the ClSiH +
a
b
From ref 16e. Replaces earlier value;16e see the text.
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Figure 3. Molecular geometries of important species on the ClSiH + Me3SiH potential energy surface at the G3MP2B3 level. Bond lengths are in
angstroms and angles in degrees.
a
a
b
Table 3. Electronic Energies, Enthalpies, and Entropies of the Complex, Transition States, and Product for Reaction of
ClSiH + Me3SiH/D Calculated at the G3MP2B3 Level
ClSiH + Me3SiH
ClSiH + Me3SiD
species
ΔE°(0 K)
ΔH°(298 K)
S°(298 K)
ΔE°(0 K)
ΔH°(298 K)
S°(298 K)
reactants
CX
0
0
587.2
461.8
440.3
432.3
430.9
441.1
0
0
590.4
465.0
443.4
434.8
433.1
444.6
−36.68
+10.83
−9.22
−21.79
−36.11
+10.39
−11.24
−23.71
−180.50
−36.42
+11.60
−8.36
−21.12
−179.50
−35.85
+11.11
−10.45
−23.18
−180.31
TS2s
TS2a
TS3
product
−179.68
a
b
Values in kilojoules per mole. Values in joules per kelvin per mole.
Me3SiH reaction system,16e so only a brief summary is
presented here. The calculations (G3MP2B3 level) reveal the
presence of an intermediate complex as well as provide the
structures of the separated reactants and product Me3SiSiH2Cl
and several transition states. The structures are shown in Figure
3.35 The complex (CX) has a structure in which the attached
ClSiH moiety has a syn configuration to the Si−H bond in the
substrate, but it is unsymmetrical (C1 symmetry), so there are
two (mirror image) minima. There are three transition states,
TS2s and TS2a, which lead from the complex to the product,
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and TS3, which connects the two forms of the intermediate
complex. TS2s, while retaining the syn configuration, has
essentially transferred a H atom (H* in Figure 3) from the
substrate to silylene so that it is now more symmetric (Cs) and
rather resembles a radical pair, with the SiH2Cl fragment in the
process of inverting. This process is sufficiently energy
demanding that TS2s has a positive energy relative to the
reactants and cannot provide an accessible pathway for the
rearrangement. TS2a has the ClSiH in an anti configuration
and, although slightly unsymmetric, has a negative energy and
therefore provides the lower energy route to the disilane
product. TS3, not surprisingly, is of Cs symmetry, with the two
bridging H atoms (both H* in Figure 3) almost coplanar with
the two Si atoms. The energies and enthalpies of the stationary
points are given in Table 3. Data for the ClSiH with both
Me3SiH and Me3SiD are given to indicate the magnitude of the
small differences arising from the inclusion of zero point and
thermal energies in the two systems. The calculated energy
surface is shown in Figure 4.
Supporting Information) leads to an expression for the overall
second-order rate constant, kD, of
kD = k1D/(2 + k−1D/k3D
)
(1)
In the case of the reaction of ClSiH with Me3SiH, already
studied,16e the mechanism shown in Scheme 3 is simpler.
Scheme 3
For this mechanism the steady-state analysis leads to an
expression for the overall second-order rate constant of
kH = k1H/(1 + k−1H/k2H
)
(2)
Thus, the KIE of interest here, viz., the comparison of second-
order rate constants for the disappearance of ClSiH in the
presence of Me3SiD and Me3SiH, is the following:
kD
kH
k1D 1 + k−1H/k2H
k1H 2 + k−1D/k3D
KIE =
=
(3)
It seems reasonable to suppose, since step 1 in both systems is
barrierless, that k1H and k1D will be close in magnitude and that
to a good approximation38
kD
kH
1 + k−1H/k2H
2 + k−1D/k3D
KIE =
=
(4)
We have employed this expression in the calculations carried
out here. The RRKM exercise was thus based on the
unimolecular decomposition of the intermediate complexes
via two or three channels. This required transition-state models
for steps −1H, 2H, −1D, 3D, and also 2D since the latter
would also be occurring in the ClSiH + Me3SiD system. The
vibrational assignments and geometries for TS(2H), TS(2D),
and TS(3D) were taken from the quantum chemical
calculations as were those for the reacting species, i.e., the
complexes themselves (Me3 Si···H···SiHCl and
Me3Si···D···SiHCl). Assignments for TS(−1H) and TS(−1D)
were not available from the quantum chemical calculations
since the processes are dissociative and without a specifically
located energy maximum. They were therefore assigned
empirically. First, values for the A factors, A−1, were calculated
at each temperature using ln(A1/A−1) = ΔSo1,−1/R.39 Values for
ΔSo1,−1 were obtained from the quantum chemical calculations,
and values for A1 were assumed to be the same as those of the
rate constants for the reaction of SiH2 with Me3SiH.16c This is
justified on the grounds that SiH2, unlike ClSiH, reacts very fast
with Me3SiH (at close to the collision rate), and calculations
suggest that, for this reaction, formation of the intermediate
complex Me3Si···H···SiH2, rather than its rearrangement, is the
rate-determining step. With the resulting values for A−1, the
vibrational assignment of TS(−1) was obtained by adjusting
the transitional mode wavenumber values to get a match for the
entropy of activation, ΔS⧧−1, using
Figure 4. Potential energy surfaces for the reaction of ClSiH +
Me3SiH: solid line, calculated at the G3MP2B3 level; dashed line,
calculated to fit the RRKM-calculated KIE. Energy values in kilojoules
per mole.
RRKM Calculations. RRKM theory is conventionally used
to calculate the pressure dependence of unimolecular
dissociation reactions and their reverse association processes.36
We have frequently used it in this context to model the
pressure dependences of association reactions of silyle-
nes.6,16b,c,37 It can equally be used to model chemical activation
systems, where intermediates are formed with excess energy
and population distributions are different from those of thermal
reactions. This is the case in the present reaction system. From
the quantum chemical results the proposed mechanism is
shown in Scheme 2. In this scheme Me3Si···D···SiHCl* and
Me3Si···H···SiDCl* represent the energized species of the two
forms of the intermediate complex. This mechanism does not
include possible stabilization steps for the intermediates. A
stationary state treatment of this mechanism (see the
Scheme 2
⧧
ekT
h
−1/R
A−1
=
eΔS
The values for log A1, log A−1 and ΔS⧧−1 are shown in Table
4. The small temperature dependence of A−1 builds in
variational behavior, a common feature of these reactions.37
The vibrational assignments40 for these transition states
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to fit the rate constant values16e for ClSiH + Me3SiH
assuming step 1 occurs at close to the collision rate.
(iii) E0(3D) was adjusted empirically to ensure an approx-
imate value for k3D/k2H of ca. 7. This corresponds closely
to the average measured isotope effect.
Table 4. A Factors for Formation and A Factors and
Entropies of Activation for Decomposition of the
Intermediate Complexes in Step −1
T =
T =
317 K
T =
342 K
T =
365 K
T =
407 K
quantity
297 K
log[A1/(cm3
−9.55
−9.60
−9.66
−9.71
−9.77
The kD/kH values obtained with these modified E0 values are
shown in Table 5b. The altered E0 values themselves are given
in the footnotes. It is worth pointing out that while E0(−1) and
E0(2) are reduced, E0(3) is increased slightly. Other
combinations of E0 values were tried, but the ones shown
gave the optimal fit.
Additionally and to verify the importance of step 3, kD/kH
values were also calculated without step 3. The results are
shown in Table 6.41
molecule−1 s−1)]
log(A−1/s−1
ΔS⧧−1/(J K−1 mol−1
)
15.96
52.3
15.91
50.8
15.85
49.1
15.80
47.6
15.74
45.5
)
together with those for reaction steps 2 and 3 are given in the
Supporting Information. As well as vibrational assignments, the
TS models required values for E0, i.e., critical energies for each
channel and also a collisional deactivation model. For the latter
we used a stepladder model with ⟨ΔE⟩down = 1000 cm−1. This
follows previous practice when, as here, SF6 is the bath gas. The
minimum input energy of each complex corresponds to
E0(−1), its redissociation energy. However, the complexes
contain additional thermal energy. This was calculated as a
distribution function using f(E) = ρ(E) exp[−(E − E0(−1))/
RT] where, at each energy level E above E0(−1), ρ(E)
corresponds to the species density of states (calculated by the
RRKM program) and f(E) is the resulting population (which
was normalized by the program).
Table 6. RRKM Theory Calculated Values for kD/kH (KIE)
a
without Step 3
P/Torr T = 297 K T = 317 K T = 342 K T = 365 K T = 407 K
30
10
3
0.47
0.44
0.43
0.46
0.44
0.43
0.46
0.44
0.43
0.46
0.43
0.41
0.46
0.43
0.41
a
Using a modified energy surface; see Table 5, footnote b.
In the particular calculations undertaken, k values for each
channel, and thereby KIE values, were obtained at each of the
five temperatures of interest. This was carried out for several
different E0 scenarios. These included one with the original
quantum chemical values of E0 for each channel, as well as
others with E0 values modified to approach the conditions and
results of the experiments. It should be noted that, for each set
of calculations, E0 values for the D and H species differ from
one another because of their different zero point vibrational
energies.
DISCUSSION
■
Kinetic Findings and Comparisons. The main exper-
imental objective of the present study was to measure the gas-
phase rate constants and their temperature dependence for the
reaction of ClSiH with Me3SiD and thereby obtain kD/kH and
its temperature dependence by comparison with earlier data for
ClSiH + Me3SiH.16e This has been achieved over the
temperature range of 295−407 K. This is the first reported
gas-phase study of this isotope effect and its temperature
dependence, as well as the first of any gas-phase study for a Si−
D (Si−H) insertion reaction. The results, given in Tables 1 and
2, show that the rate constants for ClSiH + Me3SiD decrease
with increasing temperature similarly to those for ClSiH +
Me3SiH. The Arrhenius parameters for the two reactions are
fairly similar, with A factors ca. 3 orders of magnitude lower
than that for SiH2 + Me3SiH16c and activation energies
significantly more negative than those for SiH2 + Me3SiH.16c
These features indicate the strong likelihood of intermediate
complexes in the present reaction system.
The kD/kH value of ca. 7 is not only very large but almost
constant with temperature (T), although it decreases slightly as
T increases. Its value is significantly greater than the room
temperature, solution values of 0.90 0.10 for the reaction of
SiMes2 with Et3SiD/Et3SiH9 or 0.76 0.11 for the reaction of
SiMe2 with nBuMe2SiH/nBuMe2SiD.10 Since a value of less
than unity would be expected if the mechanism had been a
straightforward (single-step) process,1 this clearly shows that
the present reaction has greater complexity.
The first set of calculations, based on the quantum chemical
energy surface, gave the KIE values shown in Table 5a. Since
Table 5. RRKM Theory Calculated Values for kD/kH (KIE)
Using the Full Mechanism
P/Torr T = 297 K T = 317 K T = 342 K T = 365 K T = 407 K
a
(a) Using Quantum Chemical Calculated Critical Energies
30
10
3
5.03
3.18
2.04
5.00
3.14
2.06
4.98
3.15
2.10
5.23
3.36
2.28
5.82
3.86
2.66
b
(b) Using Empirically Adjusted Critical Energies
30
10
3
8.02
7.89
7.08
7.46
7.47
6.93
6.99
6.39
6.24
6.34
5.79
5.52
5.72
5.32
6.77
a
c
d
e
c
d
e
c
E0(−1) = 3066 (3044), E0(2) = 2295 (2346), and E0(3) =
e b
c
d
e
c
d
e
1279. E0(−1) = 1866 (1844), E0(2) = 1695 (1746), and
c
e c
d
E0(3) = 1380. Values in inverse centimeters. ClSiH + Me3SiH.
e
ClSiH + Me3SiD.
these do not match the experimental values of Table 3 and are
furthermore pressure dependent, unlike experiment, we
modified the E0 values as follows:
The small temperature dependence of kD/kH suggests that
the energy barriers to the rate-controlling steps for the Si−D
and Si−H insertion processes differ by only 1.0 kJ mol−1. While
the energy difference is clearly small, it should not be taken too
literally since, as we argue in this paper, the behavior of the
intermediate species is that of a chemically activated, rather
than thermal, intermediate. Similarly the A factor ratio, which
would mean that ΔS⧧D − ΔS⧧H = −13 J K−1 mol−1 in a purely
thermal reaction, cannot be taken too literally.
(i) E0(−1) was reduced in magnitude to reduce the
stabilization of the complex (which causes the pressure
dependence effect).
(ii) E0(2H) was adjusted empirically to ensure an approx-
imate value for k2H/k−1H in the range from 0.004 to
0.007. This numerical range corresponds to that required
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The lack of a pressure dependence of the rate constants
allows the elimination of one mechanistic possibility, viz.
more theoretical work to be done to help identify the factors
which affect the stabilities of these transition states.
RRKM Calculations and the Kinetic Isotope Effect. Our
objective was to reproduce the experimental value of kD/kH and
its temperature dependence by means of RRKM calculations
based on a plausible potential energy (PE) surface. Within the
limitations of some approximations, this has been achieved. The
starting point for the calculations was the quantum chemical PE
surface. When this was used, it was quickly apparent that
agreement with experiment was not achieved (Table 5a). The
most striking difficulty was that although the kD/kH values were
substantially larger than 1, they were significantly pressure
dependent, in disagreement with experiment. An examination
of the results showed this to arise from varying degrees of
collisional stabilization of the complexes at different pressures
up to a maximum value of 47% (ClSiH + Me3SiH, 30 Torr, 297
K). Since this clearly depended on the energy well depth of the
intermediate complex, we adjusted its value empirically,
together with the values of the energy barriers via the two
reaction channels. This was done by maintaining the same zero
point energy differences between the D and H species (as
obtained from the quantum chemical calculations). The
resulting best fit values (Table 5b) are in reasonable agreement
with experiment (Table 1) and only slightly pressure
dependent. The maximum collisional stabilizations of com-
plexes obtained from the calculations for these conditions were
7.3% (ClSiH + Me3SiH system, 297 K) and 5.3% (ClSiH +
Me3SiD, 297 K). Although these are now quite small, it is
possible that further refinement of the energy surface could
improve agreement slightly. However, the rate constant eq 1,
and therefore the kD/kH eq 4, involve several approximations
arising from simplifications of the steady-state treatment of
Scheme 2, such as treating the initial and secondary complexes
as having the same entropy and kinetic constants, so we
considered such refinement not worthwhile. Separate checks on
the assumptions and approximations underlying eq 4 showed
the values of Table 5b to be reliable within a maximum
uncertainty of 15%. Similar, i.e., relatively small, uncertainties
surround the values of kD/kH (Table 6) calculated for the
mechanism without the participation of step 3. Such
uncertainties are negligible compared to the large differences
between the values of Tables 5b and 6. It is therefore clear that
a mechanism without an additional removal step, or steps, for
the initially formed complex cannot account for the observed
isotope effect. It is also clear, via the kinetic modeling, that the
proposed mechanism is able to account for the observed
isotope effect as well as its only very slight temperature
dependence to a very good approximation.
ClSiH + Me3SiD ⇌ Me3SiSiClHD* → Me3SiH + ClSiD
↓+M
Me3SiSiClHD
where Me3SiSiClHD* represents the vibrationally excited
disilane product. If this process were to occur, it would indicate
that collisional stabilization of the disilane product was
incomplete and the measured rate constant would be pressure
dependent. The same lack of pressure dependence was found
for ClSiH + Me3SiH studied previously.16e
Quantum Chemical Calculations and the Mechanism.
The results obtained here for the energy surface and the
intermediate species involved in the ClSiH + Me3SiH reaction
have been discussed in detail previously.16e In summary, the
intermediate CX has a structure with the attacking ClSiH
moiety in a syn conformation to the substrate Si−H bond, while
the lower energy (favored) transition state (TS2a) has an anti
conformation. What is new here are the details for TS3 and the
isotopic differences. TS3 has a more symmetric Cs structure
with an almost planar Si···H···Si···H··· ring, clearly showing the
geometric equivalence of the H atoms originating from ClSiH
and Me3SiH. The low energy (−21.8 kJ mol−1) and enthalpy
(−23.7 kJ mol−1) indicate its stability and capability to allow
easy exchange of the H atoms in the ring. In our earlier
paper,16e calculations of the energy surface for the similar
reaction of ClSiH with MeSiH3 (at the same G3MP2B3 level)
revealed an equivalent TS3-like transition state with an energy
of −6.5 kJ mol−1 and an enthalpy of −9.8 kJ mol−1. These
negative values indicate that exchange reactions are also
potentially possible in that reaction system as well (and
presumably also ClSiH + Me2SiH2). The differences in energy
and enthalpy between the two systems show that these TS3-like
transition states are stabilized by Me-for-H substitution in the
substrate silane, thus making ClSiH + Me3SiH/Me3SiD a
particularly favorable reaction system for achieving this
exchange experimentally. The first theoretical evidence for
such exchange processes was obtained by Swihart and Carr,42
who found similar four-center transition states for exchange
reactions of the type
ClSiH + SiHnCl4−n ⇌ SiH2 + SiHn−1Cl5−n
ClSiH + SiHnCl4−n ⇌ SiCl2 + SiHn+1Cl3−n
These processes were calculated to have energy barriers several
kilocalories per mole above those for the insertion processes
(G2 and/or G2MP2 level), suggesting that Cl atom
substitution in the substrate silane may not be as effective at
stabilizing these TS3-like transition states as Me group
substitution. It should be noted, however, that the exchange
processes in our system are H-for-H(D), whereas those found
by Swihart and Carr42 are H-for-Cl. Therefore, strict
comparisons are not possible. Recent calculations by some of
us43 have shown that, in the ClSiH + H2 reaction system, H-for-
H exchange reactions are less energy demanding than H-for-Cl.
This latter system may also not be a good guide because it is
not accompanied by a Si−H insertion process. All this suggests
that a search for experimental evidence for these processes in
the ClSiH + chlorosilane systems might well be worthwhile to
extend our knowledge of substituent effects on the exchange
process discovered in the present work. There is clearly also
The adjustments of energy barriers and the intermediate
complex well depth required to model the isotope effect are
depicted in the PE surface shown in Figure 4. The comparison
with the quantum chemical PE surface shows a qualitatively
similar pattern of barriers. In quantitative terms, the complex
well depth is reduced by 14.3 kJ mol−1 (39%), the secondary
barrier, E0(2), is reduced by 7.1 kJ mol−1 (26%), and the
complex label-scrambling barrier, E0(3), is increased by 1.2 kJ
mol−1 (8%). It is worth pointing out that the increase of E0(3)
relative to E0(−1) and E0(2), is a necessary consequence of the
very small temperature dependence of kD/kH. Indeed E0(3)
cannot be too small or step 3 would cease to be the rate-
controlling process for ClSiH + Me3SiD. These necessary
changes in the energy barrier values are not particularly large,
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Journal of the American Chemical Society
Article
and we regard the differences as lying within reasonable bounds
of the accuracy of the G3MP2B3 method.
ACKNOWLEDGMENTS
■
This work was supported by the award of an RSC Journals
grant to R.B. R.B. also thanks the Ministerio de Educacion y
Ciencia (Spain) for support under Project CTQ2006-10512.
Thanks are also due to Professor David Cardin for the gift of a
sample of Me3SiCl.
One further modeling assumption deserves comment, that is,
the use of the values of A1 (Table 4) corresponding to those of
the rate constants for SiH2 + Me3SiH.16c We have justified this
on the grounds that, in this reaction, the first step in the
mechanism is rate determining. However, it is not known
whether, despite this, the values for k1 for ClSiH + Me3SiH
should be identical in magnitude with these. Two arguments
suggest that they should be high. First, the bottleneck in the
overall insertion reaction of ClSiH with Me3SiH is, without
doubt, the second step (from the Arrhenius parameters of
Table 2). Secondly, we found that the use of lower values for
A−1, implying lower values for A1, produced poorer agreement
with measured isotope effect values (see the preliminary
communication25). This supports the view that, in comparing
the Si−H insertion behavior of ClSiH and SiH2, the substantial
reduction of reactivity brought about by the Cl-for-H
substitution arises almost entirely in the second step of the
reaction, viz., the rearrangement of the complex rather than its
formation. A collision theory estimate of the value of the
encounter rate constant is 5.87 × 10−10 cm3 molecule−1 s−1 (see
the Supporting Information), so this suggests that ClSiH reacts
with Me3SiH at ca. 48% of the collision rate at 298 K.
REFERENCES
■
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SUMMARY AND CONCLUSION
■
We have measured a uniquely high value for the kinetic isotope
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ASSOCIATED CONTENT
* Supporting Information
■
S
Derivation of eqs 1−3, full microscopic reversibility equation,
values for the vibrational wavenumbers for the silylene adduct
(intermediate complexes), TS(−1H), TS(2H), TS(−1D),
TS(2D), and TS(3D), collision number parameters, and
quantum chemical energies, imaginary wavenumbers, and
atomic coordinates for all species. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
r.becerra@iqfr.csic.es:; r.walsh@reading.ac.uk
■
Notes
(17) (a) Apeloig, Y. In The Chemistry of Organic Silicon Compounds,
The authors declare no competing financial interest.
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Vol. 1, Chapter
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(39) Care has to be exercised in the use of this equation because A−1
is usually expressed in concentration units and ΔS°1,−1 employs values
based on the standard state of 1 bar. See the Supporting Information.
(40) A reviewer has pointed out that some of the low-wavenumber
vibrations might more realistically be modeled as internal rotations.
The only obvious potential internal rotations in the structures for TS2
and TS3 are those of the Me groups in the Me3Si part of the
molecules. However, since these are common to both transition states,
we do not think the calculated isotope effects will be affected.
(41) The values at 297 K differ slightly from that (0.73 at 10 Torr) of
our preliminary communication25 because of the modified E0 values.
(42) Swihart, M. T.; Carr, R. W. J. Phys. Chem. A. 1997, 101, 7434.
(43) Boganov, S. E.; Promyslov, V. M.; Faustov, V. I.; Egorov, M. P.;
Nefedov, O. M. Izv. Akad. Nauk. Ser. Khim. 2011, 2107; Russ. Chem.
Bull. (Engl. Transl.) 2012, in press.
(44) Unlike ClSiH, ClSiD has no vibronic band coincident with any
line of our Ar ion laser monitor.
(32) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Stephens, P.
J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994,
98, 11623.
(33) Hehre, W. A.; Radom, L.; Pople, J. A. Ab Initio Molecular Orbital
Theory; Wiley: New York, 1986.
(34) Baboul, A. G.; Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. J.
Chem. Phys. 1999, 110, 7650.
(35) The label TS1 has not been used here because it was reserved
for the variational transition state for formation of the complex from
reactants (see Scheme 2).
(36) Holbrook, K. A.; Pilling, M. J.; Robertson, S. H. Unimolecular
Reactions, 2nd ed.; Wiley: Chichester, U.K., 1996.
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