Time-Resolved Gas-Phase Kinetic, Quantum Chemical and RRKM

Dec 22, 2009 - tion processes alone, we have undertaken RRKM calculations30 of the reverse ..... harder to break the initial OsC bond in the oxetane c...
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J. Phys. Chem. A 2010, 114, 784–793

Time-Resolved Gas-Phase Kinetic, Quantum Chemical and RRKM Studies of Reactions of Silylene with Cyclic Ethers Rosa Becerra,*,† J. Pat Cannady,‡ Olivia Goulder,§ and Robin Walsh*,§ Instituto de Quimica-Fisica ‘Rocasolano’, C.S.I.C., C/Serrano 119, 28006 Madrid, Spain, Dow Corning Corporation, P.O. Box 994, Mail CO1232, Midland, Michigan, 48686-0994, and Department of Chemistry, UniVersity of Reading, Whiteknights, P.O. Box 224, Reading, RG6 6AD, U.K. ReceiVed: August 27, 2009; ReVised Manuscript ReceiVed: NoVember 18, 2009

Time-resolved kinetic studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reactions with oxirane, oxetane, and tetrahydrofuran (THF). The reactions were studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas, at four or five temperatures in the range 294-605 K. All three reactions showed pressure dependences characteristic of third-body-assisted association reactions with, surprisingly, SiH2 + oxirane showing the least and SiH2 + THF showing the most pressure dependence. The second-order rate constants obtained by extrapolation to the high-pressure limits at each temperature fitted the Arrhenius equations where the error limits are single standard deviations: ∞ log(koxirane ⁄cm3molecule-1s-1) ) (-11.03 ( 0.07) + (5.70 ( 0.51) kJ mol-1 ⁄ RT ln 10 ∞ log(koxetane ⁄cm3molecule-1s-1) ) (-11.17 ( 0.11) + (9.04 ( 0.78) kJ mol-1 ⁄ RT ln 10 ∞ log(kTHF ⁄cm3molecule-1s-1) ) (-10.59 ( 0.10) + (5.76 ( 0.65) kJ mol-1 ⁄ RT ln 10

Binding-energy values of 77, 97, and 92 kJ mol-1 have been obtained for the donor-acceptor complexes of SiH2 with oxirane, oxetane, and THF, respectively, by means of quantum chemical (ab initio) calculations carried out at the G3 level. The use of these values to model the pressure dependences of these reactions, via RRKM theory, provided a good fit only in the case of SiH2 + THF. The lack of fit in the other two cases is attributed to further reaction pathways for the association complexes of SiH2 with oxirane and oxetane. The finding of ethene as a product of the SiH2 + oxirane reaction supports a pathway leading to H2SidO + C2H4 predicted by the theoretical calculations of Apeloig and Sklenak. Introduction Silylenes are of importance because they are implicated in the thermal and photochemical breakdown mechanisms of silicon hydrides and organosilanes and they are key intermediates in CVD. Time-resolved kinetic studies, carried out in recent years, have shown that the simplest silylene, SiH2, reacts rapidly and efficiently with many chemical species.1-3 Examples of its reactions include SisH bond insertions, CdC, and CtC π-bond additions.4 Because it has an empty p orbital in its 1A1 ground state, SiH2 is particularly reactive with lone-pair donor molecules such as H2O (D2O),5-7 MeOH (CD3OD),5 and Me2O.8,9 There is little doubt that such reactions, in their initial steps, form donor-acceptor complexes with zwitterionic character. Originally, such complexes were invoked by Weber and his group10-15 to account for their findings in kinetic and mechanistic studies of SiMe2 in solution. Their existence was supported by the theoretical calculations of Raghavachari et al.16 Soon after that, a number of specific complexes of silylenes were isolated in frozen matrices by the groups of Ando17,18 and West.19,20 They have also been directly detected by time-resolved kinetic means in solution by Levin et al.21 and more recently * Corresponding authors. † Instituto de Quimica-Fisica ‘Rocasolano’. ‡ Dow Corning Corporation. § University of Reading.

by Moiseev and Leigh.22,23 In our own laboratories,24,25 complexes of SiMe2 with Me2O and with tetrahydrofuran (THF) were invoked to explain biexponential kinetic behavior in theirgas phase reactions. Complexes of SiH2 itself with O-donor molecules have not been seen directly, but from gas-phase kinetic studies, the evidence includes (i) reversibility of reaction9 and (ii) third-body-assisted association processes,5-9 both consistent with formation of weakly bound adducts with binding energies of 54-65 kJ mol-1 (H2Si · · · OH2)6 and 88 kJ mol-1 (H2Si · · · OMe2)9 consistent with theoretically calculated values.6,9,16,26-28 In the gas phase, the complexes themselves appear to have relatively long lifetimes and, for the most part, do not readily rearrange to other products despite the apparently straightforwardlooking isomerization process 1 shown below:

It appears that the energy barriers to the 1,2 migration of X (X ) H or Me) from O to Si are too high to be easily surmounted. (An H2O catalyzed version of reaction 1 (X ) H) has been observed,7 viz., a bimolecular reaction. This is the analogue of solvent catalyzed process 1 in solution.)6,16,26-28

10.1021/jp908289h  2010 American Chemical Society Published on Web 12/22/2009

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Other unimolecular processes also have barriers that rule out their occurrence.6,28 One way in which such complexes might be encouraged to react further is by incorporation of strain energy into their structures. To this end, we decided to embark on a kinetic study of the reaction of SiH2 with oxirane (cycloCH2CH2Os) and oxetane (cyclo-CH2CH2CH2Os). In order to provide a benchmark, we also included the reaction of SiH2 with THF (cyclo-CH2CH2CH2CH2Os), a relatively unstrained cyclic ether. These reactions have not previously been investigated experimentally. In addition, we have undertaken quantum chemical calculations of the binding energies of SiH2 with each of these molecules. Apeloig and Sklenak29 have shown, in a previous study of SiH2 + oxirane, that there is a relatively lowenergy pathway leading to formation of silanone, viz.:

SiH2 + oxirane f complex f H2SiO + C2H4

(2)

The occurrence of reaction 2 is consistent with the findings of a previous study of SiMe2 + oxirane from our laboratories.25 As well as the measurement of rate constants for these reactions, a key aspect of the present study is to obtain their pressure dependences and to undertake modeling via RiceRamsperger-Kassel-Marcus (RRKM) theory30 in order to test the consistency with quantum-chemically calculated binding energies of the complexes. For book-keeping purposes, we use the following reaction numbering system:

SiH2 + oxirane f products

(3)

SiH2 + oxetane f products

(4)

SiH2 + tetrahydrofuran f products

(5)

Experimental Section Equipment, Chemicals and Method. The apparatus and equipment for these studies have been described in detail previously.31,32 Only essential and brief details are therefore included here. SiH2 was produced by the 193 nm flash photolysis of phenylsilane (PhSiH3) by using a Coherent Compex 100 exciplex laser. (An Oxford Lasers KX2 exciplex laser was used in some early experiments.) Photolysis pulses (beam cross section 4 cm × 1 cm) were fired into a variable-temperature quartz reaction vessel with demountable windows at right angles to its main axis. SiH2 concentrations were monitored in real time by means of a Coherent 699-21 single-mode dye laser pumped by an Innova 90-5 argon ion laser and operating with Rhodamine 6G. The monitoring laser beam was multipassed 36 times along the vessel axis, through the reaction zone, to give an effective path length of 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 17259.50 cm-1, corresponding to the known RQ0,J (5) strong rotation ˜ 1B1(0,2,0) r X ˜ 1A1(0,0,0) vibronic transition32,33 in the SiH2 A absorption band. Light signals were measured by a dual photodiode/differential amplifier combination, and signal decays were stored in a transient recorder (Datalab DL910) interfaced to a BBC microcomputer. This was used to average the decays of 5-15 photolysis laser shots (at a repetition rate of 0.5 or 1 Hz). The averaged decay traces were processed by fitting the data to an exponential form by using a nonlinear least-squares package. This analysis provided the values for first-order rate constants, kobs, for removal of SiH2 in the presence of known partial pressures of substrate gas. Gas mixtures for photolysis were made up containing between 1.6 and 6.9 mTorr of PhSiH3, variable pressures of substrate and inert diluent (SF6) to total pressures of 1-100 Torr (1 Torr

) 133.3 N m-2). The substrate pressure ranges were as follows: oxirane, 0-940 mTorr; oxetane, 0-680 mTorr; THF, 0-800 mTorr. Pressures were measured by capacitance manometers (MKS, Baratron). All gases used in this work were frozen and rigorously pumped to remove any residual air prior to use. PhSiH3 (99.9%) was obtained from Ventron-Alfa (Petrarch). Oxirane was obtained from Cambrian gases (99.7%). Oxetane was obtained from Aldrich (99.7%). THF was obtained from BDH (99.5%). Sulfur hexafluoride, SF6, (no GC-detectable impurities) was obtained from Cambrian Gases. GC purity checks were carried out both with a 3 m silicone oil column (OV101) operated at 60 °C and with a 3 m Porapak Q operated at 120 °C. The latter column was also used for end-product analyses. N2 was used as carrier gas, and detection was by FID. Detection limits for impurity peaks were better than 0.1% of the principal component. The ethers used in this study show weak absorptions at 193 nm, and it was found in preliminary experiments, by GC analysis, that decomposition of a few percent occurred under the conditions of these studies. Without a longer-wavelength absorbing precursor for SiH2, we were unable to avoid this problem. The consequences of this are considered in the Results section. Ab Initio Calculations. The electronic-structure calculations were performed with the Gaussian 98 software package.34 All structures were determined by energy minimization at the MP2)Full/6-31G(d) level. Stable structures, corresponding to energy minima, were identified by possessing no negative eigenvalues of the Hessian. [Transition states which would have been identified by having one negative eigenvalue were not sought in this study.] The standard Gaussian-3 (G3) compound method35 was employed to determine final energies for all local minima. Harmonic frequencies were obtained from the values calculated at the HF/6-31G(d) level adjusted by the correction factor 0.893 appropriate to this level.36 Results Kinetics. Preliminary experiments established that, for given reaction gas mixtures in each of the three reaction systems, decomposition decay constants, kobs, were not dependent on the exciplex laser energy within the normal routine range of variation (50-80 mJ/pulse). There was also no dependence on the number of photolysis laser shots (up to 15 shots). The constancy of kobs (five shot averages) showed that there was no effective depletion of reactants. Higher pressures of precursor were required at the higher temperatures because signal intensities decreased with increasing temperature. However, for the purposes of rate constant measurement at a given temperature, the precursor pressure was kept fixed. For each reaction system, at each temperature of study and at 10 Torr total pressure, a series of experiments was carried out to investigate the dependence of kobs on reactant partial pressure (at least five different values). The purpose of these experiments was to establish the second-order nature of the kinetics. In addition, again for each reaction system at each temperature of study, another series of runs was carried out in which total pressures were varied by addition of SF6 to values in the range 1-100 Torr, in order to investigate the dependences of rate constants on total pressure. The data were obtained in the same way as those at 10 Torr, although in these experiments, only three or four substrate partial pressures were tried at each total pressure (because secondorder behavior was established at 10 Torr). The pressure range was limited by practical considerations. Above ca. 100

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Figure 1. Second-order plots for reaction of SiH2 + oxirane: temperatures for each set of data are marked in the figure.

Figure 2. Pressure dependence of second-order rate constants for reaction 3, SiH2 + oxirane, in the presence of SF6 at five temperatures (indicated). Curves are eyeball fits (see text).

TABLE 1: Second-Order Rate Constants for SiH2 + Oxirane over the Range 294-605 K, at 10 Torr Total Pressure and Infinite Pressure (by Extrapolation)

c

T/K

ka,b

k∞a,c

294 356 397 481 605

9.23 ( 0.29 6.25 ( 0.35 4.61 ( 0.17 2.86 ( 0.12 1.92 ( 0.16

10.0 ( 1.0 6.3 ( 0.6 5.0 ( 0.5 3.5 ( 0.35 3.2 ( 0.3

a Units: 10-11 cm3 molecule-1 s-1. Estimated (10%.

b

Single standard deviations.

Torr, transient signals became too small to be measured reliably, and below 1 Torr, pressure measurement uncertainties became significant. The particular results for each reaction system are given below. Oxirane and oxetane absorb 193 nm radiation weakly and undergo small amounts of photochemical decomposition (see below). The percentage product yields are at most a few percent, and although products such as C2H4 are reactive toward SiH2, the small amounts produced are unlikely to affect the SiH2 decays beyond existing experimental error. Kinetics of SiH2 + Oxirane. This reaction was investigated over the temperature range 294-605 K. The second-order rate plots are shown in Figure 1 for the five temperatures studied. It can be seen that reasonably linear plots are obtained, and the second-order rate constants at 10 Torr, obtained by least-squares fitting to these plots, are given in Table 1. The error limits are single standard deviations. It should be noted that the rate constants decrease with increasing temperature. The pressure dependences of these rate constants are plotted in Figure 2, in log-log plots for convenience. Although the rate constants show some pressure dependence, it is relatively small. We were not able to model this pressure dependence (see later); therefore, the fits are empirical. With this in mind, the values for the high-pressure limiting rate constants, also shown in Table 1, clearly have a higher uncertainty (estimated at (10%). Figure 3 shows an Arrhenius plot of the rate constants obtained here both at 10 Torr and at infinite pressure. A linear least-squares fit to the infinite pressure values corresponds to the Arrhenius equation, where the uncertainties are again single standard deviations:

Figure 3. Arrhenius plots of second-order rate constants for reaction 3, SiH2 + oxirane in SF6: O, 10 Torr; b, infinite pressure. Line is LSQ fit to k∞ values.

log(k∞3 /cm3molecule-1s-1) ) (-11.03 ( 0.07) + (5.67 ( 0.50) kJ mol-1 /RT ln 10 Because of the unknown uncertainties involved in extrapolation, we would estimate that these Arrhenius parameters may have maximum uncertainties of (0.5 (log A∞) and (3.6 kJ mol-1 (Ea). Kinetics of SiH2 + Oxetane. This reaction was investigated over the temperature range 296-594 K. The second-order rate plots are shown in Figure 4 for the five temperatures studied. It can be seen that reasonably linear plots are obtained, and the second-order rate constants at 10 Torr, obtained by least-squares fitting to these plots, are given in Table 2. It may be noted that the rate constants again decrease with increasing temperature. The pressure dependences of these rate constants are plotted in Figure 5. The rate constants show more pressure dependence than for the SiH2 + oxirane system, but we were again unable to model it (see later); therefore, the fits are empirical. The values for the high-pressure limiting rate constants, also shown in Table 2, again have a higher uncertainty (estimated). Figure 6 shows an Arrhenius plot of the rate constants obtained here both at 10 Torr and at infinite pressure. A linear least-squares fit to the infinite pressure values corresponds to the Arrhenius equation, where the uncertainties are again single standard deviations:

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log(k∞4 /cm3molecule-1s-1) ) (-11.17 ( 0.11) + (9.04 ( 0.78) kJ mol-1 /RT ln 10 Because of the unknown uncertainties involved in extrapolation, we would estimate that these Arrhenius parameters may have maximum uncertainties of (0.7 (log A∞) and (5.0 kJ mol-1 (Ea). Kinetics of SiH2 + THF. This reaction was investigated over the temperature range 294-398 K. The temperature range for this study was more limited because decay traces showed end absorptions above 400 K. The second-order rate plots

Figure 6. Arrhenius plots of second-order rate constants for reaction 4, SiH2 + oxetane in SF6: 4, 10 Torr; 2, infinite pressure. Line is LSQ fit to k∞ values.

Figure 4. Second-order plots for reaction of SiH2 + oxetane: temperatures for each set of data are marked in the figure.

are shown in Figure 7 for the four temperatures studied. It can be seen that reasonably linear plots are obtained, and the second-order rate constants at 10 Torr, obtained by leastsquares fitting to these plots, are given in Table 3. The error limits are single standard deviations. It may be noted that the rate constants again decrease with increasing temperature. The pressure dependences of these rate constants are plotted in Figure 8. The rate constants show more pressure dependence than for the SiH2 + oxetane, and in this case, we were able to model it (see later); therefore, the fits are those calculated by RRKM theory.30 The values for the high-

TABLE 2: Second-Order Rate Constants for SiH2 + Oxetane over the Range 296-594 K, at 10 Torr Total Pressure and Infinite Pressure (by Extrapolation)

c

T/K

ka,b

k∞a,c

296 340 398 477 594

15.7 ( 1.0 8.77 ( 1.00 7.04 ( 0.21 4.08 ( 0.19 2.38 ( 0.09

25 ( 2.5 16 ( 1.6 12.5 ( 1.3 6.3 ( 0.6 4.0 ( 0.4

a Units: 10-11 cm3 molecule-1 s-1. Estimated (10%.

b

Single standard deviations.

Figure 7. Second-order plots for reaction of SiH2 + THF: temperatures for each set of data are marked in the figure.

TABLE 3: Second-Order Rate Constants for SiH2 + THF over the Range 294-398 K, at 10 Torr Total Pressure and Infinite Pressure (by Extrapolation)

Figure 5. Pressure dependence of second-order rate constants for reaction 4, SiH2 + oxetane, in the presence of SF6 at five temperatures (indicated). Curves are eyeball fits (see text).

ka,b

k∞a,c

294 323 357 398

10.14 ( 0.35 6.09 ( 0.17 3.37 ( 0.12 1.73 ( 0.08

28 ( 2.8 21 ( 2.1 18.6 ( 1.9 14.8 ( 1.5

Units: 10-11 cm3 molecule-1 s-1. Estimated (10%. a

c

T/K

b

Single standard deviations.

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Figure 10. Ab initio, MP2)full/6-31G(d) calculated geometries of the complexes formed by SiH2 with oxirane, oxetane, and THF. Selected distances are given in angstroms, and angles are given in degrees. For simplicity, H atoms have been omitted from the rings.

Figure 8. Pressure dependence of second-order rate constants for reaction 5, SiH2 + THF, in the presence of SF6 at five temperatures (indicated). Curves are RRKM fits (see text).

Figure 9. Arrhenius plots of second-order rate constants for reaction 5, SiH2 + THF in SF6: 0, 10 Torr; 9, infinite pressure. Line is LSQ fit to k∞ values.

pressure limiting rate constants, also shown in Table 3, have uncertainties related to this fitting. Figure 9 shows an Arrhenius plot of the rate constants obtained here both at 10 Torr and at infinite pressure. A linear least-squares fit to the infinite pressure values corresponds to the Arrhenius equation, where the uncertainties are again single standard deviations:

log(k∞5 /cm3molecule-1s-1) ) (-10.59 ( 0.10) + (5.76 ( 0.65) kJ mol-1 /RT ln 10 The uncertainties involved in extrapolation performed by using RRKM theory are less than those without its aid. Thus, we would estimate that these Arrhenius parameters may have maximum uncertainties of (0.3 (log A∞) and (2.0 kJ mol-1 (Ea). End-Product Analyses. These could only be carried out for experiments at room temperature. SiH2 + Oxirane. A mixture of 0.5 Torr PhSiH3, 1.2 Torr oxirane, and 50 Torr SF6 was subjected to 150 shots of 193 nm laser radiation (70 mJ/pulse) and then analyzed by GC (Porapak Q column at 120 °C). Under these conditions, one major product

peak was observed (apart from benzene from PhSiH3 photolysis) which eluted at 2.0 min and was identified as C2H4. Under these conditions, its peak corresponded to 30% of the residual oxirane (based on peak area). Blank experiments showed that only 0.3% of C2H4 was formed from oxirane alone. SiH2 + Oxetane. A mixture of 0.6 Torr PhSiH3, 1.0 Torr oxetane, and 10 Torr SF6 was subjected to 200 shots of 193 nm laser radiation (50 mJ/pulse) and then analyzed by GC (Porapak Q column at 150 °C). Despite the formation of small amounts of some small molecules (e.g. C2H4) from oxetane alone, no product peaks were detected with retention times between those of oxetane itself and benzene, suggesting that stable end products of SiH2 with oxetane were not formed under these conditions. Quantum Chemical (ab initio) Calculations. These were limited to calculation of the structures and stabilities of the adducts, that is the donor-acceptor complexes, of SiH2 to each of the three cyclic ethers of interest, viz., oxirane, oxetane, and THF. Exploration of the potential energy surfaces for decomposition of these complexes was beyond the scope of the present study. Structures were determined at the MP2)Full/6-31G(d) level, and energies were calculated at the G3 level. Structures are shown in Figure 10, and the energies are shown in Table 4. It can be seen from Figure 10 that the Si atom of the SiH2 group is not coplanar with the CsOsC plane of any of the cyclic ethers, consistent with the Si bonding to one of the lone pairs of the O atom. The orientation of the SiH2 group is skew to the ring37a in all cases, presumably because this minimizes nonbonded interactions.37b The Si · · · O bond lengths are all ca. 0.4 Å longer than those of a normal SisO bond (e.g. 1.63 Å in (SiH3)2O)) and reflect the relatively weaker nature of the donor-acceptor bonds. The energy values (Table 4) indicate that Si · · · O bonds in the complexes of oxetane and THF are comparable and slightly stronger than that of the complex with oxirane. Interestingly, the latter complex has the longest Si · · · O bond. RRKM Calculations (General). In order to judge whether the observed pressure dependences are consistent with association processes alone, we have undertaken RRKM calculations30 of the reverse decomposition processes of the complexes. This was to enable us to assess the potential significance of further mechanistic steps of the complexes in these reactions. A general TABLE 4: Theoretical Values for Energy, Enthalpy and Free Energy of Complexes of SiH2 with Cyclic Ethers, Calculated at the G3 Level Species

∆E(0 K)

∆H(298 K)

∆G(298 K)

SiH2 + oxirane H2Si · · · oxirane SiH2 + oxetane H2Si · · · oxetane SiH2 + THF H2Si · · · THF

0 -77 0 -97 0 -92

0 -79 0 -99 0 -94

0 -39 0 -56 0 -53

Studies of Reactions of Silylene with Cyclic Ethers SCHEME 1

J. Phys. Chem. A, Vol. 114, No. 2, 2010 789 TABLE 5: Molecular and Transition-State Parameters for RRKM Calculations for Decomposition of the SiH2 · · · THF Complex at 294 K SiH2 · · · THF complex TS-a(5) -1

ν˜ /cm

mechanism for reactions 3-5 is shown in Scheme 1. If the mechanism involves only steps (a), (-a) and collisional stabilization, RRKM theory should be able to provide a fit. But if step (b) is occurring, this will not be the case. The RRKM calculations were carried out in combination with a collisional deactivation model (also called the master-equation method) in a manner similar to that of previous work.6,7,31,38-52 Because experimental details of the structures and vibrational wavenumbers of the molecules of interest here are not available, we have taken most of the necessary information from the output of the theoretical calculations. These provide the vibrational wavenumbers of the silylene complexes but not the transition states. The vibrational assignments of the transition states were obtained by the same procedure as previously,6,7,31,38-52 based on adjustment of the vibrational wavenumbers of each complex to match the A factor for its decomposition.53 The latter were calculated by use of the microscopic reversibility relationship, ln(A-a/Aa) ) ∆So-a,a/R, where A-a and Aa are the decomposition and combination A factors (i.e., A-a is what is required, Aa is the experimental, infinite pressure, value measured here (see below)) and ∆So-a,a is the entropy change. The values for ∆So-a,a were also obtained from the thermodynamic output of the theoretical calculations, although because of the approximations used in the procedure, they were corrected for the vibrational wavenumber factor (0.893). Uncertainties in entropy values derived in this way are unlikely to be beyond ca. (4 J K-1 mol-1; therefore, values of log(A-a/s-1) should be good to (0.2. There is one complication in this procedure, viz., obtaining log Aa values requires the use of the calculated RRKM curves to match experiment in order to extrapolate rate constants to infinite pressure. This is dealt with by refinement, viz., an initial (approximate) extrapolation is made, leading to a first-round choice of parameters. RRKM-calculated curves are then fitted to experimental data by matching their curvature, which in turn leads to improved values for the extrapolated infinite pressure rate constants and thereby an improved, second-round choice of Arrhenius parameters, Aa and Ea(a). The wavenumbers of the transitional modes for each reactant molecule (i.e. each silylene complex) were adjusted systematically at each temperature to match the entropy of activation and, thereby, the A-a value. Although the value of A-a was fixed, this had the effect of introducing variational character, because the derived wavenumber values were different at each temperature. Details of assignments and the choice of critical energy for each reaction are discussed below. We have assumed, as previously, that geometry changes between reactant and transition state do not lead to significant complications. In modeling the collisional deactivation process, we have used a weak collisional (stepladder) model,30 because there is overwhelming evidence against the strong-collision assumption.54 The average energy removal parameter, 〈∆E〉down, which determines the collision efficiencies, was taken as 12.0 kJ mol-1 (1000 cm-1) for SF6 for each reaction system. Further details of the calculations for each reaction system are considered below. For convenience, those for reaction 5 are considered first. RRKM Calculations for SiH2 + THF. The vibrational assignment for the donor-acceptor complex and for its decom-

reaction coordinate/cm-1 path degeneracy Eo(critical energy)/kJ mol-1 collision number (in SF6): ZLJ/10-10 cm3 molecule-1 s-1

2930(8) 1912(2) 1485(4) 1346(4) 1235(2) 1170(3) 1074(1) 1009(1) 980(1) 942(1) 865(5) 725(1) 670(1) 668(1) 555(1) 236(1) 219(1) 179(1) 134(1) 77(1) 35(1) 725 1 78 5.17

2930(8) 1912(2) 1485(4) 1346(4) 1235(2) 1170(3) 1074(1) 1009(1) 980(1) 942(1) 865(5) 668(1) 555(1) 219(1) 179(1) 158(1) 54(1) 31(1) 18(1) 10(1)

TABLE 6: Temperature-Dependent Parameters Used in RRKM Calculations for Decomposition of the SiH2 · · · THF Complex T/K 357

398

TS-a(5) wavenumbers/cm-1 158 159 54 55 31 32 18 19 10 11

294

323

162 57 34 21 11

164 58 35 22 12

ZLJ(SF6)/10-10 cm3 molecule-1 s-1 5.17 5.23

5.31

5.40

position transition state, TS-a(5), at room temperature (294 K), are shown in Table 5. Some of the higher-value vibrational wavenumbers have been grouped together (both for the complex and the transition state). This does not affect the results. The wavenumber values for the transitional modes at other temperatures are given in Table 6. Fitting to the data was accomplished by using Eo(5) as an adjustable parameter and by matching the curvatures of the calculated pressure-dependent curves to experiment. The curves at all four temperatures were used for this fitting. The resultant optimal value for Eo(5) was 78 kJ mol-1. Despite an element of judgment involved in this process, we estimate that the maximum uncertainty in this value is (10 kJ mol-1. Adjusted for thermal energy, the value for Eo(5) corresponds to E-a(5) ) 83 ( 10 kJ mol-1. This can be converted to ∆H°(298 K) by using the microscopic reversibility relationship, ∆H°(298 K) ) E-a(5) - Ea(5) + RT. The value of 92 ( 10 kJ mol-1 was obtained for ∆H°(298 K). This is in excellent agreement with the value of 94 kJ mol-1 from the quantum chemical calculations. The results of the calculations are shown in Figure 8 where they are compared with experiment. RRKM Calculations for SiH2 + Oxirane. Details of the vibrational assignment for the donor-acceptor complex and for

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Figure 11. Comparison of pressure dependence of rate constants for SiH2 + oxirane at 294 K. Experiment, O; RRKM theory, line.

Figure 12. Comparison of pressure dependence of rate constants for SiH2 + oxetane. Experiment: O, 296 K; g, 594 K. RRKM theory lines at same temperatures (indicated).

its decomposition transition state, TS-a(3), at room temperature (294 K), are given in the Supporting Information. The Eo(3) value (67 kJ mol-1) was chosen to be consistent with the theoretically calculated value for ∆H°(298 K) (75 kJ mol-1). The results of the calculation at 294 K are compared with experimental results in Figure 11. Because there is clearly no agreement with the experimental values, no further calculations were carried out on this system. RRKM Calculations for SiH2 + Oxetane. Details of the vibrational assignment for the donor-acceptor complex and for its decomposition transition state, TS-a(4), at 296 and 594 K, are given in the Supporting Information. The Eo(4) value (82 kJ mol-1) was chosen to be consistent with the theoretically calculated value for ∆H°(298 K) (99 kJ mol-1). The results of the calculation at 296 K are compared with experimental results in Figure 12. Because the difference with experiment was small, a further calculation was carried out at 594 K. This is also compared with experimental results in Figure 12, where there is clearly no agreement. No further calculations at other temperatures were carried out on this system.

Kinetic Comparisons. The main experimental purpose of the present work was to study the kinetics of the reactions of SiH2 with cyclic ethers for the first time and to investigate the temperature and pressure dependences of the second-order rate constants. This has been accomplished. All three reactions studied are found to be pressure dependent and to have negative temperature dependences, that is, rate constants which decrease with increasing temperature. This is characteristic of most silylene association reactions.1-3 There is no previous work on reactions 3-5 with which to compare these results. However, there is kinetic data for the reaction of SiH2 with Me2O,9 the prototype silylene + ether reaction. Comparisons are tricky, however, because all reactions of this type are pressuredependent, and the true bimolecular rate constants are only obtainable by extrapolation to high pressures. This has been done in all the cases cited here, and the infinite pressure Arrhenius parameters obtained are shown in Table 7. Because of the uncertainties of extrapolation, maximum error limits are given. For the three reactions studied here, the A factors are all similar in magnitude but slightly lower than those (log(A/cm3 molecule-1 s-1) ) ca. -10.0) found for SiH2 addition reactions to π-bonds,39-41,45,51,52 the other general class of silylene association reactions. The values for Ea are slightly more negative than those (-1.7 to -3.4 kJ mol-1) for the SiH2 additions to π-bonds. Although the value for SiH2 + oxetane (reaction 4) looks rather more negative than the others (reactions 3 and 5), the rate-constant data for this reaction were the hardest to extrapolate; therefore, the error limits are the largest. For the reference reaction, viz., SiH2 + Me2O, the Arrhenius parameters9 appear anomalous. Not only is the A factor exceptionally high, but the activation energy is positive. To obtain these values, once again, extrapolation of rate constants was necessary, in this case to considerably higher pressures (106-107 Torr) than those needed for reactions 3-5 (103-105 Torr). The Arrhenius parameters become more uncertain the lengthier the extrapolation, and although Alexander, King, and Lawrance (AKL)9 modeled their pressure dependence by using RRKM theory, we have shown that, in a similar study of SiH2 + H2O,6 extrapolation of such data is very dependent on precise details of the transition-state models. Thus, we suspect that the Arrhenius parameters for SiH2 + Me2O may be in error (see below). For reactions 3-5, the data are consistent with single-step reactions TABLE 7: Arrhenius Parameters for Elementary Silylene Reactions with Ethersa reaction

log(A/cm3 molecule-1 s-1)

Ea/kJ mol-1

ref

SiH2 + oxirane SiH2 + oxetane SiH2 + THF SiH2 + Me2O

-11.0 ( 0.5 -11.2 ( 0.7 -10.6 ( 0.3 -7.6 ( 0.4

-5.7 ( 3.6 -9.0 ( 5.0 -5.8 ( 2.0 +9.3 ( 2.8

this work this work this work 9

a

High-pressure limiting values.

TABLE 8: Rate Constants, Lennard-Jones Collision Numbers, and Collision Efficiences for Elementary Silylene Reactions with Ethers at Room Temperature reaction SiH2 SiH2 SiH2 SiH2 a

+ + + +

oxirane oxetane THF Me2O

ka,b

ZLJb

efficiency/%

1.0 2.5 2.8 8.09

5.23 5.79 5.20 5.04

19 43 45 160

High-pressure limiting values. b Units: 10-10 cm3 molecule-1 s-1.

Studies of Reactions of Silylene with Cyclic Ethers with no energy barrier. The small negative activation energies probably arise through conservation of angular momentum considerations such as those known to occur in radical association processes.30 We have also calculated the Lennard-Jones collision numbers at 298 K for these reactions (from parameters given in the Supporting Information) and thereby the collision efficiencies. These are shown in Table 8. The collision efficiencies for the reactions 4 and 5 compare reasonably well with values for silylene addition reactions, which lie in the range 64-82%,52,55 but the 19% value for reaction 3 seems a bit low. Because the reactive site in these cyclic ethers represents a fraction of the surface space of these ethers which diminishes as they increase in size, it might have been expected that the efficiency for reaction 3 would be greater than that for the other two reactions. The reason for this is not obvious. Once again, the value for SiH2 + Me2O looks anomalous, but AKL9 have argued that its reaction cross-section may be larger than calculations based on Lennard-Jones values because of long-range interactions. AKL9 have presented a model in which the centrifugal potential associated with rotation of the complexes, in cases where they possess weak binding energies, can lead to positive energy barriers in certain situations. However, although we cannot refute this model, we remain unconvinced of its validity (mainly because, in our experience of heavy carbene reactions,1-3 no other example needing such a model has been found). What is needed is a more direct measurement of the true (limiting high pressure) rate constant for SiH2 + Me2O (and, indeed, SiH2 with other O-donors). One other comparison is worth mentioning. In earlier studies, Baggott et al.25 measured rate constants for the analogous reactions of SiMe2. However, strict comparisons with the present work are not possible, because studies were (a) limited to room temperature (295 K), (b) carried out in Argon diluent and (c) pressure-dependence studies were only carried out for SiMe2 + oxirane. With this in mind, values for the rate constants for SiMe2 (in 5 Torr Ar; units, cm3 molecule-1 s-1) were 4.2 × 10-12 (oxirane), 3.3 × 10-11 (oxetane), and 2.9 × 10-12 (THF). These are all smaller than their SiH2 analogues (Tables 1-3) as expected for the generally less reactive SiMe2.2,3 In solution, the rate constant for SiMe2 + THF is 2.9 × 10-11 cm3 molecule-1 s-1,2,3 which shows that solvent effects can enhance the rate constants for these processes. Quantum Chemical Calculations and Stabilities of Complexes. The only previous calculation on any of these systems was carried out by Apeloig and Sklenak (AS)29 for the complex of SiH2 + oxirane. The structure and geometry of the H2Si · · · oxirane complex found by us is very similar to that obtained by AS at the QCISD/6-31G** and MP2/631G** levels.56 Similarly, our value for the binding energy (∆E(0 K), see Table 4) of 77 kJ mol-1 is in good agreement with those found by AS, viz., 71 kJ mol-1 (QCISD/6-31G** level) and 84 kJ mol-1 (MP2/6-31G** level). There are no values in the literature for comparision for the other two complexes. However, for reference, the prototype, unstrained complex of SiH2 with MeOMe has calculated values for its binding energy of 84.3 kJ mol-1 (MP2/6-311++G(d,p) level)28 and 82.9 kJ mol-1(MP2/6-311+G** level),9 which are in reasonable agreement with experiment9 (88.4 kJ mol-1). For the cyclic ethers, strain enthalpies are available from group additivity57 and have values (in kJ mol-1) of 115 (oxirane), 110 (oxetane), and 28 (THF). It would appear from these numbers that strain in the cyclic rings of these ethers

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has almost no effect on the binding energies of these complexes, because both oxirane and oxetane are much more strained than THF and Me2O has no strain at all. What is more illuminating are the values for the orbital ionization energies obtained from photoelectron spectroscopy. The lowest ionization energy is that for an oxygen lone-pair electron in all cases. The values (in eV) are 10.57 (oxirane),58 9.63 (oxetane),59 9.74 (THF),58 and 9.70 (Me2O).58 The significantly higher value for oxirane indicates the greater stability of its nO orbital and presumably, therefore, its lesser availability for bonding. This may explain why the binding energy of the H2Si · · · oxirane complex is only ca 80% of the binding energies of the other two cyclic complexes, although the H2Si · · · OMe2 complex lies awkwardly in between. Pressure-Dependences, RRKM Calculations and Mechanisms. As a generalization, we should expect that, for three members of a class of molecules undergoing the same association process, rate constant pressure-dependences would get less as the molecules increase in size. This is because, for the initially formed, vibrationally excited adducts, approximately the same (released) energy is distributed over increasing vibrational phase space, leading to longer lifetimes and a greater likelihood of being collisionally quenched at a given pressure. This reduces the probability of redissociation of an adduct and puts the reaction nearer to its bimolecular limit. Reactions 3-5 of SiH2 with oxirane, oxetane, and THF exhibit the opposite trend! The reaction of SiH2 with oxirane shows the least pressure dependence, whereas the reaction of SiH2 with THF shows the most. Among the three reactions, only 5 has been successfully modeled here by RRKM theory (Figure 8). For reaction 3, SiH2 + oxirane, RRKM modeling fails by large factors, which increase as pressure is reduced, even at 294 K (Figure 11). For reaction 4, SiH2 + oxetane, RRKM modeling gives a reasonably close fit at 296 K but again fails significantly at 594 K (Figure 12). There is, however, a fairly simple explanation for this, viz., that for the complexes of SiH2 with oxirane and oxetane, there are decomposition pathways, as well as redissociation; that is, kb (in Scheme 1) is not negligible for either reaction. For the reaction of SiH2 with oxirane, it is significant that we have found C2H4 as a substantial product. Apeloig and Sklenak29 have already shown that there is a low-barrier process from the H2Si · · · oxirane complex leading to H2SidO + C2H4. The mechanism proceeds by sequential OsC bond fission, via the intermediacy of the •SiH2OCH2CH2• diradical, as shown in Scheme 2. Starting from SiH2 + oxirane, the process is barrierless overall. This is consistent with our kinetic finding of a negative activation energy. The slight pressure dependences indicates that only small fractions of initially formed complexes are stabilized at any temperature and that the majority decomposes under all experimental conditions. The reaction mechanism for SiH2 with oxirane parallels that found earlier for SiMe2 with oxirane.25 It is not difficult to believe that the H2Si · · · oxetane complex should also have a relatively easy decomposition pathway, on account of the strain energy in the oxetane ring. Following the

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Becerra et al.

SCHEME 3

work of Gu and Weber,12 who investigated the products of SiMe2 + oxetane, the mechanism shown in Scheme 3 seems likely. We were unable to detect any end products at room temperature from the reaction of SiH2 with oxetane. However, because the experimental rate-constant pressure-dependence measurements were quite closely fitted by the RRKM modeling at 296 K, this suggests that there was little or no decomposition of H2Si · · · oxetane at this temperature. The lack of fit at other temperatures shows that decomposition to other products takes place to an increasing extent as the temperature is raised. This implies a small effective energy barrier for this process. The slight differences in strain energy of the 3-and 4-membered rings may mean that it is slightly harder to break the initial OsC bond in the oxetane complex than in the oxirane one, although there are clearly other mechanistic differences in the decomposition pathways of these two complexes. It would appear that the factors governing the behavior of silylene · · · oxetane complexes are quite subtle because SiMe2 is able to react with oxetane in solution at room temperature to produce products.12 Of course, this may be due to solvent effects, although in the gas phase, the reaction of SiMe2 with oxetane at 298 K was found to be particularly fast.25 The fact that the pressure-dependent kinetic behavior of the SiH2 + THF reaction 5 conforms to RRKM theory (with the calculated binding energy) shows that, when the cyclic ether has little or no strain energy, there is insufficient driving force for further reaction, either isomerization or decomposition of the complex. This is consistent with what is known about the reaction of SiH2 with acyclic ethers.9,28 The Me2Si · · · THF complex is similarly stable, both in solution23 and in the gas phase.25 Conclusion The experimental kinetics studies carried out here show that SiH2 reacts fairly rapidly with all three cyclic ethers, viz., oxirane, oxetane, and THF, in pressure-dependent reactions to form the Lewis acid-base association complexes. This is supported by quantum chemical calculations. For SiH2 + oxirane, the detection of C2H4 as product shows that the complex is largely decomposed. For SiH2 + oxetane, disagreement between RRKM calculations and experiment also indicates that the complex decomposes at temperatures above ambient. For SiH2 + THF, agreement between RRKM calculations and experiment shows that the complex is stable. The existence of breakdown pathways for the SiH2 complexes of oxirane and oxetane (but not THF) can be attributed to the strain in the small rings. Strain, on the other hand, appears to play no role in determining the magnitudes of the binding energies of the complexes. Acknowledgment. R.B. and R.W. thank Dow-Corning for a grant in support of the experimental work. R.B. thanks the Ministerio de Educacion y Ciencia for support under Project

CTQ2006-10512. We also thank Sarah Bowes for help with some of the experiments. Supporting Information Available: Details of vibrational assignments for the SiH2 · · · oxirane and SiH2 · · · oxetane complexes and their decomposition transition states, Lennard-Jones parameters for molecules of interest. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jasinski, J. M.; Becerra, R.; Walsh, R. Chem. ReV. 1995, 95, 1203. (2) Becerra, R.; Walsh, R. Kinetics & mechanisms of silylene reactions: A prototype for gas-phase acid/base chemistry. In Research in Chemical Kinetics; Compton, R. G., Hancock, G., Eds.; Elsevier: Amsterdam, 1995; Vol. 3, p 263. (3) Becerra, R.; Walsh, R. Phys. Chem. Chem. Phys. 2007, 9, 2817. (4) Gaspar, P. P.; West, R. Silylenes. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, 1998; Vol. 2, Chapter 43, p 2463. (5) Alexander, U. N.; King, K. D.; Lawrance, W. D. J. Phys. Chem. A 2002, 106, 973. (6) Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A 2003, 107, 11049. (7) Becerra, R.; Goldberg, N.; Cannady, J. P.; Almond, M. J.; Ogden, J. S.; Walsh, R. J. Am. Chem. Soc. 2004, 126, 6816. (8) Becerra, R.; Carpenter, I. W.; Gutsche, G. J.; King, K. D.; Lawrance, W. D.; Staker, W. S.; Walsh, R. Chem. Phys. Lett. 2001, 333, 83. (9) Alexander, U. N.; King, K. D.; Lawrance, W. D. Phys. Chem. Chem. Phys. 2001, 3, 3085. (10) Gu, T. Y.; Weber, W. P. J. Organomet. Chem. 1980, 195, 29. (11) Tseng, D.; Weber, W. P. J. Am. Chem. Soc. 1980, 102, 1451. (12) Gu, T. Y.; Weber, W. P. J. Am. Chem. Soc. 1980, 102, 1641. (13) Steele, K. P.; Weber, W. P. J. Am. Chem. Soc. 1980, 102, 6095. (14) Steele, K. P.; Weber, W. P. Inorg. Chem. 1981, 20, 1302. (15) Steele, K. P.; Tseng, D.; Weber, W. P. J. Organomet. Chem. 1982, 231, 291. (16) Raghavachari, K.; Chandraskhar, J.; Gordon, M. S.; Dykema, K. J. J. Am. Chem. Soc. 1984, 106, 5853. (17) Ando, W.; Hagiwara, K.; Sekiguchi, A. Organometallics 1987, 6, 2270. (18) Ando, W.; Hagiwara, K.; Sekiguchi, A.; Sakakibari, A.; Yoshida, H. Organometallics 1988, 7, 558. (19) Gillette, G. R.; Noren, G. H.; West, R. Organometallics 1987, 6, 2617. (20) Gillette, G. R.; Noren, G. H.; West, R. Organometallics 1989, 8, 487. (21) Levin, G.; Das, P. K.; Bilgrien, C.; Lee, C. L. Organometallics 1989, 8, 1206. (22) Moiseev, A. G.; Leigh, W. J. J. Am. Chem. Soc. 2006, 128, 14442. (23) Moiseev, A. G.; Leigh, W. J. Organometallics 2007, 26, 6277. (24) Baggott, J. E.; Blitz, M. A.; Lightfoot, P. D. Chem. Phys. Lett. 1989, 154, 330. (25) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Lightfoot, P. D.; Walsh, R. Int. J. Chem. Kinet. 1992, 24, 127. (26) Su, S.; Gordon, M. S. Chem. Phys. Lett. 1993, 204, 306. (27) Zachariah, M. R.; Tsang, W. J. Phys. Chem. 1995, 99, 5308. (28) Heaven, M. W.; Metha, G. F.; Buntine, M. A. J. Phys. Chem. A 2001, 105, 1185. (29) Apeloig, Y.; Sklenak, S. Can. J. Chem. 2000, 78, 1496. (30) Holbrook, K. A.; Pilling, M. J.; Robertson, S. H. Unimolecular Reactions, 2nd ed.; Wiley: Chichester, 1996. (31) Becerra, R.; Frey, H. M.; Mason, B. P.; Walsh, R.; Gordon, M. S. J. Chem. Soc., Faraday Trans. 1995, 91, 2723. (32) Baggott, J. E.; Frey, H. M.; King, K. D.; Lightfoot, P. D.; Walsh, R.; Watts, I. M. J. Phys. Chem. 1988, 92, 4025. (33) Jasinski, J. M.; Chu, J. O. J. Chem. Phys. 1988, 88, 1678. (34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, Jr., J. A.;

Studies of Reactions of Silylene with Cyclic Ethers Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; B. Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, R.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; AlLaham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzales, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.9; Gaussian Inc.: Pittsburgh, PA, 1998. (35) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.; Pople, J. A. J. Chem. Phys. 1998, 109, 7764. (36) Pople, J. A.; Scott, A. P.; Wong, M. W.; Radom, L. Israel J. Chem. 1993, 33, 345. (37) (a) This means that one of the Si-H bonds lies above and between the two O-C bonds when the complex is viewed along the Si-O bond. (b) A second structure for the H2Si · · oxirane complex was found, in which the SiH2 group is anti- to the oxirane ring plane. This is 4 kJ mol-1 less stable than the skew structure. (38) Baggott, J. E.; Frey, H. M.; Lightfoot, P. D.; Walsh, R.; Watts, I. M. J. Chem. Soc., Faraday Trans. 1990, 86, 27. (39) Becerra, R.; Walsh, R. Int. J. Chem. Kinet. 1994, 26, 45. (40) Al-Rubaiey, N.; Walsh, R. J. Phys. Chem. 1994, 98, 5303. (41) Al-Rubaiey, N.; Carpenter, I. W.; Walsh, R.; Becerra, R.; Gordon, M. S. J. Phys. Chem. A 1998, 102, 8564. (42) Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A 1999, 103, 4457. (43) Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A 2001, 105, 1897. (44) Becerra, R.; Cannady, J. P.; Walsh, R. Phys. Chem. Chem. Phys. 2001, 3, 2343. (45) Al-Rubaiey, N.; Becerra, R.; Walsh, R. Phys. Chem. Chem. Phys. 2002, 4, 5072. (46) Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A 2004, 108, 3987.

J. Phys. Chem. A, Vol. 114, No. 2, 2010 793 (47) Becerra, R.; Bowes, S.-J.; Ogden, J. S.; Cannady, J. P.; Almond, M. J.; Walsh, R. J. Phys. Chem. A 2005, 109, 1071. (48) Becerra, R.; Bowes, S.-J.; Ogden, J. S.; Cannady, J. P.; Adamovic, I.; Gordon, M. S.; Almond, M. J.; Walsh, R. Phys. Chem. Chem. Phys. 2005, 7, 2900. (49) Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A 2006, 110, 6680. (50) Becerra, R.; Carpenter, I. W.; Gordon, M. S.; Roskop, L.; Walsh, R. Phys. Chem. Chem. Phys. 2007, 9, 2121. (51) Becerra, R.; Cannady, J. P.; Dormer, G.; Walsh, R. J. Phys. Chem. A 2008, 112, 8665. (52) Becerra, R.; Cannady, J. P.; Dormer, G.; Walsh, R. Phys. Chem. Chem. Phys. 2009, 11, 5331. (53) In this procedure, the internal rotations around the SisO bonds have been treated as harmonic vibrations. Because these are among the modes in the transition states which are adjusted to match the activation entropies, ∆S‡, and thereby the A factors, this approximation should make little difference. (54) Hippler, H.; Troe, J. In AdVances in Gas Phase Photochemistry and Kinetics; Ashfold, M. N. R., Baggott, J. E., Eds.; Royal Society of Chemistry: London, 1989, Vol. 2, Chapter 5, p. 209. (55) Becerra, R.; Walsh, R. J. Organomet. Chem. 2001, 636, 49. (56) In ref 29, the parameters listed in Table 2 clearly show that the conformation of the SiH2 group relative to the oxirane ring is skew, although elsewhere in the paper (pictorial representations in Scheme 4 and Figure 3), it appears to be syn- (cis-). (57) Benson, S. W.; Cruickshank, F. R.; Golden, D. M.; Haugen, G. R.; O’Neal, H. E.; Rodgers, A. S.; Shaw, R.; Walsh, R. Chem. ReV. 1969, 69, 279. (58) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules; Halsted: New York, 1981. (59) Mollere, P. D.; Houk, K. N. J. Am. Chem. Soc. 1977, 99, 3226.

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