Theoretical study on the reaction of silylenoid H2SiLiF with HF

Chemical Physics 323 (2006) 185–192 www.elsevier.com/locate/chemphys

Theoretical study on the reaction of silylenoid H2SiLiF with HF Ju Xie, Dacheng Feng *, Shengyu Feng, Jie Zhang Institute of Theoretical Chemistry, Shandong University, Jinan 250100, PeopleÕs Republic of China Received 1 April 2005; accepted 29 August 2005 Available online 6 October 2005

Abstract The reactions of the two most stable isomers, the three-membered ring (Ra) and the p-complex (Rb), of silylenoid H2SiLiF with HF have been studied by G3(MP2) method, respectively. The insertion into H–F bond and the H2-elimination channels were identified on each reaction surface. Natural bond orbital (NBO) analysis has been performed to study the effects of charge transfer and to understand the nature of different interactions between atoms and groups. Furthermore, the theoretical forward reaction rate constants in the temperature range 200–1200 K were computed by canonical variational transition state theory with small-curvature tunneling correction (CVT/SCT) method. It is concluded that three insertion channels of Ra and Rb (Ra-I, Rb-I1 and Rb-I2) and the H2-elimination channel of Ra (Ra-E) would be four competitive reaction channels, the H2-elimination channel of Rb (Rb-E) could be negligible, and silane H3SiF should be the major product. The small-curvature tunneling (SCT) correction effect plays an important role for the calculation of rate constants for the reactions.  2005 Elsevier B.V. All rights reserved. Keywords: Silylenoid H2SiLiF; G3(MP2) method; Insertion; Elimination

1. Introduction Silylenoid (R1R2SiMX), a compound in which an electropositive metal (M) and a leaving group (X, usually halogen) are bound to the same silicon atom, had been confirmed to be existent and predicted to be active intermediates in some organosilicon reactions [1–3]. In contrast to extensive experimental and theoretical studies on carbenoids [4], there are only a few reports on silylenoids [5,6]. In 1995, the experimental aspects of silylenoid chemistry in (alkoxysilyl)lithium compounds were reported by Tamao and Kawachi [7]. They found that [(tert-butoxy)diphenylsilyl]lithium, Ph2SiLi(OBu-t), not only behaves as a silylenoid but also has ambiphilic reactivity. Then further experimental studies on silylenoids have been carried out. Most recently, Lee et al. [8] reported the syntheses of stable halosilylenoids (Tsi)X2SiLi(Tsi = C(SiMe3)3,X = Br,Cl) at room temperature. Thus, a breakthrough has been made in the research of silylenoids. *

Corresponding author. Tel.: +86 531 8365748; fax: +86 531 8564464. E-mail address: [email protected] (D. Feng).

0301-0104/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.08.053

Recent theoretical calculations indicate that some reactions [6c,6d] of silylenoids resemble those of silylene, preliminarily revealing the applicability of silylenoids for preparation of the new silicon-bonded and heterocyclic silicon compounds. However, for some important reactions of silylenoids, the theoretical studies are rare. To exploit further the reaction of silylenoid, we wish to report a theoretical investigation into the reaction pathways of the simplest silylenoid H2SiLiF with HF. It is hopeful that this study will provide a better understanding of the reactivity of silylenoid, and would be helpful to the experimental research of this reaction. 2. Computational methods The geometries and frequencies of all stationary points (reactants, precursor complexes, transition states, intermediates and products) were firstly optimized at the HF/631g(d) level. All structures were then reoptimized at MP2(full)/6-31G(d) level, followed by single-point calculations carried out at QCISD(t,fc)/6-31G(d)//MP2(full)/631G(d) and MP2(fc)/G3(MP2)large//MP2(full)/6-31G(d)

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levels, respectively. Finally, G3(MP2) theory [9] was used for calculations of structural energies. To verify that transition states actually connect to the expected reactants, intermediates and products for each reaction channel, intrinsic reaction coordinate (IRC) [10] calculations were performed at the MP2(full)/6-31G(d) level with a stepsize of 0.05 (amu)1/2 bohr. Natural bond orbital (NBO) [11] analyses at the MP2(full)/6-31G(d) level were then used by the NBO 3.1 program [12] included in Gaussian 03 series of programs [13]. To obtain the theoretical rate constants, we performed the canonical variational transition state theory (CVT) [14] calculation with the small-curvature tunneling (SCT) contribution [15] using the Polyrate 9.0 program [16]. The rate constants are calculated at 10 temperatures using mass-scaled Cartesian coordinate. 3. Results and discussion Since the ground state of H2Si is known to be a singlet [17] and its calculated singlet–triplet gap is large (85.7 kJ/ mol at G3MP2 level), only the singlet silylene complex is considered in this work. In the singlet silylene (Fig. 1), the pairing of the valence electrons creates empty p atomic-like molecular orbital at the silicon atomic center that are available to accept electron density from electron-rich reaction partners. When silylene H2Si coexists with alkali halide LiF, they are impossible to separate from each other and will exist in the form of silylenoid H2SiLiF. The calculation [6a] has shown that silylenoid H2SiLiF has four equilibrium isomers and their energies are in the order of the SiH2Li+F ion pair (three-membered ring) < the H2Si:FLi (p-complex) < the ‘‘classical’’ SiH2LiF (tetrahedral) < the

H2Si:FLi complex (r-complex). The three-membered ring and p-complex are two basic structures in which H2SiLiF exists and takes part in chemical reactions and will be experimentally detectable. In this study, we explore the reactions of the three-membered ring (Ra) and the p-complex (Rb) structures (Fig. 1) with HF. We calculated the potential energy diagram for these reactions, the results of which are presented in Fig. 2. In the sections below we present the results for, and discuss the reaction potential energy surfaces of, each sub-titled reaction. 3.1. Reactions of the three-membered ring structure (Ra) with HF The calculations [6a] have shown that the three-membered ring structure is the most basal isomer, and all the other isomers can interconvert with each other through it. The three-membered ring structure (Ra) of H2SiLiF can be considered as a silylene complex. Li+ cation and F anion link to the orbital and the unoccupied p orbital of singlet H2Si, respectively. In Ra, the electron cloud distributions of the HOMO and LUMO are quite different from each other. The components of HOMO mainly concentrate on Si atom (the orbital) and two H atoms, while those of LUMO concentrate on Li atom with a little part of them on Si atom (the p orbital). Our calculation results (Fig. 2) indicate that the reactions of Ra with HF proceed via two reaction athways, the insertion into H–F bond channel (Ra-I) and the H2 elimination channel (Ra-E):

aTS1

aIM1

P1 + LiF

Ra+HF aTS2

aP2 + H 2

(Ra-I) (Ra-E)

The optimized geometric parameters and the corresponding structures at the MP2(full)/6-31G(d) level are given in Fig. 3. The total energies and relative energies of reactants, products, intermediates and transition states are listed in Table 1. E(kJ/mol) aTS2 100

bTS3 aTS1

50 0

bTS1

bTS2

Ra+HF Rb+HF

bPC

-50 -100 -150 -200

Fig. 1. Frontier molecular orbital analysis of silylene H2Si, the threemembered ring (Ra) and the p-complex (Rb) isomers of silylenoid H2SiLiF, HOMO (middle) and LUMO (right).

aP2+H2

bP2+H2

P1+LiF a(b)IM1

bIM2

Fig. 2. Potential energy profiles for reactions of Ra and Rb with HF molecule at G3(MP2) level.

J. Xie et al. / Chemical Physics 323 (2006) 185–192

3.1.1. Insertion reaction Ra-I Silylenoid Ra could be regarded as two moieties, H2SiF and Li+ with net natural charge 0.871 and 0.871, respectively. The p orbital on Si atom is not vacant because of the electron donating of F atom, F ! Si. The r orbital (occupancy 1.900) on Si atom is similar to that of silylene H2Si. Therefore, the H end of H–F bond attacks the orbital firstly with HF approaching Ra, and the electrons are partially transferred to the s orbital of H atom. The F end of H–F bond then interacts with the p orbital on Si atom from the back of the F atom in Ra. The reaction reaches transition state aTS1 (see Fig. 3, the hydrogen and fluorine in HF were marked as H* and F*, respectively). ˚ with a bond orIn aTS1, the H*–Si distance is 1.696 A der of 0.560. However, the distance between Si and F* ˚ , corresponding to a bond order of atoms is 2.109 A

0.078, that is, the interaction between them is very weak. In comparison with HF molecule, the breaking H*–F* ˚ with the bond in aTS1 is elongated distinctly by 0.220 A bond order decreased to 0.204. On the other hand, compared with the structure Ra, there is little variation in the structural parameters of H2SiLiF moiety in aTS1 (see Fig. 3). The natural charge of Si atom increased from 0.444 to 0.844. Meanwhile, the natural charges of H* and F* decrease obviously (see Table 2). That is to say, silylenoid Ra shows nucleophilic behaviour in the process from the beginning of the insertion to the transition state aTS1. The activation barriers for this reaction is 38.3 kJ/mol at the G3(MP2) level. After getting over the transition state aTS1, Si–H* bond is formed gradually with leaving of Li from Si and breaking of H*–F* bond. The three-membered ring in Ra has been F

F

2.387 (0.290)

99.6

H

1.503 Si (0.707)

1.751 (0.075)

1.804 (0.324)

H2 H1

1.746 (0.089)

1.807 (0.337)

H

187

2.380 (0.272)

Si

Li 1.696 2.109 (0.560) (0.078) 1.154 (0.204) H*

Ra

Li

aTS1

F*

F 1.972 (0.241)

2

H1H

H 1.618 (0.531) Si

Li

Si 1.655 (0.480)

H

1.655 (0.123)

1.541 (0.672)

91.9

H*

F*

1.480 (0.734)

F*

H*

aIM1 P1

2 0.970 H (0.451) 1.679 H* (0.419) 1.206 (0.226)

F H1 Si

1.983 (0.789)

F*

F 1.882 (0.262)

Li

F*

1.640 (0.494)

aTS2

1.788 (0.337)

H1

Li

Si aP2

2.017 (0.089)

2.376 (0.114)

46.8

Si

1.747 (0.082)

H

1.570 (0.633)

88.6

H

H2

H1

Li

Li

1.811 (0.337)

Rb

1.743 (0.086)

83.9

F

Si

1.793 (0.350)

F

2.507 (0.098)

H*

bPC

0.945 (0.467) F*

˚ ; bond angles, degree) and atom–atom overlap-weighted NAO bond order in parentheses at MP2(full) Fig. 3. Optimized geometries (bond lengths, A /6-31g(d) level.

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J. Xie et al. / Chemical Physics 323 (2006) 185–192

Li

H2

2.112 (0.092)

F*

1.551 (0.658)

H1

1.704 (0.088) 92.3

Si

139.2 95.6 1.613 1.217 (0.187) H* (0.598)

1.835 (0.324)

F*

1.655 (0.480) 99.2

F

H2 H1

1.484 87.9 (0.736) 1.649 (0.489)

F* Li

(0.214)

1.765 (0.383)

Si

H*

bTS2

F*

1.917 (0.263)

1.735 (0.088)

F

bIM2

Li

F*

1.760 (0.096)

1.682 (0.112) 1.742 H1 (0.376)

Li

1.680 (0.526)

H

(0.489)

F

bIM1

H*

1.612 1.727 H 1 (0.583) (0.092) 94.6 Si 1.762 F 1.605 (0.381) (0.629) 85.1 2.124 95.9 (0.078) *

H2 * H 0.943 1.229

1.972 (0.241)

1.470 (0.748)

Li

1.235 F * (0.170)

1.655 (0.123)

Si

bTS1

H2

Li

H2 1 H

1.801 (0.340)

F

92.7

H1 2.015 (0.178)

Si

85.1

F

Si

bTS3

bP2

Si 92.51 1.519 (0.673)

H

H

H

0.934 (0.477)

Li

F

1.567 (0.223)

F

H

0.738 (0.762)

H

Fig. 3 (continued)

destroyed, and the Li–F bond is shortening, close to that in ionic compound LiF. Then intermediate aIM1 is formed (see Fig. 3). Geometry of the intermediate aIM1 is analogous to a trigonal bipyramidal complex. Two F atoms are located at two peaks, respectively. The process from the transition state aTS1 to the intermediate aIM1 is also that of inversion of the triangular cone ‘‘umbrella’’ formed by Si atom and three H atoms, and that of F* reacting with Si from back of F atom with the departing of F from Si. Therefore, this process is similar to SN2 type nucleophilic substitution mech˚ ) and the F*–Si bond anism. In aIM1, the H*–Si (1.541 A ˚ (1.655 A) formed with the bond order 0.672 and 0.480, respectively. In addition, the distance between F and Si ˚ , and the interaction between them has beatoms is 1.972 A come very weak. Relative to the sum energies of Ra and HF, the G3(MP2) energy of aIM1 is 231.0 kJ/mol, and the insertion reaction is high exothermic. We investigated the variation of energies for optimized geometries which were obtained by freezing a certain

Si–F distance and optimizing the other geometric parameters at MP2(full)/6-31G(d) level. The calculations show that the energies increase continuously with elongating of the Si–F distance. Fig. 4 shows the dissociation of aIM1 with a non-barrier process. When LiF separates away completely from Si atom, product H3SiF (P1) can be obtained. The relative G3(MP2) energies of sum of H3SiF and LiF is 167.7 kJ/mol in Ra-I reaction. The intrinsic reaction coordinate (IRC) calculations on Ra-I reaction has been performed at the MP2(full)/631g(d) level. Fig. 5.1 shows the energy changes and the variations of Si–F*, Si–H*, Si–F, and Si–Li distances along the reaction coordinate in reaction path. It is very clear that the Si–F* and Si–H* bond distances decrease along the reaction path, and Si–F and Si–Li distances increase along the reaction path. These show that aTS1 is related to the reactants, Ra and HF, and the insertion intermediate aIM1, and this channel is an insertion of Ra into H–F bond with the dissociation of LiF from Si atom.

J. Xie et al. / Chemical Physics 323 (2006) 185–192 Table 1 Total energies (a.u.) and relative energies (kJ/mol, in parentheses) for reactants R, precursor complexes PC, transition states TS, intermediates IM and products P

189

Energy/a.u. -437.6412

-437.6416

Structure

MP2(full)/6-31G(d)

G3(MP2)

Ra Rb LiF H2

397.28635 397.29088 107.12946 1.14414

397.53276 397.53948 107.29215 1.17014

HF + Ra aTS1 aIM1 P1 + LiF aTS2 aP2 + H2

497.47052 497.46517 497.57426 497.54256 497.44437 497.53686

(0.0) (14.0) (272.4) (189.2) (68.6) (174.2)

497.89155 497.87698 497.97955 497.95544 497.85862 497.94604

(0.0) (38.3) (231.0) (167.7) (86.4) (143.1)

HF + Rb bPC bTS1 bIM1 bTS2 bIM2 P1 + LiF bTS3 bP2 + H2

497.47504 497.48600 497.45466 497.57426 497.44703 497.56296 497.54256 497.44025 497.55789

(0.0) (28.8) (53.5) (260.5) (73.6) (230.8) (177.3) (91.3) (217.5)

497.89826 497.90842 497.87006 497.97955 497.86466 497.96995 497.95544 497.85974 497.95874

(0.0) (26.7) (74.1) (213.4) (88.2) (188.2) (150.1) (101.1) (158.8)

-437.6420

-437.6424

-437.6428

-437.6432 2.4

2.6

2.8

Structure

Si

F

Li

H1

H2

Ra HF LiF

0.444

0.768

0.871

0.274

0.274

aTS1 aIM1 P1 aTS2 aP2 Rb bPC bTS1 bIM1 bTS2 bIM2 bTS3 bP2

0.924

0.924

0.844 1.561 1.488 0.703 0.980

0.775 0.829

0.860 0.947

0.803 0.778

0.664 0.641 0.942 1.561 1.041 1.676 0.936 1.023

0.780 0.771 0.783 0.829 0.758 0.756 0.814 0.790

F*

0.888 0.843

0.238 0.265 0.270 0.240

0.940 0.948 0.958 0.947 0.953 0.938 0.947 0.946

0.411 0.386 0.255 0.265 0.266 0.286 0.464 0.389

0.411 0.386 0.406 0.265 0.480 0.566 0.261

3.4

r/angstrom

Energy/a.u. -497.46

2.6

aTS1 E

r(Si-Li)

-497.48

2.4

-497.50

2.2

H* -497.52

0.560

0.560

0.707 0.724 0.679 0.690 0.715

0.254 0.425 0.270 0.401

0.602 0.712 0.724 0.720 0.720 0.709 0.790

0.557 0.258 0.425 0.231 0.286 0.365

r(Si-F)

-497.54

0.238 0.265 0.270 0.259 0.330

3.2

Fig. 4. Changes of energy with LiF dissociating apart from aIM1 at MP2(full)/6-31g(d) level.

Ra+HF

Table 2 Atomic natural charges

3.0

r(Si-F)/angstrom

r(Si-F*) r(Si-H*)

-497.56

2.0

1.8

1.6

aIM1 1.4

-497.58 -4

-2

0

2

4

Reaction coordinate/amu1/2-Bohr

3.1.2. H2-elimination reaction Ra-E The H2-elimination pathway is initiated by an interaction between the p orbital on Si atom and a lone pair of electrons on F atom in HF molecule. Then a hydrogen (with positive charge) migration takes place from F atom in HF moiety to H2 atom (with negative charge) through a four-center transition state aTS2 (Fig. 3). In aTS2, the ˚ with a bond order of 0.189. Si–F* distance is 1.983 A 2 ˚ with a bond orThe forming H*–H bond length is 0.970 A der 0.451. The breaking H*–F* and Si–H2 distances are ˚ , which are much longer than those in 1.206 and 1.679 A reactants HF and Ra, respectively. The natural charges of Si and H2 atoms in aTS2 increased by 0.259 and

Fig. 5.1. Changes of energies and bond distances along the Ra-I reaction path at MP2(full)/6-31g(d) level.

0.034, and those of H* and F* atoms decreased by 0.159 and 0.130, respectively. The activation barrier for Ra-E channel is 86.4 kJ/mol, which is 48.2 kJ/mol higher than that of Ra-I channel. After passing through the transition state aTS2, the reaction leads to the separate products, the three-membered ring silylenoid HF2SiLi (aP2) and H2 molecule. The reaction is exothermic by 143.1 kJ/mol, which is less than that of Ra-I channel by 24.7 kJ/mol. To understand more about this feature of the mechanism, intrinsic reaction coordinate (IRC) was traced at the MP2(full)/6-31g(d) level of theory starting from the saddle point (Fig. 5.2). It can be seen that the Si–F* and Si–H* bond distances decrease monotonically before point 0.0 and then Si–H* bond distance increases on the IRC path. Whereas, the Si–H2 distance is almost constant before point 0.5 and then it sharply increases. These show

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J. Xie et al. / Chemical Physics 323 (2006) 185–192

r/angstrom

Energy/a.u.

3.8

2

-497.44

r(Si-H )

aTS2

3.6

E

3.4

-497.46

r(Si-H*)

Ra+HF

3.2 3.0

-497.48

2.8 2.6 2.4

-497.50

2.2 2.0

-497.52

1.8

r(Si-F*)

1.6

-497.54

aP2+H 2 -10

-8

-6

-4

-2

0

2

4

6

8

10

1.4

12

Reaction coordinate/amu1/2-Bohr Fig. 5.2. Changes of energies and bond distances along the Ra-E reaction path at MP2(full)/6-31g(d) level.

that aTS2 is related to the reactants (Ra and HF) and the products (aP2 and H2), and this channel is an insertiondehydrogenation reaction. 3.2. Reactions of the p-complex structure (Rb) with HF The p-complex structure (Rb) of silylenoid H2SiLiF can also be regarded as a silylene complex with ionic compound LiF (see Fig. 1). Rb is formed by donating a part of electrons of F atom in LiF toward unoccupied p orbital of Si atom in singlet H2Si. In fact there also exist weak interactions among Li atom (with positive charge) and two H atoms (with negative charge). This means that there are two four-membered rings H-Si–F–Li, which make Rb more stable than Ra by 17.6 kJ/mol (G3(MP2)). In Rb, the main part of HOMO is the r orbital (occupancy 1.988), and it is exposed (Fig. 1). Therefore structure Rb would show obvious nucleophilic in the r orbital direction. The calculational results indicate that the reactions of Rb with HF proceed via three reaction pathways (Fig. 2), two insertion into H–F bond reactions (Rb-I1 and RbI2), and a H2-elimination reaction (Rb-E), as depicted in the following: bTS1

bIM1

P1+LiF (Rb-I1)

bTS2

bIM2

P1+LiF (Rb-I2)

bPC Rb+HF

bTS3

bP2+H2

(Rb-E)

3.2.1. Insertion reactions Rb-I1 and Rb-I2 The insertion reactions are initiated by an interaction between the occupied r orbital on Si atom and the s orbital on H atom of H–F bond, leading to the precursor complex bPC (Fig. 3) bound by 26.7 kJ/mol at G3(MP2) level. ˚ InbPC, the Si–H* bond is long, calculated to be 2.507 A

with a bond order of 0.098. As the H* atom further approaches Si atom with electrostatic interaction, the F* end of H–F bond interacts with Si atom. There exist two insertion pathways from bPC that lead to the formation of silane H3SiF (P1) via transition states bTS1 and bTS2, followed by insertion intermediates bIM1 and bIM2 (Fig. 2), respectively. Structure bTS1 is formed with F* atom interacting with Si atom from the back of F atom in H2SiLiF moiety. In bTS1, \F*SiF is 169.7. Compared with Rb and HF molecules, the natural charge of Si atom in bTS1 increased by 0.278, the natural charges of H* and F* atoms decreased by 0.302 and 0.152, respectively. It is indicate that a part of the r electrons are transferred to H*F* moiety, and Rb shows nucleophilic property. Compared with structure Rb, there is much variation in the structural parameters of H2SiLiF moiety in bTS1. Dihedral angle of \LiFSiH2 decreased from 44.0 to 0.5. The F*–H* distance is ˚ , 0.283 A ˚ longer than that in HF. The Si–F* and 1.217 A ˚ with Si–H* bond lengths in bTS1 are 2.112 and 1.613 A bond order of 0.092 and 0.598, respectively. The relative energy of bTS1 is 74.1 kJ/mol at the G3(MP2) level. After getting over the bTS1, an intermediate bIM1 forms with departing of the LiF moiety from the Si atom and the forming of Si–F* and Si–H* bonds. It is interesting to note that the geometry of bIM1 is same to that of aIM1. The G3(MP2) energy of bIM1 is 213.4 kJ/mol lower than the sum energies of Rb and HF. Structure bTS2 is formed with F* atom migrating to Si atom in the nearly perpendicular to the F–Si bond direction (Fig. 3). In bTS2, \F*SiF is 85.1. The forming ˚ with a bond order of 0.629. The Si–H* bond is 1.605 A ˚ . The Si–F* distance breaking H*–F* distance is 1.235 A ˚ . Dihedral angle of \LiFSiH2 is 0.52. is long, 2.124 A Compared with Rb and HF molecules, the natural charge of Si atom in bTS2 increased by 0.377, the natural charge of H* atom decreased by 0.329, and that of F* atom decreased by 0.160 as well as (Table 2). The activation barrier for Rb-I2 reaction is higher than that of Rb-I1 by 14.2 kJ/ mol. Beyond bTS2, the insertion intermediate bIM2 formed (Fig. 3). In bIM2, the Si–H* and Si–F* distances ˚ shorter than those in bTS2, respecare 0.121 and 0.475 A tively. The formation of Si–H* and Si–F* bonds, and the cleavage of F*–H bond lead to the structure of bIM2 a penta-coordination around the silicon atom. G3(MP2) energy of bIM2 is 188.2 kJ/mol lower than the sum of energies of Rb and HF, less than that of Rb-I1 pathway by 25.2 kJ/mol but this reaction is still highly exothermic. The intermediates bIM1 and bIM2 can further decompose to free H3SiF (P1) and LiF with the similar non-barrier process as the decomposing of aIM1 in Ra-I reaction. It could be presumed that P1 formed via Rb-I1 and Rb-I2 pathways, respectively, would be enantiomorphs if the two hydrogen atoms substituted by different atoms or groups, respectively. It is very significative to prepare new silane species. Sum of the relative G3(MP2) energies of P1 and LiF is 150.1 kJ/mol.

J. Xie et al. / Chemical Physics 323 (2006) 185–192

3.2.2. H2-elimination channel Rb-E The reaction may proceed directly via H2-elimination with a transition state bTS3 to obtain a four-membered ring silylenoid HSiF2Li (bP2) and a H2 molecule (Fig. 2). In the transition state bTS3, the F* atom interacts with ˚ with a bond the Si atom, and the Si–F* distance is 2.015 A 2 ˚ longer than order of 0.178. The Si–H distance is 0.172 A that in the structure Rb; the forming H*–H2 bond is ˚ with a bond order of 0.489. The electron transfers 0.943 A Si ! F* and H2 ! H* exist in bTS3, which make the natural charge of Si and H2 atoms increase to 0.936 and 0.261 (Table 2), respectively. The transition state bTS3 can decompose directly to a four-membered ring structure of silylenoid HSiF2Li (bP2) and H2 molecule. The sum of energies for bP2 and H2 molecule is 158.8 kJ/mol lower than the sum of energies for reactants, Rb and HF. 3.3. Rate constant The canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) is an efficient method to calculate the rate constant. In this work, we used this method to calculate the rate constants for the reaction of silylenoid H2SiLiF with HF over a temperature range from 200 to 1200 K to obtain a better understanding of these reactions. In order to calculate the constant, 30 points were selected near the saddle point along the minimum energy path (MEP) at MP2(full)/6-31 G(d) level, 15 points in the reactant zone and 15 points in the product zone. The calculated CVT/SCT rate constants are listed in Table 3 for these five reactions. It can be seen from Table 3 that the calculated rate constant increases with rise of temperature. To compare further the CVT/SCT rate constants among these reactions, for example, at 300 K, the Rb-I1, Rb-I2, Ra-I and Ra-E rate constants are 3.65 · 1016, 1.30 · 1016, 5.75 · 1017 and 6.17 · 1017 cm3 molecule1 S1, respectively, while the Rb-E rate constant is 1.55 · 1026 cm3 molecule1 S1. At 1000 K, the Rb-I1 rate constant is 3.99, 8.11 and 12.33 times larger than the Rb-I2, Ra-I and Ra-E rate constants, respectively. The Rb-E rate constant is much smaller than those of other four reactions. To conclude, the Rb-I1,RbTable 3 The CVT/SCT rate constants for the reaction of H2SiLiF with HF over the temperature range of 200–1200 K (in cm3 molecule1 S1) T (K)

Ra-I

Ra-E

Rb-I1

Rb-I2

Rb-E

200 300 400 500 600 700 800 900 1000 1200

4.69E  17 5.75E  17 7.69E  17 1.13E  16 1.72E  16 2.59E  16 3.84E  16 5.55E  16 7.82E  16 1.45E  15

6.30E  17 6.17E  17 7.10E  17 8.91E  17 1.19E  16 1.65E  16 2.38E  16 3.48E  16 5.14E  16 1.10E  15

2.98E  16 3.65E  16 5.09E  16 7.52E  16 1.15E  15 1.77E  15 2.74E  15 4.21E  15 6.34E  15 1.35E  14

1.17E  16 1.30E  16 1.65E  16 2.25E  16 3.21E  16 4.70E  16 7.01E  16 1.06E  15 1.59E  15 3.53E  15

1.17E  26 1.55E  26 2.37E  26 3.81E  26 6.24E  26 1.03E  25 1.71E  25 2.83E  25 4.66E  25 1.23E  24

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I2, Ra-I and Ra-E will be four competitive reaction channels, the Rb-E channel could be negligible, and silane H3SiF should be the major product. In addition, aTS1 is the less energetic saddle point but the rate constant of Ra-I is a little smaller than that of Rb-I1, which means the small-curvature tunneling correction effect plays an important role for the reaction besides the barrier height. 4. Concluding remarks The insertion and H2-elimination reactions of silylenoid H2SiLiF (Ra and Rb) with HF have been investigated theoretically by G3(MP2) method and CVT/SCT method. Our calculation results indicate that the reactions of Ra with HF proceed via two reaction pathways, the insertion into H–F bond channel (Ra-I) and the H2 elimination channel (Ra-E). The reactions of Rb with HF proceed via three reaction pathways, two insertion into H–F bond reactions (Rb-I1 and Rb-I2), and a H2-elimination reaction (Rb-E). Detailed comparison is made between the structures and energies of all five reactions. The rate constant calculations predict that Rb-I1, Rb-I2, Ra-I and Ra-E will be four competitive reaction channels, the Rb-E channel could be negligible, and silane H3SiF should be the major product. We expect that the calculation results presented in this work are advisable and can make some useful predictions for experiments. Acknowledgments The authors thank Professor Donald G. Truhlar for providing the POLYRATE-9.0 program. This work was supported financially by the National Nature Science Foundation of China (No. 20373034), PhD Special Research Foundation of Chinese Education Department, and High Performance Computational Center of Shandong University. References [1] [2] [3] [4] [5]

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