Unusual Isomers of Disilacyclopropenylidene (Si2CH2) - The Journal

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J. Phys. Chem. A 2010, 114, 7102–7109

Unusual Isomers of Disilacyclopropenylidene (Si2CH2) Qunyan Wu,†,‡ Qiang Hao,‡,§ Yukio Yamaguchi,‡ Qianshu Li,†,| De-Cai Fang,§ and Henry F. Schaefer III*,‡ Institute of Chemical Physics, Beijing Institute of Technology, Beijing, P. R. China 100081, Center for Computational Quantum Chemistry, UniVersity of Georgia, Athens, Georgia 30602, College of Chemistry, Beijing Normal UniVersity, Beijing, P. R. China 100875, and Center for Computational Quantum Chemistry, South China Normal UniVersity, Guangzhou, P. R. China 510631 ReceiVed: December 31, 2009; ReVised Manuscript ReceiVed: May 25, 2010

Nine electronic singlet state structures of Si2CH2 have been systematically investigated by high level theoretical methods. This research employed coupled cluster (CC) methods with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] using the correlation-consistent polarized valence cc-pVXZ/cc-pV(X+d)Z (X ) D, T, and Q) basis sets. Full valence complete active space self-consistentfield (CASSCF) wave functions were used for the interpretation of geometries and physical properties. Among the nine singlet stationary points, six structures (1S-6S) are found to be minima, two structures (7S and 8S) are transition states, and one structure (9S) is a second-order saddle point. The existence of the two peculiar hydrogen bridged isomers, 1S (Si · · · H · · · Si) and 4S (agostic CH · · · Si) is established. Extensive focal point analyses are used to obtain complete basis set (CBS) limit energies. For the six lowest-lying singlet minima, after focal point analyses, the energy ordering and energy differences (in kcal mol-1, with the zero-point vibrational energy corrected values in parentheses) are predicted to be 1S [0.0 (0.0)] < 3S [14.7 (14.5)] < 4S [25.1 (25.3)] < 5S [28.2 (26.0)] < 6S [45.0 (45.4)] < 2S [73.8 (72.0)]. Their relative energies are strikingly different from those for the isovalent parent C3H2 molecule. Geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities are reported for all equilibrium structures. 1. Introduction The singlet state of cyclopropenylidene (1S) is known to be the global minimum on the C3H2 potential energy surface (PES).1-4 Cyclopropenylidene is abundant in molecular clouds in interstellar space and has played an important role in the chemistry of the interstellar medium.5 The first laboratory detection of 1S was achieved by Reisenauer, Maier, Riemann, and Hoffmann in 1984.1 The identification of 1S was enabled by the ab initio prediction of its vibrational frequencies and infrared (IR) intensities provided by Lee and Bunge.2 Shortly thereafter, Reisenauer and Maier were able to show that upon irradiation 1S is photoisomerized into triplet propynylidene (propargylene, 2T) and in a second photostep into propadienylidene (vinylidenecarbene, 3S) as presented in Scheme 1.3,4 Vinylidenecarbene has also been detected in interstellar space;6 however, cyclopropenylidene appears to be the most abundant of all interstellar hydrocarbons. The chemistry of silicon-containing molecules in interstellar clouds is far from fully understood. Although silicon is present in interstellar space and its cosmic abundance is roughly twice that of sulfur, few silicon-containing molecules have been detected up to now. The study of SixCy species7-15 is very challenging in view of the complexity of the structures and spectroscopy of these species. These molecules are also important within the context of the silicon carbide industry. When considering hydrogenated silicon-carbon species (SixCyHz), very little is known from a spectroscopic point of view;

we note in particular SiC2H,16 silicon methylidine SiCH,17-19 and silylidene H2CSi.20,21 Despite predictions that such systems should be detectable in circumstellar envelopes and in molecular clouds, none of them has been detected yet. Maier and Reisenauer have reported the matrix spectroscopic identification of four SiC2H2 molecules.22-24 The SiC2H2 molecular system is isovalent to the C3H2 system, but with one Si atom. The global minimum of the SiC2H2 structures is known to be 1-silacyclopropenylidene (4S).25 The 4S isomer has been generated via pulsed flash pyrolysis of 2-ethynyl-1, 1, 1-trimethyldisilane. By subsequent photolysis 4S can be isomerized into ethynylsilanediyl (5S), vinylidenesilanediyl (6S), and silacyclopropyne (7S), as shown in Scheme 2. The identification of the SiC2H2 isomers and their 13C and D isotopomers was based on the comparison of their experimental and theoretical infrared (IR) spectra.22-27 The equilibrium geometries for the related Si2CH2 structures are not fully determined. The Si2CH2 molecular system is isovalent to the C3H2 and SiC2H2 systems and involves two Si atoms. In 1990, Jemmis, Prasad, Tsuzuki, and Tanabe28 reported a theoretical study of trivalent boron and divalent silicon including Si2CH2. They studied 10 isomers of Si2CH2 and predicted that 1S (in Scheme 3) is the global minimum at the MP2/6-31G*//HF/3-21G* level of theory. In 2004, Ikuta and SCHEME 1: The Three Most Important Structures for C3H2 Isomers

* To whom correspondence should be addressed. E-mail: [email protected]. † Beijing Institute of Technology. ‡ University of Georgia. | South China Normal University. § Beijing Normal University.

10.1021/jp912280z  2010 American Chemical Society Published on Web 06/11/2010

Unusual Isomers of Disilacyclopropenylidene SCHEME 2: The Four Most Important Structures for Singlet SiC2H2 Isomers

J. Phys. Chem. A, Vol. 114, No. 26, 2010 7103 2. Electronic Structure Considerations Among the various electronic states of Si2CH2 isomers the global minimum is 1S.28,29 Structure 1S has the dominant electron configuration

[core]6a127a125b228a122b129a126b22

1

A1

(1)

C2V

where [core]() 1a121b222a123a122b224a121b121a223b225a124b22) denotes the eleven lowest-lying core (Si: 1s, 2s, 2p-like and C: 1s-like) orbitals. The electron configurations and symmetries of the other eight structures are described as follows:

SCHEME 3: The Nine Structures for Singlet Si2CH2

2S:[core]6a125b227a128a126b222b129a12

1

A1

C2V

(2) 3S:[core]10a′211a′212a′213a′214a′23a′′215a′2

1

(3)

4S:[core]10a′211a′212a′213a′214a′215a′23a′′2

1

(4)

A′ Cs A′ Cs

5S:[core]8a129a1210a123b2211a123b124b22

1

C2V (5)

6S:[core]8a129a123b2210a1211a123b124b22

1

C2V (6)

7S:[core]6a127a125b228a126b222b129a12

A1 A1

1

A1

C2V

(7) 8S:[core]6a127a122b125b228a126b223b12

1

A1

C2V

(8) 9S:[core]8a129a1210a123b2211a124b223b12

Wakamatsu29 studied the geometric structures and isomeric stabilities of various stationary points of Si2CH2 neutral and its cation and anion with coupled cluster theory. They located five equilibrium structures and one isomerization transition state for neutral Si2CH2 at the cc-pVTZ CCSD(T) level of theory. Ikuta and Wakamatsu29 predicted the global minimum to be an isomer with a fascinating three-center Si · · · H · · · Si bond (1S in Scheme 3). They also predicted an unprecedented equilibrium structure (4S in Scheme 3), which they ascribed to two-electron threecenter bonding. Their structure 4S was predicted to lie 24.6 kcal mol-1 [with the aug-cc-pVTZ CCSD(T) method] above the global minimum. In the present research the Si2CH2 molecule, which is isovalent to C3H2 and SiC2H2 yet contains two Si atoms, has been systematically investigated employing coupled cluster (CC) methods with the correlation consistent polarized valence family of basis sets. Systems with more than one Si atom have been known to provide isomers with peculiar chemical bonds.28,29 In this regard, two other fascinating examples are the theoretical predictions and experimental identifications of the dibridged30-33 and monobridged31,34,35 structures of the Si2H2 molecule. A total of nine Si2CH2 singlet state isomers are here found to be stationary points, as shown in Scheme 3. This is demonstrably the most systematic and reliable study on the low-lying structures of the Si2CH2 molecules to date. Analogous high level theoretical predictions have stimulated experimental investigations in the fields of organo-silicon chemistry, interstellar chemistry, chemical dynamics, and high-resolution spectroscopy.

1

A1

C2V (9)

3. Theoretical Procedures In this research the correlation-consistent polarized valence cc-pVXZ (C, H, Si) and cc-pV(X+d)Z (Si) (where X ) D, T, and Q) basis sets developed by Dunning and co-workers36-38 were utilized. The cc-pV(X+d)Z basis sets are formed by adding a high-exponent d function to the cc-pVXZ sets for the silicon atoms.38 Zeroth-order descriptions of all stationary points were obtained using restricted Hartree-Fock (RHF) self-consistent field (SCF) theory for these closed-shell singlet state molecules. Dynamic correlation effects were included using coupled cluster methods with single and double excitations (CCSD)39,40 and CCSD with perturbative triple excitations [CCSD(T)].41,42 The correlated wave functions were constructed by freezing the 11 lowest-lying core (Si: 1s, 2s, 2p-like and C:1s-like) orbitals. To interpret geometrical parameters and physical properties at a correlated level, full valence complete active space SCF (CASSCF) wave functions43-45 with 14 electrons in 14 molecular orbitals (14e/14MO) were constructed at the 9 different cc-pV(Q+d)Z CCSD(T) optimized geometries. Focal point analyses (FPA)46-50 using the HF, CCSD, CCSD(T), and CCSDT levels of theory have been performed employing the correlation consistent polarized valence family of basis sets of Dunning and co-workers36 to obtain energetic values extrapolated to the complete basis set (CBS) limit. Geometries used for the focal point analysis were optimized at the cc-pV(Q+d)Z CCSD(T) level of theory. The total energy extrapolation was partitioned in two terms. The first term

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Wu et al.

Figure 1. Predicted geometries for the nine Si2CH2 structures at the cc-pV(Q+d)Z CCSD(T) level of theory. Bond lengths are in Å.

corresponds to the total SCF energy and was fitted to the functional form51

ESCF(X) ) A + Be-CX

(10)

where A, B, and C are the fitting parameters from SCF energies, and X is the cardinal number corresponding to the maximum angular momentum of the basis set. The correlation energy was extrapolated using the formula52

ECORR(X) ) E + FX-3

(11)

where E and F are the fitting parameters from correlation energies, and X is the cardinal number mentioned above. The structures of the stationary points were optimized using analytic derivative methods.53-55 Dipole moments, harmonic vibrational frequencies, and corresponding IR intensities were determined analytically. Electronic structure computations were carried out using the ACESII (Mainz-Austin-Budapest version),56 MOLPRO,57 PSI2,58 and PSI359 suites of quantum chemistry packages.

4. Results and Discussion The optimized geometries and the predicted total energies and physical properties for the nine Si2CH2 structures at the cc-pV(Q+d)Z CCSD(T) level of theory are presented in Figure 1 and Table 1, respectively. The geometries for the nine structures at all levels of theory are provided in Figures S1-S9, and the corresponding total energies and physical properties are given in Tables S1-S9 as Supporting Information. 4.1. CASSCF Wave Functions. The cc-pV(Q+d)Z CASSCF wave functions at the cc-pV(Q+d)Z CCSD(T) optimized geometries are described in terms of CASSCF natural orbitals (NOs). The reference configurations (Φ1) and several excited configuration state functions (CSFs) with relatively large CI coefficients (|CI| g 0.050) are presented in Table S11. The electron occupation numbers (Nocc) in the active MOs of CASSCF wave functions are provided in Table S12. The dominant excited configurations for structures 1S and 2S in Table S11 may be interpreted in the following manner. 4.1.1. 1S. For the 1A1 state of the unconventional structure 1S, three prominent doubly excited configurations, Φ2[(2b1)2f (3b1)2], Φ3[(2b1)2f(2a2)2], and Φ4[(9a1)2f(7b2)2], contribute to the CASSCF wave function. 4.1.2. 2S. For the ground state of the symmetrical disilacyclopropenylidene 2S there are four important doubly excited configu-

Unusual Isomers of Disilacyclopropenylidene

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TABLE 1: Theoretical Predictions of the Total Energy (in hartree), Dipole Moment (in debye), Harmonic Vibrational Frequencies (in cm-1), Infrared Intensities (in parentheses in km mol-1), and Zero-Point Vibrational Energy (ZPVE in kcal mol-1) for the Nine Si2CH2 Structures at the cc-pV(Q+d)Z CCSD(T) Level of Theory structures

energy

µe

1S

-617.301 436 0.302

2S

-617.181 984 3.260

3S

-617.277 508 1.006

4S

-617.261 564 0.969

5S

-617.256 606 1.834

6S

-617.229 520 0.737

7S

-617.258 459 0.795

8S

-617.207 981 1.463

9S

-617.122 113 5.200

ω1

ω2

ω3

ω4

ω5

ω1(a1) C-H str 3252(1.5) ω1(a1) Si-H s-str 2210(6.2) ω1(a′) C-H str 3199(1.9) ω1(a′) C-H4 str

ω2(a1) SiH s-str 1560(9.2) ω2(a1) Si-C s-str 854(4.6) ω2(a′) Si-H str 2191(104.8) ω2(a′) C-H5 str

ω3(a1) Si-C s-str 893(15.3) ω3(a1) SiSiH s-bend 721(8.6) ω3(a′) C-Si s-str 940(14.5) ω3(a′) CH2 scissor

ω4(a1) Si-Si str 446(0.9) ω4(a1) Si-Si str 513(0.8) ω4(a′) SiCH bend 868(75.9) ω4(a′) C-Si s-str

3137(5.0) ω1(a1) SiH2 s-str 2259(111.9) ω1(a1) CH2 s-str 3128(0.9) ω1(a1) CH2 s-str 3074 ω1(a1) CH2 s-str 3028 ω1(a1) SiH2 s-str 2260

2404(37.1) ω2(a1) Si-C a-str 1400(320.5) ω2(a1) CH2 scissor 1384(0.0) ω2(a1) CH2 scissor 1310 ω2(a1) CH2 scissor 1407 ω2(a1) Si-C str 963

1565(10.0) ω3(a1) SiH2 scissor 990(113.8) ω3(a1) Si-C str 894(24.6) ω3(a1) Si-C s-str 699 ω3(a1) Si-C s-str 713 ω3(a1) SiH2 scissor 919

795(16.6) ω4(a1) Si-C s-str 573(0.8) ω4(a1) Si-Si str 479(33.8) ω4(a1) Si-Si str 515 ω4(a1) Si-Si str 240 ω4(a1) Si-Si str 459

ω5(b1) C-H oop 892(10.7) ω5(a2) SiSiH oop 363(0.0) ω5(a′) C-Si a-str 742(18.5) ω5(a′) CH2 rock+ Si-Si str 626(14.1) ω5(b1) SiH2 wag 609(32.7) ω5(b1) CH2 wag 720(23.7) ω5(a2) CH2 twist 263 ω5(a2) CH2 twist 814 ω5(b1) SiH2 wag 371

rations, Φ2[(9a1)2f(7b2)2], Φ3[(2b1)2f(2a2)2], Φ4[(2b1)2f (3b1)2], and Φ5[(8a1)2f(3b1)2], that contribute to the CASSCF wave function. The excited configurations for the remaining seven structures shown in Table S11 may be explained in a manner similar to the wave functions for 1S and 2S. For all structures, the leading CI coefficients are sufficiently large for single-reference theoretical treatments to be used. We note that correlation effects are largest for structure 9S. The 4b1 and 5b2 natural orbitals occupation numbers in Table S12 are 0.216 and 0.130, due largely to the Φ2[3b12f4b12] and Φ3[4b22f5b22] doubly excited configurations, respectively. 4.2. Geometries. 4.2.1. 1S (in Figure S1). For the hydrogen bridged structure 1S, a two-electron three-centered chemical bond has been deduced via contour plots of the localized molecular orbital (LMO) relating to the Si · · · H · · · Si bond by Ikuta and Wakamatsu.29 The Si-C, Si-Si, and C-H bond lengths decrease with the basis set size and increase with the level of theoretical sophistication. Here, the two CSFs, Φ2 and Φ3 mentioned above, are closely related to the latter phenomenon. The 2b1 MO describes an out-of-plane SiCSi π bonding, and the 3b1 and 2a2 MOs represent an out-of-plane Si-C and Si-Si π antibonding, respectively. The Φ2 and Φ3 double excitations, therefore, increase the Si-C and Si-Si bond distances. With the addition of an extra tight d function on the Si atom the Si-C and Si-Si bond distances are 0.002 and 0.004 Å shorter than at the ccpVQZ CCSD(T) level of theory, respectively. The Si-Si bond length [2.539 Å at the cc-pV(Q+d)Z CCSD(T) levels of theory] is 0.212 Å longer than the typical Si-Si single bond (2.327 Å in the H3Si-SiH3 molecule60). This is the longest Si-Si distance predicted here among the eight Si2CH2 structures involving the Si-Si bond. 4.2.2. 2S (in Figure S2). The Si-C, Si-Si, and Si-H bond lengths for the symmetrical disilacyclopropenylidene 2S decrease with basis set size, although they increase with the completeness of correlation treatment. This latter feature is mainly attributed to the doubly excited configurations, Φ2-

ω8

ω9

ω6(b1) ω7(b2) SiH oop SiH bend 495(18.3) 1256(318.6) ω6(b1) ω7(b2) SiSiH oop Si-H a-str 401(6.4) 2207(160.7) ω6(a′) ω7(a′) Si-Si+Si-C str Si-Si str 569(0.1) 346(43.7) ω6(a′) ω7(a′) C-Si a-str CH2 rock

ω6

ω7

ω8(b2) C-H bend 799(3.4) ω8(b2) Si-C a-str 720(24.6) ω8(a′′) SiCH oop 786(69.7) ω8(a′′) CH2 wag

ω9(b2) Si-C a-str 787(8.0) ω9(b2) SiSiH a-bend 314(37.6) ω9(a′′) CSiH oop 396(1.4) ω9(a′′) CH2 twist

538(42.3) ω6(b1) SiCSi oop 29(7.6) ω6(b1) CSiSi oop 17(0.1) ω6(b1) CH2 wag 733 ω6(b1) CH2 a-str 3123 ω6(b1) SiSiC oop 107i

729(66.9) ω8(b2) SiH2 rock 655(63.3) ω8(b2) CH2 rock 680(0.7) ω8(b2) Si-C a-str 504 ω8(b2) CH2 wag 712 ω8(b2) SiH2 rock 469

323(0.1) ω9(b2) SiCSi bend 106(14.6) ω9(b2) CSiSi bend 68(2.9) ω9(b2) CH2 rock 343i ω9(b2) Si-C a-str 575i ω9(b2) SiSiC bend 89i

421(0.6) ω7(b2) SiH2 a-str 2268(77.5) ω7(b2) CH2 a-str 3227(0.9) ω7(b2) CH2 a-str 3113 ω7(b1) CH2 rock 983 ω7(b2) SiH2 a-str 2288

ZPVE 14.84

11.87

14.35

15.07

12.71

15.15

14.60

15.75

11.05

[(9a1)2f(7b2)2], Φ3[(2b1)2f(2a2)2], and Φ4[(2b1)2f(3b1)2], reported in Table S11. The 2b1 and 9a1 MOs describe an out-ofplane SiSiC π bonding and an in-plane Si-Si π bonding, whereas the 7b2, 2a2, and 3b1 MOs represent an in-plane Si-Si σ antibonding, an out-of-plane Si-Si π antibonding, and nonbonding, respectively. Therefore, the double excitations (Φ2-Φ4), from the π bonding MOs to the π and σ antibonding MOs, necessarily elongate the Si-C and Si-Si bond distances. The Si-Si bond distance (2.089 Å at the same level) is 0.085 Å shorter than that of the “standard” double bond length (2.174 Å in the trans-bent Si2H4 molecule61), indicating the Si-Si bond has strong double bond character. This is the shortest Si-Si distance among the eight Si2CH2 structures incorporating a Si-Si bond. 4.2.3. 3S (in Figure S3). The Si-Si bond length for the unsymmetrical disilacyclopropenylidene 3S increases with more advanced treatments of correlation effects due primarily to the Φ3[(3a′′)2f(4a′′)2] excitation: the 3a′′ MO is related to the SiSiC π bonding, and the 4a′′ MO is assigned to Si-Si π antibonding character. The Si-Si bond distance of 2.271 Å is slightly shorter than the typical Si-Si single bond distance (2.327 Å) of the Si2H6 molecule.60 The Si(2)-C bond distance is 1.748 Å, which implies double bond character. 4.2.4. 4S (in Figure S4). The fascinating structure 4S is a transition state at the SCF level of theory (see below); however, when electron correlation effects are taken into account it becomes a minimum. This isomer was deduced to have a twoelectron three-centered chemical bond Si · · · H · · · C from the localized orbital analysis presented by Ikuta and Wakamatsu.29 We are rather inclined to describe 4S as involving an agostic CH · · · Si interaction. We draw this conclusion because the Si · · · H distance (1.754 Å) is quite extended with respect to a normal Si-H bond. Moreover, the C-H distance (1.157 Å) in 4S is only slightly extended with respect to a normal C-H bond. The C-Si(1) and Si-Si bond length increase with the completeness of correlation treatment due to the Φ2[(3a′′)2f(4a′′)2] and Φ3[(15a′)2f(5a′′)2] excitations. It should be noted that a nonbonded Si(2)-H(5) distance decreases considerably with the

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basis set size and level of sophistication, owing to the Φ4[(14a′)2f(16a′)2] double excitation. 4.2.5. 5S (in Figure S5). The double excitations from the π bonding (3b1 and 4b2) MOs to the π antibonding (5b2, 4b1, and 5b1) MOs (Φ2-Φ4 in Table S11) inevitably elongate the two Si-C bond distances in correlated descriptions of 5S. The Si(1)-C (1.697 Å) and Si(2)-C (1.682 Å) bond lengths are somewhat shorter than a standard SidC double bond (1.71 Å).62,63 These distances strongly support our naming of 5S as silavinylidene-silanediyl. 4.2.6. 6S (in Figure S6). The Si-C bond distance (1.744 Å) is longer than those of the Si-C bonds of 5S at the same level of theory, suggesting a weaker SidC bond. The Si-Si bond distance (2.180 Å) is close to the typical double bond length (2.174 Å in the trans-bent Si2H4 molecule61), indicating the Si-Si bond of 6S has a double bond character. These structural predictions support our giving the name silylidenesilanediyl to 6S. 4.2.7. 7S (in Figure S7). Structure 7S has two imaginary vibrational frequencies with SCF theory (see below), although it has one imaginary vibrational frequency when electron correlation effects are taken into account, indicating a true transition state. The Si-C single bond length (1.927 Å) is 0.049 Å shorter than Si(2)-C (1.976 Å) for the less symmetrical equilibrium geometry 4S, whereas it is 0.105 Å longer than Si(1)-C (1.822 Å) of 4S. The C-H bond length (1.092 Å) is shorter than that of C-H(5) (1.157 Å) and longer than that of C-H(4) (1.088 Å) of 4S. The Si-Si bond distance (2.225 Å) is 0.051 Å longer than that of the double bond length (2.174 Å in the trans-bent Si2H4 molecule61), suggesting that the Si-Si bond has weak double bond character. Thus, the naming (planar disilacyclopropyne) of 7S conforms to electron counting, however, the silicon-silicon connection is not a triple bond. 4.2.8. 8S (in Figure S8). Structure 8S has one imaginary vibrational frequency (see below), indicating a transition state on the Si2CH2 PES. The Si-C bond distance (1.919 Å) is close to the standard value for a single Si-C bond length (1.92 Å).62,63 The Si-Si bond length (2.392 Å) is 0.065 Å longer than a typical Si-Si single bond (2.327 Å in H3Si-SiH360). Although we have named this structure twisted disilapropyne via simple electron counting, it is clear that the Si-Si interaction is far from being a triple bond. 4.2.9. 9S (in Figure S9). Structure 9S has one imaginary vibrational frequency with SCF theory (see below), although it has two imaginary vibrational frequencies at the CC levels of theory, indicating a second-order saddle point. The Si-Si bond length (2.193 Å) is 0.019 Å longer than that of the “standard” double bond length (2.174 Å in the trans-bent Si2H4 molecule61), suggesting that the Si-Si bond has double bond character. The Si-C distance (1.750 Å) is characteristic of a longish double bond. 4.3. Dipole Moments. The dipole moment of a molecule sensitively reflects the electronegativities of the constituent atoms and their structural configurations. The standard electronegativities (Pauling scale) of our three constituent atoms are Si (1.8), C (2.5), and H (2.1).64 4.3.1. 1S (in Table S1). The dipole moment of structure 1S is predicted to be 0.30 debye at the cc-pV(Q+d)Z CCSD(T) level of theory, being necessarily directed along the C2(b) axis with sign -HSi2CH+, contrary to expectations from the Pauling electronegativities. This value is the smallest in magnitude among our nine Si2CH2 structures, which may be attributed to the fact that the two H atoms reside at the two ends of the molecule. The theoretical dipole moment is smaller with the

Wu et al. coupled cluster theories than with the SCF method. This phenomenon is associated with the Φ4[(9a1)2f(7b2)2] excitation in Table S11, where the electrons shift from the 9a1 MO (parallel to the C2 axis) to the 7b2 MO (perpendicular to the C2 axis). This interpretation is supported by the fact that the CASSCF natural orbital electron occupation numbers (Nocc) in Table S12 are Nocc ) 1.949 (9a1) and Nocc ) 0.049 (7b2) compared to Nocc ) 2.0 (9a1) and Nocc ) 0.0 (7b2) at the SCF level of theory. Although the magnitude of the dipole moment is not too large, it may be possible to detect the exotic structure 1S with a multicentered chemical bond (Si · · · H · · · Si) by microwave (MW) spectroscopic techniques. 4.3.2. 2S (in Table S2). The dipole moment of 2S is predicted to be 3.26 debye, which is similar to that of the analogous C3H2 isomer (cyclopropenylidene, 3.41 debye with the same method). This similarity may be attributed to the fact that the C and H atoms reside at the two extremities of the molecule. The dipole moment of 2S is necessarily along the C2(b) axis with the expected sign -CSi2H2+, and it decreases with inclusion of correlation effects due to the Φ5[(8a1)2f(3b1)2] excitation. The 8a1 MO is, in the simplest picture, the lone-pair orbital localized on the C atom (parallel to the C2 axis), whereas the 3b1 MO (nonbonding) is assigned to the nonbonding 2p (perpendicular to the C2 axis) orbital of the C atom. The double excitation Φ5 lowers the polarity of the molecule along the C2 axis, decreasing the magnitude of the dipole moment. This is consistent with the electron shift from the 8a1 MO (Nocc ) 1.957) to the 3b1 MO (Nocc ) 0.059) shown in Table S12. 4.3.3. 3S (in Table S3). The dipole moment of the asymmetric disilacyclopropenylidene 3S (µ ) 1.01 debye; µa ) 0.945, µb ) 0.344) is much smaller than that for the symmetrical 2S. This result may be attributed to the fact that the electropositive Si and H atoms reside at the opposite ends of the molecule. The qualitative +Si-H- and -C-H+ bond polarities are clearly opposing each other. 4.3.4. 4S (in Table S4). The unique structure 4S has a relatively small dipole moment of 0.97 debye (µa ) 0.517, µb ) 0.819). As with 3S, the small magnitude of this dipole moment is a consequence of the electropositive atoms (Si and H) occupying the extremities of the molecule. However, the dipole moment may be large enough for future detection of the peculiar 4S structure with its interesting CH · · · Si agostic interaction via MW spectroscopy. 4.3.5. 5S (in Table S5). With our most reliable cc-pV(Q+d)Z CCSD(T) method the dipole moment of 5S is predicted to be 1.83 debye, the direction being necessarily along the C2(a) axis with sign +H2SiCSi-. This relatively large dipole moment may be related to existence of the two electropositive atoms of the SiH2 group. The theoretical dipole moment is smaller at the coupled cluster levels than with the SCF method. 4.3.6. 6S (in Table S6). The dipole moment of 6S is 0.74 debye, being necessarily along the C2(a) axis with sign + H2CSi2-, contrary to the Pauling electronegativities. This dipole moment is much smaller than that of 5S, which may be attributed to the fact that the electropositive Si and H atoms inhabit in the two opposite ends of the molecule. 4.3.7. 7S (in Table S7). Structure 7S also has a relatively small dipole moment of 0.80 debye, a value similar to the magnitude of the related unsymmetrical agostic structure 4S. The direction of the dipole moment is along the C2(b) axis with the unconventional sign +H2CSi2-. 4.3.8. 8S (in Table S8). The dipole moment of the twisted disilacyclopropyne structure 8S is 1.46 debye, necessarily along the C2(b) axis, with sign +H2CSi2-, contrary to the Pauling

Unusual Isomers of Disilacyclopropenylidene electronegativities. The dipole moment of 8S is much more sensitive to the level of sophistication than that of 7S. This phenomenon may be rationalized by the significantly smaller electron occupation number of the 9a1 MO (Nocc ) 1.903, along the C2 axis) for 8S, compared to the corresponding value of the 9a1 MO (Nocc ) 1.951, along the C2 axis) for 7S as presented in Table S12. 4.3.9. 9S (in Table S9). With our most reliable cc-pV(Q+d)Z CCSD(T) method the dipole moment of the silylidene carbene 9S is predicted to be 5.20 debye, the direction being necessarily along the C2(a) axis with sign +H2SiSiC-. This value is the largest in magnitude among the nine structures, with the more electronegative hydrogen and carbon atoms positioned at opposite ends of the molecule, one might have expected some cancellation of polarities. 4.4. Harmonic Vibrational Frequencies and Infrared (IR) Intensities. Among the nine low-lying electronic singlet stationary points of Si2CH2 located in this study, six structures (1S-6S) have been found to be minima, two structures (7S and 8S) are transition states (first-order saddle points), and the last structure (9S) is a second-order saddle point. 4.4.1. 1S (in Table S1). For the hydrogen-bridged structure 1S, all the predicted harmonic vibrational frequencies decrease with treatment of correlation effects, reflecting the elongated bond distances. Specifically the C-H stretching [ω1 (a1)] vibrational frequency decreases 134 cm-1 from the SCF method to the cc-pV(Q+d) CCSD(T) level. Since the vibrational mode for ω7 (b2) (Si-H bending) is determined to have by far the strongest IR intensity [319 km mol-1 at the same level of theory] and ω3 (a1), ω5 (b1), and ω6 (b1) to have considerable IR intensities, it should be possible to identify structure 1S under appropriate laboratory conditions. 4.4.2. 2S (in Table S2). Although structure 2S is energetically the highest lying (see below) among the six equilibrium structures, there are no imaginary vibrational frequencies, confirming that it is a minimum on the singlet Si2CH2 PES. All the predicted harmonic vibrational frequencies of 2S decrease with correlation effects due to the lengthened bond distances. The vibrational mode for Si-H antistretching [ω7 (b2)] is determined to have a strong IR intensity (161 km mol-1). 4.4.3. 3S (in Table S3). The asymmetric disilacyclopropenylidene structure 3S is the true equilibrium geometry, since there are no imaginary vibrational frequencies. The Si-Si stretching frequency (346 cm-1) is lower than that (513 cm-1) of the symmetrical 2S, reflecting a longer Si-Si bond distance. Among the nine IR active frequencies, two modes [ω2 (a′) and ω4 (a′)] have strong IR intensities (105 and 76 km mol-1, respectively). 4.4.4. 4S (in Table S4). The CH2 twist mode ω9 (a′′) of the fascinating agostic structure 4S presents an imaginary vibrational frequency at the SCF level of theory [199i cm-1 at the cc-pV(Q+d)Z RHF level of theory]. However, this frequency becomes real when electron correlation effects are taken into account [323 cm-1 at the cc-pV(Q+d)Z CCSD(T) level of theory]. The C-H(5) stretching frequency [ω2 (a′)] significantly decreases with an improved treatment of correlation effects, due to elongation of the bond distance upon formation of the Si · · · H(5)-C interaction. Isomer 4S presents noticeable IR intensities for all the vibrational modes except ω7 (a′) and ω9 (a′′). 4.4.5. 5S (in Table S5). For silavinylidene-silanediyl, all the predicted harmonic vibrational frequencies decrease with correlation effects due to the elongated bond distances. All vibrational modes except the Si-C symmetric stretching ω4 (a1)

J. Phys. Chem. A, Vol. 114, No. 26, 2010 7107 and SiCSi bending motions [ω6 (b1) and ω9 (b2)] are predicted to have strong IR intensities. 4.4.6. 6S (in Table S6). Note that Table S6 is longer than Tables S1-S5 and S7-S9. In Table S6 only, the harmonic vibrational frequencies with the correlation-consistent polarized core-valence basis sets (cc-pCVXZ where X ) D and T)38 are also included. For the cc-pCVXZ basis sets only two deep core orbitals (Si: 1s-like) were frozen in order to construct the coupled cluster wave functions. The harmonic vibrational frequency for the CSiSi out-of-plane mode [ω6 (b1)] of structure 6S is very low [17 cm-1 with the cc-pV(Q+d)Z CCSD(T) method and 36 cm-1 with the cc-pCVTZ CCSD(T) method]. This isomer may have a bent equilibrium structure with Cs point group symmetry at even higher levels [e.g., CCSDT, CCSDT(Q), etc] of theory. The vibrational modes ω3, ω4, and ω5 have significant IR intensities. 4.4.7. 7S (in Table S7). With SCF theory, for the planar disilacyclopropyne, the CH2 twist mode ω5 (a2) [195i cm-1 at the cc-pV(Q+d)Z RHF level of theory] and CH2 rock mode ω9 (b2) (339i cm-1 at the same level) have imaginary vibrational frequencies. The former frequency becomes real when electron correlation effects are taken into account [263 cm-1 at the ccpV(Q+d)Z CCSD(T) level of theory]. The eigenvector of the ω9 (b2) mode lowers the molecular symmetry and leads to the equilibrium Cs symmetry agostic structure (4S). The two CH2 stretching frequencies [ω1 (a1) ) 3074 cm-1 and ω7 (b2) ) 3113 cm-1 at the same level of theory] of 7S are closer to the C-H(4) stretching frequency [ω1 (a′) ) 3137 cm-1 at the same level] for 4S, rather than the C-H(5) stretching frequency [ω2 (a′) ) 2404 cm-1] of 4S. 4.4.8. 8S (in Table S8). The twisted disilacyclopropyne structure 8S has an imaginary vibrational frequency for the Si-C antisymmetric stretching mode (575i cm-1), indicating a transition state. The eigenvector of the ω9 (b2) mode leads to the linear structure 6S. The vibrational frequency for the Si-Si stretching mode of structure 8S (240 cm-1) is considerably lower than the corresponding values of 6S (479 cm-1) and 7S (515 cm-1), reflecting the longer Si-Si bond distance. 4.4.9. 9S (in Table S9). With SCF theory, for structure 9S, the SiSiC out-of-plane bending mode ω6 (b1) has an imaginary vibrational frequency [148i cm-1 at the cc-pV(Q+d)Z RHF level]. However, the SiSiC in-plane bending mode ω9 (b2) also becomes imaginary when electron correlation effects are taken into account [89i cm-1 at the cc-pV(Q+d)Z CCSD(T) level], indicating a second-order saddle point. The eigenvectors of the two SiSiC bending modes lead to two different Cs structures. These two Cs structures are not located in this study, since structure 9S is energetically high lying (112.5 kcal mol-1 above the global minimum 1S). The Si-Si stretching frequency of 9S [459 cm-1 at the same level] is close to the corresponding frequency of 6S (479 cm-1 at the same level). 4.5. Energetics. The relative energies of the eight Si2CH2 structures (2S-9S) with respect to 1S at the 18 levels of theory are presented in Table S10. Focal point analyses (FPA) employing the HF, CCSD, CCSD(T), and CCSDT levels of theory with the correlation-consistent polarized valence family of basis sets (cc-pVXZ, X ) D, T, Q, 5, and 6) for the six equilibrium structures are provided in Tables 2-6. The unconventional hydrogen-bridged structure 1S is seen to be the global minimum on the singlet Si2CH2 PES. The second lowest-lying isomer is predicted to be the unsymmetrical disilacyclopropenylidene 3S. The energy difference between isomers 1S and 3S based on the FPA is reported in Table 3. At the HF level of theory the energy separation is predicted to be

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Wu et al.

TABLE 2: Incremented Focal Point Analysis of the Relative Energy (in kcal mol-1) Between Isomers 1S and 2Sa basis set

∆E[HF]

cc-pVDZ cc-pVTZ cc-pVQZ cc-pV5Z cc-pV6Z CBS limit

+80.14 +77.76 +76.65 +76.03 +75.88 [+75.84]

+δ[CCSD] +δ[CCSD(T)] +δ[CCSDT] +3.64 +1.39 +0.38 -0.02 -0.21 [-0.46]

-1.39 -1.48 -1.53 -1.57 -1.59 [-1.62]

-0.11 +0.02 [+0.02] [+0.02] [+0.02] [+0.02]

) ∆E

basis set

∆E[HF]

[+82.28] [+77.69] [+75.53] [+74.45] [+74.11] [+73.79]


+25.53 +24.89 +23.79 +23.38 +23.30 [+23.28]

a The symbol δ denotes the increment in the relative energy (∆E) with respect to the preceding level of theory in the hierarchy HF f CCSD f CCSD(T) f CCSDT. Square brackets signify results obtained from basis set extrapolations or additivity assumptions. Final prediction is boldfaced.

TABLE 3: Incremented Focal Point Analysis of the Relative Energy (in kcal mol-1) Between Isomers 1S and 3S basis set

∆E[HF]


+13.96 +14.02 +13.96 +13.88 +13.88 [+13.88]

+δ[CCSD] +δ[CCSD(T)] +δ[CCSDT] +2.47 +1.37 +1.10 +0.97 +0.92 [+0.84]

+0.00 +0.01 +0.02 +0.02 +0.01 [+0.01]

-0.03 +0.00 [+0.00] [+0.00] [+0.00] [+0.00]

) ∆E [+16.40] [+15.40] [+15.08] [+14.87] [+14.81] [+14.73]


∆E[HF]


+29.95 +30.77 +31.26 +31.44 +31.48 [+31.49]

+δ[CCSD] +δ[CCSD(T)] +δ[CCSDT] -4.26 -5.32 -5.29 -5.30 -5.31 [-5.31]

-0.79 -1.07 -1.15 -1.18 -1.19 [-1.20]

+0.09 +0.14 [+0.14] [+0.14] [+0.14] [+0.14]

TABLE 5: Incremented Focal Point Analysis of the Relative Energy (in kcal mol-1) Between Isomers 1S and 5S

) ∆E [+25.00] [+24.51] [+24.96] [+25.10] [+25.12] [+25.11]

13.9 kcal mol-1 at the CBS limit. This value increases by 0.8 kcal mol-1 with inclusion of correlation effects. At the CCSDTCBS limit the energy difference between the two lowest-lying isomer becomes 14.7 kcal mol-1. Including the zero-point vibrational energy (ZPVE) corrections at the cc-pV(Q+d)Z CCSD(T) level of theory, the relative energy becomes 14.5 kcal mol-1. The third lowest-lying isomer is predicted to be the fascinating agostic structure 4S. Although structure 4S has an imaginary vibrational frequency [199i cm-1 at the cc-pV(Q+d)Z RHF level of theory] at the SCF level of theory, it is a genuine minimum when electron correlation effects are taken into account. The FPA for the relative energy between isomers 1S and 4S is presented in Table 4. The energy difference is predicted to be 31.5 kcal mol-1 at the CBS limit with the HF method. It is seen that structure 4S is energetically lowered (relative to 1S) by 5.4 kcal mol-1 with correlation effects. The final extrapolated energy difference is 25.1 (25.3) kcal mol-1, where the energy in the parentheses indicates the ZPVE corrected value. The fourth lowest-lying equilibrium structure, the H2SidCdSi: structure 5S, is located 28.2 (26.0) kcal mol-1 above structure 1S with the CCSDT-CBS level of theory (in Table 5). The fifth lowest-lying minimum is predicted to be the other linear structure 6S, lying 45.0 (45.4) kcal mol-1 above structure 1S with the same level of theory (in Table 6). 2S is the highest lying equilibrium structure located in this research (in Table 2). Its relative energy with respect to 1S is predicted to be 73.8 (72.0) kcal mol-1 at the CBS limit with the CCSDT method. The relative energy of 2S most strikingly displays the difference between Si2CH2 and its all-carbon parent. For C3H2, the analogous cyclopropenylidene structure (1S) lies lowest in energy among its isomers. For Si2CH2 we predict the cyclo-

+δ[CCSD] +δ[CCSD(T)] +δ[CCSDT] +3.76 +4.79 +4.78 +4.90 +4.90 [+4.90]

-0.67 -0.23 -0.12 -0.09 -0.08 [-0.07]

-0.02 +0.06 [+0.06] [+0.06] [+0.06] [+0.06]

) ∆E [+28.59] [+29.50] [+28.51] [+28.24] [+28.18] [+28.18]


∆E[HF]


+46.61 +49.29 +49.99 +50.44 +50.53 [+50.55]

+δ[CCSD] +δ[CCSD(T)] +δ[CCSDT] -4.46 -4.37 -3.70 -3.38 -3.33 [-3.26]

-1.70 -1.79 -1.76 -1.75 -1.76 [-1.77]

-0.56 -0.54 [-0.54] [-0.54] [-0.54] [-0.54]

) ∆E [+39.88] [+42.59] [+43.98] [+44.77] [+44.90] [+44.98]

propenylidene structure of C2V symmetry to be the least favored among isomers considered. Structure 7S has two imaginary vibrational frequencies at the SCF level; however, it has only one imaginary vibrational frequency when electron correlation effects are taken into account. This transition state is located 27.0 kcal mol-1 above the global minimum 1S. The eigenvector of the imaginary vibrational frequency [CH2 rocking mode ω9(b2)] leads to the energetically lower-lying agostic structure 4S. The energy barrier for the isomerization reaction connecting the two mirror images of the 4S isomer (4Sf7Sf4S′) is determined to be only 1.95 (1.48) kcal mol-1. Thus, the CH2 rocking motion is met with little energetic resistance. Structure 8S has only one imaginary vibrational frequency at all levels of theory. This transition state lies 58.6 kcal mol-1 above the global minimum. The eigenvector of the imaginary vibrational frequency [Si-C antisymmetric stretching mode, ω9 (b2)] leads to the lower-lying linear structure 6S. Structure 9S has one imaginary vibrational frequency at the SCF level; however, it has two imaginary vibrational frequencies when electron correlation effects are taken into account. This secondorder saddle point is located 112.5 kcal mol-1 above the global minimum 1S. For the electronic singlet states of the isovalent C3H2 and SiC2H2 systems, cyclopropenylidene (1S, cyclic structure) and silacyclopropenylidene (4S, cyclic structure) are found to be the global minima, respectively. However, for the Si2CH2 system the unconventional hydrogen bridged structure (1S) is predicted to be the global minimum. As to the cyclopropenylidene-like structures of the Si2CH2 system, the asymmetric cyclic silylene structure (:Si, 3S) lies energetically lower than the corresponding carbene structure (:C, 2S) by about 60 kcal mol-1. For the three noncyclic structures (5S, 6S, and 9S), isomer 5S with a terminal CdSi: bond lies below isomer 6S with a terminal SidSi: bond by about 17 kcal mol-1 and is located below structure 9S with a terminal SidC: bond by about 84 kcal mol-1. These theoretical findings for the electronic singlet states of the Si2CH2 molecules indicate the following structural information: (1) Si often resists the formation of multiple bond(s). (2) Si has a tendency to form terminal silylenes (Si:). Terminal carbenes (C:) are less favorable. (3) For multi Si atom systems hydrogen bridged structures are often predicted. 5. Concluding Remarks Ab initio electronic structure theory has been employed in order to systematically investigate the electronic singlet state

Unusual Isomers of Disilacyclopropenylidene potential energy surface of the Si2CH2 system. Among the nine Si2CH2 stationary points located in this research, six structures (1S-6S) are found to be minima, two structures (7S and 8S) are transition states (first-order saddle points), and one structure (9S) is a second-order saddle point. After focal point analyses, the energy ordering and energy differences (with the zero-point vibrational energy corrected values in parentheses) for the lowest lying six singlet minima are predicted to be 1S [0.0 (0.0)] < 3S [14.7 (14.5)] < 4S [25.1 (25.3)] < 5S [28.2 (26.0)] < 6S [45.0 (45.4)] < 2S [73.8 (72.0)] kcal mol-1. The existence of the multicentered (hydrogen bridged) structure, 1S (Si · · · H · · · Si), and the agostic structure 4S (CH · · · Si), is firmly established. We hope the reliable theoretical predictions for the Si2CH2 molecules presented in this research will encourage future experimental investigations in the fields of organo-silicon chemistry, interstellar chemistry, chemical dynamics, and high-resolution spectroscopy. Acknowledgment. Q. W. and Q. H. gratefully acknowledge the support provided by the China Scholarship Council (CSC) [2008] 3019 and the University of Georgia Center for Computational Quantum Chemistry for hospitality during their oneyear visit. We thank Dr. Andrew C. Simmonett and Dr. Justin M. Turney for many helpful discussions. We are indebted to the 111 Project (B07012) in China and the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division (No. DEFG02-97-ER14748) for support of this research. Supporting Information Available: Nine tables of physical properties for the nine Si2CH2 structures (1S-9S) at all levels of theory. Three tables involving relative energies, CASSCF CI coefficients, and CASSCF electron occupation numbers. Nine figures of the nine Si2CH2 optimized geometries (1S-9S) at all levels of theory. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Reisenauer, H. P.; Maier, G.; Riemann, A.; Hoffmann, R. W. Angew. Chem., Int. Ed. Engl. 1984, 23, 641. (2) Lee, T. J.; Bunge, A.; Schaefer, H. F. J. Am. Chem. Soc. 1985, 107, 137. (3) Maier, G.; Reisenauer, H. P.; Schwab, W.; Carsky, P.; Hess, B. A.; Schaad, L. J. J. Am. Chem. Soc. 1987, 109, 5183. (4) Maier, G.; Reisenauer, H. P.; Schwab, W.; Carsky, P.; Spirko, V.; Hess, B. A.; Schaad, L. J. J. Chem. Phys. 1989, 91, 4763. (5) Thaddeus, P.; Vrtilek, J. M.; Gottlieb, C. A. Astrophys. J. 1985, 299, L63. (6) Cernicharo, J.; Gottlieb, C. A.; Gue´lin, M.; Killian, T. C.; Paubert, G.; Thaddeus, P.; Vrtilek, J. M. Astrophys. J. 1991, 368, L39. (7) (a) Grev, R. S.; Schaefer, H. F. J. Chem. Phys. 1984, 80, 3552. (b) Michalopoulos, D. L.; Geusic, M. E.; Langridge-Smith, P. R. R.; Smalley, R. E. J. Chem. Phys. 1984, 80, 3556. (8) Trucks, G. W.; Bartlett, R. J. J. Mol. Struct. 1986, 135, 423. (9) Fitzgerald, G. B.; Bartlett, R. J. Int. J. Quantum Chem. 1990, 38, 121. (10) Mu¨hlha¨user, M.; Froudakis, G. E.; Zdetsis, A.; Peyerimhoff, S. D. Chem. Phys. Lett. 1993, 204, 617. (11) Presilla-Marquez, J. D.; Rittby, C. M. L.; Graham, W. R. M. J. Chem. Phys. 1996, 104, 2818. (12) Hunsicker, S.; Jones, R. O. J. Chem. Phys. 1996, 105, 5048. (13) Rintelman, J. M.; Gordon, M. S. J. Chem. Phys. 2001, 115, 1795. (14) Bertolus, M.; Finocchini, F.; Millie´, P. J. Chem. Phys. 2004, 120, 4333. (15) Pradhan, P.; Ray, A. K. J. Mol. Struct. 2005, 716, 109. (16) Flores, J. R. Chem. Phys. Lett. 2004, 392, 196. (17) Smith, T. C.; Li, H.; Hostutler, D. A.; Clouthier, D. J.; Merer, A. J. J. Chem. Phys. 2001, 114, 725. (18) Sari, L.; Gonzales, J. M.; Yamaguchi, Y.; Schaefer, H. F. J. Chem. Phys. 2001, 114, 4472. (19) Cireasa, R.; Cossart, D.; Vervloet, M.; Robbe, J. M. J. Chem. Phys. 2000, 112, 10806. (20) Izuha, M.; Yamamoto, S.; Saito, S. J. Chem. Phys. 1996, 105, 4923. (21) Damrauer, R.; Nobel, A. L. Organometallics 2008, 27, 1707.

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Unusual Isomers of Disilacyclopropenylidene (Si2CH2) - The Journal

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