Theoretical study on the structures, isomerization, and stability of [Si, C, N, S] isomers

Computational and Theoretical Chemistry 965 (2011) 123–130

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Theoretical study on the structures, isomerization, and stability of [Si, C, N, S] isomers Tao Ting-Ting, Zhou Zhong-Jun, Yang Yu-Hong, Liu Hui-Ling ⇑, Huang Xu-Ri ⇑, Sun Chia-Chung State Key Laboratory of Theoretical & Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 24 November 2010 Received in revised form 24 January 2011 Accepted 24 January 2011 Available online 28 January 2011 Keywords: Potential energy surface SiCNS Stability Isomers

a b s t r a c t The structures, energies, spectral parameters, and stabilities of the doublet [Si, C, N, S] radical are explored at the density functional theory and ab initio levels. Eighteen isomers including chainlike, three-membered ring, four-membered ring and cagelike structures are located, connected by 25 interconversion transition states at the B3LYP/6-311G(d) level. The structures of the kinetically stable isomers and their relevant transition states are further optimized at the QCISD/6-311G(2d) level. At the CCSD(T)/ 6-311 + G(2d)//QCISD/6-311G(2d)+ZPVE level, the global lowest-lying isomer NCSiS1 is a bent structure. Additionally, the chainlike isomers CNSiS2, SiNCS3, SiSCN4, SiSNC5, and SiCNS6 also process considerable kinetic barriers (more than 10.0 kcal/mol). All the six isomers may be experimentally or astrophysically observable. The bonding natures of all the six isomers are analyzed seriously. The calculated results are compared with those of analogous molecules [Si, Si, N, S], [Si, C, P, S], and [Si, C, N, O]. Implications of the computational structures and spectroscopies are also discussed. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Silicon, carbon, nitrogen, and sulfur chemistries have drawn considerable attention from various fields. Silicon, as an attractive material for microelectronics, has been applied in photovoltaics [1], synthesis, Gap/SiOX nanocables [2], characteristic of nano-silicon (C–nano-SiC–B4C, SiC nanotubes (SiCNTs)) [3,4]. Carbon and nitrogen are crucial organic elements widely existed and applied in the study of nucleic acid [5–9]. Sulfur compounds also largely lie in oil, animals and plants [10,11]. Another particular aspect is their possible roles in astrophysical chemistry. By this time, so many silicon-, carbon-, nitrogen-, sulfur-containing molecules and ions, such as SiCn (n = 1–8), CnS (n = 1–3), CnO (n = 1–3 and 5), CO+, SO+, NP, NO, NS, and SO have been detected in the dense molecular clouds or circumstellar shells [12–14]. Furthermore, copious experimental and theoretical investigations have been performed on the SiS, SiC7, Si2NS, SiCN+, CN, CnS (n = 1–6), NCnS (n = 1–7), NS+, and NS species [15–23], which have been expected to be carriers of some interstellar bands. In this paper, we did a lot of efficient theoretical calculations on the spectroscopic characterizations of [Si, C, N, S] species which may be detected in the interstellar medium to resolve several questions: (1) which is the lowest lying structure among the [Si, C, N, S] isomers? (2) What

⇑ Corresponding authors. E-mail addresses: [email protected] (L. Hui-Ling), xurihuang@gmail. com (H. Xu-Ri). 2210-271X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.comptc.2011.01.032

are the bonding natures of these stable isomers? (3) Which dissociations may integrate and form into the stable species? (4) What are the similarities and discrepancies between the [Si, C, N, S] species and the analogical [Si2, N, S], [Si, C, P, S], and [Si, C, N, O] molecules?

2. Theoretical computational methods All computations are carried out using the GAUSSIAN 03 [24] and NBO 3.0 [25] program packages. The optimized geometries and harmonic vibrational frequencies of the local minima and transition states are initially obtained at the B3LYP/6-311G(d) [26–29] level followed by CCSD(T)/6-311 + G(2d) [30] single-point energy calculations using the B3LYP/6-311G(d) optimized geometries. To confirm whether the obtained transition states connect the right isomers or fragments, the intrinsic reaction coordinate (IRC) [31,32] calculations are performed at the B3LYP/6-311G(d) level. Further, for the relevant species, the structures and frequencies are refined at the QCISD/6-311G(2d) [33,34] level and the singlepoint energies at the CCSD(T)/6-311 + G(2d) level, respectively. The zero-point vibrational energies (ZPVE) at the B3LYP and QCISD levels are included in the final relative energy consideration. For conciseness, the levels CCSD(T)/6-311 + G(2d)//B3LYP/6311G(d) + ZPVE and CCSD(T)/6-311 + G(2d)//QCISD/6-311G(2d) + ZPVE are simplified as CCSD(T)//B3LYP and CCSD(T)//QCISD. The negative barrier transition states are calculated at the method G3B3 [35,36].

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Fig. 1. The scheme for the isomeric species search.

Fig. 2. Optimized geometries of [Si, C, N, S] isomers at the B3LYP/6-311G(d) level and the geometrical parameters in bracket are at the QCISD/6-311G(2d) level. Bond length and angle are in angstroms and degrees, respectively.

T. Ting-Ting et al. / Computational and Theoretical Chemistry 965 (2011) 123–130

3. Results and discussion To the [Si, C, N, S] molecule, we considered various isomeric forms as many as possible, including five types of isomers, i.e., chainlike species (I), three-membered ring species (II), four-membered ring species (III), branched-chain species (IV), and cagelike species (V), as depicted in Fig. 1. For conciseness, the computational results are organized as follows: The optimized geometries of the [Si, C, N, S] isomers and transition states are shown in Figs. 2 and 3, respectively. For the

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kinetically stable isomers, the resonance structures have been shown in Fig. 4. The schematic potential energy surface (PES) of the [Si, C, N, S] is presented in Fig. 5. The dissociation curves of the isomer 1, 2, and 4 are shown in Fig. 6. The harmonic vibrational frequencies as well as the infrared intensities, dipole moments, and rotational constants of the stable isomers of [Si, C, N, S] species are listed in Table 1. The relative energies of isomers and transition states are listed in Table 2. The relative energies of various dissociation fragments of [Si, C, N, S] are summarized in Table 3.

Fig. 3. Optimized geometries of [Si, C, N, S] transition states at the B3LYP/6-311G(d) level and the geometrical parameters in bracket are at the QCISD/6-311G(2d) level. Bond length and angle are in angstroms and degrees, respectively. (The italic values of TS2/5 are calculated at the MP2/6-311G(2d) level).

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3.1. [Si, C, N, S] isomers On the PES, 18 minimum isomers (m) and 25 interconversion transition states (TS m/n) are obtained at the B3LYP/6-311G(d) level. Among all the 18 isomers, NCSiS1 (0.0, 0.0), CNSiS2 (4.0, 4.2), SiNCS3 (6.9, 7.0) SiSCN4 (18.3, 18.3), SiSNC5 (45.2, 45.6), SiCNS6

(47.7, 48.4), SiSNC7 (57.8), SiNSC8 (82.9), SiCSN9 (103.4), and CSiNS10 (139.0) are all chainlike structures. Among the ten isomers, SiNCS3 and SiCNS6 are of C1v symmetry with 2P electronic states. Bent structures NCSiS1, CNSiS2, SiSCN4, SiSNC5, SiNSC8, SiCSN9, and CSiNS10 are of Cs symmetry with 2A0 electronic states. Only bent SiSNC7 is of Cs symmetry with a 2A00 electronic state. The first and the second italic values in parentheses are the relative

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 4. The resonance structures for the relevant species. (a) For isomer 1, (b) for isomer 2, (c) for isomer 3, (d) for isomer 4, (e) for isomer 5, and (f) for isomer 6.

Fig. 5. Schematic potential energy surface (PES) of [Si, C, N, S] molecule at the CCSD(T)/6-311 + G(2d)//B3LYP/6-311G(d) + ZPVE level. The values in bracket were obtained at the CCSD(T)/6-311 + G(2d)//QCISD/6-311G(2d) + ZPVE level, and the value of TS2/5 in bracket is calculated at the MP2/6-311G(2d) level. The italic values are calculated at the G3B3 level. The unit of energy is in kcal/mol for all the species.

Table 1 Harmonic vibrational frequencies (cm1), infrared intensities (km/mol) (in parentheses), dipole moment (Debye), and rotational constants (GHz) of [Si, C, N, S] structures at the B3LYP/6-311G(d) level.

a

Species

Frequencies(infrared intensity)

NCSiS1 NCSiS1a CNSiS2 CNSiS2a SiNCS3 SiNCS3a SiSCN4 SiSCN4a SiSNC5 SiSNC5a SiCNS6 SiCNS6a

115(6) 121(7) 119(4) 113(5) 117(1) 89(1) 126(4) 131(4) 120(2) 128(3) 144(1) 143(0)

237(5) 233(5) 162(0) 133(1) 432(4) 455(2) 371(4) 363(4) 215(0) 233(0) 375(0) 393(0)

At the QCISD/6-311G(2d) level.

326(5) 327(3) 277(3) 269(1) 496(67) 484(111) 405(11) 405(4) 285(8) 302(5) 400(0) 429(60)

529(32) 535(41) 560(19) 571(28) 509(5) 515(2) 449(43) 471(52) 456(12) 480(24) 449(78) 452(1)

686(58) 700(55) 723(137) 741(156) 1012(0) 1003(4) 653(0) 646(1) 560(47) 608(38) 955(2) 914(4)

2263(31) 2176(89) 2094(455) 2092(478) 1993(864) 2003(1527) 2268(18) 2252(4) 2108(236) 2021(223) 1950(606) 2098(12)

Dipole moment

Rotational constant

2.0755 2.1739 1.7039 1.6702 1.3072 1.5595 3.6639 3.5791 3.4924 3.5351 1.5761 3.2112

20.753 21.816 24.869 27.0143

12.446 11.992 14.343 13.600 0 0

2.298 2.244 2.415 2.374 1.627 1.621 2.837 2.885 2.859 2.936 1.578 1.557

2.069 2.034 2.201 2.183 1.627 1.621 2.310 2.325 2.384 2.415 1.578 1.557

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and S-cNCSi13 are of Cs symmetry with 2A0 electronic states. CcSiNS14, C-cSiSN15, and C-cSiNS16 are of C1 symmetry.cSiNCS17 (26.5) is the only four-membered ring structure, which is of Cs symmetry with a 2A00 electronic state. And there is just one cagelike isomer cageSiCNS18 (70.4), which is of C1 symmetry. 3.2. [Si, C, N, S] isomerization and dissociation Considering that the lowest isomerization or dissociation barriers control the kinetic stability of isomers, we considered as many isomerization and dissociation pathways as possible. In general, with higher barrier, the isomer should have higher kinetic stability. From the relative energies of the dissociation fragments of [Si, C, N, S] (listed in Table 3), fragments SiS(1R) + CN(2R) (51.1 kcal/mol) attracts our interest because its relative energy is lower than others. Intuitively, the diatomic + diatomic consociation reactions of the radical-molecule SiS/CN may easily lead to the formation of isomers NCSiS1, CNSiS2, and SiSCN4. As shown in Fig. 6, the fragments SiS and CN can associate without a barrier to form NCSiS1, CNSiS2, and SiSCN4. The relative energies of the other dissociation products are high enough (about more than 100 kcal/mol at the CCSD(T)//B3LYP level) for us to believe it is the isomerization barriers that control the possibilities of isomerization formation. On the PES (shown in Fig.5), we can see that six chainlike isomers NCSiS1, CNSiS2, SiNCS3, SiSCN4, SiSNC5, and SiCNS6 may be of interest due to their relative higher conversion barriers (21.8 kcal/mol for 1 ? 2, 17.8 kcal/mol for 2 ? 1, 54.8 kcal/mol for 3 ? 1, 13.5 kcal/mol for 4 ? 1, 28.6 kcal/mol for 5 ? 2, and 22.2 kcal/mol for 6 ? 7) at the CCSD(T)//B3LYP level. Such kinetic stabilities ensure that 1–6 could exist in the low temperature conditions of interstellar space (such as in the dense interstellar clouds). At the CCSD(T)//B3LYP level, The remaining species isomerization barriers can be listed as: 7.6 kcal/mol (7 ? 2), 13.4 kcal/ mol (8 ? 3), 12.5 kcal/mol (9 ? 8), 5.8 kcal/mol (17 ? 2), 1.9 kcal/mol (12 ? 6), 1.5 kcal/mol (13 ? 12), 1.2 kcal/mol (16 ? 18), 0.1 kcal/mol (11 ? 2), 0.0 kcal/mol (14 ? 15), 1.3 kcal/mol (10 ? 12), and 5.9 kcal/mol (15 ? 14). The zero and negative barriers on the PES are treated with higher energy prediction method G3B3 (0.37 (14 ? 15), 5.46 (10 ? 12), and 0.38 kcal/mol (15 ? 14)). Considering the low isomerization barriers and high relative energy, these species are of less interest as observable species both in the laboratory and space. For isomers 14 and 15, although we did not find other transition state which can connect them to the PES, their existence is less likely because of higher energies (137.4 and 143.3 kcal/mol, respectively). 3.3. Properties of the isomers 1–6

Fig. 6. Dissociation curves of the isomers at the B3LYP/6-311G(d) level. (a) For isomer 1, (b) for isomer 2, and (c) for isomer 4.

energies at the CCSD(T)//B3LYP and CCSD(T)//QCISD levels, respectively (in kcal/mol). Six three-member ring isomers can be located on the PES. They are C-cNSiS11 (68.2), S-cNSiC12 (75.5), S-NCSi13 (86.9), C-cSiNS14 (137.4), C-cSiSN15 (143.3), and C-cSiNS16 (152.1). Among them, CcNSiS11 is of Cs symmetry with a 2A00 electronic state. S-cNSiC12

From what have been discussed above, we know only six isomers 1–6 process considerable thermodynamic stabilities, which have more possibilities to be detected in the laboratory or in interstellar space than other species. We now analyze all the six isomers0 structures and bonding natures at the B3LYP/6-311G(d) level. The lowest-lying isomer NCSiS1 has a 2A0 electronic state. Its SSi (1.976 Å) bonding lies between the typical S–Si (2.166 Å) and S@Si (1.956 Å) bond length. The length of SiC (1.855 Å) bond is closer to the normal Si–C (1.884 Å) bond length. Its terminal CN (1.159 Å) bond length is much close to the regular C„N (1.149 Å). Based on the bond length and the spin density distribution (0.423, 0.572, 0.018, and 0.022e for S, Si, C, and N, respectively), the species NCSiS1 is described as the conjugated forms shown in Fig. 4a. The symbols ‘‘.’’ and ‘‘|’’ denote the single electron and lone-pair electrons, respectively. The resonance structures are confirmed by the natural bond orbital (NBO) analysis.

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Table 2 Relative (kcal/mol) energies of the [Si, C, N, S] isomers and transition states at the B3LYP/6-311G(d) and single-point CCSD(T)/6-311 + G(2d) levels. For the relevant isomers, the CCSD(T)/6-311 + G(2d)//QCISD/6-311G(2d) values are also included. The symbols in parenthesis of the column denote the electronic states. Species

Erel (B3LYPb)

DZPVE (B3LYPb)

Erel(CCSD(T)c //B3LYPb)

Total 1

Erel (QCISDd)

DZPVE (QCISDd)

Erel(CCSD(T)c //QCISDd)

Total 2

a

0.0 3.6 1.6 19.8 47.6 38.8 60.5 85.2 102.6 135.8 70.5 67.5 80.6 141.5 145.7 153.5 26.9 73.6 23.0 58.8 32.5 86.5 41.3 45.8 74.0 66.4 70.5 31.5 76.4 90.1 91.4 104.9 106.3 119.9 93.5 68.0 70.0 120.8 120.4 135.8 80.9 141.5 154.1

0.0 0.3 0.6 0.2 0.6 0.2 0.6 1.1 1.1 1.1 1.3 0.4 0.8 2.0 2.0 2.0 0.1 1.3 1.0 0.0 0.6 1.0 0.9 1.2 1.3 1.4 1.5 0.7 2.0 1.3 0.9 2.3 1.8 1.7 1.7 0.5 0.7 1.6 1.7 1.4 1.6 2.1 2.6

0.0 4.3 6.3 18.1 45.8 47.5 58.4 84.0 104.5 140.1 69.5 75.9 87.7 139.4 145.3 154.1 26.6 71.7 22.8 61.7 32.4 87.6 43.8 43.3 75.1 66.8 69.8 33.0 74.6 90.7 94.6 98.6 103.6 121.0 97.5 70.4 78.1 117.5 120.0 139.1 90.0 139.5 155.9

0.0 4.0 6.9 18.3 45.2 47.7 57.8 82.9 103.4 139.0 68.2 75.5 86.9 137.4 143.3 152.1 26.5 70.4 21.8 61.7 31.8 86.6 42.9 42.1 73.8 65.4 68.3 32.3 72.6 89.4 93.7 96.3 101.8 119.3 95.8 69.9 77.4 115.9 118.3 137.7 88.4 137.4 153.3

0.0 3.3 6.4 17.9 45.4 48.9

0.0 0.2 0.6 0.3 0.3 0.7

0.0 4.4 6.4 18.0 45.9 47.7

0.0 4.2 7.0 18.3 45.6 48.4

NCSiS1 (2A0 ) CNSiS2 (2A0 ) SiNCS3(2P) SiSCN4(2A0 ) SiSNC5(2A0 ) SiCNS6(2P) SISNC7(2A00 ) SiNSC8(2A0 ) SiCSN9(2A0 ) CSiNS10(2A0 ) C-cNSiS11(2A00 ) S-cNSiC12(2A0 ) S-cNCSi13 (2A0 ) C-cSiNS14 C-cSiSN15 C-cSiNS16 cSiNCS17(2A00 ) cageSiCNS18 TS1/2 TS1/3 TS1/4 TS1/12 TS2/2 TS2/4 TS2/5 TS2/7 TS2/11 TS2/17 TS2/18 TS3/3(a) TS3/3(b) TS3/8(a) TS3/8(b) TS3/8(c) TS3/13 TS6/7 TS6/12 TS8/9(a) TS8/9(b) TS10/12 TS12/13 TS14/15 TS16/18

Erel(G3B3f) 0.00

130.35

134.20 134.19

22.8 64.4 34.2

0.9 0.2 0.7

22.8 61.7 32.6

21.9 61.9 31.9

89.9

2.9

75.2

72.3e

72.0

0.1

71.0

70.9

135.81 134.57

Total 1: The relative energy at the CCSD(T)/6-311 + G(2d)//B3LYP/6-311G(d) + ZPVE level. Total 2: The relative energy at the CCSD(T)/6-311 + G(2d)//QCISD/6-311G(2d) + ZPVE level. a The total energies of the reference isomer 1 at the B3LYP/6-311G(d) and QCISD/6-311G(2d) levels are 780.5714564 au and 779.3375831 au. At the CCSD(T)/6311 + G(2d)//B3LYP/6-311G(d) level is 779.371584 au, at the CCSD(T)/6-311 + G(2d)//QCISD/6-311G(2d) level is 779.3717891 au. The ZPVE at the B3LYP/6-311G(d) and QCISD/6-311G(2d) levels are 5.93994 and 5.84965 au, respectively. b The basis set is 6-311G(d) for B3LYP. c The basis set is 6-311 + G(2d) for CCSD(T). d The basis set is 6-311G(2d) for QCISD. e The italic values of TS2/5 are calculated at the MP2/6-311G(2d) and CCSD(T)/6-311 + G(2d)//MP2/6-311G(2d) levels. f The method is G3B3. The reference isomer 1 at the G3B3 level is -780.1805171 au.

For the isomer CNSiS2, the second-lowest species with a 2A0 electronic state, its SSi (1.975 Å) bond value is longer than the double S@Si (1.956 Å) bond and shorter than the single S–Si (2.166 Å) bond. The internal SiN (1.744 Å) bond length is close to the normal single Si–N (1.728 Å) bond length. The terminal CN (1.184 Å) bond length is a little longer than the typical C„N (1.149 Å) bond length. According to the spin density distribution (0.393, 0.635, 0.067, and 0.039e for S, Si, N, and C, respectively), we conclude the resonance structure shown in Fig. 4b. For the species SiNCS3, its SC (1.578 Å) bond length is shorter than S@C (1.615 Å) and longer than S„C (1.541 Å) bond length. Its CN (1.207 Å) bond length is also between C@N (1.266 Å) and C„N (1.149 Å) bond length. The terminal NSi (1.707 Å) bond length is closer to the single N–Si (1.728 Å) bond than the double N@Si (1.601 Å) bond. The bond length and the spin density distri-

bution (0.106, 0.125, and 0.170, and 0.939e for S, C, N, and Si, respectively) indicate that the isomer SiNCS3 can be mainly described as the forms in Fig. 4c. For the isomer SiSCN4 with a 2A0 electronic state, its SiS (2.175 Å) bond length is slightly longer than the single Si–S (2.166 Å) bond length. The internal SC (1.712 Å) bond length lies between S–C (1.834 Å) bond and S@C (1.615 Å) bond. The last CN (1.158 Å) bond length is close to the C„N (1.149 Å) bond. Based on the bond lengths and the spin density distribution (1.002, 0.0573, 0.055, and 0.001 for Si, S, C, and N, respectively), the isomer SiSCN4 is described as the form shown in Fig. 4d. For the species SiSNC5 with a 2A0 electronic state, its SiS (2.128 Å) bond length also lies between the typical S–Si (2.166 Å) bond and S@Si (1.956 Å) bond. Its SN (1.708 Å) bond length is closer to the single S–N (1.737 Å) bond than S@N (1.579 Å) bond. The

T. Ting-Ting et al. / Computational and Theoretical Chemistry 965 (2011) 123–130 Table 3 Relative (kcal/mol) energies of dissociation fragments of the [Si, C, N, S] structures at the B3LYP/6-311G(d) and single-point CCSD(T)/6-311G(2d) levels. Species

(B3LYPb)

DZPVE (B3LYPb)

CCSD(T)c//B3LYPb

Totala

3

168.4 106.0 63.7 168.4 85.8 201.2 87.2 237.8 286.1 197.7 89.6 138.7 225.8 119.6 138.7 257.5

2.8 2.4 1.8 2.5 1.3 2.5 1.6 3.7 4.1 3.3 0.9 2.1 2.9 1.2 2.1 2.9

162.3 98.9 52.9 166.7 82.6 195.4 82.6 226.9 257.0 191.7 89.2 134.9 216.0 118.0 135.0 247.0

159.5 96.5 51.1 164.2 81.3 192.9 81.0 223.2 252.9 188.4 88.3 132.8 213.1 116.8 132.9 244.1

2

SiC + NS SiN + 1CS SiS + 2CN 2 SiNS + 3C 2 SiCN + 3S 3 SiCS + 2N 2 SiNC + 3S 2 SiSN + 3C 3 SiSC + 2N 2 NSiS + 3C 2 NCS + 3Si 2 NCS + 3Si 2 NSC + 3Si 2 CNS + 3Si 2 CNS + 3Si 3 CSiS + 2N 2 1

Total: The relative energy at the CCSD(T)/6-311 + G(2d)//DFT/B3LYP/6311G(d) + ZPVE level. a The isomer 1 is the reference species for the dissociation fragments, and the total energy of isomer 1 at CCSD(T)//B3LYP level is listed in Table 2. b The basis set is 6-311G(d) for B3LYP. c The basis set is 6-311 + G(2d) for CCSD(T).

terminal NC (1.179 Å) bond length is longer than N„C (1.149 Å) bond and shorter than N@C (1.266 Å) bond. As the bond length and the spin density distribution (0.938, 0.019, 0.029, and 0.014 for Si, S, N, and C), we show the species SiSNC5 in Fig. 4e. For the isomer SiCNS6, its SN (1.582 Å) bond length is much close to the S@N (1.579 Å). The NC (1.194 Å) bonding is longer than N„C (1.149 Å) and shorter than N@C (1.266 Å). The last CSi (1.789 Å) bond length lies between the normal single Si–C (1.884 Å) bond length and the double Si@C (1.707 Å) bond length. The distribution of the spin density is 0.800, -0.126, 0.157, and 0.168 for Si, C, N, and S). Its single electron located mainly on the Si atom. Thus, the isomer SiCNS6 can be viewed as the form in Fig. 4f. 3.4. Interstellar and laboratory implications It is well known that three types of large-scale structures composed mainly the interstellar medium. They are often called ‘‘clouds’’, including diffuse clouds, translucent clouds, and dense clouds [37]. In this paper, six isomers 1–6 can be viewed as thermodynamically and kinetically stable species on the PES. We performed calculations on [Si, C, N, S] at room temperature (298.15 k), which is different from the temperature of diffuse clouds (100–120 k) [38], translucent clouds (50–100 k) [39], and dense clouds (10–15 k) [40]. Because of the decrease of the temperature from 298.15 k to 10 k may make these species kinetically stable, we expect that the six isomers 1–6 can also be stable under interstellar conditions. The fragments SiS and CN have been detected in interstellar space, and, as shown in Table 3, the fragments 1SiS and 2CN can associate to form the isomer 1, 2, and 4 with no barrier (shown in Fig.6), especially in some dark interstellar dense clouds with ultralow temperature and much collisional occasion. So, the observation of isomer 1, 2, and 4 in the interstellar space is promising. In order to the future identification of the [Si, C, N, S] isomers either in the experiment or in interstellar space, the calculations of vibrational frequencies, dipole moments, and rotational constants for the isomers 1–6 at the B3LYP/6-311G(d) and QCISD/6311G(2d) levels are shown in Table 1. At the QCISD/6-311G(2d) level, the dominant frequencies of the six isomers are 686, 2094, 1993, 449, 2108, 1950 cm1, with the corresponding infrared

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intensities 58, 455, 864, 43, 236, 606 km/mol, respectively. Moreover, the isomers 4 and 5 have large dipole moments of 3.6639 D and 3.4924 D, which can be identified by the microwave detection in the future. 3.5. Reliability of methods In order to more accurate prediction of the structures and energies, we performed the higher levels QCISD/6-311G(2d) and CCSD(T)/6-311 + G(2d)//QCISD/6-311G(2d) + ZPVE calculations for the six kinetically stable isomers 1–6, and the five related transition states TS1/2, TS1/3, TS1/4, TS2/5, TS6/7. Except TS2/5, the structural parameters on QCISD/6-311G(2d) and B3LYP/6311G(d) levels agree well with each other as exhibited in Figs. 2 and 3. At the CCSD(T)/6-311 + G(2d)//B3LYP/6-311G(d) + ZPVE and CCSD(T)/6-311 + G(2d)//QCISD/6-311G(2d) + ZPVE levels, the relative energies of the stable isomers are also very close as shown in Table 2. Since TS2/5 has not been obtained at the QCISD/6311G(2d) level, we used the MP2/6-311G(2d) [41] value, and it agree well with the B3LYP/6-311G(d) calculation. The higher level energy prediction method G3B3 was employed to these transitions with the negative barrier (listed in Table 2). As a result, it is found that the negative values are changed into positive ones. 3.6. Comparison with the analogues [Si2, N, S], [Si, C, P, S], and [Si, C, N, O] It is well useful to compare [Si, C, N, S] with its isovalent analogues [Si2, N, S] [17], [Si, C, P, S] [42], and [Si, C, N, O] [43] which have been studied experimentally or theoretically. All of the four species are composed by two IVA atoms, one VA atom and one VIA atom. Among them, [Si, C, N, S] and [Si, C, N, O] composed of two 1st cycle atoms and two 2nd cycle atoms; [Si2, N, S] and [Si, C, P, S] composed of one 1st cycle atom and three 2nd cycle atoms. As shown in the previous investigations, the stable isomers on the [Si, C, N, S] and [Si, C, N, O] PES are all chainlike structures, while the stable isomers on the PES of [Si2, N, S] and [Si, C, P, S] are not only chainlike structures, but three-membered ring isomers. The reason may be that the 2nd cycle atoms are easy to form r bond and the 1st cycle atoms are easy to form p bond. 4. Conclusions A detailed doublet potential energy surface of the [Si, C, N, S] is theoretically investigated at the density functional theory and ab initio levels. Eighteen isomers are connected by 25 interconversion transition states. Six linear isomers NCSiS1, CNSiS2, SiNCS3, SiSCN4, SiSNC5, and SiCNS6 are predicted to possess high kinetic stabilities and promising candidates in the future laboratory and astrophysical detection. The possible formation strategies and the implications of the stable isomers in the experiment and space are also discussed in detail. Additionally, the similarities and differences among the analogues molecular [Si2, N, S], [Si, C, P, S], [Si, C, N, O], and [Si, C, N, S] are discussed. The theoretical results in this paper are expected to be useful for the future laboratory and interstellar identification of various [Si, C, N, S] isomers. Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 21073075). References [1] S.W. Boettcher, J.M. Spurgeon, M.C. Putnam, E.L. Warren, D.B. Turner-Evans, M.D. Kelzenberg, J.R. Maiolo, H.A. Atwater, N.S. Lewis, Science 327 (2010) 185. [2] S.Y. Bae, H.W. Seo, H.C. Choi, D.S. Han, J. Park, J. Phys. Chem. B 109 (2005) 8496.

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