Theoretical study of the global potential energy surface of the [CH3,N,C,S] system in singlet and triplet states

Chemical Physics Letters 430 (2006) 13–20 www.elsevier.com/locate/cplett

Theoretical study of the global potential energy surface of the [CH3,N,C,S] system in singlet and triplet states Zhen Fu a, Xiu-mei Pan b

a,b,*

, Ze-sheng Li b, Chia-chung Sun b, Rong-shun Wang

a

a Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun, Jilin 130023, PR China

Received 17 April 2006; in final form 18 June 2006 Available online 1 September 2006

Abstract The global potential energy surface (PES) of the [CH3,N,C,S] system in singlet and triplet states, involving 16 isomers and 15 transition structures, is studied at DFT(B3LYP), MP2 and QCISD levels. It is shown that the chainlike singlet isomer CH3NCS is the most stable species among all the isomers and the branched-C(CH3)NS has the lowest energy among the triplet species. The stability of these isomers, their isomerizations and dissociations are discussed and theoretical results are consistent with the available experimental ones. Ó 2006 Elsevier B.V. All rights reserved.

1. Introduction

2. Computational methods

Methylisothiocyanate (CH3NCS, MITC) is the sulfur analogue of methylisocyanic CH3NCO, which is the primary breakdown product of metamsodium, and a potential replacement fumigant pesticide for methyl bromide [1]. Due to its toxicity and high potential for volatilization, scientists have paid great efforts these years to investigate the structures [2–8] of CH3NCS and its isomers CH3SCN, CH3CNS, and their photochemical behaviors [9–17] including the mechanism and the rate for their removal mechanism. However, to the best of our knowledge, there is no high level computational study to explore the potential energy surface of [CH3,N,C,S] system up to now. In this letter, the global PES of singlet and triplet states is obtained and further used to discuss the isomeric structures, relevant energetics and the mechanism of isomerizations and dissociations of the isomers.

All computations are carried out using GAUSSIAN-98 program [18]. The isomers, transition structures and their vibrational frequencies are obtained at 6-311+G(d,p)DFT(B3LYP) and MP2 levels, respectively. Intrinsic reaction coordinate calculations are carried out at the same levels. The DFT(B3LYP)/6-311+G(d,p) geometries are further employed to calculate single-point energies using B3LYP/aug-cc-pVTZ and QCISD/6-311+G(d,p) levels, respectively.

* Corresponding author. Address: Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China. Fax: +86 431 5099511. E-mail address: [email protected] (X.-m. Pan).

0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.07.103

3. Results and discussion The optimized geometries, including 16 isomers and 15 corresponding transition structures found in [CH3,N,C,S] system at different levels of theory are shown in Figs. 1– 3. Total energies and relative energies for these species are summarized in Tables 1 and 2, respectively. The PES of the isomers’ connections relative to the global minimum CH3NCS 11 for the singlet and triplet states are given in Fig. 4. For discussing conveniently, the energy of CH3NCS 1 1 is set zero for reference. The main frequencies and intensities of 11 at B3LYP/6-311+G(d,p) and MP2/6311+G(d,p) levels are presented in Table A [19] of Supporting information, which are in generally good

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Fig. 1. Optimized geometrical parameters for structure of isomers on the single [CH3,N,C,S] PES at the B3LYP/6-311+G(d,p) and MP2/6-311+G(d,f) (in ˚ and °, the parenthesis) compared with the experiments [5,2] and previous theoretical [7] results (in bond italics). The bond lengths and angles are in A respectively. CM denoted as the C atom of methyl.

agreement with the observed microwave spectra [2] and IR [20] frequencies and this means that our calculation at B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) levels can provide reliable results for the system at least as regards stable species. 3.1. Isomerization and dissociation for singlet state 3.1.1. Isomerization There are nine minima on the singlet [CH3,N,C,S] PES as shown in Fig. 1, and their associated energies are included in Table 1. The relative QCISD energies for these isomers increases in the following order (barriers in parenthesis, kcal/mol): 1 1 CH3NCS 1(0.0) < CH3SCN 2(0.6) < CH3CNS 1 3(25.6) < CH3SNC 4(29.5) < cyclic-C(CH3)NS

1

1 1

5(32.8) < cyclic-S(CH3)CN 16(48.7) < cyclic-N(CH3)CS 7(66.8) < CH3CSN 18(126.4) < CH3NSC 19(132.3).

Among these isomers, only CH3NCS 11, CH3SCN 12 and CH3CNS 13 were observed and characterized experimentally [2–6,21]. Our calculation results are in general agreement with the experimental facts. It is obvious that isomer 11 is the most stable structure, followed by isomer 12. The rearrangement of isomer 12 to 1 1 occurs by S, N-methyl transfer via TS11/12 with imaginary frequency of 447.1i cm1 and the CMNC and CMSC angles of 53.5° and 48.2°, respectively (CM denotes the carbon of methyl) (see Fig. 2). TS11/12 is located 74.8 kcal/ mol higher than isomer 11 and 74.2 kcal/mol above isomer 1 2. Thus, under some subtle conditions, the isomerization from isomer 12 to 11 may occur. Li et al. [17] predicted that the production of NH in CH3SCN plasma first undergoes

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Fig. 2. Optimized geometrical parameters for transition structures on the singlet [CH3,N,C,S] PES at the B3LYP/6-311+G(d,p) and MP2/6-311+G(d,f) ˚ and °, respectively. (in the parenthesis). The bond lengths and angles are in A

the isomerization CH3SCN 12 ! CH3NCS 11, which is consistent with our analysis. The cyclic-isomer 16 can transform into open-chain isomers 14 and 12 via TS14/16 and TS12/16, respectively. Because isomer 16 locates in a very shallow potential well (0.8 kcal/mol and 4.6 kcal/mol lower than TS14/16 and TS12/16, respectively), such low barriers cannot ensure kinetic stability of isomer 16. Consequently, we can conclude that the isomerization 14 ! 12 can be achieved

TS1 4=1 6

through a two-step mechanism CH3 SNC 1 4 ! cyclic-S TS1 2=1 6

ðCH3 ÞCN 1 6 ! CH3 SCN 1 2 with isomer 16 as intermediate, and it is exothermic by 28.9 kcal/mol. The cyclic-isomer 17 can rearrange to isomer 11 via TS11/17, which is significantly exothermic by 66.8 kcal/ mol. The TS11/17 has a CH3NCS-like structure with the ˚ . In addition, isomer 17 can N  S distance being 2.284 A 1 also rearrange to isomers 2, 13 and 14 as presented in Fig. 4. They are all exothermic paths by 66.2, 41.2,

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Fig. 3. Optimized geometrical parameters for structures of isomers and transition structures on the triplet [CH3,N,C,S] PES at the B3LYP/6-311+G(d,p) ˚ and °, respectively. and MP2/6-311+G(d,p) (in the parenthesis). The bond lengths and angles are in A

37.3 kcal/mol via TS12/17, TS13/17 and TS14/17, respectively. The isomerization 17 ! 13 happens by the methyltransfer from N to C, accompanied by the ring opening through the cleavage of C–S bond. Likewise, the transformation from isomer 17 to isomers 12 and 14 also experiences the methyl-transfer from N to S and ring opening via the rupture of N–S and C–S bond, respectively. TS13/17, TS12/17 and TS14/17 are located 26.6, 15.3 and

48.8 kcal/mol above isomer 17, all with the CMNCS dihedral angle being 180.0°. As for the direct transformation between isomer 14 and 13, although we tried our best, the transition structure is still not found. But the intercoversion can undergo the TS1 4=1 7

TS1 3=1 7

mechanism CH3 CNS 1 4 ! cyclic-NðCH3 ÞCS 1 7 ! CH3 SNC 1 3 with isomer 17 as intermediate.

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Table 1 Total energies (a.u.) and relative energies (kcal/mol, in the parenthesis) of the singlet isomers and transition structures for the isomerizations and dissociations in [CH3,N,C,S] at 6-311+G(d,p)-DFT(B3LYP), MP2 and QCISD levels Singlet species

B3LYP

MP2

QCISD

CH3NCS 11 CH3SCN 12 CH3CNS 13 CH3SNC 14 Cyclic-C(CH3)NS 15 Cyclic-S(CH3)CN 16 Cyclic-N(CH3)CS 17 CH3CSN 18 CH3NSC 19 TS14/16 TS12/16 TS13/15 TS11/12 TS11/17 TS12/17 TS13/17 TS15/17 TS14/15 TS14/17 1 CH3CN + 3S 1 CH3NC + 3S 2 CH3 + 2NCS 1 CH3NC + 1S 2 CH3S + 2CN 2 CH3 + 2CNS 3 CH3N + 1CS

530.96646 (0.0) 530.95183 (9.1) 530.92931 (23.2) 530.90735 (36.9) 530.90655 (37.4) 530.87838 (55.0) 530.85290(71.0) 530.76295(127.2) 530.74815(136.5) 530.87714(55.8) 530.87224(58.9) 530.85976(66.7) 530.84672(74.9) 530.83177(84.2) 530.82361(89.3) 530.81669(93.6) 530.79520(107.1) 530.79129(109.5) 530.78152(115.6) 530.88407(51.5) 530.84597(75.3) 530.85733(68.2) 530.82255 (90.0) 530.77764(118.0) 530.80961(98.1) 530.79108(109.6)

530.02008(0.0) 530.01734(1.7) 529.97960(25.3) 529.96364(35.3) 529.96627(33.6) 529.93553(52.9) 529.90405(72.5) 529.79006(143.8) 529.81001(131.3) 529.93098(55.7) 529.92515(59.3) 529.91850(63.5) 529.89544(77.9) 529.88384(85.2) 529.88840(82.3) 529.86282(98.3) 529.85034(106.1) 529.84473(109.6) 529.81704(126.9) 529.94765(45.3) 529.90378(72.7) 529.89304(79.4) 529.87684 (89.5) 529.82611(121.3) 529.85221(104.9) 529.83998(112.6)

530.09312(0.0) 530.09208(0.6) 530.05211(25.6) 530.04594(29.5) 530.04071(32.8) 530.01520(48.7) 529.98623(66.8) 529.89089(126.4) 529.88147(132.3) 530.01395(49.5) 530.00790(53.3) 529.99831(59.3) 529.97339(74.8) 529.96425(80.6) 529.96179(82.1) 529.94370(93.4) 529.92226(106.8) 529.91722(110.0) 529.90827(115.6) 530.02940(39.8) 529.99198(63.2) 529.98441(68.0) 529.97198 (75.7) 529.94019(95.6) 529.93567(98.4) 529.93404(99.4)

Table 2 Total energies (a.u.) and relative energies (kcal/mol, in the parenthesis) of the triplet isomers and transition structures for the isomerizations and dissociations in [CH3,N,C,S] at 6-311+G(d,p)-DFT(B3LYP), MP2 and QCISD levels Triplet species

B3LYP

MP2

QCISD

Branched-C(CH3)NS 31 CH3NCS 32 CH3CNS33 Cyclic-C(CH3)NS 34 CH3SCN 35 CH3SNC 36 Cyclic-N(CH3)CS 37 TS31/Pdiss1 TS31/Pdiss2 TS33/34 TS35/Pdiss2 TS31/32 1 CH3CN + 3S Pdiss1 2 CH3 + 2SCN Pdiss2

530.87640(56.3) 530.86313(64.6) 530.85573(69.2) 530.83463(82.4) 530.83523(82.0) 530.82368(89.3) 530.77026(122.7) 530.86659(62.4) 530.841497(78.1) 530.81887(92.3) 530.80571(100.5) 530.79377(108.0) 530.88407(51.5) 530.85733(68.2)

529.91418(66.2) 529.90463(72.2) 529.88383(85.2) 529.88186(86.4) 529.87623(89.9) 529.86939(94.2) 529.81512(128.1) 529.89841(76.0) 529.87000(93.8) 529.86199(98.8) 529.82962(119.1) 529.83387(116.4) 529.94765(45.3) 529.89304(79.4)

530.01694(47.6) 529.99318(62.5) 529.97907(71.3) 529.96786(78.3) 529.96668(79.0) 529.95387(87.1) 529.90271(119.0) 530.00399(55.7) 529.97189(75.8) 529.94599(92.0) 529.92356(106.0) 529.92085(107.7) 530.02940(39.8) 529.98441(68.0)

Isomer 15 is the most stable species among the three cyclic isomers, lying 32.8 kcal/mol. However, it is still less stable than isomers 13 and 14. Isomer 15 could convert into isomer 13 in the exothermic step (7.2 kcal/mol) via TS13/15 with 59.3 kcal/mol and the C  S distance of ˚ . Isomer 15 could also exothermically transform 2.501 A to isomer 14 through TS14/15 by 3.3 kcal/mol. This process experiences again methyl-shift from C to S and ring opening through the cleavage of C  S bond. The barrier of TS14/15 relative to isomer 15 is 77.2 kcal/mol. In view of thermodynamics, the energies of chainlike isomer 14 and

cyclic-isomer 15 are comparable with isomer 13, which means under some conditions isomers 14 and 15 may be detected in the laboratory. As Fig. 4 shows, the isomerization between the two cyclic-isomers 17 and 15 occurs through methyl-shift from N to C via TS15/17 lying 106.8 kcal/mol with the CMNCS dihedral angle of 111.4°. The 17 ! 15 isomerization is exothermic by 34.0 kcal/mol with barrier 74.0 kcal/mol for isomer 1 5 and 40.0 kcal/mol for isomer 17. As the open-chain isomers 18 and 19 lie significantly higher in energy compared to the other singlet isomers,

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Fig. 4. Profile of the potential energy surface for the [CH3,N,C,S] PES, calculated at the QCISD/6-311+G(d,p).

the possible reactions involving the two species are not discussed further. 3.1.2. Dissociation The important dissociation products for singlet are shown in Table 1, while the fragment energies are shown in Table B of Supporting information. Despite our numerous efforts, the transition structures associated with these dissociations have not been found. However, by scanning from the isomers to fragments identify that these dissociation processes might be barrierless. For isomer 12, it can dissociate into 2CH3S + 2CN with dissociation energy 95.0 kcal/mol, which in favorable agreement with experiment values (96.0 kcal/mol [14]). Moreover, it should be mentioned that for another CH3SCN 1 2 ! 2CH3 + 2NCS, the dissociation energy is 67.4 kcal/ mol (the experiment result are 70.1 and 69.0 kcal/mol, respectively [9,11]), which can be compared with the barrier 74.2 kcal/mol of isomerization CH3SCN 12 ! CH3NCS 11. However, from the energetics analysis, it is obviously that this dissociation is preferable to the isomerization. For isomer 11, it can dissociate into 1CH3NC + 3S, 2 CH3 + 2NCS and3CH3N + 1CS and their corresponding dissociation energies are 63.2, 68.0 and 99.4 kcal/mol,

respectively. (The experiment values for the first two dissociations are 76.2 and 77.7 kcal/mol, respectively [16,9].) These results are in agreement with the viewpoint proposed by Nau et al. [16] that C@S bond cleavage represents the primary decomposition pathway at low excess energies while higher energies make C–N bond cleavage possible. Isomer 13 can dissociate into 1CH3NC + 1S and 2CH3 + 2CNS, which require 50.1 and 72.8 kcal/mol, and these results are consistent with that were given by Gerbaux et al. [7] using G2(MP2, SVP) method with the corresponding revalues 58.3 and 78.9 kcal/mol. It should be noted that isomer 13 can easily decompose into 1CH3NC + 3S with energy 14.2 kcal/mol, which is quite preferable to the isomerizations concerning 13. Isomer 14 can undergo dissociation to products 2CH3 + 2CNS and 1CH3S + 2CN, which require 68.9 kcal/mol and 66.1 kcal/mol of energies, respectively. In general our theoretical results are reliable and may be helpful for further experimental investigation. 3.2. Isomerization and dissociation for triplet state As can be seen from Fig. 3, among the seven triplet isomers, five open-chain species (denoted as 31, 32, 33, 35, 36) are found. The bond angles formed by N, C, S atom of

Z. Fu et al. / Chemical Physics Letters 430 (2006) 13–20

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Table 3 Comparison of the selected energy barriers (in kcal/mol) calculated for singlet [CH3,N,C,S] and [H,N,C,S] isomerization at B3LYP/aug-cc-pVTZ level Reaction

Barriera

Reaction

Barrierb

HNCS ! HSCN HSCN ! HNCS

62.4 50.6

(1)

CH3NCS ! CH3SCN CH3SCN ! CH3NCS

(2)

Cyclic-N(CH3)CS ! CH3SCN CH3SCN ! cyclic-N(CH3)CS

44.5 106.4

Cyclic-N(H)CS ! HSCN HSCN ! cyclic-N(H)CS

15.5 80.1

(3)

Cyclic-C(CH3)NS ! CH3SNC CH3SNC ! cyclic-C(CH3)NS

70.8 72.5

Cyclic-C(H)NS ! HSNC HSNC ! cyclic-C(H)NS

32.6 43.1

(4)

CH3SNC ! cyclic-S(CH3)CN Cyclic-S(CH3)CN ! CH3SNC

18.6 0.8

HSNC ! cyclic-S(H)CN Cyclic-S(H)CN ! HSNC

19.2 0.2

(5)

CH3NCS ! cyclic-N(CH3)CS Cyclic-N(CH3)CS ! CH3NCS

79.1 9.3

HNCS ! cyclic-N(H)CS Cyclic-N(H)CS ! HNCS

86.0 9.6

a b

74.3 66.0

This work at B3LYP/aug-cc-pVZT//B3LYP/6-311+G(d,p). Ref. [22].

triplet chainlike isomers are all 120° whereas they are all linear for singlet structures. The triplet isomer cyclicS(CH3)CN has not been located, while the cyclicC(CH3)NS 34 and cyclic-N(CH3)CS 37 with the CMCSN dihedral angle of 121.2° and 21.0° are found. From Table 2, it is obvious that the most stable species structure, namely branched-C(CH3)NS 31 is located 47.6 kcal/mol above the singlet isomer 11, and the distance between ˚ with the CMCNS dihedral angle of N  S is 2.552 A 180.0°. The relative QCISD energies of the triplet isomers increase in the following order(barriers in parenthesis, kcal/mol):

[H,N,C,S] system, except singlet isomers 18 and 19. On the other hand, we also compare part of singlet barriers of [CH3,N,C,S] with [H,N,C,S] at B3LYP/aug-cc-pVTZ level of theory (see Table 3). It can be easily seen that the isomerizations concerning methyl-transfer exhibit much higher barriers than those of hydrid analogues due to the methyl’s larger steric hindrance. But for the isomerizations without methyl-transfer, they show the similar barriers as those of [H,N,C,S] system.

branched-C(CH3)NS 31(47.6) < CH3NCS 32(62.5) < CH3CNS 33(71.3) < cyclic-C(CH3)NS 34(78.3) < CH3SCN 3 5(79.0) < CH3SNC 36(87.1) < cyclic-N(CH3)CS 37(119.0).

The global PES of the [CH3,N,C,S] system in singlet and triplet states is studied at DFT(B3LYP), MP2 and QCISD levels. On the singlet surface, nine isomers (connecting by 10 transition states) are found in which only CH3NCS 11, CH3SCN 12 and CH3CNS 13 have been experimentally observed and characterized. However, in view of thermodynamics, the energies of chainlike CH3SNC 14 and cyclic-C(CH3)NS 15 are comparable with CH3CNS 13, which means that in some conditions 14 and 15 may be detected in the laboratory. On the triplet surface, seven isomers are explored among which the branched-C(CH3)NS has the lowest energy. By comparing with [H,N,C,S] system, we conclude that the singlet isomerizations concerning methyl-transfer exhibit higher barriers than that of hydrid analogues due to the methyl’s larger steric hindrance. But for the isomerizations without methyl-transfer, they show the similar barriers as those of [H,N,C,S] system.

The second most stable triplet isomer 32 can convert to 31 through one step mechanism via TS31/32 with barrier 45.2 kcal/mol. In addition, isomer 34 can rearrange to 33 through TS33/34 by the rupture of C–S bond with barrier 13.7 kcal/mol. Both 32 ! 31 and 34 ! 33 are exothermic by 14.9 and 7.0 kcal/mol, respectively. Moreover, 31 can dissociate to 1CH3CN + 3S (Pdiss1) and 2CH3 + 2NCS (Pdiss2) via TS31/Pdiss 1 and TS31/Pdiss 2, respectively. TS31/Pdiss 1 is located only 8.1 kcal/mol above 31 while TS31/Pdiss 2 is located 28.2 kcal/mol above 3 1, 35 can also dissociate into 2CH3 + 2NCS (Pdiss 2) through the rupture of CM–S bond exothermically by only 11 kcal/mol, yet it proceeds through a high-energy TS35/ Pdiss 2 lying 27.0 kcal/mol above 35.

4. Conclusions

3.3. Comparison with [H,N,C,S] system

Acknowledgements

Compared with the [H,N,C,S] system [22], the presence of methyl of isomers in [CH3,N,C,S] system has little impact on the structures formed by N, C, S atoms. Moreover, isomers in singlet and triplet PES of [CH3,N,C,S] system reveal the similar stability order as those of the

This work was financially supported by National Natural Science Foundation of China (Grant No. 20333050). The Key Subject of Science and Technology by the Ministry of Education of China, and The Innovation Foundation by Jilin University.

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