ORIGINAL PAPER Theoretical investigation on the

Chemical Papers 68 (1) 145–152 (2014) DOI: 10.2478/s11696-013-0412-y

ORIGINAL PAPER

Theoretical investigation on the reaction of HS+ with CH3 NH2 Li-Li Zhang, Hui-Ling Liu*, Hao Tang, Xu-Ri Huang State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China Received 14 January 2013; Revised 11 March 2013; Accepted 12 March 2013

The singlet and triplet potential energy surfaces for the reaction of HS+ with the simplest primary amine, CH3 NH2 , were determined at the CCSD(T)/6-311+G(d,p) level using the B3LYP/6311G(d,p) and QCISD/6-311G(d,p) geometries. All possible reaction channels were explored. The results show that three paths on the singlet potential energy surface and one path on the triplet potential energy surface are competitive. These four feasible paths provide products which are presented in the paper and they are consistent with previous experimental results. On the other hand, the stationary points involved in the most favourable path all lie below those of the reactant and thus the title reaction is expected to be rapid, which is also consistent with the experiment. c 2013 Institute of Chemistry, Slovak Academy of Sciences Keywords: HS+ with CH3 NH2 , theoretical calculations, reaction mechanism, potential energy surface (PES)

Introduction Methylamine (CH3 NH2 ), the simplest primary amine, has received considerable attention and has played an important role in biology (Hamdani et al., 2009; Xiao & Yu, 2009; Conklin et al., 2004; Choi et al., 2011), chemistry (Lu et al., 2010; Li & Oshima, 2005; Atroshchenko et al., 2005; Cho & Choi, 2011), and astrochemistry (Singh et al., 2010). Consequently, it has been the subject of many experimental and theoretical studies (Jackson et al., 2005; Ilyushin et al., 2005; Lin et al., 2011). Up to now, many experimental investigations on its quantum chemical parameters (Irgibaeva, 2004) and spectroscopic properties (Baek et al., 2003a, 2003b; Naganathappa & Chaudhari, 2010) have been reported. Several theoretical studies have been devoted also to the reaction of CH3 NH2 with OH (Tian et al., 2009), HNO2 (Tiwary & Mukherjee, 2009), Cl (Rudić et al., 2003), H (Kerkeni & Clary, 2007), and so forth (Lv et al., 2010; Kua et al., 2011; Liu et al., 2010) using the density functional theory or the ab initio methods. Smith et al. (1981) reported the reactions of Hn S+ ions with several molecular gases, such as CH3 NH2 ,

Fig. 1. Possible products of the reaction of HS+ with CH3 NH2 .

NO, NH3 , etc., using a SIFT apparatus. Their results indicated that the charge transfer does not necessarily imply a long range electron transfer, but rather that a charge transfer channel is often just one of the exit channels resulting from the interaction. It was postulated that the reaction of HS+ with CH3 NH2 can afford various products as illustrated in Fig. 1 (products distribution is given in brackets). In the present article, a detailed theoretical study on the reaction of CH3 NH2 with HS+ was carried out to investigate the reaction mechanism. This investigation can help to explain the experimental results and understand the mechanism of this series of reactions.

*Corresponding author, e-mail: hui ling [email protected]

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L. L. Zhang et al./Chemical Papers 68 (1) 145–152 (2014)

Fig. 2. Possible singlet reaction channels including optimised geometries and parameters involved in the reaction. Distances are given in nm and angles in degrees.

Fig. 3. Dissociation curves computed at the B3LYP/6-311G(d,p) (a) and MP2/6-311G(d,p) (b) level for IM1→P1-1; R is the interatomic distance.

Theoretical methods The density functional theory (DFT) calculations were carried out using the Gaussian 03 program package (Frisch et al., 2004). Geometries of the reactants, products, intermediates, and transition states involved in the reaction processes were fully optimised at the B3LYP/6-311G(d,p) level (Becke, 1993) including the vibration frequency analysis. The intrinsic reaction coordinate (IRC) method (Gonzalez &

Schlegel, 1989) was used to ensure the minimum energy paths on the potential energy surface (PES). For more accurate estimation of the energy of the stationary points, single point calculations were performed at the CCSD(T)/6-311+G(d,p) level (Pople et al., 1987) using the B3LYP/6-311G(d,p) optimised structures. Furthermore, for the most feasible channels, the structures were optimised at the QCISD/6-311G(d,p) level, followed by the CCSD(T)/6-311+G(d,p) single-point energy calculations.



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Fig. 4. Possible triplet reaction channels including optimised geometries and parameters involved in the reaction. Distances are given in nm and angles in degrees.

Fig. 5. Dissociation curves computed at the B3LYP/6-311G(d,p) (a) and MP2/6-311G(d,p) (b) level for IM3→P3-1; R is the interatomic distance.

Result and discussion In the present study, possible intermediates and relevant transition states of the title reaction are characterised. The possible singlet and triplet reaction channels and the geometries of the stationary points are shown in Figs. 2 and 4, respectively. Figs. 3 and 5 show the dissociation curves of the products. The relative energies with respect to the optimised geometries are summarised in Ta-

ble 1. Based on the interrelations between the reactants, intermediates, transition states, products, and the corresponding relative energies, the energy profile for the HS+ + CH3 NH2 reaction was determined and it is depicted in Fig. 6. Fig. 7 displays the optimised structures of the species involved in the most favourable channels at the B3LYP/6-311G(d,p) and QCISD/6-311G(d,p) (in italic) levels; the corresponding energies of both levels are listed in Table 2.


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Table 1. Total (Etotal ) and relative (Erel ) energies including zero-point vibration energies for the species of the reaction of HS+ with CH3 NH2 on the singlet and triplet potential energy surfaces Singlet

Surface

Triplet

Surface

B3LYP

CCSD(T)

(CCSD(T)+ZPVE)rel Species

B3LYP

CCSD(T)

Etotal /a.u. R IM1 TS1-2 IM1-2 TS1-3 IM1-3 P1-1 P1-2 P1-3

–494.276842 –494.4302167 –494.3346124 –494.4428932 –494.3065427 –494.3310849 –494.3359152 –494.4269893 –494.3193693

–493.5041301 –493.6411892 –493.5386618 –493.6454344 –493.5177096 –493.5508345 –493.5360067 –493.6316905 –493.5392197

(CCSD(T)+ZPVE)rel Species

Erel /(kcal mol−1 ) 0.0 –80.9 –21.0 –87.3 –7.0 –22.0 –21.5 –80.4 –16.0

Etotal /a.u. R IM3 TS3-2 IM3-2 TS3-3 IM3-3 P3-1 P3-2 P3-3

–494.276842 –494.367822 –494.3311906 –494.3355098 –494.3470646 –494.39371 –494.1987739 –494.2428339 –494.3807975

–493.5041301 –493.5684812 –493.5313609 –493.5316852 –493.5506728 –493.6074349 –493.4062722 –493.4410706 –493.5925948

Erel /(kcal mol−1 ) 0.0 –38.6 –18.6 –18.8 –29.0 –58.5 56.5 34.2 –49.5

Fig. 6. Singlet and triplet PES for the reaction of HS+ with CH3 NH2 at the CCSD(T)/6-311+G(d,p)//B3LYP/6-311G(d,p) level (including zero-point vibration energy); Erel – relative energy (kcal mol−1 ); superscripts s and t represent the singlet and the triplet, respectively.

Singlet state reaction channels When the HS+ ion approaches the CH3 NH2 molecule, the most favourable entrance intermediate is IM1 (CH3 NH2 SH+ ) because the N atom is the nucleophilic centre of CH3 NH2 which prefers to attack the S atom of HS+ . Since the initial association of IM1 is by 81.0 kcal mol−1 (1 kcal = 4.184 kJ) lower in energy than that of reactant R (CH3 NH2 + HS+ ), the IM1 creates an energy reservoir used as the reaction proceeds toward the products. The most important pathways from IM1 are shown in Fig. 2; thus it can be seen that starting from IM1, three possi-

ble channels can be identified: (i) the charge transfer −1 product P1-1 (CH3 NH+ 2 + HS) is by 21.5 kcal mol more stable than the reactant. From the dissociation curves (Fig. 5), it can be seen that there is no transition state on the path from IM1 to P1-1. The entrance intermediate IM1 can dissociate to P1-1 directly via the N—S single bond; (ii) the energy of the H2 S elimination product P1-2 (CH2 NH+ 2 + H2 S) is by 80.4 kcal mol−1 below that of the reactant. In Fig. 2 it can be seen that when the S atom abstracts the H atom of methyl, the C—N bond distance is shortened. Finally, when the H atoms of methyl are abstracted by SH, the N—S bond is broken and the C—N double bond


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Fig. 7. Optimised structures of the species involved in the most favourable channels at the B3LYP/6-311G(d,p) and QCISD/6311G(d,p) (in italic) levels. Table 2. Total (Etotal ) and relative (Erel ) energies of the critical structures for the title reaction CCSD(T)/6-311+G(d,p)//B3LYP/6-311G(d,p) Species

R P1-1 P1-2 P1-3 P3-3 IM1 IM1-2 IM1-3 IM3 IM3-3 TS1-2 TS1-3 TS3-3

CCSD(T)/6-311+G(d,p)//QCISD/6-311G(d,p)

Etotal /a.u.

Erel /(kcal mol−1 )

Etotal /a.u.

Erel /(kcal mol−1 )

–493.4345345 –493.4687567 –493.5626032 –493.4600532 –493.5134283 –493.5634796 –493.5737288 –493.4695753 –493.4960428 –493.527805 –493.4680581 –493.445687 –493.4807508

0.0 –21.5 –80.4 –16.0 –49.5 –80.9 –87.3 –22.0 –38.6 –58.5 –21.0 –7.0 –29.0

–493.4332905 –493.467703 –493.5619646 –493.4589391 –493.5123142 –493.5620142 –493.5735161 –493.4697835 –493.4960549 –493.5264241 –493.4685301 –493.4442355 –493.4773941

0.0 –21.6 –80.7 –16.1 –49.6 –80.8 –88.0 –22.9 –39.4 –58.4 –22.1 –6.9 –27.7

is formed. When H2 S is eliminated from IM1-2, a van der Waals complex is formed before the dissociation to P1-2. In TS1-2, the unique imaginary frequency is 828i cm−1 , which corresponds to the normal mode of

the transfer of H from C to S. The calculated energy barrier of 60.0 kcal mol−1 from IM1 to IM1-2 is by 0.5 kcal mol−1 higher than the energy difference between IM1 and P1-1. So, this channel is competitive


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with pathway P1-1, although less favourable; (iii) IM1 can lead to IM1-3 via the H-atom abstraction transition state TS1-3 with a barrier of 74.0 kcal mol−1 . In Fig. 1, it is noticed that the S—H bond of IM1-3 (2.362 ˚ A) is easy to be broken, which leads to the expected hydrogen migration product P1-3 (CH3 NH+ 3 + S). In TS1-3, the unique imaginary frequency is 722i cm−1 , which corresponds to the normal mode of the H—S bond breakage followed by the formation of the N—H bond. The barrier heigth of path P1-3 (74.0 kcal mol−1 ) is much higher than those of paths P1-1 (59.5 kcal mol−1 ) and P1-2 (60.0 kcal mol−1 ). It can therefore be concluded that the order of the optimal channels is: path P1-1, P1-2, and P1-3 in the singlet surface.

Triplet state reaction channels As a supplement of the singlet surface, the triplet potential energy surface was also analysed. Given the multiplicity of the reactants (HS+ (1 A or 3 A ) and CH3 NH2 ), this reaction should, in principle, take place on the triplet potential energy surface. Optimised geometries of the intermediates, transition states, and possible pathways for the reaction HS+ (3 A ) + CH3 NH2 are shown in Fig. 4 while Fig. 6 shows the energy profile. The most favourable interaction of the HS+ (3 A ) ion is that with CH3 NH2 resulting in IM3 (38.6 kcal mol−1 lower in energy than the reactant). In Fig. 3, possible pathways starting from IM3 can be seen identified as follows: (i) path P3-1 is very similar to path P1-1 and no transition state occurs between IM3 to P3-1. The dissociation curves are shown in Fig. 5. However, the process to get product CH3 NH+ 2 + HS on the triplet potential surface has some differences from that on the singlet surface. In Fig. 4 it can be seen that on triplet PES, the N—S bond length of IM3 is 2.614 ˚ A, that of H—S of P3-1 is 2.663 ˚ A; however, in IM1 and P1-1, these two bonds lengths are 1.824 ˚ A and 1.353 ˚ A, respectively. It can be considered that in IM1, the lone electron pair of S coordinates to the empty orbital of the N atom while in IM3, the connection between the S atom and the N atom can be a van der Waals force. From Fig. 4 follows that the formation of P3-1 is an endothermic process and the energy of the product is located above that of the reactant. Therefore, this is probably a very minor pathway; (ii) in Fig. 4 it is shown that path P3-2, to form CH2 NH+ 2 and H2 S via hydrogen abstraction from the CH3 group of CH3 NH2 by HS, is similar to path P1-2; however, P3-2 is located above that of the reactant as well as in P3-1, which means that it is also a very minor pathway; (iii) finally, the theoretical calculations of the triplet potential energy surface suggest that path P3-3 is a feasible process. Although the formation of CH3 NH+ 3 + S is an exothermic process on both surfaces, the energy barrier of P3-3 is the

Fig. 8. Products formation pathway.

lowest one on the singlet as well as on the triplet surfaces, so this path is competitive with the three paths on the singlet surface. Comparison with experiment In Fig. 4 it is shown that the singlet reaction pathways are more stable than the corresponding triplet ones. The feasible formation pathways leading to experimentally observed products are depicted in Fig. 8. In the energetically lowest pathway IM1→TS13→IM1-3→P1-3, the transition state TS1-3 is by 22.0 kcal mol−1 higher than the triplet transition state TS3-3; this implies the presence of an intersection point on the pathway. It is presumed that the IRC calculation method can explain the reaction mechanisms because a chemical reaction pathway can be found by IRC calculations (Zeng et al., 2003). In order to validate this statement, the coordinates and the single-point energy of every point on the intrinsic reaction coordinate (IRC) reverse direction pathways of IM1→TS1-3 and IM3→TS3-3 at the B3LYP/6311G(d,p) level, respectively, were explored. The calculation results show that there is no intersection point in the PES. According to the calculation results, the total of four product channels can be observed (Fig. 8). P11 is the most favourable product, P1-2 is the less favourable and much less competitive than P1-1. P1-3 and P3-3 are the least feasible products. Obviously, our calculation results are in agreement with the experimental findings obtained by Smith et al. (1981). Reliability assessment Additional calculations of the critical species R (CH3 NH2 + HS+ ), P1-1 (CH3 NH+ 2 + HS), P1-2 + 1 (CH2 NH+ + H S), P1-3 (CH NH 2 3 2 3 + S( A )), P3-3 + 3 (CH3 NH3 + S( A )), IM1, IM1-2, IM1-3, IM3, IM33, TS1-2, TS1-3, and TS3-3, involving the most feasible pathways using the higher level and more expensive QCISD/6-311G(d,p) method were carried out. As shown in Fig. 7, structural parameters at both levels are in good agreement with each other. It should be pointed out that P1-3 and P3-3 are very similar except for the state of the S atom. Consequently, the



structure parameters of CH3 NH+ 3 for P1-3 and P3-3 were listed in Fig. 7. It is crucial that the CCSD(T)/6311+G(d,p)//B3LYP/6-311G(d,p) relative energies of these thirteen species agree well with the corresponding CCSD(T)/6-311+G(d,p)//QCISD/6-311 G(d,p) values with the maximal discrepancy of 1.3 kcal mol−1 of TS3-3 (Table 2). Thus, the CCSD(T)/6311+G(d,p)//B3LYP/6-311G(d,p) method was proven to provide credible information on the title reaction.

Conclusions A theoretical study on the ion-molecule reaction of HS+ with CH3 NH2 was carried out on the singlet and triplet potential energy surfaces. The possible transition states as well as the relevant intermediates for this reaction were characterised at the B3LYP/6311G(d,p) level. Theoretical calculations of the triplet surface suggested that only the path P3-3 is a feasible pathway, and calculations of the singlet potential energy surface suggested that all the channels are clearly exothermic, and the most favourable channel, from the kinetic point of view, is the proton transfer process which has a lower activation barrier. Results observed from the SIFT experiments showed that the products correspond to the proton transfer process, which is in agreement with the theoretical results found for the singlet and triplet potential energy surfaces. The less competitive channel seems to be the formation of P1-2. On the other hand, the least competitive channels are the formation of P1-3 and P3-3 from IM1 and IM3, respectively. Acknowledgements. This research has been supported by the National Natural Science Foundation of China (project no. 21073075).

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