Theoretical study on the mechanism of the N(4S)CC2H5 reaction

Yong Yang*, Weijun Zhang, Shixin Pei, Jie Shao, Wei Huang, Xiaoming Gao

Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

Received 25 October 2004; accepted 28 February 2005

Available online 13 June 2005

Abstract

The reaction for N(4S)CC2H5 has been studied by DFT method. The geometries of the reactants, the intermediates, the transition states

and the products are optimized at the B3LYP/6-311G(d,p) level. The corresponding vibration frequencies are calculated at the same level.

The single-point calculations for all the stationary points are carried out at the QCISD(T)/6-311CG(d,p) level using the B3LYP/6-311G(d,p)

optimized geometries. The all energies were refined at the G3B3 level. The results of the theoretical study indicate that the major products are

the C2H4C3NH and H2CNCCH3 in the reaction. The majority of the products C2H4C3NH are formed via a direct hydrogen abstraction

pathway c1: (R)/TS1/(C). While the products H2CNCCH3 are produced via an addition/elimination pathway a: (R)/IM1/TS2/(A). In addition, the CH3CHNCH are found to be the possible products. Furthermore, the products CH3CHN and H2CN can undergo

dissociation into the products HCN and CH3CN at higher temperatures.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Potential energy surface; Mechanism; Transition states

1. Introduction

The reactions of ground state N atom with hydrocarbon

radicals are significant in a wide variety of systems

including the chemistry of the atmospheres of Titan and

Neptune [1,2], the nitrogen chemistry in hydrocarbon

combustion [3,4], and the chemistry of interstellar and

circumstellar clouds [5–7]. In many of these systems a

prominent reaction of this type is that of N(4S) with CH3

reaction. Absolute rate constants and primary reaction

pathways have been determined [8–10].

Despite the prominence of N(4S)CCH3 in some of the

above complex systems, it must be noted that C2 hydro-

carbons in general and C2H5 radical in particular are of

comparable or even greater importance in two of these

systems. Although the reaction of N(4S)C C2H5 is very

important, there is only one experimental investigation on

the absolute rate constant and the product distributions by

Stief et al. [11]. The reaction is very rapid with k(298 K)Z(1.1C0.3)!10K11 cm3 moleculeK1 sK1 and branching

ratios are estimated to be 0.65 and 0.35 for the products

0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2005.02.081

* Corresponding author. Tel.: C86 551 5593174; fax: C86 551 5591551.

E-mail address: yongyang@aiofm.ac.cn (Y. Yang).

C2H4C3NH and H2CNCCH3, respectively. To the best of

our knowledge, there is no theoretical study on the title

reaction up to now. However, there was still somewhat

uncertainty for the title reaction. Firstly, in the previous

work of Stief et al., whether this is a direct abstraction

mechanism or an addition/elimination mechanism for the

0.65 yield of the products C2H4C3NH remains an open

question [11,13]. Secondly, the products CH3CHNCH was

not observed in previous experiments, while the branching

ratio of the products CH3CHOCH is estimated to be 0.40G0.04 in the similar reaction of O(3P)CC2H5 [12]. Finally,

we expect to know whether the products can take further

dissociation into the final products HCN and CH3CN.

Therefore, a detail theoretical study on the triplet potential

surface of the N(4S)CC2H5 reaction is desirable.

In this paper, an attempt is made to elucidate the

mechanism of this reaction and to give a sound explanation

to the experimental results. We hope that our theoretical

results may provide useful information on understanding the

mechanism of this reaction.

2. Computational methods

All computations were carried out by using the

GAUSSIAN98 program [14]. The optimized geometries

Journal of Molecular Structure: THEOCHEM 725 (2005) 133–138

www.elsevier.com/locate/theochem

Y. Yang et al. / Journal of Molecular Structure: THEOCHEM 725 (2005) 133–138134

and vibration frequencies of the reactants, the intermediates,

the transition states and the products were obtained at

B3LYP/6-311G(d,p) level. Moreover, single-point calcu-

lations were performed at QCISD(T)/6-311CG(d,p) level

using the B3LYP/6-311G(d,p) optimized geometries. The

zero-point vibration energy (ZPVE) at the B3LYP/6-

311G(d,p) level was also included. To confirm that the

obtained transition states connected with the right reactants

and products, the intrinsic reaction coordinate (IRC)

calculations were performed at the B3LYP/6-311G(d,p)

level. To obtain more reliable energies, the G3B3 method

was utilized.

3. Results and discussion

The geometries of the reactants, the intermediates, the

products and the transition states are shown in Figs. 1 and 2.

Fig. 1. Optimized geometries of the reactants, the intermediates and the products

The potential energy profile of the title reaction is shown in

Fig. 3. The computed energies by QCISD(T) and G3B3

methods are listed in Table 1.

3.1. Reaction mechanism

As shown in Figs. 1–3, two kinds of reaction pathways in

the N(4S)CC2H5 reaction were revealed. They are: direct

hydrogen abstraction and C-addition/elimination pathways.

For direct hydrogen abstraction, the N(4S) atom can abstract

H atom from CH3 group of C2H5 radical, forming 3NH and

C2H4. For C-addition/elimination, the N(4S) atom can attack

C atom of CH2 group of C2H5 radical to form the adduct

isomer IM1. Furthermore, the isomer IM1 with the relative

energy 301.5 kJ/mol can be regarded as an energy rich

species and may thus undergo the subsequent reaction steps.

Six possible products (A) H2CNCCH3, (B) CH3CHNCH,

(C) C2H4C3NH, (D) CH3CNHCH, (E1) CH2CHNH(c)CH

at the B3LYP/6-311G(d,p) level. Bond lengths in (A), bond angles in (8).

Fig. 2. Optimized geometries of the transition states at the B3LYP/6-311G(d,p) level. Bond lengths in (A), bond angle in (8).

Y. Yang et al. / Journal of Molecular Structure: THEOCHEM 725 (2005) 133–138 135

and (E2) CH2CHNH(t)CH may be suggested for this

reaction, among which the products (A) H2CNCCH3 and

(C) C2H4C3NH have been experimentally observed [11].

The reaction pathways for these possible products will be

discussed on the basis of the triplet potential energy

surface of the N(4S)CC2H5 reaction. The formation of the

products CH3CNCH2 and HCNCCH4 is spin-forbidden on

the triplet potential energy surface and thus requires the

intersystem crossing process, and the details will be

presented elsewhere.

3.1.1. Formation of (A) H2CNCCH3

Only a pathway can be obtained for the formation of

products (A) as follows:

Pathway a:

(R)/IM1/TS2/(A).

As shown in pathway a, the isomer IM1 can dissociate

into H2CNCCH3 via transition state TS2 which has a Cs

symmetry. This is a C–C bond scission process. The

breaking C–C bond in TS2 is elongated by up to 0.776 A.

Fig. 3. Profile of the potential energy surface for the CH3CH2CN(4S) reaction at the G3B3 level.

Y. Yang et al. / Journal of Molecular Structure: THEOCHEM 725 (2005) 133–138136

The C–N bond is shortened to 1.269 A. Meanwhile, the CH3

and H2CN groups appear to be very similar to the final CH3

and H2CN radical. This C–C bond cleavage pathway is

endothermic by 88.4 kJ/mol, while the overall pathway

is exothermic by 213.1 kJ/mol. The barrier height is

116.9 kJ/mol with respect to IM1. Note that the energy of

TS2 is 184.6 kJ/mol lower than that of the reactants N(4S)CC2H5 and the barrier height of TS2 is lowest among all the

isomerization and dissociation transition states involved

isomer IM1. We can deduce that the most feasible pathway

starting from the isomer IM1 is surely pathway a. There has

been only one experimental investigation on the product

distribution of the N(4S)CC2H5 reaction by Stief et al. They

found that H2CNCCH3 (branching ratio of 0.35) was one of

the major products [11]. Surely, our theoretical results can

well interpret the H2CNCCH3 formation of the title

reaction.

3.1.2. Formation of (B) CH3CHNCH

Two pathways can be obtained for the formation of

products (B) as follows:

Pathway b1:

(R)/IM1/TS3/(B)

Pathway b2:

(R)/IM1/TS4/IM2/TS7/(B).

As shown in pathway b1, the isomer IM1 can dissociate

into CH3CHNCH via transition state TS3. The C–H bond

cleavage transition state TS3 has no symmetry. The

breaking C–H bond is elongated from 1.104 A in IM1 to

1.999 A. The C–N bond length is decreased from 1.416 to

1.260 A, closing to a multiple bond CbN in the product

CH3CHN. Thus, the TS3 is a product-like barrier. The C–C

bond cleavage pathway is endothermic by 124.7 kJ/mol,

while the total pathway, C2H5CN(4S)/CH3CHNCH is

exothermic by 176.8 kJ/mol. The TS3 barrier height

is 139.0 kJ/mol. Compared with pathway b1, pathway b2

is more complex. Firstly, the reaction may proceed via

an isomerization process from IM1 to IM2. The process is H

atom shifting from the C atom to the N atom via a three-

member ring barrier TS4 with 176.9 kJ/mol height. The

breaking C–H bond is 1.307 A and the forming N–H bond is

1.229 A. Although this process is endothermic by

47.6 kJ/mol, the isomer IM2 is still an energy rich

intermediate, 253.9 kJ/mol stable relative to the initial

reactants. Furthermore, CH3CHNCH can be produced from

dissociation of the IM2, via a N–H bond cleavage barrier

TS7 with 100.9 kJ/mol height. Comparing pathways b1 and

b2, it seems that pathway b1 may dominate over pathway b2

due to its lower barrier height and less reaction steps. It is

worth noting that the barrier height of TS3 is about

22.1 kJ/mol higher than that of TS2. We suggest that the

products (B) may be less important than the products (A).

However, the path b1 may have some contribution in the

title reaction because the barrier heights of TS2 and TS3 are

relatively close. However, the products (B) have not been

explored by Stief et al. [11]. It may be of interest to make a

comparison between the reactions N(4S)CC2H5 and

O(3P)CC2H5. In the reaction O(3P)CC2H5, the branching

ratio of the products CH3CHOCH has been estimated to be

0.40G0.04 [12]. Although the branching ratio of the

products CH3CHNCH may be lower than the value of

Table 1

The total energies (Hartree) and relative energies (kJ/mol) of various

species at QCISD(T)/6-311CG(d,p)//B2LYP/6-311G(d,p) and G3B3

levels

QCISD/6-311C

G(d,p)CZPVE

G3B3

NCC2H5 K133.382535 (0.0) K133.625422 (0.0)

IM1 K133.481991 (K261.1) K133.740271 (K301.5)

IM2 K133.460497 (K204.7) K133.722114 (K253.9)

IM3 K133.452113 (K182.7) K133.712189 (K227.8)

IM4 K133.453157 (K185.4) K133.713977 (K232.5)

TS1 K133.372816 (25.5) K133.623278 (5.6)

TS2 K133.435706 (K139.6) K133.695724 (K184.6)

TS3 K133.426473 (K115.4) K133.687296 (K162.5)

TS4 K133.412408 (K78.4) K133.672888 (K124.6)

TS5 K133.414565 (K84.1) K133.674816 (K129.7)

TS6 K133.424585 (K110.4) K133.685500 (K157.7)

TS7 K133.422657 (K105.3) K133.683709 (K153.0)

TS8 K133.409141 (K69.9) K133.672587 (K123.8)

TS9 K133.416293 (K88.6) K133.680036 (K143.4)

TS10 K133.450813 (K179.3) K133.712203 (K227.8)

TS11 K133.417379 (K91.5) K133.680903 (K145.7)

TS12CH K133.393569 (K29.0) K133.651364 (K68.1)

TS13CH K133.389856 (K19.2) K133.649425 (K63.0)

TS14CH K133.365826 (43.9) K133.624451 (2.5)

TS15CH K133.385870 (K8.8) K133.645170 (K51.8)

TS16CH K133.396216 (K35.9) K133.656781 (K82.3)

TS17CH K133.371759 (28.3) K133.629102 (K9.7)

TS18CH K133.372934 (25.2) K133.630719 (K13.9)

TS19CCH3 K133.399397 (K44.3) K133.658653 (K87.2)

CH3CHNCH K133.434862 (K138.4) K133.692754 (K176.8)

CH3CH2CN K133.448183 (K172.4) K133.706569 (K213.1)

C2H4CNH K133.440253 (K151.5) K133.697776 (K190.0)

CH3CNH(c)CH K133.414945 (K85.1) K133.676143 (K133.2)

CH3CNH(t)CH K133.423762 (K108.2) K133.683365 (K152.1)

CH2CHNH(c)CH K133.426806 (K116.2) K133.687831 (K163.9)

CH2CHNH(t)CH K133.482399 (K119.5) K133.688689 (K166.1)

CH3CNCHCH K133.402979 (K53.7) K133.658854 (K87.8)

CH3CHCNCH K133.410602 (K73.7) K133.665857 (K106.2)

CH3CHNCCH K133.387380 (K12.7) K133.642046 (K43.6)

Y. Yang et al. / Journal of Molecular Structure: THEOCHEM 725 (2005) 133–138 137

0.4G0.04, our theoretical results suggest the products

CH3CHNCH may have previously been omitted by Stief

et al. Obviously, further experimental and theoretical

investigations will be desirable with a focus on determining

the products CH3CHNCH.

3.1.3. Formation of (C) C2H4C3NH

Two pathways for the formation of products (C) can be

also obtained as follows:

Pathway c1:

(R)/TS1/(C)

Pathway c2:

(R)/IM1/TS5/IM3/TS6/(C).

As shown in pathway c1, the transition state TS1 has a Cs

symmetry. The angle N–H–C is nearly collinear with an

angle of 168.38. The breaking C–H bond is elongated by

0.064 A, while the forming N–H bond is 0.665 A longer

than the equilibrium distance of the 3NH radical. The bond

length of C–H and N–H increases by 5.8% and 63.7%,

respectively. In view of these structural characteristics, TS1

is more reactant-like and thus an early barrier, as could be

anticipated from the reaction exothermicity by

190.0 kJ/mol. The TS1 barrier height is 5.6 kJ/mol. For

pathaway c2, the reaction starts from IM1. H atom of the

methyl group is shifted to the N atom. The transition state

TS5 involved appears to be a four-center structure. The

barrier height for this H-shift pathway is relatively high,

171.8 kJ/mol relative to IM1. For the second reaction step,

the isomer IM3 transforms to the products (C) via transition

state TS6 with 70.1 kJ/mol barrier height. It may be of very

interesting to make a comparison between the path c1 and

path c2 because previous experiments of the reaction

N(4S)CC2H5 provide no information on whether these are

a direct abstraction pathway c1 or an addition/elimination

pathway c2 about the mechanism for products C2H4C3NH

formation [11,13]. Our results indicate that the pathway c1

leading to the products C2H4C3NH may be more preferable

due to the low barrier height and high exothermicity, while

the pathway c2 should be less competitive than the path a

and path b1 because the barrier height of the TS5 is 54.9 and

32.8 kJ/mol higher than that of TS2 and TS3. Thus, the

pathway c1 can be regarded as the major pathway and the

path c2 may have little contribution in the products

C2H4C3NH formation. It is worthy of mentioning that a

direct abstraction process is proposed to dominate the

products C2H4COH in the similar reaction C2H5CO(3P)

[15]. Surely, our results provide a good interpretation about

the mechanism of products C2H4C3NH formation. We

suggest that future experimental investigations should be

desirable to clarify the formation mechanism of

C2H4C3NH.

3.1.4. Formation of (D) CH3CNH(t)CH, (E1)

CH2CHNH(c)CH and (E2)CH2CHNH(t)CH

Three pathways for the formation of products (D), (E1)

and (E2) are also suggested as follows:

Pathway d:

(R)/IM1/TS4/IM2/TS8/(D)

Pathway e1:

(R)/IM1/TS5/IM3/TS9/(E1)

Pathway e2:

(R)/IM1/TS5/IM3/TS10/IM4/TS11/(E2).

Our results indicate that the paths d, e1 and e2 cannot

compete with the paths a and b1 due to their more

reaction steps and higher barrier height. We suggest that

the contribution of the products (D), (E1) and (E2) should

be negligible. This is in excellent agreement with

experimental fact that the products (D), (E1) and (E2)

cannot be detected [11].

3.2. Are HCN and CH3CN the direct products of the

reaction NCC2H5?

The reaction N(4S)CCH3 is believed to be a major

source of HCN in the upper atmosphere of Titan [1,10].

However, HCN and CH3CN have not been explored by Stief

Y. Yang et al. / Journal of Molecular Structure: THEOCHEM 725 (2005) 133–138138

et al. at TZ298 K [11]. It is of interest to turn to why HCN

and CH3CN were not previously explored. To answer the

question we provide some possible sources for the HCN and

CH3CN as follows.

Pathway 1:

CH3CHN/TS12(108.7 kJ/mol)/CH3CHCN(70.6 kJ/mol)

Pathway 2:

CH3CHN/TS13(113.8 kJ/mol)/CH3CNCH(89.0 kJ/mol)

Pathway 3:

CH3CHN/TS14(179.3 kJ/mol)/CH3-

CNH(t)(24.7 kJ/mol)/TS16(94.5 kJ/mol)/CH3CNH(c)(43.6 kJ/mol)/TS15(125.0 kJ/mol)/CH3CNCH(89.0 kJ/mol)

Pathway 4:

CH3CNH(t)/TS16(69.8 kJ/mol)/CH3-

CNH(c)(18.9 kJ/mol)/TS15(100.3 kJ/mol)/CH3CNCH(64.3 kJ/mol)

Pathway 5:

H2CN/TS19(125.9 kJ/mol)/HCNCH(106.9 kJ/mol).

As shown in pathway 1, the CH3CHN can dissociate into

CH3CHCN via transition state TS12. The corresponding

barrier height is 108.7 kJ/mol. For pathway 2, the C–H bond

cleavage of the CH3CHN may lead to CH3CNCH via the

transition state TS13 with the barrier height of 113.8 kJ/mol.

In pathway 5, the H2CN can dissociate into HCHCN via the

transition state TS19 with the barrier height of 125.9 kJ/mol.

From our theoretical results, it seems that the pathways 1, 2

and 5 are energetically feasible for the formation of HCN

and CH3CN because the barrier heights of TS12, TS13 and

TS19 are not very high. However, we can also find that the

pathways 1, 2 and 5 are considerably highly endothermic,

which is not preferable for the reaction. As a result, it is very

difficult that the H2CN and CH3CHN undergo further

dissociation into the final secondary products HCN and

CH3CN at lower temperatures. We suggest that at higher

temperatures the path 1, 2 and 5 should become more

important. Experiments performed at higher temperatures

would be required.

4. Conclusion

We reported a theoretical study of triplet potential

surface for the N(4S)CC2H5 reaction at the QCISD(T)/6-

311CG(d,p)//B2LYP/6-311G(d,p) and G3B3 level. The

reaction can start in two kinds of reaction pathways: direct

hydrogen abstraction and C-addition/elimination pathways.

The major pathways can be expressed as

Pathway a:

(R)/IM1/TS2/(A)

Pathway c1:

(R)/TS1/(C)

and the minor pathway can be expressed as

Path b1:

(R)/IM1/TS3/(B).

We suggest that the H2CN and CH3CHN can undergo

further dissociation into the final products HCN and CH3CN

at higher temperatures.

Acknowledgements

This work is supported by the National Natural Science

Foundation of China (G20477043) and Knowledge Creative

Program by Chinese Academy of Sciences (KJCX2-SW-

H08).

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