Theoretical study on reaction mechanism of the vinyl

radical with nitrogen atom

Yang Suna,*, Qi-yuan Zhanga, Xi-cheng Aia, Jian-ping Zhanga, Chia-chung Sunb

aThe State Key Laboratory for Structural Chemistry of Unstable and Stable Species, The Center for Molecular Science, Institute of Chemistry,

Chinese Academy of Sciences, Beijing 100080, People’s Republic of ChinabInstitute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China

Received 3 March 2004; accepted 5 August 2004

Abstract

The complex triplet potential energy surface of the C2H3N system is investigated at the UB3LYP and CCSD(T) (single-point) levels in

order to explore the possible reaction mechanism of C2H3 radical with N(4S). Eleven minimum isomers and 18 transition states are located.

Possible energetically allowed reaction pathways leading to various low-lying dissociation products are obtained. Starting from the energy-

rich reactant C2H3CN(4S), the first step is the attack of the N atom on the C atom having one H atom attached in C2H3 radical and form the

intermediate C2H3N(1). The associated intermediate 1 can lead to product P1 CH2CNCH and P23CH2C3HCN by the cleavage of C–H bond

and C–C bond, respectively. The most favorable pathway for the C2H3CN(4S) reaction is the channel leading to P1, which is preferred to that

of P2 due to the comparative lower energy barrier. The formation of P33C2H2C3NH through hydrogen-abstraction mechanism is also

feasible, especially at high temperature. The other pathways are less competitive comparatively.

q 2004 Elsevier B.V. All rights reserved.

Keywords: Potential energy surface; Vinyl radical; Isomers

1. Introduction

The vinyl radical, C2H3, is a reactive intermediate

formed during oxidation of hydrocarbons and played an

important role in the atmospheric and combustion

chemistry. The formation enthalpy of vinyl radical is

300.0G3.3 kJ/mol [1], which is much more active than its

saturated analog C2H5. The kinetics of small C2 radicals

such as vinyl has implicit importance in the atmospheric

chemistry of combustion processes and in ultralow-

temperature chemistry of dense interstellar clouds. It is of

great insignificance to learn the chemical behavior of the

C2H3 radical for environmental protection. So far, the

reactions of the C2H3 with other radicals, atoms and

molecules (H2, C2H2, CH3, CO, O2, NO, O, F, Cl) have

been studied extensively both experimentally and theoreti-

cally [2–12]. N(4S) is the ground state of nitrogen atom,

0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2004.08.019

* Corresponding author. Tel.: C86 1062563101; fax: C86 1062588930.

E-mail address: sunyang@iccas.ac.cn (Y. Sun).

and nitrogen atom reactions have been a subject of interest

among experimental and theoretical chemists for the past

decades. Nitrogen-containing compounds also play inter-

esting and important roles in atmospheric chemistry,

combustion of nitrogen-fuels, and explosion processes.

The reactions of the ground state atomic nitrogen with the

saturated or unsaturated hydrocarbon radicals are of

importance in a series of systems such as the chemistry of

the atmosphere in some planet [13,14]; the chemistry of

interstellar clouds [15–17]; nitrogen chemistry in hydro-

carbon combustion and the ‘active nitrogen’ hydrocarbon

reactions [18,19]. The reactions of the atomic nitrogen with

CH3 and C2H5 have been studied in detail before [20,21].

For the title reaction, the rate constant is determinate as

(7.7G2.9)!10K11 cm3 moleculeK1 sK1 by Payne et al.

[22] at TZ298 K and pressure of 1 Torr He. They also give

the branching ratio for the reaction and indicate that the

CH2CNCH is the primary product. Thorn et al. [23] also

investigate the reaction of C2H3CN(4S), whereas they

observed that CH3CN is the main product. For the C2H3N

system, there are some theoretical investigations, but mostly

Journal of Molecular Structure (Theochem) 686 (2004) 123–130

www.elsevier.com/locate/theochem

Y. Sun et al. / Journal of Molecular Structure (Theochem) 686 (2004) 123–130124

concentrating on the physical chemistry properties such as

formation heat and electron affinity [24–26]. Doughty et al.

[27] also got some isomers of C2H3N system, but these

information is obviously inadequate for learning the detail

mechanism for the C2H3CN(4S) reaction. Thus, a high-

level and detail potential energy surface (PES) is desirable

for the full understanding of the title reaction.

Therefore, due to the importance of the title reaction and

the rather limited knowledge about its reaction mechanism,

we decide to carry out a systematic theoretical study. A

detailed triplet PES is explored by means of density

functional theory (DFT-B3LYP) and coupled cluster

[CCSD(T)] (single-point) methods. The aim in this

investigation is to identify the product distribution and the

exact channels for each of the products and thereby to gain

insight into the reaction kinetics.

2. Computational methods

All computations are carried out using the GAUSSIAN98

program package [28]. The optimized geometries and

harmonic frequencies of the reactant, products, local

minima and transition states are obtained at the UB3LYP/

6-311G(d,p) theory level.

Vibrational frequencies are calculated at the UB3LYP/

6-311G(d,p) level to check whether the obtained stationary

point is an isomer or a first-order transition state. In order to

confirm whether the obtained transition states connect the

right reactants and products, the intrinsic reaction coordinate

calculations are performed at the UB3LYP/ 6-311G(d,p)

level. Single-point calculations are performed at the

CCSD(T)/6-311G(2df,p) theory level at the UB3LYP/6-

311G(d,p) optimized geometries. The UB3LYP/6-311G(d,p)

zero-point vibration energy (ZPVE) is also included. Unless

otherwise specified, the CCSD(T) single-point energies with

Fig. 1. Potential energy curve for the initial attack of the nitrogen atom to C2H3 ra

pointwise optimization of the remaining varied bond lengths or bond angles with e

is constrained as Cs.

inclusion of UB3LYP/ 6-311G(d,p) zero-point energies are

used in the following discussions.

3. Results and discussions

For the title reaction, various dissociation products,

including P1 CH2CNCH, P23CH2C3HCN, P3

3C2H2C3NH,

P43CH2C3HNC, P5 CH3CCN, P6 C2HCNH2 and

P7 CH2NCCH (in Fig. 1), are considered. In Table 1,

total and relative energies including ZPVE of all the seven

products as well as the reactants are listed. Note that the

energy of reactant R is set zero for reference. Eleven

intermediate isomers and 18 transition states are located.

The optimized structures of isomers and the transition

states are given in Figs. 2 and 3, respectively. The energetic

of the isomers and the transition states are listed in Tables 2

and 3, respectively. The symbol TSm/n is used to denote

the transition state connecting the isomers m and n. By

means of the products, intermediate isomers, transition

states and their corresponding relative energies, a sche-

matic PES of C2H3N in triplet is plotted in Fig. 4.

3.1. Initial association

Both singlet and triplet PES are possible for the C2H3N

isomers. And the ground state atomic nitrogen can attack the

C2H3 radicals by two ways: the direct attack on the carbon

atom with only one hydrogen atom attached or the side CaC

p bond attack. Two association adduct will be formed, i.e.

the vinyl nitrene (isomer 1 in our triplet PES) and the cyclic

isomer 2H-azirine (isomer 4 in our triplet PES). The singlet

vinyl nitrene isomer cannot be optimized at the RB3LYP/

6-311G(d,p) level of theory, and all initial configuration

lead to the 2H-azirine isomer at last. In triplet PES, The

direct N attack on the carbon atom can barrierlessly lead to

dical at the UB3LYP/6-311G(d,p) level. The calculations are performed by

very fixed internal C–N bond length. During the optimization, the symmetry

Table 1

Total (a.u.) and relative energies in parentheses (kcal/mol) as well as those including zero-point vibration energies (kcal/mol) of the reactant products for the

C2H3CN reaction

Species UB3LYP/6-311G(d,p) CCSD(T)/6-311G(2df,p) CCSD(T)/6-311G(2df,p)CZPVE

R C2H3CN K132.5255749(0.0) K132.2489698(0.0) 0.0

P1 CH2CNCH K132.6340804(K68.0) K132.3503811(K63.6) K66.9

P23CH2CHCN K132.6164891(K57.0) K132.336505(K54.9) K59.1

P3 C2H2C3NH K132.5917562(K41.5) K132.310237(K38.4) K40.7

P4 CH2CHNC K132.5928601(K42.2) K132.3118656(K39.5) K42.4

P5 CH3CCN K132.5906249(K40.8) K132.3094711(K37.9) K39.1

P6 C2HCNH2 K132.5245526(0.6) K132.2414626(4.7) 1.6

P7 CH2NCCH K132.50126(15.3) K132.2149145(21.4) 18.9

Y. Sun et al. / Journal of Molecular Structure (Theochem) 686 (2004) 123–130 125

adduct C2H3N(1), which can be confirmed by the calculated

pointwise potential curves at the B3LYP/6-311G(d,p) (in

Fig. 5) level of theory. The nitrogen atom can also attack the

CaC double bond of the C2H3 radical, but the energy of

such triplet three-membered isomer (4) is much higher than

that of C2H3N(1). This attacking process involves the

formation of the C–N bond and the activation of CaC

double bond, and it is commonly an energy-consuming

process, so the formation of triplet 2H-azirine is unfavorable

compared to the former attack ways. The energy of the

singlet 2H-azirine adduct is 15.7 kcal/mol lower than that of

the triplet vinyl nitrene at the B3LYP/6-311G(d,p) level of

theory, but no similar barrierless scan potential curves as

Fig. 5 are located for this p bond attacking ways. In

addition, the spin density is dominantly on the carbon atom

(1.027), which is much bigger than the other atoms in C2H3

radical, indicating that this carbon atom is the active site for

the C2H3 radical in the intermolecular reactions. Thus the

most reasonable initial association mechanism is the direct

attack on the carbon atom with one hydrogen atom attached

and form the triplet associated adduct 1.

3.2. Reaction pathways

Except for one pathway of the product P3, all the other

products undertake the addition–elimination mechanism, and

Fig. 2. B3LYP/6-311G(d,p) optimized geometries for reactant and

products. Bond lengths are in angstroms and angles in degrees.

they can be isomerized and dissociated from the initial

associated intermediate 1, so we can discuss the seven products

and their pathways in detail starting from the isomer 1.

Two pathways can be obtained for the formation of

product P1 CH2CNCH as follows:

Path P1ð1Þ : R C2H3 CN/C2H3Nð1Þ/P1

Path P1ð2Þ : R C2H3 CN/C2H3Nð1Þ/CH3CNð2Þ/P1

In the first pathway, P1 can be obtained by the dissociation

of one H atom via the transition state TS1/P1, and the barrier

for this process is 37.4 kcal/mol. The bond length for the

Fig. 3. B3LYP/6-311G(d,p) optimized geometries for C2H3N isomers.

Bond lengths are in angstroms and angles in degrees.

Table 2

Total (a.u.) and relative energies in parentheses (kcal/mol) as well as those including zero-point vibration energies (kcal/mol) of the isomers for the C2H3CN

reaction

Species UB3LYP/6-311G(d,p) CCSD(T)/6-311G(2df,p) CCSD(T)/6-

311G(2df,p)CZPVE

1 K132.6899679(K103.2) K132.3987072(K93.9) K90.5

2 K132.6246255(K62.2) K132.3367528(K55.1) K51.5

3 K132.6750637(93.8) K132.3804684 (K82.5) K78.9

4 K132.607076(K51.1) K132.3170104(K42.7) K39.8

5 K132.5401598(K9.2) K132.2467748(1.4) 2.2

6 K132.5844607(K36.9) K132.2966579(K29.9) K26.2

7 K132.6507256(K78.5) K132.360171(K69.8) K66.4

7 0 K132.6513557(K78.9) K132.3611808(K70.4) K66.8

7 00 K132.653024(K79.9) K132.3623581(K71.1) K67.7

8 K132.6158663(K56.6) K132.3212132(K45.3) K40.8

9 K132.6278225(K64.2) K132.3309935(K51.5) K47.1

Y. Sun et al. / Journal of Molecular Structure (Theochem) 686 (2004) 123–130126

C–H is elongated from 1.1 to 1.889 A. P1 can also be

obtained by the dissociation of one H atom in CH3CN via

TS2/P1, and the barrier is 27.0 kcal/mol, which is lower than

that of Path P1(1), whereas the formation of CH3CN(2) must

overcome a high isomerization barrier of TS1/2, which

involves a 1,3 H-shift process. The formation of such three-

membered transition state is energy-consuming, so this

pathway is unfavorable kinetically. The two pathways are

both energetically feasible, while it is obvious that Path

P1(1) will dominate over the other due to its less reaction

steps and lower barrier.

Only one pathway is located for the formation of product

P23CH2C3HCN.

Path P2 R C2H3 CN/C2H3Nð1Þ/P23CH2 C 3HCN

The C–C bond in the associated intermediate 1 has been

weakened compared with that in the C2H3, which is

elongated from 1.304 to 1.398 A, so the energy-rich

Table 3

Total (a.u.) and relative energies in parentheses (kcal/mol) as well as those includ

C2H3CN reaction

Species B3LYP/6-311G(d,p) C

TSR/P3 K132.5252074(0.2) K

TS1/2 K132.5755756(K31.4) K

TS1/4 K132.5864719(K38.2) K

TS1/7 K132.6044337(K49.5) KTS1/3 K132.6102725(K53.1) K

TS1/P1 K132.6274947(K63.9) K

TS1/P2 K132.6110059(K53.6) K

TS2/6 K132.4974926(17.6) KTS4/5 K132.5367757(K7.0) K

TS3/8 K132.559616(K21.3) K

TS3/7 0 K132.5770249(K32.3) K

TS2/P1 K132.5767272(K32.1) K

TS2/P5 K132.5780971(K32.9) K

TS7 00/8 K132.5454029(K13.5) K

TS6/P5 K132.5668462(K27.8) K

TS7 0/9 K132.570672(K28.3) K

TS7 0/7 00 K132.6110059(53.6) K

TS7/7 0 K132.6218133(60.4) K

intermediate 1 can undergo further dissociation and give

rise to the product P2 through TS1/P2. The C–C bond in

TS1/P2 is 2.291 A, whereas the other bonds such as C–H

and C–N bonds or angles between them is close to those in

CH2 and HCN, so TS1/P2 is a ‘loose’ transition state and can

be described as approximately separate CH2 and HCN. The

barrier for TS1/P2 is 42.6 kcal/mol.

Product P33C2H2C3NH can be formed by the following

two pathways through different mechanism:

Path P3ð1Þ R/P3

Path P3ð2Þ R C2H3 CN/C2H3N1/C2H2NH7/P3

The first pathway is the H-abstraction mechanism, and the

second one is the addition–elimination mechanism. In the

H-abstraction transition state TSR/P3, the angle of :CHN

is nearly 1808, and the CH bond which is to be broken is

only elongated from 1.094 to 1.168 A, while the NH bond to

be formed is 1.629 A. The conformation of TSR/P3 does not

ing zero-point vibration energies (kcal/mol) of the transition states for the

CSD(T)/6-311G(2df,p) CCSD(T)/6-311G(2df,p)CZPVE

132.2375445(7.6) 6.0

132.281498(K20.4) K20.1

132.2953092(K29.1) K23.5

132.3135871(K40.5) K39.9

132.3184876(K43.6) K43.5

132.3393152(K50.7) K53.1

132.3267737(K48.8) K47.9

132.2045336(27.8) 28.8

132.2429143(3.8) 4.6

132.2646945(K9.8) K10.2

132.2809001(K20.0) K20.9

132.2852885(K22.8) K24.5

132.2923947(K27.2) K26.7

132.247048(1.2) K0.7

132.2783942(K18.5) K17.0

132.2773797(K17.8) K16.8

132.3267737(K48.8) K49.3

132.3302044(K51.0) K49.2

Fig. 4. B3LYP/6-311G(d,p) optimized geometries for C2H3N transition

states. Bond lengths are in angstroms and angles in degrees.

Y. Sun et al. / Journal of Molecular Structure (Theochem) 686 (2004) 123–130 127

change much compared with the reactant, and the barrier for

TSR/P3 is 6.0 kcal/mol. In the addition–elimination mech-

anism, intermediate C2H3N(1) can isomerize to isomer 7 via

TS1/7, which is a four-membered transition state with a

barrier of 50.6 kcal/mol. In this energy-consuming process,

one hydrogen atom undergo 1,3 shift to the nitrogen atom on

the same side in C–C bond, then 7 can lead to P3 by the

fission of C–N bond with the dissociation energy of

25.7 kcal/mol.

There is only one pathway located for product P43CH2C3HNC

Path P4 R C2H3 CN/C2H3Nð1Þ/CH2CNHð3Þ/P4

P4 can be formed through the break of the C–N bond in

intermediate CH2CNH(3), the barrier for the isomerization

from 1 to 3 is 47.0 kcal/mol. TS1/3 is a three-membered

transition state involving the shift of hydrogen atom

between the two adjacent atoms. In TS1/3, the bond to be

breached and the bond to be formed are 1.266 and 1.249 A,

respectively, and the energy for the dissociation of isomer 3

to the product P4 is 36.5 kcal/mol.

Two pathways are located for the product P5 CH3CCN:

Path P5ð1Þ R C2H3 CN/C2H3Nð1Þ/CH3CNð2Þ/P5

Path P5ð2Þ R C2H3 CN/C2H3Nð1Þ/CH3CNð2Þ

/CH3NCð6Þ/P5

P5 can be obtained by the breach of the C–C bond and C–N

bond in CH3CN and CH3NC, respectively. The correspond-

ing transition states are TS2/P5 and TS6/P5. Intermediate

CH3CN and CH3NC are resembled to each other in

geometry, and the difference is that the carbon and nitrogen

atoms are in opposite position. The two isomers both

possess Cs symmetry, and CH3CN is more stable than

CH3NC thermodynamically. The barrier for TS2/P5 is

24.8 kcal/mol, whereas the barrier for TS6/P5 is much

lower, which is 9.2 kcal/mol, but the formation of isomer 6

must through the isomerization from isomer CH3CN, and

the barrier for this isomerization is rather high. In TS2/6, the

distance between the carbon atom in methyl with another

carbon atom and nitrogen atom is 1.723 and 1.749 A,

respectively. Such methyl migration process is very energy-

consuming due to the steric hindrance. The barrier for TS2/6

is 80.3 kcal/mol, and it is also 28.8 kcal/mol higher than the

reactant, thus the pathway Path P5(2) is unfeasible both

kinetically and thermodynamically.

There are three pathways for the formation of product P6

C2HCNH2 including:

Path P6ð1Þ R C2H3 CN/C2H3Nð1Þ/CH2CNHð3Þ

/HC2NH2ð8Þ/P6

Path P6ð2Þ R C2H3 CN/C2H3Nð1Þ/CH2CNHð3Þ

/C2H2NHð70Þ/C2HNH2ð9Þ/P6

Fig. 5. The triplet potential energy surface (PES) for the reaction C2H3CN(4S), relative energies are calculated at the CCSD(T)/6-311G(2df,p)//B3LYP/6-

311G(d,p)CZPVE level of theory.

Y. Sun et al. / Journal of Molecular Structure (Theochem) 686 (2004) 123–130128

Y. Sun et al. / Journal of Molecular Structure (Theochem) 686 (2004) 123–130 129

Path P6ð3Þ R C2H3 CN/C2H3Nð1Þ/CH2CNHð3Þ

/C2H2NHð70Þ/ ð700Þ/HC2NH2ð8Þ/P6

The energy of product P6 is a bit higher than the reactant.

Intermediate C2H2NH has three isomers, i.e. 7, 7 0 and 7 00.

The energy of the three isomers is similar, and they can

convert to each other easily with low barrier. Isomer 7 00 is

the lowest energetic isomer. Intermediate 3 can also

isomerize to 8. Similar to TS7 0/9, the hydrogen atom

connected to carbon atom can undergo 1,3 migration to

nitrogen atom on the same side. TS3/8 is also a four-

membered planar transition state, and the corresponding

barrier is 68.7 kcal/mol. TS7 00/8 is a three-membered

structure, the formation of such transition state need high

energy, and this can be proved by the high energy barrier of

67.0 kcal/mol. The energy for isomer 8 and 9 is close, and

the difference between them is that the hydrogen atoms are

connected to different carbon atoms, and they can both give

rise to the product P6 by the rupture of C–N bond, the

dissociation energy is 42.4 and 48.6 kcal/mol, respectively.

We can notice that the pathways leading to the product P6 all

need to overcome high barrier and involve complex steps,

thus the formation of P6 is less favorable kinetically.

The product P7 CH2NCCH can be formed through the

ring open process and the subsequent C–C bond dissociation

of the three-membered intermediate 4, but P7 lies

18.9 kcal/mol higher than the reactants, so P7 can be

neglected for it is unfavorable thermodynamically.

3.3. Reaction mechanism

Through the analysis in Section 3.2, we can notice that

that the product P6 and P7 lies energetically higher than the

reactants, so the pathways leading to them can be ruled out

for the discussion. The pathway Path P5(2) is also excluded

due to the factors mentioned above. The other pathways

such as follows:

Path P1ð1Þ R C2H3 CN/C2H3Nð1Þ/P1CH2CN CH

Path P1ð2Þ R C2H3 CN/C2H3Nð1Þ/CH3CNð2Þ

/P1CH2CN CH

Path P2R C2H3 CN/C2H3Nð1Þ/P23CH2 C 3HCN

Path P3ð1ÞR/P33CH2 C 3NH

Path P3ð2Þ R C2H3 CN/C2H3Nð1Þ/C2H2NHð7Þ

/P33CH2 C 3NH

Path P4 R C2H3 CN/C2H3Nð1Þ/CH2CNHð3Þ

/P43CH2 C 3HNC

Path P5ð1Þ R C2H3 CN/C2H3Nð1Þ/CH3CNð2Þ

/P5CH3 CCN

are below the reactants energetically, so they all can

contribute to the final products to some extent. By means of

these feasible pathways, let us discuss the possible

mechanism of the title reaction. Noticing that except for

Path P3(1), the common intermediate 1 are involved in the

other pathways, so some qualitative conclusions can be

drawn about the distributions of these products. As shown in

Fig. 4, the lowest dissociation or isomerization barrier

starting from isomer 1 is TS1/P1, which is 37.4 kcal/mol. In

addition, the product P1 obtained by this pathway is also the

most energetic low-lying product, so P1 is the most

favorable product both kinetically and thermodynamically.

The second pathway Path P1(2) is much less competitive

compared to Path P1(1) and would not play an important

role in the formation of P1. P2 can also be obtained from

intermediate 1 via transition state TS1/P2, and the

corresponding barrier is 42.6 kcal/mol, only 5.2 kcal/mol

higher than TS1/P1, so Path P2 and Path P1(1) can compete

with each other. The pathway in addition–elimination

mechanism for formation of product P3 is unfavorable

kinetically due to the high barrier as discussed above,

whereas the barrier for the pathway in hydrogen-abstraction

mechanism is only 6.0 kcal/mol, which is feasible for the

intermolecular hydrogen transition, and this channel will

play an important role for the title reaction at high

temperature and vice versa, can be neglected at low

temperature. The barrier for TS1/3 in the Path P4 is

9.6 kcal/mol higher than that of TS1/P1, for such three-

membered transition state is not very easy to be attained, so

product P4 will be less competitive compared to P1, P2 and

P3. The formation of P5 is also by the isomerization of

CH3CN, although CH3CN is stable energetically, but the

formation of it must undergo a 1,2 H-shift migration

process, and this is a very energy-consuming mechanism,

the barrier for it is 70.4 kcal/mol, so P5 is also a less

competitive product kinetically.

In the study of Payne et al. [22], the branching ratio for

the product CH2CNCH is 0.8, which is in agreement with

our calculation result that the P1 is the mostly kinetically

and thermodynamically preferable product. The branching

ratio for the product C2H2CNH is 0.16, which is in

coincidence with the hydrogen-abstraction mechanism of

our calculation. They also verify the branching ratio for the

isomer CH3CN is 0.04. Through the analysis above, the

formation for CH3CN is difficult kinetically in triplet PES.

The energy of singlet CH3CN is about 100 kcal/mol lower

than the triplets CH3CN, athough it has high energetic

stability but the direct formation of the singlet CH3CN is

unfeasible by the triplet mechanism, so we infer maybe a

part of CH3CN is obtained by the secondary reaction of

CH2CN with the H atom. The heat formation by our

calculation is also in good agreement with the experiment

data given by their paper. Notice that the product P23CH2C3HCN is not observed in the experiment, and this

perhaps due to the triplet carbene radical is so active that it

cannot keep stable and undergo further reactions rapidly.

Y. Sun et al. / Journal of Molecular Structure (Theochem) 686 (2004) 123–130130

We expect that the product CH2CHCN can be detected in

future experimental detection.

It is also worthy to compare the mechanism of the similar

reaction of CH3C4N with the title reaction. In the quantum

chemistry, investigation carried out by Hadjebar et al. [29],

they also identify that the reaction go through the triplet

PES, and the most favorable pathway is also the first

formation of the adduct 3NCH3 with subsequently hydrogen

dissociation and at last give rise to the product H2CNCH.

The barrier for the Path P1(1) (37.4 kcal/mol) is very close

to the activation energy of 3NCH3, which is 37 kcal/mol

obtained at high-level of theory (MPW1PW91). All this

indicated that the PES calculated by us is reliable and

reasonable.

4. Conclusions

A detailed triplet PES of the C2H3CN(4S) reaction

system is built up at the B3LYP and CCSD(T) (single-point)

level of theory. For the title reaction, the initial step is the

attack of N(4S) on the carbon atom having one H atom

attached in C2H3 radical and form the energy-rich triplet

intermediate C2H3N. The intermediate C2H3N can undergo

the dissociation of the hydrogen atom and give rise to the

primary product P1 CH2CNCH. This pathway is the most

favorable channel for the title reaction both thermodynami-

cally and kinetically. The intermediate C2H3N can also lead

to product P23CH2C3HCN by the cleavage of C–C bond,

this pathway is less competitive compared to that of P1. The

formation of P33C2H2C3NH through hydrogen-abstraction

mechanism is a feasible pathway and would be an important

channel at high temperature. The possibility for the

formation of CH3CN is small, and the pathways leading to

other products can be neglected at normal conditions. We

hope our calculated results may provide some useful

information for understanding the reaction mechanism of

the unsaturated hydrocarbon with the nitrogen atom.

Acknowledgements

This project was supported by The National Natural

Science Foundation of China (20133020, 39890390, and

20073014) and The State Key Basic Research and

Development Plan (G1998010100).

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