Chemical Physics Letters 599 (2014) 15–22

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Calculations on the complex mechanism of the HCNO + OH reaction

http://dx.doi.org/10.1016/j.cplett.2014.03.0010009-2614/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: hue.nguyen@hnue.edu.vn (H.M.T. Nguyen).

Hue Minh Thi Nguyen a,⇑, Trong Nghia Nguyen b

a Center for Computational Science and Faculty of Chemistry, Hanoi National University of Education, Hanoi, Viet Namb School of Chemical Engineering – Hanoi University of Science and Technology, Hanoi, Viet Nam

a r t i c l e i n f o

Article history:Received 27 October 2013In final form 3 March 2014Available online 11 March 2014

a b s t r a c t

Complex reaction mechanism of HCNO + OH were investigated at the CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level. The results show that this reaction has four main entrance channels. From initialadducts sixteen different pathways lead to products. The most favorable channel involves the barrierlessentrance isomer (HC(OH)CNO) which is formed by OH adding to C atom. Products of (CO + NHOH) and(CHO + HNO) are the most favorable, (H2O + NO), (H2 + OCNO), (H2 + CO + NO), (CO + NH2O) are minorpathways. Heats of reaction are predicted at the CCSD(T)/CBS level. Our results are in good agreementwith available experiment.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Fulminic acid, HCNO, has been known as an important interme-diate in NO-reburning processes for the reduction of NOx pollu-tants from fossil-fuel combustion emissions [1–3]. It was foundthat CH2 þNO! HCNOþH is a key reaction for the formation offulminic acid. Recently, HCNO was detected in dark clouds by usingthe IRAM 30 m telescope (Granada, Spain) and two 3 mm super-conductor–insulator–superconductor (SIS) receivers workingsimultaneously at the same frequency, but with orthogonal polar-izations [4]. Therefore, knowledge of the subsequent chemistry ofHCNO is of interest in atmospheric and combustion chemistry.Up to now, several HCNO reactions have been investigated eitherexperimentally or theoretically [1–11]. The OH radical is the mostimportant oxidant in the troposphere, the lowest part of the atmo-sphere (below about 10 km). Crutzen coined the term ‘detergent ofthe atmosphere’ to describe this important cleansing role of OH.Most of the trace gases found in the troposphere are oxidized byOH into water-soluble products that are washed out by rain andsnow. Specifically, OH is responsible for oxidizing carbon monox-ide (CO) and other carbon-based molecules [12].

The HCNO + OH reaction, which is identified as a crucial step,has been investigated by many research groups [1–3]. In an exper-imental study carried out by laser-induced fluorescence and IRdiode laser absorption spectroscopy, Feng et al. [1] reported thatthe reaction is fast, with k = (3.39 ± 0.3) � 10�11 cm3 molecule�1

s�1 at 296 K, and has a moderate, negative temperaturedependence. The product branchings of the HCNO + OH reactionwere also studied:

OHþHCNO! H2Oþ NCO ð3aÞ! NH2 þ CO2 ð3bÞ! HNOþHCO ð3cÞ! HCNþHO2 ð3dÞ! CH2Oþ NO ð3eÞ! COþH2NO ð3fÞ! COþH2 þ NO ð3gÞ

On the basis of detection and measurements of CO, H2CO, NO,and HNO products, the authors concluded that CO + H2NO andHNO + HCO are the major product channels, with a minor contri-bution from H2CO + NO.

The potential energy surface (PES) of this reaction has been pre-viously studied using computational methods [2,3]. One study pre-dicted a low-energy pathway to H2 + CO + NO products, channel 3g[2]. But experimental observation of very low NO yields suggestedthat this is not a major channel. In addition, the study did not coverthe channel 3f (CO + H2NO) which is the major product channel.

In another study of this reaction, CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) + ZPVE computational methods were used [3]. How-ever, according to the PES, to form the main product P3 (HNO + H-CO), reactants have to overcome transition state with relatedenergy of �8.2 kcal/mol which is higher than some other path-ways, such as P7 (CO + H2 + NO) which the experimental studyconcluded not to be main pathway because of observation of verylow NO yields [1]. Furthermore, the product channel (CH2O + NO)has a very high barrier with relative energy of 24.2 kcal/mol abovereactants. This barrier is also higher than the barriers to form mostproducts, such as NCO with the barrier of 11.2 kcal/mol above reac-tants. But experimental results indicated that the product(NO + CH2O) is a minor pathway mean while formation of NCO is

16 H.M.T. Nguyen, T.N. Nguyen / Chemical Physics Letters 599 (2014) 15–22

at most a very minor pathway in this reaction [1]. The presentstudy uses a larger basis set in an attempt to resolve these appar-ent discrepancies.

2. Computational methods

The geometries of the reactants, transition structures and prod-ucts for OH with HCNO reactions were initially optimized by usingdensity functional theory (DFT) with the hybrid B3LYP functionaland the 6-311++G(3df,2p) basis set [13–16]. Frequencies were cal-culated at the same level to check whether the obtained stationarypoint was an local isomer or a first-order transition state. The sta-tionary points were identified for local minima or transition statesaccording to vibrational analysis in which the reactants, intermedi-ates, and products possessed all real frequencies, whereas a transi-tion state had one and only one imaginary frequency. Transitionstate geometries were then used as an input for IRC calculationsto verify the connectivity of the reactants and products.

For more accurate evaluation of energies for all the species, weused higher level single-point energy calculations with the B3LYPoptimized geometries at the coupled-cluster theory CCSD(T)/6-311++G(3df,2p) level [17]. The relative energies presented in thePES’s have been corrected for zero-point vibrational energies(ZPVE, scaled). For the barrierless steps (RA ? IS1 andIS10 ? PR14), we performed scanned procedures by calculating

Figure 1. Optimized geometries of the reactants and intermediate states of the HCNO +letters given within parentheses are the point groups).

the potential energy curves at the B3LYP/6-311++G(3df,2p) leveltheory along their reaction coordinate from their equilibrium at astep size of 0.1 ÅA

0

. To confirm to reliability of the method employed,heats of reaction for HCNO + OH are calculated at the three levels:B3LYP/6-311++G(3df,2p), CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p), CCSD(T)/CBS//B3LYP/6-311++G(3df,2p) and com-pared with available experimental data [18,19].

The CCSD(T)/CBS energies were also evaluated at these geome-tries [20]. These calculations have been performed using the GAUS-

SIAN 03 software package [21].

3. Results and discussion

3.1. Potential energy surface of the OH + HCNO reaction

The optimized geometries of the reactants, important interme-diates, and some important transition states are presented in Fig-ures 1 and 2 and the optimized geometries of other transitionstates and intermediates are presented in Figure 3S, the optimizedgeometries of the products are presented in Figure 4S of the Sup-plementary Information (ESI). Reaction channels of the HCNO + OHreaction (kcal/mol) related to experiment are shown in the Figure3a, the minor channels are in Figure 3b, other important reactionchannels of OH + HCNO potential energy surface are simplifiedand shown in Figure 4 and the detailed scheme of PES including

OH reaction using B3LYP/6-311++G(3df,2p). (Length in Å and angle in degree. The

Figure 2. Optimized geometries of the products of the HCNO + OH reaction using B3LYP/6-311++G(3df,2p). (Length in Å and angle in degree. The letters given withinparentheses are the point groups).

H.M.T. Nguyen, T.N. Nguyen / Chemical Physics Letters 599 (2014) 15–22 17

other product channels are available in Figure 5S of the ESI. Thescheme of the HCNO + OH reaction is presented in Figure 6S inwhich the symbol Tx/y (T1/9, T10P11, TP11P15, . . .) is used to de-note the transition state connecting the isomers ISx (IS1, IS2, . . .) orPRx (PR10, PR11) and ISy or product PRy (PR1, PR2, . . .). Thus, T1/9is the transition state connecting both IS1 and IS9; T10P11 con-necting the IS10 and PR11; . . .

Figure 5 shows the barrierless energy paths that were obtainedby calculating the potential energy curves. Theoretical predicationof related energies DE (kcal/mol) for reactants, intermediates, maintransition states, and products of the OH + HCNO reaction in differ-ent levels of theory in Table 1. Table 2 shows a comparison of cal-culated heats of reaction for HCNO + OH with availableexperimental data. Table 3S of the ESI shows variations of Gibb freeenergies (DG) and entropies (DS). Table 4S lists frequencies of thespecies considered, Table 5S their coordinates and Table 6S theo-retical predication of single point energy (a.u.), ZPVE (a.u.), andfor reactants, intermediates, main transition states, and products

of the OH + HCNO reaction two different levels. Tables 3S, 4S, 5Sand 6S are available in the ESI.

There are four reaction channels involving an initial attack ofOH group to H, C, N and O atoms in HCNO molecule: direct attackwithout an intrinsic transition state, which is confirmed by varia-tional transition state computations along the minimum energypath (VTSTMEP) scan (Figure 5) to the only C atom to form inter-mediate IS1 (HC(OH)NO) with a relative energy of �49.3 kcal/mol. This value is in good agreement with �48.3 kcal/molpredicted by Wang et al. [3] using CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) method as well as with that of �49.9 kcal/mol pre-dicted by Miller et al. [2].

For the attack of OH group to N atom, the reactants have toovercome a barrier T0/11 to form IS11 (HCN(OH)O) with relativeenergy of 24.3 and 17.2 kcal/mol above reactants, respectively.The transition state T0/11 has only one imaginary frequency at517i cm�1 corresponding to an association of O atom in OH groupand N atom in HCNO molecule. The bond length HO� � �N is 1.729 Å

Figure 3a. The potential energy surface of the HCNO + OH reaction (kcal/mol) related to the experimental results calculated at the CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) + ZPVE level. (Length in Å and angle in degree).

-50.0

E (kcal/mol)

RA

0.0

H-CNO+OH

IS2

IS8

IS10

-30.3

-48.0

-66.9

T1/2

-2.0

T2/811.7

T2/96.4

T8/9

-0.5

T1/10

-22.3T9/10

-12.1

-18.6

13.4

T9P10

T10P15

PR7

PR10

PR13PR15

-13.7

-29.4

-58.2-61.6HC(OH)NO

HOCNOH

OCNHOH

OC(H)N(H)O

-26.1

TP10P13

(CO+NH2O)

(CO+NO+H2)

(H2+OCNO)

(OCNOH+H)

IS9

-55.8

OC(H)NOH

-49.3

IS1 T1/9

-48.7

Δ

T8/10

-4.3-8.0

T9P7

Figure 3b. Simple illustration of the potential energy surface of the HCNO + OH reaction related to minor product channels calculated at the CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) + ZPVE level. (Length in Å and angle in degree).

18 H.M.T. Nguyen, T.N. Nguyen / Chemical Physics Letters 599 (2014) 15–22

which is in good agreement with the bond length in transitionstate, and the structure of HCNO changed corresponding the form-ing of a transition state, such as the angle \CNO reduces from180.0� at HCNO molecule to 144.7� at the transition state T0/11and is equal 130.0� in the intermediate IS11 due to formation N–OH bond.

IS11 is the unique species in the PES which the single-point en-ergy calculation at the CCSD(T)/6-311++G(3df,2p) level of theorycannot be performed. Therefore, the total and relative energies ofIS11 are calculated at B3LYP/6-311++G(3df,2p) level of theory.But it is not important because this entrance has a very high energy(Figure 5S). This is similar to the reaction of OH with an isomer ofHCNO, isofulminic acid, HNCO. In which, the products related tothe addition of OH to N is very minor because of the highest barrier

[22]. For two other atoms, the association reaction and hydrogenabstraction occur via the intermediate IS7 lying 29.7 kcal/molabove reactants and product PR11 with relative energy of�14.1 kcal/mol via T0/7 and T0P1 lying 31.0 and 8.7 kcal/molabove reactants, respectively. Both of two transition states haveunique imaginary frequencies at 468i and 1154i correspondingassociation of O atom in OH group and O atom in HCNO moleculeand migration of H atom from HCNO to OH radical. The bondlengths and angles of T0/7 and T0P1 are in harmony with thoseof transition states. The barrier height at T0/7 and T0P1 agree withthose of 30.5 and 7.3 kcal/mol calculated by Miller et al. [2].

Then, atoms in these three intermediates can migrate to formother intermediates or can undergo dissociation to give products.For example in IS1, the H atom in the OH group can migrate to

(a) (b)R(C-O) (angstrom)

2.5 2 1.5

Ener

gy (k

cal/m

ol)

55 50 45 40 35 30 25 20 15 10

5 0

R(C-N) (angstrom) 3.5 3 2.5 2 1.5

Ener

gy (k

cal/m

ol)

65 60 55 50 45 40 35 30 25 20 15 10 5 0

Figure 5. The dissociation curves with respect to the dissociation of C–O bond in isomer IS1 ? RA (a), and C–N bond in isomer IS10 ? PR14 (b) at the B3LYP/6-311++G(3df,2p) level. The origin of the energies are �244.499109082 a.u. (a), and �244.523713943 a.u. (b) (Length in Å and energy in kcal/mol).

30.0

-50.0

E (kcal/mol)

RA0.0

-100.0

H-CNO+OH

IS2

IS4

IS6

IS8

IS7

IS5

IS10

IS12-30.3

-18.8

-89.9-93.2

29.7

-48.0

-66.9

-23.4

T0/731.0

T1/2-2.0

T2/811.7

T2/96.4

T8/9-0.5

T1/10-22.3

T1/1216.3

T2/1225.7

T9/10-12.1

T4/5-12.5

T4/1210.68.7

T0P1

-18.6

-4.4

-33.2

13.4

31.1

-1.4

-82.4

T5/6

T2P6

T9P9

T8P12

T10P15

T7P16

T1P4

T6P5

PR1

PR4

PR5

PR6

PR9

PR12

PR15

PR16

-14.1

-2.6

-100.5

-43.9

-76.3

-68.9

-61.6

-14.2

HC(OH)NO

HOCNOH

cy_HO-CN(H)O

OC(OH)NHOC(O)NH2

HCNO-OH

OCNHOH

OC(H)N(H)O

HO-CN(H)O

-59.1IS9

-55.8

OC(H)NOH

-49.3IS1 T1/9

-48.7

Δ

T8/10-4.3

Figure 4. Simple illustration of the potential energy surface of the HCNO + OH reaction related to other product channels calculated at the CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) + ZPVE level (Length in Å and angle in degree).

H.M.T. Nguyen, T.N. Nguyen / Chemical Physics Letters 599 (2014) 15–22 19

the O atom in NO group via T1/9 (�48.7 kcal/mol) to form IS9(OC(H)NOH, �55.8 kcal/mol), or to the N atom via T1/10(�22.3 kcal/mol) to form IS10 (OC(H)N(H)O, �66.9 kcal/mol), orto the C atom via T1P2 (16.4 kcal/mol) to be followed by N–C bondrupture yielding PR2 (CH2O + NO: �54.9 kcal/mol), and so on (seeFigures 3, 4 and 5S). After such an exhaustive search, we haveestablished a potential energy surface with 16 product channelsfrom PR1 to PR16 in which many of them have low barrier.

Therefore, the reaction is fast which is in good agreement withexperimental result in which k = (3.39 ± 0.3) � 10�11 cm3

molecule�1 s�1 at 296 K [1]. The PES shows that the majorpathways are PR11, PR14 producing CO, HCO and HNO; the minor

pathways are PR2, PR7, PR10, PR13, PR15 leading to H2, NO, H,CH2O and others are very minor products (Figure 5S).

3.1.1. Formation of CO main product, PR11 (CO + NHOH)There are other pathways giving PR11: first, OH can attack

directly to the C atom of HCNO to form IS1, trans–trans-HC(OH)NO,with a large exothermicity (�49.3 kcal/mol), to be followed by Hmigration from the OH group to the N atom in NO group via four-centered transition state T1/10 in which the unique image frequencyis 1767i, the relative energy is�22.3 kcal/mol and the bond lengthsH–N, H–O are 1.400 and 1.297 Å, respectively, corresponding to themigration of the H atom from O to N to form IS10 (OC(H)N(H)O) with

Table 1Theoretical predication of related energies DE (kcal/mol) for intermediates, transition states, and products of the OH + HCNO reaction two different levels.

Species DE (kcal/mol) DE (kcal/mol) Species DE (kcal/mol) DE (kcal/mol)B3LYP/6-311++G(3df,2p) CCSD(T)/6-311++G(3df,2p) B3LYP/6-311++G(3df,2p) CCSD(T)/6-311++G(3df,2p)

RA (HCNO + OH) 0.0 0.0 T8P11 �34.6 �36.6IS1 �50.6 �49.3 T10P15 10.6 13.4IS2 �29.3 �30.3 T10P11 �23.1 �23.2IS3 �35.8 �41.3 T2P1 16.8 16.5IS4 �13.4 �18.8 T2P7 �0.5 2.9IS6 �94.6 �93.2 T7P16 30.6 31.1IS5 �87.5 �89.9 T9P9 �6.0 �4.4IS7 30.3 29.7 T9P7 �7.3 �8.0IS8 �47.5 �48.0 T1/3 �21.8 �22.9IS9 �54.7 �55.8 T3/5 5.2 1.8IS10 �67.5 �66.9 T4/5 �8.6 �12.5IS11 17.2 — T4/12 9.6 10.6IS12 �23.1 �23.4 T8/10 �7.9 �4.4T0P1 1.3 8.7 T9/10 �11.9 �12.1T1P3 15.2 14.4 T5/6 �57.8 �59.1T1P2 15.8 16.4 T6P5 �82.5 �82.4T2P8 23.8 26.3 T1P4 �0.7 �1.4T2P6 �19.6 �18.6 T12/10 10.4 11.5T0/7 28.5 31.0 T11/9 50.4 50.8T9P10 �21.6 �18.6 PR1 (CNO + H2O) �13.2 �14.1T10P2 �10.8 �7.4 PR2 (CH2O + NO) �49.8 �54.9T10P13 2.8 5.6 PR3 (OCHNO + H) 11.3 2.7T1/2 �1.7 �2.0 PR4 (CHOH + NO) 3.0 �2.6T0/11 19.8 24.3 PR5 (CO2 + NH2) �97.5 �100.5T1/9 �48.2 �48.7 PR6 (HOCN + OH) �39.0 �43.9T2/8 13.0 11.7 PR7 (OCNOH + H) �10.9 �13.7T2/9 5.7 6.4 PR8 (HOCNO + H) 21.0 18.8T3/4 26.7 22.2 PR9 (NCO + H2O) �75.0 �76.3T8/9 �0.1 �0.5 PR10 (H2 + OCNO) �32.7 �29.4T1/10 �24.4 �22.3 PR11(CO + NHOH) �48.6 �54.2T1/12 13.7 16.3 PR12 (OCNH + OH) �68.0 �68.9T2/12 23.5 25.7 PR13 (CO + H2 + NO) �48.8 �58.2T8/10 �7.9 �4.3 PR14 (CHO + HNO) �10.6 �15.1T9/10 �11.9 �12.1 PR15 (CO + NH2O) �58.4 �61.6T8P12 �37.5 �33.2 PR16 (HCN + HO2) �11.7 �14.2

Table 2Comparison of calculated heats of reaction for HCNO + OH with available experimental dataa.

Species B3LYP/6-311++G(3df,2p) CCSD(T)/6-311++G(3df,2p) CCSD(T)/CBS Reference b

(kcal/mol) (kcal/mol) (kcal/mol) (kcal/mol)

PR1 (CNO + H2O) �13.2 �14.1 �14.3 –PR2 (CH2O + NO) �49.8 �55.4 �54.0 �55.4b

PR3 (OCHNO + H) 11.3 2.3 4.5 –PR4 (CHOH + NO) 3.0 �3.0 �2.0 –PR5 (CO2 + NH2) �97.5 �100.9 �100.0 �98.9b

PR6 (HOCN + OH) �39.0 �44.1 �44.6 �44.0b

PR7 (OCNOH + H) �10.9 �13.9 �13.1 –PR8 (HOCNO + H) 21.0 18.6 18.5 –PR9 (NCO + H2O) �75.0 �76.5 �76.9 �76.7b

PR10 (H2 + OCNO) �32.7 �29.3 �27.6 –PR11 (CO + NHOH) �48.6 �54.6 �54.1 –PR12 (OCNH + OH) �68.0 �69.2 �69.5 �68.6b

PR13 (CO + NO + H2) �48.8 �57.1 �54.4 �55.9b

PR14 (CHO + HNO) �10.6 �15.3 �14.2 �15.2b

PR15 (CO + NH2O) �58.4 �61.8 �61.3 �59.2c

PR16 (HCN + HO2) �11.7 �14.6 �15.2 �16.5b

a The exothermicity for the formation of HCNO + OH was calculated on the basis of the experimental heats of formation at 0 K.b From Refs. [20,21].c From Ref. [1].

20 H.M.T. Nguyen, T.N. Nguyen / Chemical Physics Letters 599 (2014) 15–22

relative energies of �66.9 kcal/mol. Then, IS10 can easily decom-pose, giving PR11 (CO + NHOH; �54.2 kcal/mol) via four-centeredtransition state T10P11 in which the relative energy is �23.2 kcal/mol, the unique image frequency is 1347i and the bond lengths H–C, H–O and N– are 1.345, 1.302 and 1.927 Å, respectively. This pro-cess is easy because the first step RA ? IS1 is barrierless and highlyexothermic, and the next steps IS1 ? IS10 ? PR11 proceed via the

transition state T1/10 and T10P11 which lie far below the reactantswith relative energy of�22.3 and�23.2 kcal/mol, respectively. Thisvalue is in good agreement with �21.4 and �24.2 kcal/mol and thefirst step RA ? IS1 is barierless from Wang et al. [3]. In addition,PR11 can also be produced by some other ways: e.g., OH attackingthe C atom in HCNO barrierlessly giving IS1, which isomerizes tointermediate IS2 (HOCNOH: �30.3 kcal/mol) via T1/2 (�2.0 kcal/

H.M.T. Nguyen, T.N. Nguyen / Chemical Physics Letters 599 (2014) 15–22 21

mol) in which H atom migrate from C atom to O atom in NO group ofIS1. Then, IS2 can transform to IS9 (OC(H)NOH: �55.8 kcal/mol)through transition state T2/9 (6.4 kcal/mol) in which H atom mi-grates from O atom to neighboring C atom, and IS9 can easily trans-form to IS10(OC(H)N(H)O: �66.9 kcal/mol) via T9/10 (�12.1 kcal/mol); or, H (in the OH group of IS1) can undergo migration via T1/9 (�48.7 kcal/mol) to form IS9, then, isomerizes to intermediateIS10, which finally produces PR11. In fact, IS1 and IS9 have somestructures which the relative energies are very closely together(<4.0 kcal/mol) and it can easily overcome low barrier (only20 kcal/mol) to form the other structure involving rotation of OHand NO groups. Because of the close relative energies and the com-plex PES, the process IS1 (trans–trans) ? trans–cis ? IS9 (cis) ? IS9(trans) can be regarded as a one step process IS1 ? IS9 as we dis-cussed above (see Figures 3a and b). It is clear that these processescannot be competitive comparing with the process described abovebecause T1/2 is much higher than T1/10. Accordingly, the main path-way yielding the PR11 is the following: RA ? IS1 ? IS10 ? PR11.

In the experimental study, Feng et al. were based on measure-ments of CO and NO, and concluded that (CO + H2NO) is the majorproduct channel, meanwhile (H2 + CO + NO) is not a main channel.However, that study did not cover the channel PR11 (CO + NHOH).Our PES reveals that (CO + NH2O) has the barrier with rather highrelative energy of �10.0 kcal/mol which is higher than the barrierwith relative energy of�18.6 kcal/mol giving (H2 + CO + NO), mean-while the channel PR11 (CO + NHOH) is the lowest one. So that, wewould conclude that PR11 (CO + NHOH) is the main pathway, andCO is one of the main products which is in good agreement withthe experimental results from the Feng database.

3.1.2. Formation of HNO main product, PR14 (HCO + HNO)PR14 is mainly produced when OH attacks directly into the C

atom of HCNO to form IS1 (trans–trans-HC(OH)NO: �49.3 kcal/mol). Then, H atom from the OH group migrate to the N atom inNO group via four-centered transition state T1/10 (�22.3 kcal/mol) to form IS10 (OC(H)N(H)O: �66.9 kcal/mol) to be followedby N–C bond rupture giving PR14 (HCO + HNO; �15.1 kcal/mol):RA ? IS1 ? IS10 ? PR14. As we discussed above, RA ? IS1 ? IS10is a easy process with a large exothermicity. The rich energy sub-process can help the next sub-process IS10 ? PR11 to go forwardthe product. In this step, the PES of Wang et al. [3] was constructedby CCSD(T)/6-311G(d,p)//B3LYP6-311G(d,p) computations and re-sulted in a transition state lying �8.2 kcal/mol which is ratherhigher than transition state to form other products such as(CO + NHOH), or (CO + NO + H2). Our CCSD(T)//B3LYP/6-311++G(3df,2p) PES shows that this is a barrierless step which isconfirmed by VTST MEP scan (Figure 5). So, PR14 can be competi-tive with the main pathway PR11, although PR14 (�15.1 kcal/mol)lies slight higher than T10P11 (�23.2 kcal/mol), the transition stateto form PR11 which is the lowest pathway because it has a loosevariational transition state. This is similar to HCOOH + O(1D) sys-tem in which PR8 (OCOH + OH) (�54.4 kcal/mol), is produced fromIS8 (HO–CO–OH: �162.7 kcal/mol) by barrierless step, is slighthigher than T8P6 (�119.6 kcal/mol), the transition state connectsthe IS8 and the lowest product PR6 (CO2 + H2O: �170.1 kcal/mol). Theoretical and experimental kinetic results both of themare main pathways. So, here, PR14 is also one of the main path-ways. Therefore, HCO, HNO are main products which are in goodagreement with the experiment by reported by Feng et al. [1].

3.1.3. Formation of NO, CH2O minor products, PR2 (CH2O + NO), PR4(CHOH + NO), PR10 (H2 + CONO), PR13 (H2 + CO + NO) and PR15(CO + NH2O)

PR2 can also be produced by different pathways. Firstly, RA(HCNO + OH) forms IS1 (HC(OH)CNO: �49.3 kcal/mol) to be fol-lowed by isomerization to form IS10 (OC(H)N(H)O: �66.9 kcal/

mol) as discussed. Then, H atom in NH group of IS10 migrates toC atom via three-centered transition state T10P2 and this is fol-lowed by N–C bond rupture giving PR2 (CH2O + NO: �54.9 kcal/mol). In the T10P2, the unique negative frequency is 833i, the rel-ative energy is �7.4 kcal/mol and the bond lengths H–N, H–C andN–C are 1.124, 1.611 and 1.932 Å, respectively, which are in goodagreement with the transition state corresponding the migrationof the H atom and dissociation of the N–C bond. Or, H atom inHCO group of IS1 can easily migrate to O atom of NO to yield IS9(OC(H)NOH: �55.8 kcal/mol) via T1/9 (�48.7 kcal/mol). Then, Hatom can migrate from O atom to neighboring N atom via T9/10with rather high relative energy of �12.1 kcal/mol to give IS10(OC(H)N(H)O: �66.9 kcal/mol). This step is followed by productionof PR2 (see Figures 3a and b). Alternatively, in IS1, H atom migratesfrom O to neighboring C atom and this is followed by C–N bondrupture via T1P2 in which the relative energy is 16.4 kcal/molabove the reactants, etc. (Figure 5S). Accordingly, the main path-way yielding the PR2 is: RA ? IS1 ? IS10 ? PR2. In the previoustheoretical study [3], the reported PES showed that these productshave to overcome high barrier with relative energy of 24.4 kcal/mol above reactants, because they did not consider this pathwayas discussed. This barrier is higher than the barrier to form NCOwhich experimental results indicated that its formation is at mosta very minor pathway in this reaction [1]. Our study reveals thatthe highest barrier of this pathway is through the TS T10P2 withrelative energy of only �7.4 kcal/mol which is lower than the bar-rier to form NCO, �4.4 kcal/mol. However, PR2 is still a minor path-way because the barrier is rather higher than T10P11 with relativeenergy of �23.2 kcal/mol giving PR11 (Figures 3a and b). So that,CH2O and NO are minor products which are in good agreementwith the experiment by Feng.

PR4 can be produced by barrierless association of the reactantsforming IS1 (HC(OH)NO). IS1 can dissociate the C–N bond via T1P4(�1.4 kcal/mol) to produce PR4 (�2.6 kcal/mol): RA ? IS1 ? PR4.It is clear that this pathway is less favorable because it has to over-come the transition state T1P4 which is located higher than T10P2.

PR10 is mainly produced when RA (HCNO + OH) forms IS1(HC(OH)CNO: �49.3 kcal/mol) and the latter is followed by Hmigration from OH group to O atom in NO group to yield IS9(OC(H)NOH: �55.8 kcal/mol) through T1/9 (�48.7 kcal/mol). Then,via T9P10 (�18.6 kcal/mol), IS9 is decomposed into two parts, H2

and OCNO denoted as PR10 (H2 + OCNO: �29.4 kcal/mol):RA ? IS1 ? IS9 ? PR10.

PR13 (�58.2 kcal/mol) is produced due to decomposition ofOCNO in PR10 via TP10P13 (�26.1 kcal/mol) in which H2 moleculekeeps unchanged. It is obvious that the transition state TP10P13associated with PR10 and PR13 are identical with those of transi-tion state OCNO/(CO + NO) associated OCNO and (CO + NO):RA ? IS1 ? IS9 ? PR10 ? PR13.

PR15 (CO + NH2O) is mainly produced from the PR11 when theH in OH group of the NHOH fragment undergoes 1,2-shift to formNH2O. Thus, PR11 can directly be transformed through TP11P15locating at �10.0 kcal/mol below the reactants to yield PR15(CO + NH2O: �61.6 kcal/mol): RA ? IS1 ? IS10 ? PR11 ? PR15.

It is clear that these pathways cannot be also competitive withthat involving PR11 because of the higher activation energy. As aconsequence, PR10 and PR13 are minor pathways.

3.1.4. Formation of other products, PR1 (CNO + H2O), PR3(OCHNO + H), PR5 (CO2 + NH2), PR6 (HOCN + OH), PR7 (OCNOH + H),PR8 (HOCNO + H), PR9 (NCO + H2O), PR12 (OCNH + OH), PR16(HCN + HO2)

When reactants associate to each other forming IS1 (HC(OH)-NO), its can undergoes isomerization to form IS9 (OC(H)NOH).Then, H atom in CH group of IS9 can associate with OH group viaT9P9 (�4.4 kcal/mol) to yield PR9 (NCO + H2O: �76.3 kcal/mol):

22 H.M.T. Nguyen, T.N. Nguyen / Chemical Physics Letters 599 (2014) 15–22

RA ? IS1 ? IS9 ? PR9. T9P9 with related energy of �4.4 kcal/molis much higher energy than T10P2 with related energy of�7.4 kcal/mol in minor pathway PR2. So, pathway PR9 is lessfavorable than PR2 which is in good agreement with experimentresults of Feng et al.

Similarly, other products are forming as follows: RA ? PR1;RA ? IS1 ? PR3; RA ? IS1 ? IS12 ? IS4 ? IS5 ? IS6 ? PR5; RA ? I-S1 ? IS2 ? PR6; RA ? IS1 ? IS9 ? PR7; RA ? IS1 ? IS2 ? PR8;RA ? IS1 ? IS10 ? IS8 ? PR12; RA ? IS7 ? PR16. It is obvious thatthese are very minor pathways because of the high energy content ofthe species involved.

3.2. Heats of reaction for different channels

In order to further evaluate the reliability of the values ob-tained, we calculated some parameters at the CCSD(T)/CBS//B3LYP/6-311++G(3df,2p) level, and the heats of reaction for thereaction are compared with available experimental values. The val-ues are summarized in Table 2.

The table shows that the heat of reaction predicted at theCCSD(T)/CBS level for the PR13 (CO + NO + H2) production,�55.9 kcal/mol is in excellent agreement with experimental value,�55.9 kcal/mol. The calculated heats of reaction for the productionPR2 (CH2O + NO), PR5 (CO2 + NH2), PR6 (HOCN + OH), PR9(NCO + H2O), PR12 (OCNH + OH), PR14 (CHO + HNO), PR15(CO + NH2O), and PR16 (HCN + HO2) from the reactants, �54.0,�100.0, �44.8, �77.0, �69.6, �14.3, �61.5 and �15.2 kcal/mol arealso in good agreement with experimental values of �55.4, �98.9,�44.0,�76.7,�68.6,�15.2,�59.2 and�16.5 kcal/mol, respectively.

The heats of reaction predicted at the CCSD(T)/6–311++G(3df,2p) level are rather near either those at CCSD(T)/CBSor experimental data. It is not only similar to other studies aboutreactions in gas phase [23–27], it also shows that the CCSD(T)/6-311++G(3df,2p) level of theory which we used is reasonable andthe predicted energy values for the PES are reliable.

4. Conclusion

The gas-phase mechanism for the HCNO + OH reaction has beenelucidated at the CCSD(T)//B3LYP/6-311++G(3df,2p) level. The re-sults show that the most pathways involve the reaction of HCNOwith OH can directly form HC(OH)NO intermediate state to be fol-lowed by the isomerization to form HC(O)N(H)O, then produce CO(PR11) requiring 43.7 kcal/mol activation energy or give HNO(PR14) barrierlessly. Meanwhile, PR2 (CH2O + NO), PR4 (CHOH + NO), PR10 (H2 + CONO), PR13 (H2 + CO + NO), and PR15 (CO + NH2O)are minor pathways. And, the other pathways PR1 (CNO + H2O),PR3 (OCHNO + H), PR5 (CO2 + NH2), PR6 (HOCN + OH), PR7 (OCN

OH + H), PR8 (HOCNO + H), PR9 (NCO + H2O), PR12 (OCNH + OH),and PR16 (HCN + HO2) are very minor. The present study eluci-dated that the process IS10 (OC(H)N(H)O) forming PR14(HCO + HN O) is barrierless, and PR2 (CH2O + NO) is formed via anew channel which is more reasonable than previous studies[2,3]. Our results are good agreement with experimental resultsof Feng et al. [1].

Acknowledgements

We thank the National Foundation for Science and TechnologyDevelopment (Nafosted), Viet Nam which has sponsored this workunder the project number of 104.03.2010.29.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.cplett.2014.03.001.

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