Theoretical mechanistic study on the reactions between FO2 + NO and FO + NO2
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Theoretical mechanistic study on the reactions between FO 2 + NO and FO + NO 2 Yang Sun a, * , Jingzheng Yao b , Miao Sun c , Hui Zhang c , Milin Zhang a a Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR China b Multihull Ship Technology, Key Laboratory of Fundamental Science for National Defence, Harbin Engineering University, Harbin 150001, PR China c College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, PR China article info Article history: Received 21 April 2009 Received in revised form 7 August 2009 Accepted 7 August 2009 Available online 3 September 2009 Keywords: Potential energy surface Reaction mechanism Oxygen fluoride Density functional theory (DFT) abstract The detailed singlet potential energy surface (PES) of the [FNO 3 ] system is investigated at B3LYP/6- 311+G(2d) and CCSD(T)/6-311+G(2d) (single-point) levels. Six isomers and nine transition states are located. Various possible isomerization and dissociation channels are probed in order to explore the reac- tion mechanism for FO 2 + NO. The calculated results indicate that the initial association between FO 2 and NO can lead to the adduct FOONO, followed by the concerted F-shift and cleavage of the ON bond leading to the dominant product FNO + O 2 . The formation of other products, however, is much less feasible due to energy constraints. The three feasible products and reaction pathways are also analyzed for another reac- tion, FO + NO 2 , on the basis of the [FNO 3 ] PES. This study may be helpful in understanding the reaction and chemical behaviors of oxygen fluorides. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Oxygen fluorides, FO 2 and FO, are important fluorine-containing radicals in the atmosphere. Their structures, formation heats, and physical properties have been investigated extensively both exper- imentally and theoretically [1–9], whereas comparatively few studies are done on the reactions relevant to FO 2 and FO. It is imperative to learn the reaction behavior for these oxygen fluo- rides since they are reactive intermediates in the atmosphere which may have a negative influence on ozone depletion cycles [10,11]. For example, the reaction between FO 2 with NO is an important reaction that couples the FO x and NO x families of radi- cals. The reaction competes effectively with FO x +O 3 and may play significant roles in the final fate of oxygen fluoride. The reaction between FO 2 and NO has been investigated by both Nielsen et al. and Sander et al. The two possible reaction pathways detected and discussed in their studies are as follows: FO 2 þ NO ! FNO þ O 2 FO 2 þ NO ! FO þ NO 2 Nielsen et al. gave the rate constants for the reaction as (1.47 ± 0.08) 10 12 cm 3 molecule 1 s 1 by pulse radiolysis and UV absorption spectroscopy [12], and they identified that the main channel was the first one since its FNO yield was determined as 100 ± 14%. The FO 2 + NO reaction was also studied by Sander et al. using the discharge-flow mass spectrometric technique, and the rate coefficient was determined to be (7.5 ± 0.5) 10 12 exp(688 ± 377/T) cm 3 molecule 1 s 1 [13]. They came to the sim- ilar conclusion that the first channel was the dominant one, as the branching ratio of NO 2 in the second channel is less than 0.03. Another reaction related to the [FNO 3 ] system, FO + NO 2 , was investigated only by Bedzhanyan et al. using combined EPR/IR LMR spectrometry in a flow system [14]. Three separate product channels for the FO + NO 2 reaction were observed: F + NO 3 , FONO 2 , and unknown products. Francisco et al. suggested that the FO 2 + NO and FO + NO 2 may be the unknown products on the basis of the two FOONO isomers, which were obtained at the QCISD(T)/ 6-311G(3df)//QCISD(T)/6-311G(2d) level of theory [15]. However, without the detailed PES given by high-level theoretical calcula- tion, such prediction is difficult to confirm. Except for several stud- ies on FONO 2 and FOONO, few theoretical studies have been done on the [FNO 3 ] system, and a thorough computational investigation is needed to gain a deeper understanding of the reactions related to the [FNO 3 ] system. Previous studies indicate that the fluorine-containing molecules present considerable challenges to the theoretical calculations [16–19]. The traditional ab initio method may give poor results for such systems and more accurate results require high-level the- oretical methods with large basis sets. The density functional the- ory (DFT) method has been proven to be an ideal and cheap method for such systems, and it can give reliable geometries and energies, which are in good agreement with the experiment value [20]. Therefore, the density functional theory (DFT-B3LYP) and coupled cluster [CCSD(T)] (single-point) methods are adopted in 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.08.032 * Corresponding author. E-mail address: [email protected] (Y. Sun). Journal of Molecular Structure: THEOCHEM 916 (2009) 10–16 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem
Transcript of Theoretical mechanistic study on the reactions between FO2 + NO and FO + NO2
Journal of Molecular Structure: THEOCHEM 916 (2009) 10–16
Contents lists available at ScienceDirect
Journal of Molecular Structure: THEOCHEM
journal homepage: www.elsevier .com/locate / theochem
Theoretical mechanistic study on the reactions between FO2 + NO and FO + NO2
Yang Sun a,*, Jingzheng Yao b, Miao Sun c, Hui Zhang c, Milin Zhang a
a Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR Chinab Multihull Ship Technology, Key Laboratory of Fundamental Science for National Defence, Harbin Engineering University, Harbin 150001, PR Chinac College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, PR China
a r t i c l e i n f o
Article history:Received 21 April 2009Received in revised form 7 August 2009Accepted 7 August 2009Available online 3 September 2009
Keywords:Potential energy surfaceReaction mechanismOxygen fluorideDensity functional theory (DFT)
0166-1280/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.theochem.2009.08.032
* Corresponding author.E-mail address: [email protected] (Y. Sun).
a b s t r a c t
The detailed singlet potential energy surface (PES) of the [FNO3] system is investigated at B3LYP/6-311+G(2d) and CCSD(T)/6-311+G(2d) (single-point) levels. Six isomers and nine transition states arelocated. Various possible isomerization and dissociation channels are probed in order to explore the reac-tion mechanism for FO2 + NO. The calculated results indicate that the initial association between FO2 andNO can lead to the adduct FOONO, followed by the concerted F-shift and cleavage of the ON bond leadingto the dominant product FNO + O2. The formation of other products, however, is much less feasible due toenergy constraints. The three feasible products and reaction pathways are also analyzed for another reac-tion, FO + NO2, on the basis of the [FNO3] PES. This study may be helpful in understanding the reactionand chemical behaviors of oxygen fluorides.
� 2009 Elsevier B.V. All rights reserved.
1. Introduction
Oxygen fluorides, FO2 and FO, are important fluorine-containingradicals in the atmosphere. Their structures, formation heats, andphysical properties have been investigated extensively both exper-imentally and theoretically [1–9], whereas comparatively fewstudies are done on the reactions relevant to FO2 and FO. It isimperative to learn the reaction behavior for these oxygen fluo-rides since they are reactive intermediates in the atmospherewhich may have a negative influence on ozone depletion cycles[10,11]. For example, the reaction between FO2 with NO is animportant reaction that couples the FOx and NOx families of radi-cals. The reaction competes effectively with FOx + O3 and may playsignificant roles in the final fate of oxygen fluoride.
The reaction between FO2 and NO has been investigated by bothNielsen et al. and Sander et al. The two possible reaction pathwaysdetected and discussed in their studies are as follows:
FO2 þ NO! FNOþ O2
FO2 þ NO! FOþ NO2
Nielsen et al. gave the rate constants for the reaction as(1.47 ± 0.08) � 10�12 cm3 molecule�1 s�1 by pulse radiolysis andUV absorption spectroscopy [12], and they identified that the mainchannel was the first one since its FNO yield was determined as100 ± 14%. The FO2 + NO reaction was also studied by Sander
ll rights reserved.
et al. using the discharge-flow mass spectrometric technique, andthe rate coefficient was determined to be (7.5 ± 0.5) � 10�12
exp(�688 ± 377/T) cm3 molecule�1 s�1 [13]. They came to the sim-ilar conclusion that the first channel was the dominant one, as thebranching ratio of NO2 in the second channel is less than 0.03.
Another reaction related to the [FNO3] system, FO + NO2, wasinvestigated only by Bedzhanyan et al. using combined EPR/IRLMR spectrometry in a flow system [14]. Three separate productchannels for the FO + NO2 reaction were observed: F + NO3, FONO2,and unknown products. Francisco et al. suggested that theFO2 + NO and FO + NO2 may be the unknown products on the basisof the two FOONO isomers, which were obtained at the QCISD(T)/6-311G(3df)//QCISD(T)/6-311G(2d) level of theory [15]. However,without the detailed PES given by high-level theoretical calcula-tion, such prediction is difficult to confirm. Except for several stud-ies on FONO2 and FOONO, few theoretical studies have been doneon the [FNO3] system, and a thorough computational investigationis needed to gain a deeper understanding of the reactions related tothe [FNO3] system.
Previous studies indicate that the fluorine-containing moleculespresent considerable challenges to the theoretical calculations[16–19]. The traditional ab initio method may give poor resultsfor such systems and more accurate results require high-level the-oretical methods with large basis sets. The density functional the-ory (DFT) method has been proven to be an ideal and cheapmethod for such systems, and it can give reliable geometries andenergies, which are in good agreement with the experiment value[20]. Therefore, the density functional theory (DFT-B3LYP) andcoupled cluster [CCSD(T)] (single-point) methods are adopted in
Y. Sun et al. / Journal of Molecular Structure: THEOCHEM 916 (2009) 10–16 11
this study to give the detailed PES for [FNO3] systems. The aim ofthis investigation is to identify the product distributions and theexact channel for each product, thereby revealing more about thereaction kinetics of FO2 + NO and FO + NO2.
2. Computational methods
All computations are carried out using the GAUSSIAN03 pro-gram package [21]. The optimized geometries and harmonic fre-quencies of the reactant, products, local minima and transitionstates are obtained at the B3LYP/6-311+G(2d) theory level. Vibra-tional frequencies are calculated at the B3LYP/6-311+G(2d) levelto check whether the obtained stationary point is an isomer or afirst-order transition state. In order to confirm whether the ob-tained transition states connect the right reactants and products,the intrinsic reaction coordinate calculations are performed atthe B3LYP/6-311+G (2d) level. Single-point calculations are per-formed at the CCSD(T)/6-311+G(2d) theory level with the B3LYP/6-311+G(2d) optimized geometries. The B3LYP/6-311+G(2d)zero-point vibration energy (ZPVE) is also included.
Fig. 1. B3LYP/6-311+G(2d) optimized geometries for reactant and products. Bondlengths are in angstroms and angles in degrees.
3. Results and discussions
3.1. Potential energy surface
Two reactions are involved for the [FNO3] system, so the reac-tant and products are denoted as A, B, C, and D. The optimizedgeometries of the reactant and product molecules or radicals areshown in Fig. 1. In Table 1, the total and relative energies includingZPVE of the products, the reactants as well as the isomers arelisted. Note that the energy of the reactant A (in reaction FO2 + NO)is set zero for reference. Unless otherwise specified, the B3LYPgeometries and CCSD(T) energies are used in the followingdiscussions.
There are altogether six intermediate isomers located for the[FNO3] system, and the relative energies (in kcal/mol) of these iso-mers can be given as follows: 2(0.0) < 10(13.7) < 1(17.1) < 4(26.8)< 5(41.8) < 3(60.1), with isomer 2 as the global minimum. The opti-mized structures of the isomers are given in Fig. 2. The vibrationfrequencies and infrared intensities for the FNO3 isomers are alsolisted in Table 3. Notice that there are two isomers existing forthe adduct FOONO, that is, isomer 1 and isomer 10, which can bedescribed as the perp-trans isomer and perp-cis isomer, respec-tively. Isomer 10 is more stable than 1 thermodynamically, andthe two isomers can convert to each other easily through thelow-barrier transition state. Isomers 3, 4, and 5 have weak boundstructures, and their relative energies are higher comparatively.
To make clear the interrelation between various FNO3 isomers,nine transition states are obtained. The symbol TSm/n is used todenote the transition state connecting the isomers m and n. Theoptimized structures of transition states are given in Fig. 3 andthe energetic data are listed in Table 2. Their vibration frequenciesand infrared intensities are presented in Table 4.
By means of the products, intermediate isomers, transitionstates and their corresponding relative energies, a schematic PESof FNO3 in singlet is plotted in Fig. 4. Based on the schematic, wecan discuss the stability of various [FNO3] isomers towards isomer-ization and dissociation.
3.2. Reaction mechanism for FO2 + NO
Starting from the reactant (A) FO2 + NO, three dissociation prod-ucts, including (B) FNO + O2, (C) FO + NO2, and (D) F + NO3, are con-sidered. Unlike the HO2 + NO reaction, the direct intermolecularfluorine abstraction between FO2 and NO seems difficult for the
FO2 + NO reaction, since no corresponding transition state waslocated despite numerous attempts. Therefore, the FO2 + NO reac-tion proceed through the addition–elimination mechanism. Thefirst step for this reaction is the formation of an associated adduct,followed by isomerization or dissociation of the products and otherintermediates from the adduct. This reaction mechanism is alsoconsistent with the investigation of Sander et al. [13]. We cansee from the PES that the initial step for the reaction FO2 + NO is
Table 1Total (a.u.) and relative energies in parentheses (kcal/mol) as well as those including zero-point vibration energies (kcal/mol) of the reactant, products and isomers for the FNO3
system.
Species B3LYP/6-311+G(2d) CCSD(T)/6-311+G(2d) CCSD(T)/6-311+G(2d)+ZPVE
A FO2 + NO �380.0885393(0.0) �379.3481999(0.0) �379.3383739(0.0)B FNO + O2 �380.1060251(�11.0) �379.3844856(�22.8) �379.3743336(�22.6)C FO + NO2 �380.0829214(3.5) �379.3426567(3.5) �379.3314417(4.4)D F + NO3 �380.0791865(5.8) �379.3397873(5.3) �379.3291513(5.8)1 �380.0962924(�4.9) �379.3610927(�8.1) �379.3477427(�5.9)10 �380.100782(�7.6) �379.3667811(�11.6) �379.3531301(�9.3)2 �380.1309698(�26.6) �379.3915349(�27.2) �379.3750199(�23.0)3 �380.0286733(37.6) �379.2921051(35.2) �379.2793301(37.1)4 �380.0792942(5.8) �379.3453073(1.8) �379.3323023(3.8)5 �380.0560013(20.4) �379.3229922(15.8) �379.3084612(18.8)
Fig. 2. B3LYP/6-311+G(2d) optimized geometries for FNO3 isomers. Bond lengths are in angstroms and angles in degrees.
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Fig. 3. B3LYP/6-311+G(2d) optimized geometries for FNO3 transition states. Bond lengths are in angstroms and angles in degrees.
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the attack of the FO2 radical on the N-end of NO, and the associatedintermediate FOONO can be formed barrierlessly. As mentioned
above, there are two isomers for the associated intermediateFOONO, and most FNO3 local minima and products can be isomer-
Table 2Total (a.u.) and relative energies in parentheses (kcal/mol) as well as those including zero-point vibration energies (kcal/mol) of the transition states for the FNO3 system.
Species B3LYP/6-311+G(2d) CCSD(T)/6-311+G(2d) CCSD(T)/6-311+G(2d)+ZPVE
TS1/B �380.0779219(6.7) �379.3355039(7.9) �379.3229179(9.7)TS1/10 �380.0872175(0.8) �379.3449758(4.1) �379.3321018(3.9)TS1/2 �379.9915324(60.9) �379.2594098(55.7) �379.2476248(56.9)TS1/2(2) �380.0380475(31.7) �379.3123701(22.5) �379.2997831(24.2)TS4/B �380.0790605(5.9) �379.3455559(1.6) �379.3325869(3.6)TS2/3 �379.9552426(83.6) �379.2204056(80.2) �379.2090486(81.2)TS1/5 �380.0443932(27.7) �379.3109175(23.4) �379.2980575(25.3)TS1/3 �380.0262801(39.1) �379.2757438(37.5) �379.2757438(39.3)TS3/4 �379.9247687(102.8) �379.207992(88.0) �379.197343(88.5)
Table 3Harmonic vibration frequencies (cm�1) with infrared intensities in parentheses (km/mol) for FNO3 isomers at the B3LYP/6-311+G(2d) level.
Species Harmonic frequencies (infrared intensities)
1 102(0) 188(5) 282(96) 323(28) 511(9) 644(196) 801(37) 1078(29) 1929(466)10 130(0) 209(1) 295(47) 330(65) 486(19) 671(115) 720(4) 1160(85) 1989(410)2 162(0) 294(2) 459(29) 652(4) 728(10) 814(156) 993(47) 1342(225) 1803(428)3 52(0) 95(0) 109(0) 229(3) 347(1) 649(356) 1003(162) 1320(350) 1801(353)4 71(0) 154(0) 212(3) 335(2) 360(33) 481(40) 606(79) 1408(389) 2077(600)5 144(14) 160(2) 274(0) 448(5) 453(3) 673(300) 939(110) 1337(284) 1948(290)
Table 4Harmonic vibration frequencies (cm�1) with infrared intensities in parentheses (km/mol) for FNO3 transition states at the B3LYP/6-311+G(2d) level.
Species Harmonic frequencies (infrared intensities)
TS1/B �250(6) 135(1) 271(17) 278(8) 390(32) 461(81) 523(27) 1395(352) 2069(710)TS1/10 �150(1) 134(3) 219(17) 376(49) 479(11) 560(116) 692(80) 1123(63) 1972(604)TS1/2 �766(62) 75(0) 229(1) 242(2) 302(1) 662(1) 958(247) 1126(73) 1578(217)TS1/2(2) �131(89) 63(0) 122(32) 312(3) 385(81) 755(141) 1037(232) 1188(159) 1662(252)TS4/B �80(0) 174(0) 226(4) 276(3) 361(26) 541(77) 600(36) 1441(435) 2073(630)TS2/3 �140(5) 188(3) 257(4) 319(36) 350(64) 449(23) 658(46) 774(41) 1989(571)TS1/5 �475(146) 148(0) 228(2) 298(2) 496(10) 802(37) 1014(57) 1072(59) 1585(256)TS1/3 �93(1) 22(1) 129(2) 231(2) 372(5) 650(454) 1012(204) 1315(376) 1811(298)TS3/4 �555(285) 24(0) 174(43) 212(1) 319(110) 357(0) 778(41) 952(19) 1857(415)
Fig. 4. The singlet potential energy surface (PES) for the reaction FNO3 system, relative energies are calculated at the CCSD(T)/6-311+G(2d)//B3LYP/6-311+G(2d)+ZPVE levelof theory.
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Y. Sun et al. / Journal of Molecular Structure: THEOCHEM 916 (2009) 10–16 15
ized and dissociated from the perp-trans adduct 1. Thus, we candiscuss the three products and their pathways in detail startingfrom the isomer 1.
3.2.1. Formation of (B) FNO(1A0) + O2(1Dg)Four pathways can be located for the formation of product (B)
as follows:
PathB(1): A ? 1 ? TS1/B?BPathB(2): A ? 1 ? TS1/3?3 ? TS3/4 ? 4 ? TS4/B ? BPathB(3): A ? 1 ? TS1/2 ? 2 ? TS2/3 ? 3 ? TS3/4 ? 4 ?TS4/B ? BPathB(4): A ? 1 ? TS1/2(2) ? 2 ? TS2/3 ? 3 ? TS3/4 ? 4 ?TS4/B ? B
As shown in PathB(1), the perp-trans isomer 1 can lead to theproduct (B) via the four-member transition state TS1/B. In TS1/B,The F atom is transferred from O1-atom to N-atom with the con-certed break of the O2–N bond, and the migrating fluorine is1.855 Å away from the origin O1-atom and 2.123 Å away fromthe terminus N-atom. The barrier height for TS1/B is 15.6 kcal/mol, and this reaction pathway is also consistent with the extrap-olation that ‘‘the FNO + O2 could be proceed through a 4-centerrearrangement” proposed by Nielsen et al. [12].
The production of O2 in (B) is worth noting as the molecularoxygen has unique electronic structures. The ground state ofmolecular oxygen is triplet O2(X3R), but the reaction FO2 + NO can-not proceed through the triplet PES since the reactant FO2 and ad-duct FOONO cannot be located in triplet state. Thus, the directformation of the triplet O2 (3R) molecule is spin forbidden, andthe singlet O2 should be obtained first. Two spin states are possiblefor the singlet oxygen; these are the two electronically excitedstates for the molecular oxygen. The lower excited state of the sin-glet oxygen is O2(a1Dg), and it has one antibonding pg-orbital dou-bly occupied by the two electrons. The other singlet oxygen,O2(b1Rg
+), has two degenerate antibonding pg-orbitals, which aresingly occupied by the two unpaired electrons with opposite spin.The O2(1Rg
+) state is very short lived and relaxes quickly to thelowest lying excited state O2(1Dg), so the singlet oxygen is usuallyreferred to the O2(1Dg) state. Population analysis of the calculatedresult also indicates that the singlet oxygen in product (B) is the O2
(1Dg) state as only one antibonding pg-orbital is occupied, and theother pg-orbital is vacant, so the two valence electrons are pairedand populated in the same pg-orbital. Moreover, since O2(1Rg
+) isthe second excited state, and it lies 15.1 kcal/mol above theO2(1Dg) state [22], the singlet oxygen state in product (B) can alsobe confirmed by energy analysis. From the profile of IRC, we cansee that the energy of product (B) FNO(1A0) + O2(1Dg) is consistentwith the forward path of TS1/B. This indicates that the dissociationof the adduct FOONO does lead to the singlet oxygen O2(1Dg). It isinteresting to compare TS1/B with the corresponding HNO3 transi-tion state, which is optimized at MP2/cc-pVTZ level by Dixon et al.[23]. In their studies, the peroxynitrous acid HOONO can alsodecompose to the product HNO + O2 (1Dg) via a similar four-mem-ber transition state, and this indicates that these two systems showsome common characteristics in their reaction mechanisms.
In the reaction channels PathB(2), PathB(3), and PathB(4), theweakly bounded isomer 4 can also give rise to the product (B) bythe migration of fluorine from the O2-atom to the N-atom viaTS4/B. However, as shown in Fig. 4, these pathways are all less pos-sible kinetically because some energetically high-lying transitionstates such as TS3/4 are involved; hence, these reaction channelscan be ignored due to their high barriers and multiple steps. There-fore, the most favorable pathway for product (B) is PathB(1), andnote that although the relative energy of singlet oxygen is higher
than that of triplet oxygen, product (B) is still the most energeti-cally low-lying product (�22.6 kcal/mol) for the FO2 + NO reaction.
3.2.2. Formation of (C) FO(2P) + NO2 (2A1)Product (C) can be formed by the following pathways:
PathC(1): A ? 1 ? CPathC(2): A ? 10 ? CPathC(3): A ? 1 ? TS1/2 ? 2 ? CPathC(4): A ? 1 ? TS1/2(2) ? 2 ? C
As shown in PathC(1) and PathC(2), the direct fission of the O–O bond in adduct FOONO can lead to the product FO + NO2, and thecorresponding dissociation energy for these two channels is10.3 kcal/mol and 13.7 kcal/mol, respectively. Product (C) can alsobe obtained by the break of the O1–N bond in isomer 2 FONO2,which is formed by the isomerization of intermediate 1. Just likethe RO group migration in ROONO, the migration of FO can alsotake place in the perp-trans isomer FOONO. However, the barrierheight of TS1/2 in PathC(3) is 62.8 kcal/mol, and it is the mostenergetically high-lying transition state that is linked to adduct1. TS1/2(2) in PathC(4), with a barrier height of 30.1 kcal/mol, isanother transition state that connects intermediates 1 and 2, butthe relative energy of TS1/2(2) is still higher than the rate deter-mining transition state TS1/B. Thus, PathC(3) and PathC(4) are lessfeasible due to their high barrier and complex steps. However, thePathC(1) and PathC(2) are also unfavorable since the product (C) is4.4 kcal/mol endothermic to the reactant, so FO + NO2 will not ac-count for much in the final product distribution.
3.2.3. Formation of (D) F(2P) + NO3(2A)There are two pathways for product (D):
PathD(1): A ? 1 ? TS1/2 ? 2 ? DPathD(2): A ? 1 ? TS1/2(2) ? 2 ? D
Product (D) can be obtained by the direct fission of the F–Obond in intermediate 2, and the corresponding dissociation energyis 28.8 kcal/mol. As discussed above, the formation of intermediate2 is unfavorable kinetically, and product (D) is 5.8 kcal/mol endo-thermic to the reactant, so the two pathways are also less feasible.Therefore, product (D) can be ruled out in the analysis of the finalproduct distribution for reaction FO2 + NO.
From the analyses above, we can notice that the main product ofthe reaction FO2 + NO is (B) FNO + O2 because it is the most energet-ically low-lying product, and the PathB(1) is the most favorablepathway leading to product (B) due to its low reaction barrier. Theother pathways can be neglected for the FO2 + NO reaction since theyare much less competitive both kinetically and thermodynamically.
3.3. Reaction mechanism for FO + NO2
From the PES given by Fig. 4, we can find that FO + NO2 is also areaction which proceeds from the addition-elimination mecha-nism, and four products are possible for this reaction: FO2 + NO,FONO2, F + NO3 and FNO + O2. Notice that two associated adducts,FONO2 and FOONO, can be formed barrierlessly in the FO + NO2
reaction, as the FO radical can attack either the N-atom or the ter-minal O-atom of NO2. The corresponding association energies forthe two adducts are 27.4 kcal/mol and 10.3 kcal/mol, respectively.Starting from the reactant, the possible pathways for the reactionFO + NO2 can be summarized as follows:
Path1: C ? FONO2
Path2: C ? 1 ? (A) FO2 + NO
16 Y. Sun et al. / Journal of Molecular Structure: THEOCHEM 916 (2009) 10–16
Path3: C ? 1 ? TS1/B ? (B) FNO + O2
Path4: C ? 2 ? (D) F + NO3
All the pathways are feasible in terms of their energy require-ments since they are well below the reactant, so we will discussthese reaction pathways in detail in the following. Besides thesepathways, there are also other reaction channels leading to the fourproducts, but these reaction channels are unfeasible due to theirhigh barrier height and complicated reaction steps as mentionedin Section 3.2, and these reaction channels will not be discussedin detail further.
In the reaction channel of Path1, FONO2 can be formed by thedirect association of the reactant. The fluorine nitrate FONO2 isthe most energetically low-lying isomer for the [FNO3] system,and it is also a stable molecule which was firstly synthesized byG.H. Cady in 1934 [24]. As an important fluorine reservoir in thestratosphere, fluorine nitrate has been the subject of many studiesboth experimentally and theoretically [25–32]. Thus, FONO2 is afeasible product for the FO + NO2 reaction and its association en-ergy is 27.4 kcal/mol.
Product (A) and (B) can both be obtained by the dissociation ofthe common intermediate 1 FOONO, which is another possibleassociated adduct of the reactant. The corresponding relative en-ergy for the two products is 5.9 and 15.6 kcal/mol, respectively.The barrier height of TS1/B is 9.7 kcal/mol higher than the dissoci-ation energy of product (A) in Path2, so the formation of product(A) is more advantageous kinetically than product (B), thoughproduct (B) is more favorable thermodynamically. Thus, the un-known product observed by Bedzhanyan et al. is more likely tobe FO2 + NO. However, the formation of product (B) is still possibledue to the small difference in energies between these two path-ways; this could be one of the reasons for the third product thatthe work of Bedzhanyan et al. failed to identify [14].
The direct fission of the F–O1 bond in the fluorine nitrate FONO2
can give rise to product (D), F + NO3, and the corresponding disso-ciation energy is 28.8 kcal/mol. The relative energy of product (D)is �1.4 kcal/mol and it is the most energetically high-lying productfor the FO + NO2 reaction.
Finally, a comparison is needed to analyze the contributions ofthese reaction channels to the ultimate product distributions. Asnoted above, the fluorine nitrate FONO2 is the most energeticallylow-lying product for FO + NO2 reaction, but the formation ofFONO2 should be less competitive since the activated intermediateis difficult to be stabilized in the gas phase, especially under low-pressure environments such as the stratosphere, where the bimo-lecular reaction pathway is more preferable. Thus, majority of theassociated intermediate FONO2 dissociates to product (D) F + NO3.The relative energy of another associated intermediate FOONO ishigher than that of FONO2, but its dissociated product (A) FO2 + NOis more favorable thermodynamically than product (D), so it is alsoa competitive reaction pathway for the FO + NO2 reaction, and thisis consistent with the product distributions determined by theexperimental investigation.
4. Conclusion
A detailed theoretical investigations have been performed toshed light on the possible production pathways for FO2 + NO andFO + NO2 reactions. The structures and vibration frequencies forthe [FNO3] systems are obtained, and the singlet potential energy
surface is built up at the CCSD(T)/6-311+G(2d)//B3LYP/6-311+G(2d) level of theory. From the analysis of the calculation,the main points of the two multi-channel reactions can be summa-rized as follows:
(1) The first step for FO2 + NO reaction is the formation of theassociated intermediate FOONO, then the perp-trans isomer ofFOONO can decompose to the most favorable product (B) FNO + O2
via a four-center transition state. The other pathways and productsare of no importance for the FO2 + NO reaction.
(2) Starting from the reactant, two associated intermediates,FONO2 and FOONO, are formed barrierlessly for FO + NO2 reaction.The subsequent decomposition of the two intermediates can leadto product (A) FO2 + NO and (D) F + NO3, respectively. The twoproducts may compete with each other as the main products.The associated adduct FONO2 can also be stabilized as a feasibleproduct, but its yield will not account for much in the final productdistributions under low-pressure environment.
The calculation results are in good agreement with the productdistribution of the experiment, and may provide some useful infor-mation for understanding the reaction mechanism of FOx and NOx
radicals.
Acknowledgement
This work is supported by Harbin Engineering University Fun-damental Research Funding Project (No. 002100260727).
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