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Reaction of O(3P) with ClONO2: a MP2 computation
Journal of Molecular Structure (Theochem) 663 (2003) 25–33 www.elsevier.com/locate/theochem
Reaction of O(3P) with ClONO2: a MP2 computation Wei Shena,b, Ming Lia,*, Dianyong Tanga a
Department of Chemistry, Southwest-China Normal University, Chongqing 400715, PR China b Department of Chemistry, Fuling Normal University, Chongqing 408003, PR China Received 15 April 2003; accepted 27 June 2003
Abstract The reaction of O(3P) with ClONO2 were studied by means of the density functional method at the B3PW91/6-31G(d) level and the correlation energy correction method at the MP2/6-311G(2df) level. Geometries, energies, and vibrational frequencies of reactants, transition states, intermediates and products for the examined reaction were examined. The reliable energies were also computed by employing the quadratic CI calculation at the QCISD/6-311G(2df) level. The reaction mechanism was investigated. q 2003 Published by Elsevier B.V. Keywords: Chlorine nitrate; O(3P); Mechanism; QCISD; MP2
1. Introduction There is considerable interest in the mechanisms of chemical reactions that take place in the chemically perturbed region of the Antarctic ozone hole. The importance of the chemical reactions taking place on the surface of polar stratospheric clouds (PSCs) is firmly established [1 – 13]. It is determined that chlorine nitrate (ClONO2) and HCl, two major stratospheric reservoirs of Cl [14], are involved in the chemical reactions on the PSCs, and that these reactions lead to the release of Cl in the more reactive forms of Cl2 and HOCl. Therefore, the property and reaction of Chlorine nitrate, ClONO2, are always attached importance to [10 –12]. Chlorine nitrate is formed from the reaction of ClO with NO2 in the presence of a third body [15] and is removed via * Corresponding author. E-mail address: [email protected] (M. Li). 0166-1280/$ - see front matter q 2003 Published by Elsevier B.V. doi:10.1016/S0166-1280(03)00545-1
photolytic, heterogeneous, or free radical reactions. In general, the reactions of ClONO2 with free radicals are less important than its photolytic and heterogeneous reactions in the lower stratosphere. There are many investigations on the reaction of ClONO2 with O(3P) [16 –20], but a few for its reaction mechanism. ClONO2 reacts with O(3P) as follows: Oð3 PÞ þ ClONO2 ! ClONO þ O2
ðaÞ
Oð3 PÞ þ ClONO2 ! ClO þ NO3
ðbÞ
Oð3 PÞ þ ClONO2 ! OClO þ NO2
ðcÞ
3
Oð PÞ þ ClONO2 ! ClOO þ NO2
ðdÞ
Oð3 PÞ þ ClONO2 ! ClO þ NO þ O2
ðeÞ
Oð3 PÞ þ ClONO2 ! ClNO2 þ O2
ðfÞ
3
Oð PÞ þ ClONO2 ! ClNO þ O3
ðgÞ
Oð3 PÞ þ ClONO2 ! Cl þ NO2 þ O2
ðhÞ
26
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
The four latter reaction channels in the above list are the subsequent steps for the four former reaction channels. Cai et al. [21] studied the mechanism for the reaction of O(3P) with ClONO2 by employing the density functional method at the B3LYP/6-31G(d) level, but only are the first two reaction channels involved in their investigation. In order to analyze the reaction mechanism in detail, therefore, the four former reaction channels in the above list are studied by means of the quantum chemical methods in the present work.
3. Results and discussion The energies for all the species are summarized in Table 1. Their optimized geometries are described, respectively, in Figs. 1 and 4. The electron density contours and the BCP for some species are illustrated in Figs. 2 and 5. Figs. 3 and 6 show, respectively, the profiles of the PES for the reaction of O(3P) with Cl of ClONO2 and the reaction of O(3P) with O of ClONO2. 3.1. On the reaction of O(3P) with Cl
The geometries of the reactants, products, intermediates and transition states for the examined
For the reaction of O(3P) with Cl of ClONO2 in the gas phase, there are several reaction channels through which the different reaction products are generated. These reaction channels are shown as follows:
reaction are optimized by means of the density functional method at the B3PW91/6-31G(d) level and the correlation energy correction method at the MP2/6-311G(2df) level. Computations of the vibrational frequencies for all the species are also performed at the same computational levels. All the species are positively identified for local minima with zero of the number of imaginary frequencies and for transition states with the sole imaginary frequency. To obtain more reliable energies of various species on the potential energy surface (PES), the QCISD single point energies are calculated at the QCISD/6-311g(2df) level, with the optimized geometries obtained from the MP2/6311g(2df) optimization. In addition, the wave functions obtained from the MP2/6-311g(2df) optimization are used to compute the electron densities of the bond critical points (BCP) for the intermediates and the transition states by means of the AIM2000 programmer [22].
As presented, the reaction of O(3P) with Cl of ClONO2, as the first step, leads to the intermediate 1IM. 1IM generates the intermediates 1IM2, 2IM2 and 3IM, respectively, with passing through the transition states 1TS, 2TS and 3TS. 3IM passes through the transition states 4TS and 5TS and leads to the intermediates 4IM2 and 5IM2. Further, the intermediates 1IM2, 2IM2, 4IM2 and 5IM2 generate the reaction products. The optimized geometries of the reactants, products, transition states and intermediates for the reaction of O(3P) with Cl of ClONO2 are shown in Fig. 1 and their electron density contours are illustrated in Fig. 2. Geometries. In the first reaction pathway, the OCl –ONO2 distances in 1IM, 1TS and 1IM2 are 0.1708, 0.1902 and 0.1920 nm, respectively. The O –Cl –O bond angles are 113.9, 107.9 and 115.28. The electron densities of the BCP for the OCl – ONO2 bonds in 1IM, 1TS and 1IM2 are 0.2126,
2. Computation methods
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33 Table 1 Energy differences, DE (kcal/mol), for various species in the O(3P) þ ClONO2 reaction
Rea 1IM 1TS 1IM2 P1 2TS 2IM2 P2 3TS 3IM 4TS 4IM2 5TS 5IM2 P3 1IM0 1TS0 P4(cis) 2TS0 2IM20 P5(trans) 3IM0 3TS0 P4(trans) 4TS0 4IM20 P5(cis)
B3PW91 6-31G(d)
MP2 6-311G(2df)
MP2 þ ZPE
QCISD 6-311G(2df)
0 222.8 219.7 225.8 224.7 24.9 213.1 45.1 214.2 248.5 228.0 235.9 241.0 246.7 239.6 20.5 17.2 214.0 23.9 2.1 31.6 7.9 8.1 28.1 15.4 1.4 12.5
0 233.5 229.0 231.5 217.2 231.6 232.6 24.9 221.5 248.1 228.2 231.1 245.0 251.8 230.7 7.0 12.1 218.6 7.1 4.8 86.0 17.00 11.1 214.7 2.4 228.7 60.8
0 231.9 227.2 230.2 214.2 230.8 231.2 2.5 220.4 246.2 227.3 230.8 243.7 247.7 227.8 9.8 11.9 220.6 9.5 3.8 82.3 17.7 11.1 216.9 5.2 227.5 57.7
0 216.9 212.8 215.5 29.3 9.0 210.3 58.6 0.6 237.2 217.2 223.5 220.3 242.6 232.8 24.4 29.5 215.1 54.2 17.7 31.9 16.8 26.3 212.0 33.0 17.1 14.1
a Total energies of O( 3P) þ ClONO2 : B3PW91/6-31G(d), 2 511597.63; MP2/6-311G(2df), 2 511067.82; MP2 þ ZPE, 2511057.93; QCISD/6-311G(2df), 2511075.14 kcal/mol.
0.1408 and 0.0602, respectively. It is clear that the Cl – ONO2 bond in ClONO2 is greatly weakened with the reaction going on. Finally, the fracture of the OCl – ONO2 bond in 1IM2 results in the reaction products, ClO þ NO3 (P1). The second reaction channel leads to the fracture of the OClO – NO2 bond in 1IM. The OClO – NO2 distances in 2TS and 2IM2 are, respectively, 0.1750 and 0.1963 nm. Compared with 0.1570 nm in 1IM, they are increased considerably. The electron densities of the BCP for the OClO – NO2 bonds in 2TS and 2IM2 are, respectively, 0.0404 and 0.0942, which are much smaller than 0.2317 in 1IM. This result implies that the OClO – NO2 bonds in 2TS and 2IM2
27
are quite weak. The fracture of the OClO –NO2 bond in 2IM2 leads to the products, OClO þ NO2 (P2). The transition state 3TS involves an O – O – Cl – O – N five-membered ring. As shown in Fig. 1, the O – ON – O and Cl – ON – O distances are 0.2340 and 0.2264 nm, respectively. The fracture of the Cl – ON – O bond in 3TS leads to the intermediate 3IM. The O2N – OOCl and O2NO – ClO bonds in 3IM are, respectively, 0.1606 and 0.1360 nm. It is shown in Fig. 2 that there is a ring-critical point (RCP) in the transition state 3TS and the electron density for this RCP is 0.0159. It may imply that the O –O – Cl – O – N five-membered ring in 3TS is of stability. Furthermore, the electron density of the BCP for the O2NO – ClO bond in 3IM is 0.3646, which is much greater than 0.0628 in 3TS. Obviously, the O2NO – ClO bond in 3IM is quite stable. The fracture of the O2NO – ClO bond or the O2N – OOCl bond in 3IM, with passing through the transition state 4TS or 5TS, leads to the intermediate 4IM2 or 5IM2. In the 3IM –4TS – 4IM2 reaction, the O2NO –ClO distances in 3IM, 4TS and 4IM2 are 0.1360, 0.1780 and 0.2216 nm, respectively. The electron densities of the O2NO – ClO BCP are 0.3646, 0.1318 and 0.0470, respectively. These results show that the O2NO – ClO bond in 3IM is weakened considerably with the reaction going on. For the 3IM –5TS –5IM2 reaction, the O2N – OOCl distances in 3IM, 5TS and 5IM2 are 0.1606, 0.1790 and 0.2010 nm. The electron densities of the O2N –OOCl BCP are 0.2193, 0.1111 and 0.0885, respectively. It is obvious that the O2N –OOCl bond in 3IM is also weakened greatly with the reaction going on. Further, the fracture of the O2NO –ClO bond in 4IM2 or the O2N – OOCl bond in 5IM2 generates the reaction products ClO þ NO3 (P1) or ClOO þ NO2 (P3). Reaction mechanism. The QCISD PES for the reaction of O(3P) with Cl of ClONO2 is shown in Fig. 3. The reaction of O(3P) with ClONO2 leads to the intermediate 1IM. As demonstrated in Fig. 3 and Table 1, this reaction step is exothermic and barrierless. The energy of 1IM is lower than that of the reactants O(3P) þ ClONO2 (Re) by 2 16.9 kcal/ mol. There are three reaction channels for 1IM found. These three channels pass, respectively, through the transition states 1TS, 2TS and 3TS and lead to the intermediates 1IM2, 2IM2 and 3IM. The QCISD activating energies for 1TS, 2TS and 3TS are,
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W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
Fig. 1. The optimized structures of all the species for the reaction of O(3P) with Cl of ClONO2 (bond lengths in nm, bond angles in degree).
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
29
Fig. 2. Two-dimensional electron density contours in the reaction of O(3P) with Cl of Cl)BO2 and electron densities of some selected BCPs at the MP2/6-311g(2df) level.
respectively, 4.1, 25.9 and 17.5 kcal/mol. The intermediate 1IM2, via a barrierless and endothermic process, leads directly to the free radicals ClO þ NO3 (P1) and the QCISD energy difference for this endothermic process is 6.2 kcal/mol. The intermediate 2IM2 leads to the free radicals OClO þ NO2 (P2) and
the QCISD energy difference for the barrierless process is 68.9 kcal/mol. Two reaction channels for 3IM are found. They pass, respectively, through the transition states 4TS and 5TS and generate the intermediates 4IM2 and 5IM2. The activating energies for 4TS and 5TS are 20.0 and 16.9 kcal/mol,
30
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
Fig. 3. Energy profile for the reaction of O(3P) with Cl of ClONO2.
respectively. The formation of the free radicals ClO þ NO3 (P1) or ClOO þ NO2 (P3) generated, respectively, from 4IM2 or 5IM2 is barrierless and endothermic. The corresponding energy differences are 14.2 kcal/mol for P1 and 9.8 kcal/mol for P3. Furthermore, it is found that the activating energy for 1TS is much lower than the activating energies for other transition states are. Therefore, the reaction of O(3P) with Cl of ClONO2 leads mainly to the free radicals ClO and NO3, which is in agreement with the experiment 15. 3.2. On the reaction of O(3P) with O For the reaction of O(3P) with O of ClONO2, The cis- and trans-products are generated. The channels for this reaction are shown as follows:
3
As illustrated, the reaction of O( P) with O of ClONO2 results in the intermediates 1IM0 and 3IM0 . 1IM0 leads to the products OO þ ClONO(cis), passing through the transition state 1TS0 , and to the intermediate 2IM20 that leads to the products OCl þ OONO(trans), passing through the transition state
2TS0 . Like 1IM0 , 3IM0 also has two reaction channels. One leads to the products OO þ ClONO(trans) and the other to the intermediate 4IM20 that gives the products OCl þ OONO(cis) passing through the transition state 4TS0 . The optimized geometries for all the species are shown in Fig. 4 and their electron density contours are shown in Fig. 5. Geometries. The OO –NO2Cl distances in 1IM0 and 1TS0 are 0.1390 and 0.1570 nm, respectively. The O –O – N bond angle are 116.38 for 1IM0 and 113.88 for 1TS0 . The electron densities of the OO –NO2Cl BCP are 0.3460 for 1IM0 and 0.1817 for 1TS0 . It is clear that the OO – NO2Cl bond in 1TS0 is weakened greatly, compared with that in 1IM0 . The fracture of the OO –NO2Cl bond in 1TS0 results in the products P4(cis), O2 þ ClONO(cis). The second reaction channel for 1IM0 leads to the intermediate 2IM20 . From 1IM0 to 2IM20 , the ClO – NOO2 distance is increased and the ClO – O2NO distance decreased. The ClO – O2NO distance is decreased to 0.2140 nm in 2IM20 , whereas the ClO –NOO2 distances in 1IM0 , 2TS0 and 2IM20 are 0.1810, 0.1938 and 0.2000 nm, respectively. As shown in Fig. 4, the intermediate 2IM20 involves an O –O – N three-membered ring. It is demonstrated in Fig. 5 that there is a RCP in the O –O –N threemembered ring for 2IM20 and the electron density for this RCP is 0.0409. This result implies that 2IM20 with the O –O –N three-membered ring is stable. The fracture of the ClO – NOO2 and ClO –OONO bonds in 2IM20 result in the reaction products P5(trans), ClO þ OONO(trans).
The OO –NO2Cl bonds in 3IM0 and 3TS0 are 0.1330 and 0.1605 nm. The electron densities of the OO – NO2Cl BCP are 0.3258 for 3IM0 and 0.1956 for 3TS0 . The OO – NO2Cl bond in 3TS0 is weakened greatly, compared with that in 3IM0 . The fracture of the OO –NO2Cl bond in 3TS0 leads to the products P4(trans), O2 þ ClONO(trans).
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
31
Fig. 4. The optimized structures of all the species for the reaction of O(3P) with O of ClONO2 (bond lengths in nm, bond angles in degree).
The ClO – NOO2 distances in 3IM0 , 4TS0 and 4IM20 are 0.1400, 0.1640 and 0.2060 nm, respectively. The ClO – OONO distance is decreased to 0.1940 nm. Like 2IM20 , the intermediate 4IM20 also has an O –O –N three-membered ring. As shown in Fig. 5, there is also a RCP in the O –O – N threemembered ring for 4IM20 and the electron density for
this RCP is 0.0456. The fracture of the ClO – NOO2 and ClO – OONO bonds in 4IM20 results in the reaction products P5(cis), ClO þ OONO(cis). Reaction mechanism. The QCISD PES for the reaction of O(3P) with O of ClONO2 is shown in Fig. 6. The reaction of O(3P) with ClONO2 leads to the intermediates 1IM0 and 3IM0 . As demonstrated in
32
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
Fig. 5. Two-dimensional electron density contours in the reaction of O(3P) with O of ClONO2 and electron densities of some selected BCPs at the MP2/6-311g(2df) level.
Fig. 6 and Table 1, the formation of 1IM0 is endothermic and barrierless. The energy of 1IM0 is higher than that of the reactants O(3P) þ ClONO2 (Re) by 24.4 kcal/mol. There are two reaction channels for 1IM0 found. One generates the products OO þ ClONO(cis) (P4(cis)), passing through the transition state 1TS0 , and the other, passing through the transition state 2TS0 , leads to the intermediate 2IM20 that gives the products OCl þ OONO(trans) (P5(trans)). The QCISD activating energies for 1TS0 and 2TS0 are 5.1 and 29.8 kcal/ mol. The intermediate 2IM20 , via a barrierless and endothermic process, generates directly the free radicals OCl þ OONO(trans) (P5(trans)) and the QCISD energy difference for this endothermic process is 14.2 kcal/mol. Like 1IM 0 , the formation of 3IM 0 is also endothermic and barrierless. The energy of 3IM0 is
higher than that of the reactants O(3P) þ ClONO2 (Re) by 16.8 kcal/mol. One of two reaction channels for 3IM0 generates the products OO þ ClONO(trans) (P4(trans)), passing through the transition state 3TS0 ,
Fig. 6. Energy profile for the reaction of O(3P) with O of ClONO2.
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
and the other, passing through the transition state 4TS0 , leads to the intermediate 4IM20 that gives the products OCl þ OONO(cis) (P5(cis)). The QCISD activating energies for 3TS0 and 4TS0 are 9.5 and 16.2 kcal/mol. The intermediate 4IM20 , via a barrierless and exothermic process, generates directly the free radicals OCl þ OONO(cis) (P5(cis)) and the QCISD energy difference for this exothermic process is 2 3.0 kcal/mol. Furthermore, it is known that the energy differences between all the transition states for the reaction of O(3P) with O of ClONO2 and the reactants O(3P) þ ClONO2 (Re) are large. They are, respectively, 29.5 for 1TS0 , 54.2 for 2TS0 , 26.3 for 3TS0 , and 33.0 kcal/mol for 4TS0 . Compared with the reaction of O(3P) with Cl of ClONO2, therefore, the reaction of O(3P) with O of ClONO2 is difficult. 4. Conclusions For the reaction of O(3P) with ClONO2, eight different reaction channels, including nine transition states and 10 reaction intermediates, are found. O(3P) may react with Cl and O of ClONO2, respectively. The reaction of O(3P) with O of ClONO2 is difficult, compared with that of O(3P) with Cl of ClONO2. The reaction of O(3P) with Cl of ClONO2 leads mainly to the free radicals ClO and NO3, which is in agreement with the experiment. Acknowledgements This work was supported by Natural Science Foundation of Chongqing City, People’s Republic of China (Grant No. 2002-7473).
33
References [1] M.A. Tolbert, M.J. Rossi, R. Malhotra, D.M. Golden, Science 238 (1987) 1258. [2] M.J. Molina, T.L. Tso, L.T. Molina, F.C. Wang, Science 238 (1987) 1253. [3] M.J. Rossi, R. Malhotra, D.M. Golden, Geophys. Res. Lett. 14 (1987) 127. [4] M.A. Tolbert, M.J. Rossi, D.M. Golden, Geophys. Res. Lett. 15 (1988) 847. [5] M.T. Leu, Geophys. Res. Lett. 15 (1988) 851. [6] M.A. Quinlan, C.M. Reihs, D.M. Golden, M.A. Tolbert, J. Phys. Chem. 94 (1990) 3255. [7] D.J. Hofmann, S.J. Oltmans, Geophys. Res. Lett. 19 (1992) 2211. [8] D.R. Hanson, A.R. Ravishankara, J. Geophys. Res. 96 (1991) 5081. [9] M.T. Leu, S.B. Moore, L.E. Keyser, J. Phys. Chem. 95 (1991) 7763. [10] M.J. Prather, Nature 355 (1992) 534. [11] D.R. Hanson, A.R. Ravishankara, J. Phys. Chem. 96 (1992) 7674. [12] J.P.D. Abbatt, M.J. Molina, J. Phys. Chem. 96 (1992) 7674. [13] S.C. Wofsy, M.J. Molina, R.J. Salawitch, L.E. Fox, M.B. McElroy, J. Geophys. Res. 93 (1988) 2442. [14] F.S. Rowland, Annu. Rev. Phys. Chem. 42 (1991) 731. [15] L. Goldfarb, M.H. Harwood, J.B. Burkholder, A.R. Ravishankara, J. Phys. Chem. A 102 (1998) 8556. [16] M.J. Kurylo, Chem. Phys. Lett. 49 (1977) 467. [17] W.S. Smith, C.C. Chou, F.S. Rowland, Geophys. Res. Lett. 4 (1977) 517. [18] G.S. Tyndall, C.S. Kegley-Owen, J.J. Orlando, J. Chem. Soc., Faraday Trans. 93 (1997) 2675. [19] A.R. Ravishankara, D.D. Davis, G. Smith, Geophys. Res. Lett. 4 (1977) 7. [20] L.T. Molina, J.E. Spencer, M.J. Molina, Chem. Phys. Lett. 45 (1977) 158. [21] X.P. Cai, D.C. Fang, X.Y. Fu, Acta. Phys. Chem. Sinica 16 (2000) 689. [22] F. Biegler-Ko¨nig, J. Scho¨nbohm, R. Derdau, D. Bayles, R.F.W. Bader, AIM 2000. Version 1, 2000.
Reaction of O(3P) with ClONO2: a MP2 computation Wei Shena,b, Ming Lia,*, Dianyong Tanga a
Department of Chemistry, Southwest-China Normal University, Chongqing 400715, PR China b Department of Chemistry, Fuling Normal University, Chongqing 408003, PR China Received 15 April 2003; accepted 27 June 2003
Abstract The reaction of O(3P) with ClONO2 were studied by means of the density functional method at the B3PW91/6-31G(d) level and the correlation energy correction method at the MP2/6-311G(2df) level. Geometries, energies, and vibrational frequencies of reactants, transition states, intermediates and products for the examined reaction were examined. The reliable energies were also computed by employing the quadratic CI calculation at the QCISD/6-311G(2df) level. The reaction mechanism was investigated. q 2003 Published by Elsevier B.V. Keywords: Chlorine nitrate; O(3P); Mechanism; QCISD; MP2
1. Introduction There is considerable interest in the mechanisms of chemical reactions that take place in the chemically perturbed region of the Antarctic ozone hole. The importance of the chemical reactions taking place on the surface of polar stratospheric clouds (PSCs) is firmly established [1 – 13]. It is determined that chlorine nitrate (ClONO2) and HCl, two major stratospheric reservoirs of Cl [14], are involved in the chemical reactions on the PSCs, and that these reactions lead to the release of Cl in the more reactive forms of Cl2 and HOCl. Therefore, the property and reaction of Chlorine nitrate, ClONO2, are always attached importance to [10 –12]. Chlorine nitrate is formed from the reaction of ClO with NO2 in the presence of a third body [15] and is removed via * Corresponding author. E-mail address: [email protected] (M. Li). 0166-1280/$ - see front matter q 2003 Published by Elsevier B.V. doi:10.1016/S0166-1280(03)00545-1
photolytic, heterogeneous, or free radical reactions. In general, the reactions of ClONO2 with free radicals are less important than its photolytic and heterogeneous reactions in the lower stratosphere. There are many investigations on the reaction of ClONO2 with O(3P) [16 –20], but a few for its reaction mechanism. ClONO2 reacts with O(3P) as follows: Oð3 PÞ þ ClONO2 ! ClONO þ O2
ðaÞ
Oð3 PÞ þ ClONO2 ! ClO þ NO3
ðbÞ
Oð3 PÞ þ ClONO2 ! OClO þ NO2
ðcÞ
3
Oð PÞ þ ClONO2 ! ClOO þ NO2
ðdÞ
Oð3 PÞ þ ClONO2 ! ClO þ NO þ O2
ðeÞ
Oð3 PÞ þ ClONO2 ! ClNO2 þ O2
ðfÞ
3
Oð PÞ þ ClONO2 ! ClNO þ O3
ðgÞ
Oð3 PÞ þ ClONO2 ! Cl þ NO2 þ O2
ðhÞ
26
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
The four latter reaction channels in the above list are the subsequent steps for the four former reaction channels. Cai et al. [21] studied the mechanism for the reaction of O(3P) with ClONO2 by employing the density functional method at the B3LYP/6-31G(d) level, but only are the first two reaction channels involved in their investigation. In order to analyze the reaction mechanism in detail, therefore, the four former reaction channels in the above list are studied by means of the quantum chemical methods in the present work.
3. Results and discussion The energies for all the species are summarized in Table 1. Their optimized geometries are described, respectively, in Figs. 1 and 4. The electron density contours and the BCP for some species are illustrated in Figs. 2 and 5. Figs. 3 and 6 show, respectively, the profiles of the PES for the reaction of O(3P) with Cl of ClONO2 and the reaction of O(3P) with O of ClONO2. 3.1. On the reaction of O(3P) with Cl
The geometries of the reactants, products, intermediates and transition states for the examined
For the reaction of O(3P) with Cl of ClONO2 in the gas phase, there are several reaction channels through which the different reaction products are generated. These reaction channels are shown as follows:
reaction are optimized by means of the density functional method at the B3PW91/6-31G(d) level and the correlation energy correction method at the MP2/6-311G(2df) level. Computations of the vibrational frequencies for all the species are also performed at the same computational levels. All the species are positively identified for local minima with zero of the number of imaginary frequencies and for transition states with the sole imaginary frequency. To obtain more reliable energies of various species on the potential energy surface (PES), the QCISD single point energies are calculated at the QCISD/6-311g(2df) level, with the optimized geometries obtained from the MP2/6311g(2df) optimization. In addition, the wave functions obtained from the MP2/6-311g(2df) optimization are used to compute the electron densities of the bond critical points (BCP) for the intermediates and the transition states by means of the AIM2000 programmer [22].
As presented, the reaction of O(3P) with Cl of ClONO2, as the first step, leads to the intermediate 1IM. 1IM generates the intermediates 1IM2, 2IM2 and 3IM, respectively, with passing through the transition states 1TS, 2TS and 3TS. 3IM passes through the transition states 4TS and 5TS and leads to the intermediates 4IM2 and 5IM2. Further, the intermediates 1IM2, 2IM2, 4IM2 and 5IM2 generate the reaction products. The optimized geometries of the reactants, products, transition states and intermediates for the reaction of O(3P) with Cl of ClONO2 are shown in Fig. 1 and their electron density contours are illustrated in Fig. 2. Geometries. In the first reaction pathway, the OCl –ONO2 distances in 1IM, 1TS and 1IM2 are 0.1708, 0.1902 and 0.1920 nm, respectively. The O –Cl –O bond angles are 113.9, 107.9 and 115.28. The electron densities of the BCP for the OCl – ONO2 bonds in 1IM, 1TS and 1IM2 are 0.2126,
2. Computation methods
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33 Table 1 Energy differences, DE (kcal/mol), for various species in the O(3P) þ ClONO2 reaction
Rea 1IM 1TS 1IM2 P1 2TS 2IM2 P2 3TS 3IM 4TS 4IM2 5TS 5IM2 P3 1IM0 1TS0 P4(cis) 2TS0 2IM20 P5(trans) 3IM0 3TS0 P4(trans) 4TS0 4IM20 P5(cis)
B3PW91 6-31G(d)
MP2 6-311G(2df)
MP2 þ ZPE
QCISD 6-311G(2df)
0 222.8 219.7 225.8 224.7 24.9 213.1 45.1 214.2 248.5 228.0 235.9 241.0 246.7 239.6 20.5 17.2 214.0 23.9 2.1 31.6 7.9 8.1 28.1 15.4 1.4 12.5
0 233.5 229.0 231.5 217.2 231.6 232.6 24.9 221.5 248.1 228.2 231.1 245.0 251.8 230.7 7.0 12.1 218.6 7.1 4.8 86.0 17.00 11.1 214.7 2.4 228.7 60.8
0 231.9 227.2 230.2 214.2 230.8 231.2 2.5 220.4 246.2 227.3 230.8 243.7 247.7 227.8 9.8 11.9 220.6 9.5 3.8 82.3 17.7 11.1 216.9 5.2 227.5 57.7
0 216.9 212.8 215.5 29.3 9.0 210.3 58.6 0.6 237.2 217.2 223.5 220.3 242.6 232.8 24.4 29.5 215.1 54.2 17.7 31.9 16.8 26.3 212.0 33.0 17.1 14.1
a Total energies of O( 3P) þ ClONO2 : B3PW91/6-31G(d), 2 511597.63; MP2/6-311G(2df), 2 511067.82; MP2 þ ZPE, 2511057.93; QCISD/6-311G(2df), 2511075.14 kcal/mol.
0.1408 and 0.0602, respectively. It is clear that the Cl – ONO2 bond in ClONO2 is greatly weakened with the reaction going on. Finally, the fracture of the OCl – ONO2 bond in 1IM2 results in the reaction products, ClO þ NO3 (P1). The second reaction channel leads to the fracture of the OClO – NO2 bond in 1IM. The OClO – NO2 distances in 2TS and 2IM2 are, respectively, 0.1750 and 0.1963 nm. Compared with 0.1570 nm in 1IM, they are increased considerably. The electron densities of the BCP for the OClO – NO2 bonds in 2TS and 2IM2 are, respectively, 0.0404 and 0.0942, which are much smaller than 0.2317 in 1IM. This result implies that the OClO – NO2 bonds in 2TS and 2IM2
27
are quite weak. The fracture of the OClO –NO2 bond in 2IM2 leads to the products, OClO þ NO2 (P2). The transition state 3TS involves an O – O – Cl – O – N five-membered ring. As shown in Fig. 1, the O – ON – O and Cl – ON – O distances are 0.2340 and 0.2264 nm, respectively. The fracture of the Cl – ON – O bond in 3TS leads to the intermediate 3IM. The O2N – OOCl and O2NO – ClO bonds in 3IM are, respectively, 0.1606 and 0.1360 nm. It is shown in Fig. 2 that there is a ring-critical point (RCP) in the transition state 3TS and the electron density for this RCP is 0.0159. It may imply that the O –O – Cl – O – N five-membered ring in 3TS is of stability. Furthermore, the electron density of the BCP for the O2NO – ClO bond in 3IM is 0.3646, which is much greater than 0.0628 in 3TS. Obviously, the O2NO – ClO bond in 3IM is quite stable. The fracture of the O2NO – ClO bond or the O2N – OOCl bond in 3IM, with passing through the transition state 4TS or 5TS, leads to the intermediate 4IM2 or 5IM2. In the 3IM –4TS – 4IM2 reaction, the O2NO –ClO distances in 3IM, 4TS and 4IM2 are 0.1360, 0.1780 and 0.2216 nm, respectively. The electron densities of the O2NO – ClO BCP are 0.3646, 0.1318 and 0.0470, respectively. These results show that the O2NO – ClO bond in 3IM is weakened considerably with the reaction going on. For the 3IM –5TS –5IM2 reaction, the O2N – OOCl distances in 3IM, 5TS and 5IM2 are 0.1606, 0.1790 and 0.2010 nm. The electron densities of the O2N –OOCl BCP are 0.2193, 0.1111 and 0.0885, respectively. It is obvious that the O2N –OOCl bond in 3IM is also weakened greatly with the reaction going on. Further, the fracture of the O2NO –ClO bond in 4IM2 or the O2N – OOCl bond in 5IM2 generates the reaction products ClO þ NO3 (P1) or ClOO þ NO2 (P3). Reaction mechanism. The QCISD PES for the reaction of O(3P) with Cl of ClONO2 is shown in Fig. 3. The reaction of O(3P) with ClONO2 leads to the intermediate 1IM. As demonstrated in Fig. 3 and Table 1, this reaction step is exothermic and barrierless. The energy of 1IM is lower than that of the reactants O(3P) þ ClONO2 (Re) by 2 16.9 kcal/ mol. There are three reaction channels for 1IM found. These three channels pass, respectively, through the transition states 1TS, 2TS and 3TS and lead to the intermediates 1IM2, 2IM2 and 3IM. The QCISD activating energies for 1TS, 2TS and 3TS are,
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W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
Fig. 1. The optimized structures of all the species for the reaction of O(3P) with Cl of ClONO2 (bond lengths in nm, bond angles in degree).
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
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Fig. 2. Two-dimensional electron density contours in the reaction of O(3P) with Cl of Cl)BO2 and electron densities of some selected BCPs at the MP2/6-311g(2df) level.
respectively, 4.1, 25.9 and 17.5 kcal/mol. The intermediate 1IM2, via a barrierless and endothermic process, leads directly to the free radicals ClO þ NO3 (P1) and the QCISD energy difference for this endothermic process is 6.2 kcal/mol. The intermediate 2IM2 leads to the free radicals OClO þ NO2 (P2) and
the QCISD energy difference for the barrierless process is 68.9 kcal/mol. Two reaction channels for 3IM are found. They pass, respectively, through the transition states 4TS and 5TS and generate the intermediates 4IM2 and 5IM2. The activating energies for 4TS and 5TS are 20.0 and 16.9 kcal/mol,
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W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
Fig. 3. Energy profile for the reaction of O(3P) with Cl of ClONO2.
respectively. The formation of the free radicals ClO þ NO3 (P1) or ClOO þ NO2 (P3) generated, respectively, from 4IM2 or 5IM2 is barrierless and endothermic. The corresponding energy differences are 14.2 kcal/mol for P1 and 9.8 kcal/mol for P3. Furthermore, it is found that the activating energy for 1TS is much lower than the activating energies for other transition states are. Therefore, the reaction of O(3P) with Cl of ClONO2 leads mainly to the free radicals ClO and NO3, which is in agreement with the experiment 15. 3.2. On the reaction of O(3P) with O For the reaction of O(3P) with O of ClONO2, The cis- and trans-products are generated. The channels for this reaction are shown as follows:
3
As illustrated, the reaction of O( P) with O of ClONO2 results in the intermediates 1IM0 and 3IM0 . 1IM0 leads to the products OO þ ClONO(cis), passing through the transition state 1TS0 , and to the intermediate 2IM20 that leads to the products OCl þ OONO(trans), passing through the transition state
2TS0 . Like 1IM0 , 3IM0 also has two reaction channels. One leads to the products OO þ ClONO(trans) and the other to the intermediate 4IM20 that gives the products OCl þ OONO(cis) passing through the transition state 4TS0 . The optimized geometries for all the species are shown in Fig. 4 and their electron density contours are shown in Fig. 5. Geometries. The OO –NO2Cl distances in 1IM0 and 1TS0 are 0.1390 and 0.1570 nm, respectively. The O –O – N bond angle are 116.38 for 1IM0 and 113.88 for 1TS0 . The electron densities of the OO –NO2Cl BCP are 0.3460 for 1IM0 and 0.1817 for 1TS0 . It is clear that the OO – NO2Cl bond in 1TS0 is weakened greatly, compared with that in 1IM0 . The fracture of the OO –NO2Cl bond in 1TS0 results in the products P4(cis), O2 þ ClONO(cis). The second reaction channel for 1IM0 leads to the intermediate 2IM20 . From 1IM0 to 2IM20 , the ClO – NOO2 distance is increased and the ClO – O2NO distance decreased. The ClO – O2NO distance is decreased to 0.2140 nm in 2IM20 , whereas the ClO –NOO2 distances in 1IM0 , 2TS0 and 2IM20 are 0.1810, 0.1938 and 0.2000 nm, respectively. As shown in Fig. 4, the intermediate 2IM20 involves an O –O – N three-membered ring. It is demonstrated in Fig. 5 that there is a RCP in the O –O –N threemembered ring for 2IM20 and the electron density for this RCP is 0.0409. This result implies that 2IM20 with the O –O –N three-membered ring is stable. The fracture of the ClO – NOO2 and ClO –OONO bonds in 2IM20 result in the reaction products P5(trans), ClO þ OONO(trans).
The OO –NO2Cl bonds in 3IM0 and 3TS0 are 0.1330 and 0.1605 nm. The electron densities of the OO – NO2Cl BCP are 0.3258 for 3IM0 and 0.1956 for 3TS0 . The OO – NO2Cl bond in 3TS0 is weakened greatly, compared with that in 3IM0 . The fracture of the OO –NO2Cl bond in 3TS0 leads to the products P4(trans), O2 þ ClONO(trans).
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
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Fig. 4. The optimized structures of all the species for the reaction of O(3P) with O of ClONO2 (bond lengths in nm, bond angles in degree).
The ClO – NOO2 distances in 3IM0 , 4TS0 and 4IM20 are 0.1400, 0.1640 and 0.2060 nm, respectively. The ClO – OONO distance is decreased to 0.1940 nm. Like 2IM20 , the intermediate 4IM20 also has an O –O –N three-membered ring. As shown in Fig. 5, there is also a RCP in the O –O – N threemembered ring for 4IM20 and the electron density for
this RCP is 0.0456. The fracture of the ClO – NOO2 and ClO – OONO bonds in 4IM20 results in the reaction products P5(cis), ClO þ OONO(cis). Reaction mechanism. The QCISD PES for the reaction of O(3P) with O of ClONO2 is shown in Fig. 6. The reaction of O(3P) with ClONO2 leads to the intermediates 1IM0 and 3IM0 . As demonstrated in
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W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
Fig. 5. Two-dimensional electron density contours in the reaction of O(3P) with O of ClONO2 and electron densities of some selected BCPs at the MP2/6-311g(2df) level.
Fig. 6 and Table 1, the formation of 1IM0 is endothermic and barrierless. The energy of 1IM0 is higher than that of the reactants O(3P) þ ClONO2 (Re) by 24.4 kcal/mol. There are two reaction channels for 1IM0 found. One generates the products OO þ ClONO(cis) (P4(cis)), passing through the transition state 1TS0 , and the other, passing through the transition state 2TS0 , leads to the intermediate 2IM20 that gives the products OCl þ OONO(trans) (P5(trans)). The QCISD activating energies for 1TS0 and 2TS0 are 5.1 and 29.8 kcal/ mol. The intermediate 2IM20 , via a barrierless and endothermic process, generates directly the free radicals OCl þ OONO(trans) (P5(trans)) and the QCISD energy difference for this endothermic process is 14.2 kcal/mol. Like 1IM 0 , the formation of 3IM 0 is also endothermic and barrierless. The energy of 3IM0 is
higher than that of the reactants O(3P) þ ClONO2 (Re) by 16.8 kcal/mol. One of two reaction channels for 3IM0 generates the products OO þ ClONO(trans) (P4(trans)), passing through the transition state 3TS0 ,
Fig. 6. Energy profile for the reaction of O(3P) with O of ClONO2.
W. Shen et al. / Journal of Molecular Structure (Theochem) 663 (2003) 25–33
and the other, passing through the transition state 4TS0 , leads to the intermediate 4IM20 that gives the products OCl þ OONO(cis) (P5(cis)). The QCISD activating energies for 3TS0 and 4TS0 are 9.5 and 16.2 kcal/mol. The intermediate 4IM20 , via a barrierless and exothermic process, generates directly the free radicals OCl þ OONO(cis) (P5(cis)) and the QCISD energy difference for this exothermic process is 2 3.0 kcal/mol. Furthermore, it is known that the energy differences between all the transition states for the reaction of O(3P) with O of ClONO2 and the reactants O(3P) þ ClONO2 (Re) are large. They are, respectively, 29.5 for 1TS0 , 54.2 for 2TS0 , 26.3 for 3TS0 , and 33.0 kcal/mol for 4TS0 . Compared with the reaction of O(3P) with Cl of ClONO2, therefore, the reaction of O(3P) with O of ClONO2 is difficult. 4. Conclusions For the reaction of O(3P) with ClONO2, eight different reaction channels, including nine transition states and 10 reaction intermediates, are found. O(3P) may react with Cl and O of ClONO2, respectively. The reaction of O(3P) with O of ClONO2 is difficult, compared with that of O(3P) with Cl of ClONO2. The reaction of O(3P) with Cl of ClONO2 leads mainly to the free radicals ClO and NO3, which is in agreement with the experiment. Acknowledgements This work was supported by Natural Science Foundation of Chongqing City, People’s Republic of China (Grant No. 2002-7473).
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