J. Phys. Chem. A 2010, 114, 4951–4956
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Direct Ab Initio MD Study on the Electron Capture Dynamics of Hydroperoxy Radical (HOO)-Water Complexes Hiroto Tachikawa* DiVision of Materials Chemistry, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan ReceiVed: January 21, 2010; ReVised Manuscript ReceiVed: February 17, 2010
The electron capture dynamics of hydroperoxy radical-water complexes have been investigated by means of direct density functional theory (DFT) molecular dynamics (MD) method to elucidate the solvation (hydration) effect on the reaction mechanism. The complexes composed of HOO and one to four water molecules, HOO(H2O)n (n ) 1-4), were considered to be the hydrated HOO system. After the electron capture of n ) 1, only solvent reorientation of H2O around HOO- occurred, and a stable complex (HOO--H2O) was formed within 100-300 fs. In the case of n ) 2-4, a proton of H2O was transferred from H2O to OOH-, whereas H2O2 and OH-(H2O)n-1 were found as products. It was suggested that the HOO radical adsorbed on water cluster is efficiently converted in the H2O2 without activation barrier after the electron capture of HOO. Time scales of proton transfer were calculated to be 200-300 fs. The mechanism of electron capture of HOO in polar stratospheric cloud was discussed on the basis of theoretical results. 1. Introduction The hydroperoxy radical (HOO) dominates directly the concentration of hydrocarbons, a sulfur compound, and a nitrogen oxide in atmosphere.1 Also, the HOO radical behaves as a source of H2O2 in the atmosphere, which is formed by the self-bimolecular reaction of HOO2
HOO + HOO f H2O2 + O2
(1)
It is known that this reaction possesses an activation barrier and proceeds via several intermediates.3,4 The activation energy of this reaction is estimated to be 5.8 kcal/mol from ab initio calculation.3 It was found from a laboratory experiment that water enhances the reaction efficiency of reaction 1.4 Kanno et al. measured that the complex formation energy of the HOO-H2O 1:1 complex is 7.5 kcal/mol.5 Also, they found that the water molecule enhances the reaction rate of self-reaction HOO + HOO (reaction 1). However, activation energy needs to proceed the reaction. Therefore, the complex composed of HOO and water molecules plays an important role in the H2O2 formation reaction. The structure of the smallest complex, HOO-H2O 1:1 complex, was determined experimentally by Suma et al.6 They found that a hydrogen atom of HOO orients to the oxygen atom of H2O and a hydrogen atom of H2O orients to the dangling oxygen atom of HOO. This structure is supported by their theoretical calculation at the CCSD/aug-cc-pVTZ level. In polar stratospheric cloud (PSC),7 it is known that atmospheric molecules adsorbed on surface of water ice. PSC consists of water ice or nitric acid ice particles with major composition of H2O and has several kilometers of thickness at very low temperature. Recently, on the basis of data from satellite, balloon, ground-state measurement, and laboratory experiment, Lu and Sanche8 and Lu and Madey9 proposed a model for ozone depletion caused by cosmic-ray ionization of halocarbon on PSCs.8,9 From their model, it is suggested that cosmic-ray causes ionization of water ice in * Corresponding author. E-mail:
[email protected]. Fax: +81 11706-7897.
PSCs, and an electron is generated. The electron attacks the HOO radical adsorbed on the ice, and then the reaction of HOO- occurs following the electron attachment to halocarbon. Therefore, the electron capture process of atmospheric molecules adsorbed on water ice is important in PSC because the molecule takes a complex with water molecules. In the present study, electron capture dynamics of HOO-water clusters HOO(H2O)n (n ) 1-4) were investigated by means of direct ab initio molecular dynamics (MD) method based on the DFT level to elucidate the effect of water on the electron capture process of the HOO radical. In particular, we focus our attention on the effects of the number of water molecules on the reaction mechanism. The complexes composed of HOO and one to four water molecules were examined in the present study. 2. Method of Calculation The geometries of the complexes composed of HOO and water molecules, expressed by HOO(H2O)n (n ) 1-4), were fully optimized at the B3LYP/6-311++G(d,p) level of theory. The harmonic vibrational frequency of the complex at the optimized point was calculated to elucidate the stability of the complex. Direct DFT-MD calculation was carried out at the B3LYP/ 6-311++G(d,p) level of theory throughout. The neutral states of HOO(H2O)n (n ) 1-4) were fully optimized by the energy gradient method. The trajectories for anionic system were run from the neutral complexes on the assumption of vertical electron attachment. The electronic state of the system was monitored during the simulation. We carefully confirmed that the electronic state is kept during the reaction. The velocities of atoms at the starting point were assumed to zero (i.e., momentum vector of each atom is zero). The equations of motion for n atoms in a molecule are given by
dQj ∂H ) dt ∂Pj
10.1021/jp100588z 2010 American Chemical Society Published on Web 03/19/2010
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J. Phys. Chem. A, Vol. 114, No. 14, 2010
Tachikawa Verlet algorism. No symmetry restriction was applied to the calculation of the energy gradients. The time step size was chosen as 0.10 fs, and a total of 10 000 or 20 000 steps was calculated for each dynamics calculation. The drift of the total energy is confirmed to be 1 system because the OH- ion can be stabilized by water molecules as OH-(H2O)n-1 (n > 1). Therefore, the present study strongly indicated that the reaction mechanism of HOO is dependent on the number of water molecules around the HOO
Tachikawa radical. Also, it was suggested that H2O2 can be formed without barrier from HOO on ice induced by electron capture. Acknowledgment. The author acknowledges partial support from a Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (JSPS) (project no. 21550002). Supporting Information Available: Optimized structures and results for dynamics calculations of HOO(H2O)3. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Kukui, A.; Ancellet, G.; Le Bras, G. J. Atmos. Chem. 2008, 61, 133. (2) Stone, D.; Rowley, D. M. Phys. Chem. Chem. Phys. 2005, 7, 2156. (3) Anglada, J. M.; Olivella, S.; Sole, A. J. Phys. Chem. A 2007, 111, 1695. (4) Kanno, N.; Tonokura, K.; Tezaki, A.; Koshi, M. J. Phys. Chem. A 2005, 109, 3153. (5) Kanno, N.; Tonokura, K.; Koshi, M. J. GeoPhys. Res. 2006, 111, D20312. (6) Suma, K.; Sumiyoshi, Y.; Endo, Y. Science 2006, 311, 1278. (7) Toon, O. B.; Turco, R. P. Sci. Am. 1991, 264, 68. (8) Lu, Q.-B.; Sanche, L. Phys. ReV. Lett. 2001, 87, 07850. (9) Lu, Q. B.; Madey, T. T. Surf. Sci. 2000, 451, 238. (10) Tachikawa, H.; Kawabata, H. J. Phys. Chem. C 2009, 113, 7603. (11) Tachikawa, H.; Iyama, T.; Kato, K. Phys. Chem. Chem. Phys. 2009, 28, 6008. (12) Tachikawa, H.; Orr-Ewing, A. J. J. Phys. Chem. A 2008, 112, 11575. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; HeadGordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.5; Gaussian, Inc.: Pittsburgh, PA, 1998.
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