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Ab initioMO/statistical theory prediction of the OH + HONO reaction rate: evidence for the negative temperature dependence |
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PhysChemComm,
Volume 3,
Issue 13,
2000,
Page 71-78
W. S. Xia,
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摘要:
W. S. Xia† and M. C. Lin* Department of Chemistry, Emory University, Atlanta, GA30030, USA. E-mail: chemmcl@emory.edu; Fax: 404-727-6586; Tel: 404-727-2825 Received 26th September 2000, Accepted 4th December 2000, Published 13th December 2000 The reaction of OH radical with cis- and trans-HONO has been investigated by ab initio molecular orbital and variational transition-state theory calculations. The overall mechanism has been elucidated and found to be quite complex. The bimolecular rate constants for the major reaction paths have been calculated and compared with existing, conflicting kinetic data. The total rate constant for the production of H2O and NO2 was concluded to have noticeable negative temperature dependence below 1000 K. The following two expressions are recommended for atmosphere and combustion modeling applications: k1 = 4.12 × 1012 (T/300)–0.8 cm3 mol–1 s–1, 200–500 K; k1 = 1.77 × 107 T 1.5 exp [+1260/T] cm3 mol–1 s–1, 500–2000 K.The predicted rate constant for the low-temperature regime agrees quantitatively with the result of Burkholder et al. (Int. J. Chem. Kinet., 1992, 24, 711) measured at 298 < T < 373 K. 2O + NO2 (1) Ab initio MO/statistical theory prediction of the OH + HONO reaction rate: evidence for the negative temperature dependence The activation energy for the reaction was found to be – 2.2 kJ mol–1, clearly at odds with that of Jenkin and Cox mentioned above. On account of the aforementioned results, two conflicting rate constant expressions have been recommended: k1 = 3.76 × 1012 T1.0 exp (–68/T) cm3 mol–1 s–1 for 300–2500 K by Tsang and Herron10 for high-temperature combustion modeling applications, and k1 = 1.63 × 1012 exp (+260/T) cm3 mol–1 s–1 at 290– 380 K by Atkinson et al.11 for atmospheric modeling applications.In order to reconcile the above conflicting results and to elucidate the mechanism for the reaction, which may involve both cis- and trans-isomers, we have carried out a detailed ab initio molecular orbital (MO) calculation to identify the key product channels. We have also performed statistical-theory calculations to obtain the rate constants for these channels in order to resolve the existing controversy vis-à-vis the effect of temperature. 1. Introduction The reaction of hydroxyl radical with nitrous acid is very important to the chemistry of troposphere where HONO may be formed by lightning or chemical reactions in the polluted environment.The reaction is also pivotal to our understanding of high-temperature combustion of nitrate esters, nitramines and energetic materials containing [H, N, O]-species.1,2 There have been limited but conflicting kinetic data3–9 for the OH + HONO reaction, which has been invariably assumed to occur by the direct abstraction reaction: OH + HONO H1 = –105 kJ mol–1. The mechanism for this important process, to our knowledge, has not been theoretically investigated and no reliable prediction of the rate constant has been made to resolve the controversy. with the The rate constant for reaction (1) was first reported by Fifer3 employing mixtures of NO, NO2 and H2O heated with incident-shocks in the temperature range 1000–1400 K, monitoring NO2 concentration spectroscopically.Kinetic modeling of NO2 concentration profiles based on the complex chemistry involved gave rise to a T, P-independent rate constant, k1 = (1.55 ± 0.5) × 1012 cm3 mol–1 s–1. In the temperature range of interest to atmospheric chemistry, Cox and co-workers4–8 in a series of studies, some by relative-rate methods, reported the rate constant at ambient temperature to fall within the range of (1–4) × 1012 cm3 mol–1 s–1 , independent of pressure. In a separate set of experiments, carried out in the temperature range 278–342 K, Jenkin and Cox obtained k1 = 1.1 × 1013 exp (–390/T) cm3 mol–1 s–1 with a small positive activation energy, 3.2 kJ mol–1.8 More recently, Burkholder et al.9 determined the rate constant by LIF (laser-induced fluorescence) for OH detection and reported k1 = (1.7 ± 0.8) × 1012 exp [(+260 ± 70)/T] cm3 mol–1 s–1 for the temperature range 298–373 K.DOI: 10.1039/b007803o PhysChemComm, 2000, 13 2. Computational methods 2.1. Ab initio calculations Geometry optimization and vibrational frequency calculations were carried out at the hybrid density functional B3LYP method13 with the 6-311G(d) basis set.14 In order to confirm that a specific transition state connects the designated local minima, we also performed intrinsic reaction coordinate (IRC) calculations15 at the B3LYP/6-311G(d) level of theory. The potential energy hypersurface (PES) was calculated with the G2M method,16 which approximates the high level RCCSD(T)/6-311+G(3df,2p) method, using a series of single-point calculations to improve the energies through basis set expansion, electron energy correlation and systematic error corrections.In this study, the following formalism applies,E[G2M(cc3)] = E E(HLC) + ZPE[B3LYP/6-311G(d)], where bas E E E(cc) + ’ + Ebas = E[PMP4/6-311G(d,p)] E(+) = E[PMP4/6-311+G(d,p)] – E[PMP4/6-311G(d,p)] E(2df) = E[PMP2/6-311G(2df,p)] – E[PMP2/6- 311G(d,p)] E(cc) = E[CCSD(T)/6-311G(d,p)] – E[PMP4/6- 311G(d,p)] ' = E[MP2/6-311+G(3df,2p)] – E[MP2/6-311G(2df,p)] – E[MP2/6-311 + G(d,p)] + E[MP2/6-311G(d,p)]. ZPE stands for zero-point energy corrections. The empirical "higher level correction" is given in hartrees by E(HLC) = 0.001(–5.63n – 0.19n ) for open shells and E(HLC) = 0.001(–5.45n – 0.19n ) for closed shells, where n and n are the numbers of
ISSN:1460-2733
DOI:10.1039/b007803o
出版商:RSC
年代:2000
数据来源: RSC
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