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J. Phys. Chem. 1996, 100, 100-110
Electron Affinity of Hydrogen Peroxide and the [H2,O2]•- Potential Energy Surface. A Comparative DFT and ab Initio Study Jan Hrusˇa´ k* J. HeyroVsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejsˇkoVa 3, CZ 18223 Prague 8, Czech Republic
Heike Friedrichs and Helmut Schwarz* Institut fu¨ r Organische Chemie, Technische UniVersita¨ t Berlin, Strasse des 17.Juni 135, D-10623 Berlin, Germany
Holy Razafinjanahary and Henry Chermette* Institut de Physique Nucle´ aire de Lyon, UniVersite´ Claude Bernard, 43, BouleVard du 11 NoVembre 1918, F-69622 Villeurbanne Cedex, France ReceiVed: July 17, 1995; In Final Form: September 28, 1995X
Standard ab initio and density functional calculations have been applied to the reaction O•- + H2O f [H2O2]•f OH- + OH•. While two intermediates are found as minima on the anionic potential energy surface, neither of them is directly related to the structure of neutral hydrogen peroxide. The results of different combinations of exchange and correlation functionals are systematically compared to each other and to MP2, MP4SDTQ, and CCSD(T) calculated data. The role of the basis set and the Hartree-Fock exchange in the hybrid DFT scheme is discussed. While for the two minima a reasonable agreement between all the methods was found, the geometries of the located transition structures strongly depend on the method and basis set used.
Introduction The investigation of anion-molecule reactions in the gas phase is a long standing problem, closely connected with the development of advanced experimental techniques. Due to the fact that the generation of anions often yields fairly low ion intensities and, thus, makes anion-neutral reactions difficult to observe, most studies have been dealing with positive ions. One anionic system that has attracted particular attention over the past 20 years and that is not yet completely understood concerns the reaction of O•- with H2O. As early as 1969 cross sections for that reaction were measured1 and attempts were made to elucidate the reaction mechanism that leads to the products OH• and OH-; also the rate constant for that particular reaction was determined.2 The question arose whether the reaction follows a hydrogen-stripping mechanism or a compound mechanism1 involving a long-lived H2O2•- intermediate. Numerous experiments were conducted for that purpose including energy and time of flight analysis,3 selected ion flow tube experiments,4 pulsed high-pressure mass spectrometry,5 and kinetic energy release (KER) measurements.6 In the context of atmospheric chemistry focusing on the degradation of ozone, the neutral counterpart for the reaction, i.e.
O(3P, 1D) + H2O(1A1) f 2OH•(2Π)
(1)
which serves as a major source of tropospheric OH radicals, has also been investigated.7,8 In addition to the classical hydrogen peroxide, an “oxywater” complex has been postulated. However, the experimental verification of these intermediates and the elucidation of the reaction mechanism are still lacking. X
Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0100$12.00/0
Similarly, for the analogous anionic reaction, the detailed pathways of rearrangement and the elucidation of the involved structures of [H2,O2]•- still pose a challenge for experimentalists and theoreticians. By using isotopically labeled compounds, Lifshitz9 observed the following reactions
O•- + H218O f 18O•- + H216O
(2)
O•- + H218O f 16OH- + 18OH•
(3)
O•- + H218O f 18OH- + 16OH•
(4)
16
16
16
in an ion beam collision chamber at low kinetic energies. The exchange of the oxygen atoms (eq 2) clearly pointed toward a symmetric intermediate in which the oxygen atoms were equally likely to accept the negative charge in the decomposition step. Lifshitz therefore envisioned the reaction sequence to proceed via two intermediates. First, an [O•- H2O] cluster (1) is formed, which then most likely rearranges into an [OH- OH•] ion dipole complex (2). The latter dissociates to the products OH- and OH•. The complexation energy in 1 was estimated to be about -30 kcal/mol9 (compared to separate H2O + O•-) and in 2 to be about -25 kcal/mol (relative to OH- and OH•). A requirement for the barrier of the reaction 1 f 2 is that it must be low enough to allow the observed oxygen atom equilibration (eqs 3 and 4). On the basis of an RRKM calculation,9 the isomers 1 and 2 also possess lifetimes that are long enough to allow such a process. Investigating the photodissociation of (CO3•-)‚H2O, Bowers and co-workers10 in 1991 reported the formation of CO2 and [O•- H2O]. Their experimental results were supported by ab initio MO calculations (MP2/6-31G(d,p) and MP2/6-31+G(d)), and structures 1 and 3 (Chart 1) were suggested for the [O•- H2O] product. © 1996 American Chemical Society
Electron Affinity of Hydrogen Peroxide CHART 1: MP2/6-31G(d) Optimized Structures (Bond Length in Å) for the Coordination of the Oxygen Anion to Water (Taken from Ref 10)
Formally, 1 corresponds to the oxygen water adduct postulated earlier by Lifshitz.9 However, Lifshitz’s second structure (2) was not reported by these authors. Bowers’ second isomer (3), which possesses C2V symmetry, according to our calculations, corresponds to a transition structure (Vide infra). Recently, Buntine et al.11 have investigated the photodissociation of the (O2•-)‚H2O complex. At a photoexcitation energy of 5.83 eV product signals were observed for O•-, OH-, and [O•- H2O]. The dissociation energy of the complex [O•- H2O] into O•- and H2O was bracketed to 25.5-29.9 kcal/mol,12 in agreement with the previous estimate by Lifshitz9 (
