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Chemical Physics Letters 448 (2007) 46–48 www.elsevier.com/locate/cplett
Putative mechanisms of peroxybicarbonate formation Alice Maetzke a, Svend J. Knak Jensen b
, Imre G. Csizmadia
a Department of Chemistry, Langelandsgade 140, University of Aarhus, DK-8000 Aarhus C, Denmark Lash Miller Chemical Laboratories, 80 St. George Street, University of Toronto, Toronto, Ontario, Canada M5S 3H6
Received 22 June 2007; in ﬁnal form 21 September 2007 Available online 29 September 2007
Abstract The mechanism for the formation of the oxidant peroxymonocarbonate from hydrogen peroxide and bicarbonate/carbon dioxide in aqueous solution is studied by electron structure- and statistical mechanics techniques. It is found that the reaction proceeds through a single transition state. The activation energy is 83.3 kJ/mol at the highest level of theory [CCSD(T)/aug-cc-pvdz//B3LYP/aug-cc-pvdz]. However, it is reduced by 17% when the counter ion (viz. NHþ 4 ) is incorporated in the calculations. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction The oxidizing properties of hydrogen peroxide are increased in the presence of activators. The activation of hydrogen peroxide is typically achieved through the formation of peroxyacids, which generally are more reactive than hydrogen peroxide itself. The reaction of hydrogen peroxide with bicarbonate/carbon dioxide is an example of such activation. It has recently been investigated by spectroscopic and kinetic techniques, see, e.g., [1–3] and references therein. It is concluded that the oxidant peroxymonocarbonate anion, HCO 4 , is formed in the reaction: HCO 3 þ H2 O2 ! HCO4 þ H2 O
The production of peroxymonocarbonate is reported to be completed within minutes in mixed alcohol and water solvents with pH close to neutral. Since hydrogen peroxide is an oxidant, which may be found in physiological systems, and bicarbonate/carbon dioxide is abundant in aerobic systems the reaction may play an important role in oxidative stress warranting investigations of the relevant
Corresponding author. Fax: +45 8619 6199. E-mail address: [email protected]
(S.J. Knak Jensen).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.065
reactions of peroxymonocarbonate. Here we report an investigation of the reaction mechanism for (1) using electron structure calculations. 2. Computational details The calculations use the software package GAUSSIAN 03 . The chosen level of theory is the Lee, Yang and Parr correlation functional (B3LYP) [5,6] implementation of the density functional theory. The calculations use the basis sets 6-311+G(d,p) and aug-cc-pvdz to probe the variation of the theoretical data with the basis set. The transition state was obtained using the Synchronous Transit-Guided Quasi-Newton method . The nature of the transition state was veriﬁed by a single imaginary frequency and intrinsic reaction coordinates (IRC) calculations were performed to identify the species linked by the transition state. The geometries optimized at the various B3LYP levels in the gas-phase were used to perform single point SCRF energy calculations in aqueous solution in order to estimate the thermodynamic properties of the species. Tomasi’s standard polarisable continuum model (PCM) [8–10] was used with the atomic radii taken from the Universal Force Field (UFF) . The reliability of the PCM calculations is assessed in the discussion. The energies of the species were also estimated using the coupled cluster technique on geometries derived from the
A. Maetzke et al. / Chemical Physics Letters 448 (2007) 46–48
B3LYP calculations, B3LYP/aug-cc-pvdz.
3. Results and discussion In Fig. 1 we show a reaction path that leads from a complex, RI, of the reactants to a planar product complex, PI, via a transition state, TSI. The dominant motion in the rearrangement mode in TSI involves a hydrogen atom from the hydrogen peroxide moiety. This atom oscillates between two oxygen atoms associated with the hydroxyl group (O3) and the hydrogen peroxide moiety (O5), respectively. The oscillation is coupled to the C–O3 and C–O5 bonds in an asymmetric way, i.e., when the H-atom moves towards O3 the C–O3 bond becomes longer while the C– O5 bond is shortened and likewise in the case of O5. We have also considered the presence of a NHþ 4 counter ion, which may facilitate the reaction by oﬀering the leaving group (H2O) a possibility to engage in hydrogen bonding. The transition state was obtained by placing a NHþ 4 ion in various positions in TSI deemed energetically favourable and optimizing the whole complex to a transition state followed by IRC calculations. The path with the lowest activation energy is shown in Fig. 2. The dominant motion in TSII is a ‘Grotthus-type’ motion of the atoms in the eight membered ring, indicated by the double-headed arrows in Fig. 2. TSII is linked to the product complex, PII, which looks like a complex of H2O, NH3 and monoperoxycarbonic acid, H2CO4, and to a reactant complex RII, consisting of H2O2, CO2, NH3, and H2O. This corresponds to the reaction: H2 O2 þ CO2 þ NH3 þ H2 O ! H2 CO4 þ NH3 þ H2 O
Fig. 2. Reaction path II for reaction (2). The symbols are explained in Fig. 1.
In Table 1 we have summarized the activation energies for the forward- and the reverse reaction paths in Figs. 1 and 2 calculated with the B3LYP and CCSD(T) methods with various basis sets. Table 1 shows that there is very little diﬀerence between activation energies from the two methods in the case of reaction (1) whereas for reaction (2) the CCSD(T) method gives estimates which are consistently larger than those of the B3LYP method by as much as 18 kJ/mol. An assessment of the reliability of the data in Table 1 may be obtained by comparing the energy diﬀerence between the product- and the reactant complex (Di ¼ DiR DiF , i ¼ I and II) to the reaction enthalpy ðDH ¼ 8 6 kJ=molÞ. DH° is estimated from the measured temperature dependence of the equilibrium constant . Table 1 shows that the best agreement between calculated and observed reaction energies is obtained for the reaction, that incorporates the NHþ 4 ion. It appears from Table 1 that inclusion of the counter ion reduces the activation energy for both the forward and reverse reaction. At the highest level of theory the reduction for the forward reaction amounts to 17%.
Table 1 Energy diﬀerences (kJ/mol) for reaction paths in Figs. 1 and 2 calculated at various levels of quantum mechanical approximations in aqueous phase (PCM)
Fig. 1. Reaction path I for reaction (1). The structures have been calculated at the B3LYP/aug-cc-pvdz level of theory. The double-headed arrows indicate the dominant motion in the rearrangement mode and dashed lines indicate hydrogen bonds. The relative energies of the various ˚. structures are not to scale. The distances are in A
Level of theory
B3LYP/6-311+G(d,p) B3LYP/aug-cc-pvdz CCSD(T)/aug-cc-pvdz//B3LYP/ 6-311+G(d,p) CCSD(T)/aug-cc-pvdz//B3LYP/ aug-cc-pvdz
79.5 92.2 24.6 57.7 32.4 25.3 90.5 105.7 15.2 52.6 42.3 10.3 82.4 98.2 15.8 67.4 50.4 17.0
Di ¼ DiR DiF , i ¼ I and II.
DIIF DIIR DII
99.4 16.1 69.3 50.8 18.5
A. Maetzke et al. / Chemical Physics Letters 448 (2007) 46–48
Electron structure calculations have been used to ﬁnd a reaction path with a single transition state for the production of peroxymonocarbonate from hydrogen peroxide and bicarbonate/carbon dioxide. Using the polarized continuum model, the activation energy for the forward reaction is estimated to be 83.3 kJ/mol in aqueous solution at the highest theoretical level investigated [CCSD(T)]. This activation energy is reduced by 17% if the NHþ 4 counter ion is included in calculations.
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Acknowledgement This work was supported by a grant, HDW-0107-13-(AU), from the Danish Center for Scientiﬁc Computing.