Ab initio studies of ClOx reactions. VIII. Isomerization and decomposition of ClO2 radicals and related bimolecular processes

Chlorine Dioxide: An Exception that Proves the Rules of Localized Chemical Bonding

We employ natural bond orbital (NBO) and natural resonance theory (NRT) tools to analyze the enigmatic properties of the C2v-symmetric isomer of chlorine dioxide radical (ClO2), whose many challenges to Pauling-type localized bonding concepts were recognized by Linus Pauling himself. Although spin-contamination is minimal in this species, ClO2 exhibits an unusually strong form of "different Lewis structures for different spins" bonding pattern, intrinsically outside the framework of "maximal pairing" concepts. We show how the novel spin-unpaired donor-acceptor interactions lead to weakened bonding in the supramolecular domain of polyradical (ClO2)n homoclusters and aqueous ClO2(H2O)n heteroclusters. Despite feeble binding energies and large inter-radical separations, the polyradical clusters are found to maintain coherent spin patterns in each cluster component, attesting to the quantal donor-acceptor nature of their interactions and the cooperative and anticooperative couplings that govern intra- and intermolecular spin distributions in such spin-clusters.

High resolution ro-vibrational analysis of molecules in doublet electronic states: the ν1 fundamental of chlorine dioxide (16O35Cl16O) in the X2B1 electronic ground state

We report the spectrum of the ν1 fundamental of chlorine dioxide centered in the infrared atmospheric window at 945.592 cm-1 measured with essentially Doppler limited resolution at an instrumental line width of 0.001 cm-1 using the Zürich prototype ZP2001 Bruker IFS 125 HR Fourier transform infrared spectrometer. The ro-vibrational line analysis is carried out with an improved effective Hamiltonian and a newly developed computer code ROVDES for the ro-vibrational spectra of open-shell free radical molecules including spin-rotation interactions. Accurate values of rotational, centrifugal and spin-rotational parameters were determined for 16O35Cl16O in the vibronic ground state X2B1 from more than 3500 ground state combination differences. The 7239 assigned transitions for the ν1 fundamental with Nmax = 76 and Kmaxa = 26 provide a set of 32 accurate effective Hamiltonian parameters for the ν1 fundamental (v1v2v3) = (100) (21 rotational and centrifugal distortion parameters and 11 spin-rotational interaction parameters). This effective Hamiltonian (A - reduction and Ir - representation) reproduces 1703 upper state energies from the experiment with a root-mean-square deviation drms = 1.67 × 10-4 cm-1 and the 7239 transition wavenumbers with drms = 3.45 × 10-4 cm-1. Our results provide a considerable improvement over previous results with which we compare and should provide a benchmark for theoretical studies with applications to atmospheric spectroscopy and laser chemistry, which are discussed in relation to our spectra.

Basis Set Effects in the Description of the Cl-O Bond in ClO and XClO/ClOX Isomers (X = H, O, and Cl) Using DFT and CCSD(T) Methods

The performance of a group of density functional methods of progressive complexity for the description of the ClO bond in a series of chlorine oxides was investigated. The simplest ClO radical species and the two isomeric structures XClO/ClOX for each X = H, Cl, and O were studied using the PW91, TPSS, B3LYP, PBE0, M06, M06-2X, BMK, and B2PLYP functionals. Geometry optimizations and reaction enthalpies and enthalpies of formation for each species were calculated using Pople basis sets and the (aug)-cc-pVnZ Dunning sets, with n = D, T, Q, 5, and 6. For the calculation of enthalpies of formation, atomization and isodesmic reactions were employed. Both the precision of the methods with respect to the increase of the basis sets, as well as their accuracy, were gauged by comparing the results with the more accurate CCSD(T) calculations, performed using the same basis sets as for the DFT methods. The results obtained employing composite chemical methods (G4, CBS-QB3, and W1BD) were also used for the comparisons, as well as the experimental results when they are available. The results obtained show that error compensation is the key for successful description of molecular properties (geometries and energies) by carefully selecting the method and basis sets. In general, expansion of the one-electron basis set to the limit of completeness does not improve results at the DFT level, but just the opposite. The enthalpies of formation calculated at the CCSD(T)/aug-cc-pV6Z for the species considered are generally in agreement with experimental determinations and the most accurate theoretical values. Different sources of error in the calculations are discussed in detail.

Mechanism and Rate of Thermal Decomposition of Hexachlorocyclopentadiene and Its Importance in PCDD/F Formation from the Combustion of Cyclodiene Pesticides

Thermal decomposition of hexachlorocyclopentadiene (HCCP) has been studied in inert gas and under oxidative conditions in a silica flow reactor at a residence time of 5.0 s between 690 and 923 K and 1 atm pressure. Pyrolysis was initiated by Cl bond fission to form pentachlorocyclopentadienyl radical; two such radicals then combined to undergo a series of intramolecular rearrangements and Cl fissions, producing principally octachloronaphthalene (8ClNP) and Cl₂. This process has been studied by quantum chemical calculation, and a reaction potential energy surface has been developed. The rate constant of initial Cl atom fission has been calculated by canonical variational transition state theory as k = 1.45 × 10¹⁵ exp(-222 ± 9 kJ mol^-1/RT) s^-1 between 500 and 2000 K. A minimal kinetic model was developed to model the decomposition and major products. Oxidative decomposition was studied in nitrogen with O₂ contents of 1, 6, 12, and 20 mol %. Increasing O₂ to 6-8% increased the rate of decomposition of HCCP and decreased the yield of 8ClNP. Above 823 K, hexachlorobenzene (HCB) and CO became major products. The oxidative reaction has also been studied quantum chemically. At high O₂ content (>∼10%), the rate of decomposition of HCCP declined as did yields of 8ClNP and HCB, but CO yields increased.

Gas Phase Thermal Oxidation of Endosulfan and Formation of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans

This paper investigates the thermal decomposition of technical endosulfan under oxidative conditions and the subsequent formation of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans (PCDD/F, dioxins), and their precursors. Both quantum chemical calculations and laboratory experiments were employed to investigate the pathways of oxidation of endosulfan. The laboratory scale apparatus used consists of a tubular reactor and product collection system and analysis train. The results report the effect of temperature (523-923 K) and O2 concentration on PCDD/F formation in a N2 bath gas at a residence time of 5 s. The decomposition of endosulfan produces two types of PCDD/F precursors, involving all chlorinated benzenes (CBz) and chlorinated phenols (CPh). Oxidation of endosulfan favors the formation of PCDF over PCDD. Octachlorodibenzofuran is the most abundant homologue group detected in all experiments. The maximum emission factor for PCDD/F was observed at 923 K and O2 content of 6% and corresponds to 64 ng TEQ-WHO2005 per mg of endosulfan and a total dioxin concentration of 1131 ng/mg of endosulfan.

Perchlorate formation on Mars through surface radiolysis-initiated atmospheric chemistry: A potential mechanism

Recent observations of the Martian surface by the Phoenix lander and the Sample Analysis at Mars indicate the presence of perchlorate (ClO₄^-). The abundance and isotopic composition of these perchlorates suggest that the mechanisms responsible for their formation in the Martian environment may be unique in our solar system. With this in mind, we propose a potential mechanism for the production of Martian perchlorate: the radiolysis of the Martian surface by galactic cosmic rays, followed by the sublimation of chlorine oxides into the atmosphere and their subsequent synthesis to form perchloric acid (HClO₄) in the atmosphere, and the surface deposition and subsequent mineralization of HClO₄ in the regolith to form surface perchlorates. To evaluate the viability of this mechanism, we employ a one-dimensional chemical model, examining chlorine chemistry in the context of Martian atmospheric chemistry. Considering the chlorine oxide, OClO, we find that an OClO flux as low as 3.2 × 10⁷ molecules cm^-2 s^-1 sublimated into the atmosphere from the surface could produce sufficient HClO₄ to explain the perchlorate concentration on Mars, assuming an accumulation depth of 30 cm and integrated over the Amazonian period. Radiolysis provides an efficient pathway for the oxidation of chlorine, bypassing the efficient Cl/HCl recycling mechanism that characterizes HClO₄ formation mechanisms proposed for the Earth but not Mars.

Dynamics of the O + ClO reaction: reactive and vibrational relaxation processes

Classical trajectories have been integrated to study the O + ClO reaction, both reactive and vibrational energy transfer processes, for the range of temperatures 100 ≤ T/K ≤ 500 using momentum Gaussian binning. The employed potential energy surface is the recently proposed single-sheeted double many-body expansion potential energy surface for the (2)A" ground-state of ClO2 based on multireference ab initio data. A capture-type regime with a room-temperature rate constant of (17.8 ± 0.5) × 10(-12) cm(3) s(-1) and temperature dependence of k(T/K)/cm(3) s(-1) = 22.4 × 10(-12) × T(-0.81) exp(-39.2/T) has been found. Although the value reported here is half of the experimental and recommended one, tentative explanations are given. Other dynamical attributes are also examined for the title reaction, with state-to-all and state-to-state vibrational relaxation and excitation rate constants reported for temperatures of relevance in stratospheric chemistry.

Ab initio chemical kinetic study on the reactions of ClO with C2H2 and C2H4

The mechanisms for the reactions of ClO with C(2)H(2) and C(2)H(4) have been investigated at the CCSD(T)/CBS level of theory. The results show that in both systems, the interaction between the Cl atom of the ClO radical and the triple and double bonds of C(2)H(2) and C(2)H(4) forms prereaction van der Waals complexes with the O-Cl bond pointing perpendicularly toward the π-bonds, both with 2.1 kcal/mol binding energies. The mechanism is similar to those of the HO-C(2)H(2)/C(2)H(4) systems. The rate constants for the low energy channels have been predicted by statistical theory. For the reaction of ClO and C(2)H(2), the main channels are the production of CH(2)CO + Cl (k(1a)) and CHCO + HCl (k(1b)), with k(1a) = 1.19 × 10(-15)T(1.18) exp(-5814/T) and k(1b) = 6.94 × 10(-21) × T(2.60) exp(-6587/T) cm(3) molecule(-1) s(-1). For the ClO + C(2)H(4) reaction, the main pathway leads to C(2)H(4)O + Cl (k(2a)) with the predicted rate constant k(2a) = 2.13 × 10(-17)T(1.52) exp(-3849/T) in the temperature range of 300-3000 K. These rate constants are pressure-independent below 100 atm.

Kinetic Study of the ClOO + NO Reaction Using Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy was used to study the reaction of ClOO with NO in 50-150 Torr total pressure of O2/N2 diluent at 205-243 K. A value of k(ClOO+NO) = (4.5 +/- 0.9) x 10(-11) cm3 molecule(-1) s(-1) at 213 K was determined (quoted uncertainties are two standard deviations). The yield of NO(2) in the ClOO + NO reaction was 0.18 +/- 0.02 at 213 K and 0.15 +/- 0.02 at 223 K. An upper limit of k(ClOO+Cl2) < 3.5 x 10(-14) cm3 molecule(-1) s(-1) was established at 213 K. Results are discussed with respect to the atmospheric chemistry of ClOO and other peroxy radicals.

Ab Initio Studies of ClOx Reactions: Prediction of the Rate Constants of ClO+NO for the Forward and Reverse Processes

The mechanisms for ClO+NO and its reverse reactions were investigated by means of ab initio molecular orbital and statistical theory calculations. The species involved were optimized at the B3LYP/6-311 +G(3df) level, and their energies were refined at the CCSD(T)/6-311+ G(3df)//B3LYP/6-311 + G(3df) level. Five isomers and the transition states among them were located. The relative stability of these isomers is ClNO2 > cis-ClONO > trans-ClONO > cis-OClNO>trans-OClNO. The heats of formation of the three most-stable isomers were predicted using isodesmic reactions by different methods. The predicted bimolecular reaction rate constant shows that, below 100 atm, the formation of Cl+NO2 is dominant and pressure-independent. The total rate constant can be expressed as: k(ClO+NO)= 1.43 x 10(-9)T(-083)exp(92/ T) cm3 molecule(-1)s(-1) in the temperature range of 200-1000 K, in close agreement with experimental data. For the reverse reaction, Cl+NO2-->ClNO2 and ClONO (cis and trans isomers), the sum of the predicted rate constants for the formation of the three isomers and their relative yields also reproduce the experimental data well. The predicted total third-order rate constants in the temperature range of 200-1000 K can be represented by: k0(He) = 4.89 x 10(-6)T(-5.85) exp(-796/T) cm6 molecule(-1)s(-1) and k0(N2) =5.72 x 10(-15)T(-5.80) exp(-814/T) cm6 molecule(-1)s(-1). The predicted high- and low-pressure limit decomposition rates of CINO2 in Ar in the temperature range 400-1500 K can be expressed, respectively, by: k-(ClNO2) = 7.25 x 10(19)T(-1.89) exp(-16875/T) s(-1) and kd(ClNO2) = 2.51 x 10(38)T(-6.8) exp(-18409/T) cm3 molecule(-1) s(-1). The value of k0(ClNO2) is also in reasonable agreement with available experimental data.