Negative-ion photoelectron spectroscopy, gas-phase acidity, and thermochemistry of the peroxyl radicals CH(3)OO and CH(3)CH(2)OO

The stability of oxygen-centered radicals and its response to hydrogen bonding interactions

The stability of various alkoxy/aryloxy/peroxy radicals, as well as TEMPO and triplet dioxygen (3 O2 ) has been explored at a variety of theoretical levels. Good correlations between RSEtheor and RSEexp are found for hybrid DFT methods, for compound schemes such as G3B3-D3, and also for DLPNO-CCSD(T) calculations. The effects of hydrogen bonding interactions on the stability of oxygen-centered radicals have been probed by addition of a single solvating water molecule. While this water molecule always acts as a H-bond donor to the oxygen-centered radical itself, it can act as a H-bond donor or acceptor to the respective closed-shell parent.

Experimental Compilation and Computation of Hydration Free Energies for Ionic Solutes

Although charged solutes are common in many chemical systems, traditional solvation models perform poorly in calculating solvation energies of ions. One major obstacle is the scarcity of experimental data for solvated ions. In this study, we release an experiment-based aqueous ionic solvation energy data set, IonSolv-Aq, that contains hydration free energies for 118 anions and 155 cations, more than 2 times larger than the set of hydration free energies for singly charged ions contained in the 2012 Minnesota Solvation Database commonly used in benchmarking studies. We discuss sources of systematic uncertainty in the data set and use the data to examine the accuracy of popular implicit solvation models COSMO-RS and SMD for predicting solvation free energies of singly charged ionic solutes in water. Our results indicate that most SMD and COSMO-RS modeling errors for ionic solutes are systematic and correctable with empirical parameters. We discuss two systematic offsets: one across all ions and one that depends on the functional group of the ionization site. After correcting for these offsets, solvation energies of singly charged ions are predicted using COSMO-RS to 3.1 kcal mol-1 MAE against a challenging test set and 1.7 kcal mol-1 MAE (about 3% relative error) with a filtered test set. The performance of SMD is similar, with MAE against those same test sets of 2.7 and 1.7 kcal mol-1. These results underscore the importance of compiling larger experimental data sets to improve solvation model parametrization and fairly assess performance.

Calculated and Empirical Values of Vibronic Transition Dipole Moments of Reactive Chemical Intermediates for Determination of Concentrations

Absorption spectroscopy has long been known as a technique for making molecular concentration measurements and has received enhanced visibility in recent years with the advent of new techniques, like cavity ring-down spectroscopy, that have increased its sensitivity. To apply the method, it is necessary to have a known molecular absorption cross section for the species of interest, which typically is obtained by measurements of a standard sample of known concentration. However, this method fails if the species is highly reactive, and indirect means for attaining the cross section must be employed. The HO₂ and alkyl peroxy radicals are examples of reactive species for which absorption cross sections have been reported. This work explores and describes for these peroxy radicals the details of an alternative approach for obtaining these cross sections using quantum chemistry methods for the calculation of the transition dipole moment upon whose square the cross section depends. Likewise, details are given for obtaining the transition moment from the experimentally measured cross sections of individual rovibronic lines in the near-IR Ã-X̃ electronic spectrum of HO₂ and the peaks of the rotational contours in the corresponding electronic transitions for the alkyl (methyl, ethyl, and acetyl) peroxy radicals. In the case of the alkyl peroxy radicals, good agreement for the transition moments, ≈20%, is found between the two methods. However, rather surprisingly, the agreement is significantly poorer, ≈40%, for the HO₂ radical. Possible reasons for this disagreement are discussed.

Damage of amino acids by aliphatic peroxyl radicals: a kinetic and computational study

Absolute second-order rate coefficients for the reaction of the N- and C-protected amino acids tyrosine (Tyr), tryptophan (Trp), methionine (Met) and proline (Pro) with triethylamine-derived aliphatic peroxyl radical TEAOO˙, which was used as a model for lipid peroxyl radicals, were determined using laser flash photolysis. For Ac-Tyr-OMe a rate coefficient of 1.4 × 10⁴ M^-1 s^-1 was obtained, whereas the reactions with Ac-Trp-OMe and Ac-Met-OMe were slower by a factor of 4 and 6, respectively. For the reaction with Ac-Pro-OMe only an upper value of 10³ M^-1 s^-1 could be determined, suggesting that Pro residues are not effective traps for lipid peroxyl radicals. Density functional theory (DFT) calculations revealed that the reactions proceed via radical hydrogen atom transfer (HAT) from the Cα position, indicating that the rate is determined by the exothermicity of the reaction. In the case of Ac-Tyr-OMe, HAT from the phenolic OH group is the kinetically preferred pathway, which shuts down when hydrogen bonding with an amine occurs. In an alkaline environment, where the phenolic OH group is deprotonated, the reaction is predicted to occur preferably at Cβ, likely through a proton-coupled electron transfer (PCET) mechanism.

Correcting the Experimental Enthalpies of Formation of Some Members of the Biologically Significant Sulfenic Acids Family

Sulfenic acids are important intermediates in the oxidation of cysteine thiol groups in proteins by reactive oxygen species. The mechanism is influenced heavily by the presence of polar groups, other thiol groups, and solvent, all of which determines the need to compute precisely the energies involved in the process. Surprisingly, very scarce experimental information exists about a very basic property of sulfenic acids, the enthalpies of formation. In this Article, we use high level quantum chemical methods to derive the enthalpy of formation at 298.15 K of methane-, ethene-, ethyne-, and benzenesulfenic acids, the only ones for which some experimental information exists. The methods employed were tested against well-known experimental data of related species and extensive CCSD(T) calculations. Our best results consistently point out to a much lower enthalpy of formation of methanesulfenic acid, CH₃SOH (Δ_fH⁰(298.15K) = -35.1 ± 0.4 kcal mol^-1), than the one reported in the NIST thermochemical data tables. The enthalpies of formation derived for ethynesulfenic acid, HC≡CSOH, +32.9 ± 1.0 kcal/mol, and benzenesulfenic acid, C₆H₅SOH, -2.6 ± 0.6 kcal mol^-1, also differ markedly from the experimental values, while the enthalpy of formation of ethenesulfenic acid CH₂CHSOH, not available experimentally, was calculated as -11.2 ± 0.7 kcal mol^-1.

Photoelectron Spectroscopy and High-Level Ab Initio Calculations of the Iodide-Methylperoxy Radical Complex

Anion photoelectron spectroscopy has been used to investigate the structure and dynamics of CH₃OOI^- van der Waals complexes. Peaks within the photoelectron spectrum are attributed to photodetachment to the perturbed ²P_3/2 state of I···CH₃OO (3.46 eV) and the two ²P states of bare iodine. A broad feature at 1.7-2.4 eV is attributed to detachment to the excited singlet states from two O₂^-···CH₃I complexes. This represents the first anion photoelectron spectroscopy of a halide-bound methylperoxy radical species. Complex structures have been optimized using MP2/aug-cc-pVQZ with single-point energies at W1w theory for ground-state complexes and NEVPT2 for photodetachment to excited O₂. Interactions are dominated by electrostatics, with the anion species interacting with the methyl pocket of the solvating molecule, suggesting conversion via an S_N2 mechanism, and excess energy leading to complex dissociation within the timescale of mass spectrometry. The calculated W1w Gibbs energies suggest that while an electron transfer (ET) pathway to conversion is available, it is comparatively unfavored.

Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications

We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XH_n in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H₂), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H₂) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

Alkoxy Radicals See the Light: New Paradigms of Photochemical Synthesis

Alkoxy radicals are highly reactive species that have long been recognized as versatile intermediates in organic synthesis. However, their development has long been impeded due to a lack of convenient methods for their generation. Thanks to advances in photoredox catalysis, enabling facile access to alkoxy radicals from bench-stable precursors and free alcohols under mild conditions, research interest in this field has been renewed. This review comprehensively summarizes the recent progress in alkoxy radical-mediated transformations under visible light irradiation. Elementary steps for alkoxy radical generation from either radical precursors or free alcohols are central to reaction development; thus, each section is categorized and discussed accordingly. Throughout this review, we have focused on the different mechanisms of alkoxy radical generation as well as their impact on synthetic utilizations. Notably, the catalytic generation of alkoxy radicals from abundant alcohols is still in the early stage, providing intriguing opportunities to exploit alkoxy radicals for diverse synthetic paradigms.

Synthesis, Electronic Spectroscopy, and Photochemistry of Methacrolein Oxide: A Four-Carbon Unsaturated Criegee Intermediate from Isoprene Ozonolysis

Ozonolysis of isoprene, one of the most abundant volatile organic compounds in the earth's atmosphere, generates the four-carbon unsaturated methacrolein oxide (MACR-oxide) Criegee intermediate. The first laboratory synthesis and direct detection of MACR-oxide is achieved through reaction of photolytically generated, resonance-stabilized iodoalkene radicals with oxygen. MACR-oxide is characterized on its first π* ← π electronic transition using a ground-state depletion method. MACR-oxide exhibits a broad UV-visible spectrum peaked at 380 nm with weak oscillatory structure at long wavelengths ascribed to vibrational resonances. Complementary theory predicts two strong π* ← π transitions arising from extended conjugation across MACR-oxide with overlapping contributions from its four conformers. Electronic promotion to the 1¹ππ* state agrees well with experiment, and results in nonadiabatic coupling and prompt release of O ¹D products observed as anisotropic velocity-map images. This UV-visible detection scheme will enable study of its unimolecular and bimolecular reactions under thermal conditions of relevance to the atmosphere.

Hydrogen peroxide as a hydride donor and reductant under biologically relevant conditions

Some ruthenium-hydride complexes react with O2 to yield H2O2, therefore the principle of microscopic reversibility dictates that the reverse reaction is also possible, that H2O2 could transfer an H- to a Ru complex. Mechanistic evidence is presented, using the Ru-catalyzed ABTS˙- reduction reaction as a probe, which suggests that a Ru-H intermediate is formed via deinsertion of O2 from H2O2 following coordination to Ru. This demonstration that H2O2 can function as an H- donor and reductant under biologically-relevant conditions provides the proof-of-concept that H2O2 may function as a reductant in living systems, ranging from metalloenzyme-catalyzed reactions to cellular redox homeostasis, and that H2O2 may be viable as an environmentally-friendly reductant and H- source in green catalysis.

tert-Butyl peroxy radical: ground and first excited state energetics and fundamental frequencies

The lowest adiabatic electronic transition origin and fundamental vibrational frequencies are computed, with high accuracy, for the tert-butyl peroxy radical.

Radical Thermometers, Thermochemistry, and Photoelectron Spectra: A Photoelectron Photoion Coincidence Spectroscopy Study of the Methyl Peroxy Radical

We investigated the simplest alkylperoxy radical, CH₃OO, formed by reacting photolytically generated CH₃ radicals with O₂, using the new combustion reactions followed by photoelectron photoion coincidence (CRF-PEPICO) apparatus at the Swiss Light Source. Modeling the experimental photoion mass-selected threshold photoelectron spectrum using Franck-Condon simulations including transitions to triplet and singlet cationic states yielded the adiabatic ionization energy of 10.265 ± 0.025 eV. Dissociative photoionization of CH₃OO generates the CH₃⁺ fragment ion at the appearance energy of 11.164 ± 0.010 eV. Combining these two values with Δ_fH_0K^°(CH₃) yields Δ_fH_0K^°(CH₃OO) = 22.06 ± 0.97 kJ mol^-1, reducing the uncertainty of the previously determined value by a factor of 5. Statistical simulation of the CH₃OO breakdown diagram provides a molecular thermometer of the free radical's internal temperature, which we measured to be 330 ± 30 K.

Photodissociation dynamics of the simplest alkyl peroxy radicals, CH3OO and C2H5OO, at 248 nm

The photodissociation dynamics of the simplest alkyl peroxy radicals, methyl peroxy (CH₃OO) and ethyl peroxy (C₂H₅OO), are investigated using fast beam photofragment translational spectroscopy. A fast beam of CH₃OO^- or C₂H₅OO^- anions is photodetached to generate neutral radicals that are subsequently dissociated using 248 nm photons. The coincident detection of the photofragment positions and arrival times allows for the determination of mass, translational energy, and angular distributions for both two-body and three-body dissociation events. CH₃OO exhibits repulsive O loss resulting in the formation of O(¹D) + CH₃O with high translational energy release. Minor two-body channels leading to OH + CH₂O and CH₃O + O(³P) formation are also detected. In addition, small amounts of H + O(³P) + CH₂O are observed and attributed to O loss followed by CH₃O dissociation. C₂H₅OO exhibits more complex dissociation dynamics, in which O loss and OH loss occur in roughly equivalent amounts with O(¹D) formed as the dominant O atom electronic state via dissociation on a repulsive surface. Minor two-body channels leading to the formation of O₂ + C₂H₅ and HO₂ + C₂H₄ are also observed and attributed to a ground state dissociation pathway following internal conversion. Additionally, C₂H₅OO dissociation yields a three-body product channel, CH₃ + O(³P) + CH₂O, for which the proposed mechanism is repulsive O loss followed by the dissociation of C₂H₅O over a barrier. These results are compared to a recent study of tert-butyl peroxy (t-BuOO) in which 248 nm excitation results in three-body dissociation and ground state two-body dissociation but no O(¹D) production.

Ethylperoxy radical: approaching spectroscopic accuracy via coupled-cluster theory

Highly reliable ground and excited state properties of the conformers of ethylperoxy radical are predicted using coupled-cluster theory. This research has implications for future characterization of intermediates in tropospheric and low-temperature combustion processes.

Photoelectron spectroscopy of the hydroxymethoxide anion, H2C(OH)O. J Chem Phys 2016;145:124317. [PMID: 27782682 DOI: 10.1063/1.4963225]

We report the negative ion photoelectron spectroscopy of the hydroxymethoxide anion, H₂C(OH)O^-. The photoelectron spectra show that 3.49 eV photodetachment produces two distinct electronic states of the neutral hydroxymethoxy radical (H₂C(OH)O^⋅). The H₂C(OH)O^⋅ ground state (X̃ ²A) photoelectron spectrum exhibits a vibrational progression consisting primarily of the OCO symmetric and asymmetric stretches, the OCO bend, as well as combination bands involving these modes with other, lower frequency modes. A high-resolution photoelectron spectrum aids in the assignment of several vibrational frequencies of the neutral H₂C(OH)O^⋅ radical, including an experimental determination of the H₂C(OH)O^⋅ 2ν₁₂ overtone of the H-OCO torsional vibration as 220(10) cm^-1. The electron affinity of H₂C(OH)O^⋅ is determined to be 2.220(2) eV. The low-lying à ²A excited state is also observed, with a spectrum that peaks ∼0.8 eV above the X̃ ²A state origin. The à ²A state photoelectron spectrum is a broad, partially resolved band. Quantum chemical calculations and photoelectron simulations aid in the interpretation of the photoelectron spectra. In addition, the gas phase acidity of methanediol is calculated to be 366(2) kcal mol^-1, which results in an OH bond dissociation energy, D₀(H₂C(OH)O-H), of 104(2) kcal mol^-1, using the experimentally determined electron affinity of the hydroxymethoxy radical.

An Evaluation of Gas Phase Enthalpies of Formation for Hydrogen-Oxygen (HxOy) Species

We have compiled gas phase enthalpies of formation for nine hydrogen-oxygen species (HxOy) and selected recommended values for H, O, OH, H2O, HO2, H2O2, O3, HO3, and H2O3. The compilation consists of values derived from experimental measurements, quantum chemical calculations, and prior evaluations. This work updates the recommended values in the NIST-JANAF (1985) and Gurvich et al. (1989) thermochemical tables for seven species. For two species, HO3 and H2O3 (important in atmospheric chemistry) and not found in prior thermochemical evaluations, we also provide supplementary data consisting of molecular geometries, vibrational frequencies, and torsional potentials which can be used to compute thermochemical functions. For all species, we also provide supplementary data consisting of zero point energies, vibrational frequencies, and ion reaction energetics.

Highly Oxidized RO2 Radicals and Consecutive Products from the Ozonolysis of Three Sesquiterpenes

The formation of highly oxidized multifunctional organic compounds (HOMs) from the ozonolysis of three sesquiterpenes, α-cedrene, β-caryophyllene, and α-humulene, was investigated for the first time. Sesquiterpenes contribute 2.4% to the global carbon emission of biogenic volatile organic compounds (BVOCs) and can be responsible for up to 70% of the regional BVOC emissions. HOMs were detected with chemical ionization-atmospheric pressure interface-time-of-flight mass spectrometry and nitrate and acetate ionization. Acetate ions were more sensitive toward highly oxidized RO2 radicals containing a single hydroperoxide moiety. Under the chosen reaction conditions, product formation was dominated by highly oxidized RO2 radicals which react with NO, NO2, HO2, and other RO2 radicals under atmospheric conditions. The ozonolysis of sesquiterpenes resulted in molar HOM yields of 0.6% for α-cedrene (acetate), 1.8% for β-caryophyllene (acetate), and 1.4% for α-humulene (nitrate) afflicted with an uncertainty factor of 2. Molar yields of highly oxidized RO2 radicals were identical with HOM yields measuring the corresponding closed-shell products. HOM formation from ozonolysis of α-cedrene was explained by an autoxidation mechanism initiated by ozone attack at the double bond similar to that found in the ozonolysis of cyclohexene and limonene.

Cyclopentadienone Oxidation Reaction Kinetics and Thermochemistry for the Alcohols, Hydroperoxides, and Vinylic, Alkoxy, and Alkylperoxy Radicals

Cyclopentadienone has one carbonyl and two olefin groups resulting in 4n + 2 π-electrons in a cyclic five-membered ring structure. Thermochemical and kinetic parameters for the initial reactions of cyclopentadienone radicals with O2 and the thermochemical properties for cyclopentadienone-hydroperoxides, alcohols, and alkenyl, alkoxy, and peroxy radicals were determined by use of computational chemistry. The CBS-QB3 composite and B3LYP density functional theory methods were used to determine the enthalpies of formation (ΔfH°298) using the isodesmic reaction schemes with several work reactions for each species. Entropy and heat capacity, S°(T) and Cp°(T) (50 K ≤ T ≤ 5000 K) are determined using geometric parameters, internal rotor potentials, and frequencies from B3LYP/6-31G(d,p) calculations. Standard enthalpies of formation are reported for parent molecules as cyclopentadienone, cyclopentadienone with alcohol, hydroperoxide substituents, and the cyclopentadienone-yl vinylic, alkoxy, and peroxy radicals corresponding to loss of a hydrogen atom from the carbon and oxygen sites. Entropy and heat capacity vs temperature also are reported for the parent molecules and for radicals. The thermochemical analysis shows The R(•) + O2 well depths are deep, on the order of 50 kcal mol(-1), and the R(•) + O2 reactions to RO + O (chain branching products) for cyclopentadienone-2-yl and cyclopentadienone-3-yl have unusually low reaction (ΔHrxn) enthalpies, some 20 or so kcal/mol below the entrance channels. Chemical activation kinetics using quantum RRK analysis for k(E) and master equation for falloff are used to show that significant chain branching as a function of temperature and pressure can occur when these vinylic radicals are formed.

Antioxidant Activity of Magnolol and Honokiol: Kinetic and Mechanistic Investigations of Their Reaction with Peroxyl Radicals

Magnolol and honokiol, the bioactive phytochemicals contained in Magnolia officinalis, are uncommon antioxidants bearing isomeric bisphenol cores substituted with allyl functions. We have elucidated the chemistry behind their antioxidant activity by experimental and computational methods. In the inhibited autoxidation of cumene and styrene at 303 K, magnolol trapped four peroxyl radicals, with a kinh of 6.1 × 10(4) M(-1) s(-1) in chlorobenzene and 6.0 × 10(3) M(-1) s(-1) in acetonitrile, and honokiol trapped two peroxyl radicals in chlorobenzene (kinh = 3.8 × 10(4) M(-1) s(-1)) and four peroxyl radicals in acetonitrile (kinh = 9.5 × 10(3) M(-1) s(-1)). Their different behavior arises from a combination of intramolecular hydrogen bonding among the reactive OH groups (in magnolol) and of the OH groups with the aromatic and allyl π-systems, as confirmed by FT-IR spectroscopy and DFT calculations. Comparison with structurally related 3,3',5,5'-tetramethylbiphenyl-4,4'-diol, 2-allylphenol, and 2-allylanisole allowed us to exclude that the antioxidant behavior of magnolol and honokiol is due to the allyl groups. The reaction of the allyl group with a peroxyl radical (C-H hydrogen abstraction) proceeds with rate constant of 1.1 M(-1) s(-1) at 303 K. Magnolol and honokiol radicals do not react with molecular oxygen and produce no superoxide radical under the typical settings of inhibited autoxidations.

Sequential Oxidative α-Cyanation/Anti-Markovnikov Hydroalkoxylation of Allylamines

Iron-catalyzed oxidative α-cyanations at tertiary allylamines in the allylic position are followed by anti-Markovnikov additions of alcohols across the vinylic CC double bonds of the initially generated α-amino nitriles. These consecutive reactions generate 2-amino-4-alkoxybutanenitriles from three reactants (allylamines, trimethylsilyl cyanide, and alcohols) in one reaction vessel at ambient temperature.

Understanding the antioxidant behavior of some vitamin molecules: a first-principles density functional approach

The structures, energetics, vertical and adiabatic ionization potentials, electron affinities, and global reactivity descriptors of antioxidant vitamins (both water- and fat-soluble) in neutral, positively charged, and negatively charged states were investigated theoretically. We worked within the framework of first-principles density functional theory (DFT), using the B3LYP functional and both localized (6-311G+(d,p) and plane-wave basis sets. Solvent effects were modeled via the polarizable continuum model (PCM), using the integral equation formalism variant (IEFPCM). From the computed structural parameters, ionization potentials, electron affinities, and spin densities, we deduced that these vitamins prefer to lose electrons to neutral reactive oxygen species (·OH and ·OOH), making them good antioxidants. Conceptual DFT was used to determine global chemical reactivity parameters. The computed chemical hardnesses showed that these antioxidant vitamins are more reactive than neutral reactive oxygen species (ROS), thus supporting their antioxidant character towards these species. However, in the neutral state, these vitamins do not act as antioxidants for [Formula: see text]. The reactivity of vitamins towards ROS depends on the nature of the solvent. Amongst the ROS, ·OH has the greatest propensity to attract electrons from a generic donor. The reactivities of fat-soluble vitamins towards neutral reactive oxygen species were found to be larger than those of water-soluble vitamins towards these species, showing that the former are better antioxidants.

Substituent effects on O-H bond dissociation enthalpies: a computational study

Bond dissociation enthalpies (BDEs) can exhibit dramatic variations resulting from substituent effects. The remarkable range of experimental OH bond dissociation enthalpies have been reproduced using CBS-APNO calculations with very good accuracy, so we have employed these calculations to extend the available BDE data. The effect on these BDEs of lone pairs on the atom adjacent to oxygen shows that conjugation in the product radicals is the most important interaction leading to the wide range of values. The BDE's were found to be linearly related to both the spin density at the radical center and to the change in X-O bond order in going from X-O-H to X-O·.

Disproportionation of bromous acid HOBrO by direct O-transfer and via anhydrides O(BrO)2 and BrO-BrO2. An ab initio study of the mechanism of a key step of the Belousov-Zhabotinsky oscillating reaction

The results are reported of an ab initio study of the thermochemistry and of the kinetics of the HOBrO disproportionation reaction 2HOBrO (2) ⇄ HOBr (1) + HBrO(3) (3), reaction ( R4' ), in gas phase (MP2(full)/6-311G*) and aqueous solution (SMD(MP2(full)/6-311G*)). The reaction energy of bromous acid disproportionation is discussed in the context of the coupled reaction system R2-R4 of the FKN mechanism of the Belousov-Zhabotinsky reaction and considering the acidities of HBr and HOBrO(2). The structures were determined of ten dimeric aggregates 4 of bromous acid, (HOBrO)(2), of eight mixed aggregates 5 formed between the products of disproportionation, (HOBr)(HOBrO(2)), and of four transition states structures 6 for disproportionation by direct O-transfer. It was found that the condensation of two HOBrO molecules provides facile access to bromous acid anhydride 7, O(BrO)(2). A discussion of the potential energy surface of Br(2)O(3) shows that O(BrO)(2) is prone to isomerization to the mixed anhydride 8, BrO-BrO(2), and to dissociation to 9, BrO, and 10, BrO(2), and their radical pair 11. Hence, three possible paths from O(BrO)(2) to the products of disproportionation, HOBr and HOBrO(2), are discussed: (1) hydrolysis of O(BrO)(2) along a path that differs from its formation, (2) isomerization of O(BrO)(2) to BrO-BrO(2) followed by hydrolysis, and (3) O(BrO)(2) dissociation to BrO and BrO(2) and their reactions with water. The results of the potential energy surface analysis show that the rate-limiting step in the disproportionation of HOBrO consists of the formation of the hydrate 12a of bromous acid anhydride 7 via transition state structure 14a. The computed activation free enthalpy ΔG(act)(SMD) = 13.6 kcal/mol for the process 2·2a → [14a](‡) → 12a corresponds to the reaction rate constant k(4) = 667.5 M(-1) s(-1) and is in very good agreement with experimental measurements. The potential energy surface analysis further shows that anhydride 7 is kinetically and thermodynamically unstable with regard to hydrolysis to HOBr and HOBrO(2) via transition state structure 14b. The transition state structure 14b is much more stable than 14a, and, hence, the formation of the "symmetrical anhydride" from bromous acid becomes an irreversible reaction for all practical purposes because 7 will instead be hydrolyzed as a "mixed anhydride" to afford HOBr and HOBrO(2). The mixed anhydride 8, BrO-BrO(2), does not play a significant role in bromous acid disproportionation.