Examining the ground and first excited states of methyl peroxy radical with high-level coupled-cluster theory

Theoretical analysis of the OH-initiated atmospheric oxidation reactions of imidazole

Imidazoles are present in Earth's atmosphere in both the gas-phase and in aerosol particles, and have been implicated in the formation of brown carbon aerosols. The gas-phase oxidation of imidazole (C3N2H4) by hydroxyl radicals has been shown to be preferentially initiated via OH-addition to position C5, producing the 5-hydroxyimidazolyl radical adduct. However, the fate of this adduct upon reaction with O2 in the atmospheric gas-phase is currently unknown. We employed an automated approach to investigate the reaction mechanism and kinetics of imidazole's OH-initiated gas-phase oxidation, in the presence of O2 and NOx. The explored mechanism included reactions available to first-generation RO2 radicals, as well as alkoxyl radicals produced from RO2 + NO reactions. Product distributions were obtained by assembling and solving a master equation, under conditions relevant to the Earth's atmosphere. Our calculations show a complex, branched reaction mechanism, which nevertheless converges to yield two major closed-shell products: 4H-imidazol-4-ol (4H-4ol) and N,N'-diformylformamidine (FMF). At 298 K and 1 atm, we estimate the yields of 4H-4ol and FMF from imidazole oxidation initiated via OH-addition to position C5 to be 34 : 66, 12 : 85 and 2 : 95 under 10 ppt, 100 ppt and 1 ppb of NO respectively. This work also revealed O2-migration pathways between the α-N-imino peroxyl radical isomers. This reaction channel is fast for the first-generation RO2 radicals, and may be important during the atmospheric oxidation of other unsaturated organic nitrogen compounds as well.

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.

Riddles of the structure and vibrational dynamics of HO3 resolved near the ab initio limit

The hydridotrioxygen (HO₃) radical has been investigated in many previous theoretical and experimental studies over several decades, originally because of its possible relevance to the tropospheric HO_x cycle but more recently because of its fascinating chemical bonding, geometric structure, and vibrational dynamics. We have executed new, comprehensive research on this vexing molecule via focal point analyses (FPA) to approach the ab initio limit of optimized geometric structures, relative energies, complete quartic force fields, and the entire reaction path for cis-trans isomerization. High-order coupled cluster theory was applied through the CCSDT(Q) and even CCSDTQ(P) levels, and CBS extrapolations were performed using cc-pVXZ (X = 2-6) basis sets. The cis isomer proves to be higher than trans by 0.52 kcal mol^-1, but this energetic ordering is achieved only after the CCSDT(Q) milestone is reached; the barrier for cis → trans isomerization is a minute 0.27 kcal mol^-1. The FPA central r_e(O-O) bond length of trans-HO₃ is astonishingly long (1.670 Å), consistent with the semiexperimental r_e distance we extracted from microwave rotational constants of 10 isotopologues using FPA vibration-rotation interaction constants (α_i). The D₀(HO-O₂) dissociation energy converges to a mere 2.80 ± 0.25 kcal mol^-1. Contrary to expectation for such a weakly bound system, vibrational perturbation theory performs remarkably well with the FPA anharmonic force fields, even for the torsional fundamental near 130 cm^-1. Exact numerical procedures are applied to the potential energy function for the torsional reaction path to obtain energy levels, tunneling rates, and radiative lifetimes. The cis → trans isomerization occurs via tunneling with an inherent half-life of 1.4 × 10^-11 s and 8.6 × 10^-10 s for HO₃ and DO₃, respectively, thus resolving the mystery of why the cis species has not been observed in previous experiments executed in dissipative environments that allow collisional cooling of the trans-HO₃ product. In contrast, the pure ground eigenstate of the cis species in a vacuum is predicted to have a spontaneous radiative lifetime of about 1 h and 5 days for HO₃ and DO₃, respectively.

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.

The methylsulfinyl radical CH3SO examined

Our computational investigations broaden the scope of currently available experimental results on the methylsulfinyl radical, a key atmospheric species.

The cis- and trans-formylperoxy radical: fundamental vibrational frequencies and relative energies of the X̃ 2A′′ and à 2A′ states

Acylperoxy radicals [RC(O)OO˙] play an important catalytic role in many atmospheric and combustion reactions.