Infrared absorption of gaseous CH3OO detected with a step-scan Fourier-transform spectrometer

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.

Spectroscopic characterization of two peroxyl radicals during the O2-oxidation of the methylthio radical

The atmospheric oxidation of dimethyl sulfide (DMS) yields sulfuric acid and methane sulfonic acid (MSA), which are key precursors to new particles formed via homogeneous nucleation and further cluster growth in air masses. Comprehensive experimental and theoretical studies have suggested that the oxidation of DMS involves the formation of the methylthio radical (CH₃S•), followed by its O₂-oxidation reaction via the intermediacy of free radicals CH₃SO_x• (x = 1-4). Therefore, capturing these transient radicals and disclosing their reactivity are of vital importance in understanding the complex mechanism. Here, we report an optimized method for efficient gas-phase generation of CH₃S• through flash pyrolysis of S-nitrosothiol CH₃SNO, enabling us to study the O₂-oxidation of CH₃S• by combining matrix-isolation spectroscopy (IR and UV-vis) with quantum chemical computations at the CCSD(T)/aug-cc-pV(X + d)Z (X = D and T) level of theory. As the key intermediate for the initial oxidation of CH₃S•, the peroxyl radical CH₃SOO• forms by reacting with O₂. Upon irradiation at 830 nm, CH₃SOO• undergoes isomerization to the sulfonyl radical CH₃SO₂• in cryogenic matrixes (Ar, Ne, and N₂), and the latter can further combine with O₂ to yield another peroxyl radical CH₃S(O)₂OO• upon further irradiation at 440 nm. Subsequent UV-light irradiation (266 nm) causes dissociation of CH₃S(O)₂OO• to CH₃SO₂•, CH₂O, SO₂, and SO₃. The IR spectroscopic identification of the two peroxyl radicals CH₃SOO• and CH₃S(O)₂OO• is also supported by ¹⁸O- and ¹³C-isotope labeling experiments.

Study on ozonolysis of asymmetric alkenes with matrix isolation and FT-IR spectroscopy

O₃ and alkenes are important reactants in the formation of SOA in the atmosphere. The intermediates and reaction mechanism of ozonation of alkene is an important topic in atmospheric chemistry. In this study, the low-temperature matrix isolation was used to capture the intermediates such as Primary ozonides (POZs), Criegee Intermediates (CIs), and Secondary ozonides (SOZs) generated from ozonation of 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B). The results have been identified by the vacuum infrared spectroscopy and theoretical calculation. Our results show that during the ozonation of asymmetric alkenes, two kinds of CIs and more than two kinds of SOZs were generated due to the different decomposition modes of POZs. The infrared absorption peaks of (CH₃)₂COO and CH₃CH₂C(CH₃)OO for O-O telescopic vibration was determined to be 889 cm^-1 and 913 cm^-1, respectively. Using the merged jet method, it was found that a large amount of HCHO was produced during the ozonation of 2M1B, and glyoxal and methylglyoxal were produced in the ozonation of 2M2B. Our findings highlight the importance of asymmetric alkene ozonolysis reactions in producing CIs, further improving the understanding of the generation of CIs from ozonation of alkenes.

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.

Transient infrared absorption of t-CH3C(O)OO, c-CH3C(O)OO, and alpha-lactone recorded in gaseous reactions of CH3CO and O2

A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient species produced in gaseous reactions of CH(3)CO and O(2); IR absorption spectra of CH(3)C(O)OO and alpha-lactone were observed. Absorption bands with origins at 1851+/-1, 1372+/-2, 1169+/-6, and 1102+/-3 cm(-1) are attributed to t-CH(3)C(O)OO, and those at 1862+/-3, 1142+/-4, and 1078+/-6 cm(-1) are assigned to c-CH(3)C(O)OO. A weak band near 1960 cm(-1) is assigned to alpha-lactone, cyc-CH(2)C(=O)O, a coproduct of OH. These observed rotational contours agree satisfactorily with simulated bands based on predicted rotational parameters and dipole derivatives, and observed vibrational wavenumbers agree with harmonic vibrational wavenumbers predicted with B3LYP/aug-cc-pVDZ density-functional theory. The observed relative intensities indicate that t-CH(3)C(O)OO is more stable than c-CH(3)C(O)OO by 3+/-2 kJ mol(-1). Based on these observations, the branching ratio for the OH+alpha-lactone channel of the CH(3)CO+O(2) reaction is estimated to be 0.04+/-0.01 under 100 Torr of O(2) at 298 K. A simple kinetic model is employed to account for the decay of CH(3)C(O)OO.