Unsaturated hydrocarbons are an atmospherically abundant class of species, which are emitted into the Earth's troposphere from both anthropogenic and biogenic sources. A significant tropospheric sink of alkenes is their reactions with ozone, which leads to the formation of zwitterionic reactive intermediates named carbonyl oxides, more commonly known as Criegee Intermediates (CIs). since the advent of photolytic methods to directly generate the smallest CI, CH₂OO, direct experiments by researchers across the world have revealed the reactivity of these elusive species to be much more varied and complex than previously anticipated based on ozonolysis experiments. These direct studies have shown that many CI reactions are faster than previously thought. Recent work has indicated the potentially important role of CIs in particulate formation. The fast reaction of CIs with SO₂ is estimated to contribute up to 50% of atmospheric H₂SO₄ formation - a critical precursor of sulfate aerosol. Additionally, it has been shown many bimolecular reactions of CI, such as reaction with hydroperoxides (ROOHs), lead to the formation of higher molecular weight, highly oxygenated reaction products. These species have been implicated in the formation of secondary organic aerosol (SOA). Theoretical work has also indicated that the reactions of CIs with peroxy radicals reactions might proceed rapidly, via a mechanism involving the addition of the terminal oxygen of ROO to the O-bound carbon of the CI - regenerating the -OO functional group for subsequent reaction. In this work, we have utilized novel photolytic precursors to generate select CIs and investigate their role in oligomerization reactions. We have investigated the reaction kinetics of the reaction of CIs with peroxy radicals and hydroperoxides using a multipass UV-Vis spectrometer coupled to a pulsed-laser photolysis flow reactor. We will also report on detailed kinetic investigations of water-vapor enhancement of oligomerization reactions.

Methyl vinyl ketone oxide [MVKO, (C₂H₃)(CH₃)COO] is one of the Criegee intermediates produced when atmospheric isoprene undergoes ozonolysis. The reactions between Criegee intermediates with HCOOH lead to the formation of secondary organic aerosols and impact the atmospheric compositionR. Chhantyal-Pun, B. Rotavera, M. R. McGillen, M. A. H. Khan, A. J. Eskola, R. L. Caravan, L. Blacker, D. P. Tew, D. L. Osborn, C. J. Percival, C. A. Taatjes, D. E. Shallcross, and A. J. OrrEwing. ACS Earth Space Chem. 2, 833 (2018). We recorded time-resolved infrared absorption spectra of transient species produced upon irradation at 248 nm of a flowing mixture of (Z)-(CH₂I)HCC(CH₃)I/HCOOH/O₂ at 298 K; photolysis of (CH₂I)HCC(CH₃)I in O₂ produced mainly syn-trans MVKO. Bands near 1726, 1430, 1378, 1347, 1213, 1169, 1033, and 987 cm⁻¹were assigned to the hydrogen-transfered adduct 2-hydroperoxybut-3-en-2-yl formate [HPBF, (C₂H₃)(CH₃)C(OCHO)OOH]. The observed wavenumbers and relative intensities agree with the anharmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method, but its conformation could not be definitively assigned because of the similarities in IR spectra of these isomers. We also calculated the potential-energy scheme of this reaction with the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ method. Our observation indicates that the reaction of MVKO and HCOOH follows the pathway with the smallest barrier to form HPBF. Unlike other hydrogen-transferred adducts produced in reactions of HCOOH with Criegee intermediates CH₂OO and CH₃CHOO, HPBF does not undergo dehydration because the central carbon atom in HPBF, which is connected to a methyl group and a vinyl group, does not have a hydrogen atom available for abstraction by OH to form H₂O.

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R. Chhantyal-Pun, B. Rotavera, M. R. McGillen, M. A. H. Khan, A. J. Eskola, R. L. Caravan, L. Blacker, D. P. Tew, D. L. Osborn, C. J. Percival, C. A. Taatjes, D. E. Shallcross, and A. J. OrrEwing. ACS Earth Space Chem. 2, 833 (2018)..

Sulfur atoms are integral to the intricate web of chemical interactions that influence both the Earth's atmosphere and the broader cosmos. Investigating the dynamics of sulfur-containing reactions enhances our understanding of combustion at high temperature, environmental processes, and astrochemical phenomena, contributing to advancements in multiple scientific disciplines. This study employs crossed-beam velocity map imaging and high-level ab initio/transition state theory modeling to investigate S(³P) reactions with 1,3-butadiene and isoprene under single collision conditions. Experimental findings for the butadiene reactionLi, H., Lang, J., Foley, C. D., Zádor, J., Suits, A. G., J. Phys. Chem. Lett., 2023, 14(34), 7611-7617.eveal the formation of thiophene via H₂ loss, 2H-thiophenyl radical through H loss, and thioketene through ethene loss. In the isoprene reaction,Lang, J., Foley, C. D., Thawoos, S., Behzadfar, A., Liu, Y., Zádor, J., Suits, A. G., Faraday Discuss., 2024, DOI: 10.1039/D4FD00009A.nalogous H loss radical is observed, alongside the 2H-thiophenyl radical arising from methyl loss, and methyl thiophene from H₂ loss and C₃H₄S from loss of ethene. Coupled cluster calculations on the pathways found by the automated kinetic workflow code KinBot support these results, highlighting intersystem crossing to the singlet surface, generating diverse long-lived intermediates, including 5-membered heterocycles. This behavior contrasts significantly with the analogous O(³P) reaction, primarily attributed to substantial differences in C-O and C-S bond energies. Finally, preliminary results on S(³P) interactions with 1-butene, isobutene, and 1-butyne provide valuable insights, expanding our understanding of atmospheric and astrochemical implications.

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Li, H., Lang, J., Foley, C. D., Zádor, J., Suits, A. G., J. Phys. Chem. Lett., 2023, 14(34), 7611-7617.r Lang, J., Foley, C. D., Thawoos, S., Behzadfar, A., Liu, Y., Zádor, J., Suits, A. G., Faraday Discuss., 2024, DOI: 10.1039/D4FD00009A.a

The CPUF (chirped-pulse uniform flow) technique couples a pulsed Laval flow with CP-FTmmW (Chirped-Pulse Fourier-Transform mmWave) spectroscopic method. Detection by CP-FTmmW depends on the measurement of the free induction decay of the rotational coherence, which can be attenuated by a highly collisional environment typical of a Laval flow. We recently developed an extended Laval nozzle that establishes a uniform flow within the nozzle, followed by a shock-free secondary expansion. In a recent publication by Thawoos et al.S. Thawoos, N. Suas-David, R. M. Gurusinghe, M. Edlin, A. Behzadfar, J. Lang and A. G. Suits, The Journal of Chemical Physics, 2023, 159. we detailed our first established extended Laval nozzle, which achieved a flow at 20 K. Similarly, we will delve into recent advancements in enhancing a variety of extended Laval nozzles capable of achieving comparable flow conditions across two further temperature ranges of approximately 50 K and 70 K. We chose two distinct flow conditions, one with Ar gas producing a 50 K flow (1.1×10¹⁷ molecule cm⁻³) and the other with N₂ gas producing a 70 K flow (5.5×10¹⁶ molecule cm⁻³). We established shock-free secondary expansions by lowering the target chamber pressure with both flows. The secondary expansion was characterized by the rotational spectroscopy of a dilute furan sample. Ongoing experiments involve creating extended nozzles for the established flow conditions and characterizing the flow within the nozzle by using the REMPI method.

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S. Thawoos, N. Suas-David, R. M. Gurusinghe, M. Edlin, A. Behzadfar, J. Lang and A. G. Suits, The Journal of Chemical Physics, 2023, 159.,

Chlorine atom plays important roles in reactions in the ozone layer and troposphere. Addition reaction products of chlorine and oxygen atoms on olefins, such as RCHClCH₂ or RCHCH₂O, have attracted much attention. Furthermore, investigations of addition reaction pathways and optimizations of reaction conditions contribute to the discovery of novel chemical transformations. Pure rotational transitions of CHClCHOS. Inomata, M. Yamaguchi, N. Washida, Laser-Induced Fluorescence of the CHClCHO Radical and Reaction of Oxygen Atoms with Halogenated Ethylenes, J. Phys. Chem. A, 105, 7559-7568 (2001)ave been observed using a Fourier-transform microwave spectrometer in the region of 8 GHz to 34 GHz. In this study, two isomers, trans- and cis-CHClCHO, were produced by discharging mixtures of trans- and cis-1,2-dichloroethylene with O₂ diluted in Ne, respectively. Both a- and b-type transitions were observed for the two conformers, yielding precisely determined rotational constants and the fine and hyperfine constants of chlorine and two protons. The determined molecular constants show that unpaired electron of the radical is localized on the out-of-plane π-orbitals of the carbons bonded with Cl.

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S. Inomata, M. Yamaguchi, N. Washida, Laser-Induced Fluorescence of the CHClCHO Radical and Reaction of Oxygen Atoms with Halogenated Ethylenes, J. Phys. Chem. A, 105, 7559-7568 (2001)h

The reactions between Criegee intermediates and hydrogen chloride ( HCl ) play a significant role in the atmosphere, because of their large rate coefficients. We employed a tunable cw external-cavity quantum-cascade laser coupled with a Herriott cell to probe a high-resolution absorption band of syn- CH₃CHOO , produced by photolyzing a gas mixture of 1,1-diiodoethane/ O₂ / HCl with laser light at 248 nm. We also employed a 335-nm UV laser to probe syn- CH₃CHOO and measure its concentration. By monitoring the ν₁₀ bands of syn- CH₃CHOO in region 883.13883.15 cm⁻¹, we recorded the temporal evolution of the syn- CH₃CHOO and measured the rate coefficient of syn- CH₃CHOO + HCl at 298 K and 48 Torr. The measured bimolecular rate coefficient was determined to be (4.8±1.3 )×10⁻¹¹ cm³ molecule⁻¹ s⁻¹, which is nearly identical to a value (4.8±1.0)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹ reported by Liu et al., S. Liu, X. Zhou, Y. Chen, Y. Liu, S. Yu, K. Takahashi, H. Ding, Z. Ding, X. Yang, and W. Dong, J. Phys. Chem. A 125, 8587 (2021).ho employed the laser-induced fluorescence detection of the unimolecular decomposition product OH from CH₃CHOO . We also monitored the high-resolution band of the ν₇ mode of syn- CH₃CHOO in region 1277.01283.0 cm⁻¹; the temporal profile indicated that a stable product, anti-CEHP, also contributed to the absorption. By using the model fitting to fit the temporal profile contributed by the decay and rising signals of both syn- CH₃CHOO and anti-CEHP, respectively, we derived the overall rate coefficient of the reaction syn-/anti- CH₃CHOO + HCl to be k_HCl = (8.7±1.0)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 1016 Torr and 298 K. We also found that the rate coefficient of the formation reaction CH₃CHI + O₂ to CH₃CHOO + I , (3.4±0.7)×10⁻¹² cm³ molecule⁻¹ s⁻¹, is  2.5 times smaller than those reported by Howes et al. N. U. M. Howes, Z. S. Mir, M. A. Blitz, S. Hardman, T. R. Lewis, D. Stone, and P. W. Seakins, Phys. Chem. Chem. Phys. 20, 22218 (2018).nd Sheps et al. L. Sheps, A. M. Scully, and K. Au, Phys. Chem. Chem. Phys. 16, 26701 (2014).ho employed mass spectrometry and UV absorption, respectively. We also employed a 425-nm UV laser coupled with a Herriott cell to monitor the absorption temporal profile of MACRO with HCl . The preliminary results showed that the rate coefficient of MACRO + HCl k_HCl = 7.9×10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 510 Torr and 298 K.

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S. Liu, X. Zhou, Y. Chen, Y. Liu, S. Yu, K. Takahashi, H. Ding, Z. Ding, X. Yang, and W. Dong, J. Phys. Chem. A 125, 8587 (2021).w N. U. M. Howes, Z. S. Mir, M. A. Blitz, S. Hardman, T. R. Lewis, D. Stone, and P. W. Seakins, Phys. Chem. Chem. Phys. 20, 22218 (2018).a L. Sheps, A. M. Scully, and K. Au, Phys. Chem. Chem. Phys. 16, 26701 (2014).w

Dioxygen cation, O₂⁺, is found in the lower ionosphere. In our work, O₂⁺ molecules were produced in an electric discharge with approximately 1 torr total pressure of molecular oxygen. Using a Bruker IFS 125M Fourier transform spectrometer, emission from the discharge was recorded from 10,000 – 25,000 cm⁻¹. Numerous vibrational bands of the b ⁴Σ⁻ - a ⁴Π electronic transition system were observed in the spectrum. Vibrational quantum levels v= 0-5 were observed for the a ⁴Π lower state, while v= 0-3 were observed in the b ⁴Σ⁻ upper state. The spectrum was analyzed within PGOPHER. Initially band by band analyses were performed, and ultimately Dunham parameters were determined directly in PGOPHER. The fitted molecular constants are compared to those of previous literature and a recent high-level ab initio calculation.

The odd-hydrogen radicals, OH and HO₂, play important roles in the atmospheric chemistry of Earth and Mars. In the stratosphere and mesosphere, HO_x species interconvert through reactions with odd-oxygen species (O, O₃), while their ultimate sink is the radical termination reaction, OH+HO₂→O₂+H₂O. Although this reaction has received significant attention, few studies have measured it using direct kinetic observations of both radicals. High-resolution infrared spectroscopy offers a sensitive and selective approach to monitor the concentration of both radicals in real time. Here, we present results from a study where OH and HO₂ were generated in a 1.4 m temperature-controlled flow cell using pulsed-laser-photolysis. Frequency-modulation spectroscopy was used to monitor absorption from rovibrational features in the fundamental band of OH and the first overtone band of the HO₂ OH-stretching mode. The 30 m multipass optics provided sufficient sensitivity to span a wide range of radical concentrations, allowing for the determination of bimolecular rate coefficients with few complications from secondary chemistry. This work has shown that reaction is somewhat slower than the value published in critical evaluations of kinetics data. This will impact model calculations of stratospheric ozone depletion and the interpretation of field measurements.

The oxidation of nitric oxide (NO) by the hydroxyl radical (OH) is at a prominent step in cyclic NO_X(=NO+NO₂) production and leads to the formation of HONO. The decomposition of HONO represents a significant source of OH in some environments and is the dominant OH source at dawn. The experimental measurement of the rate constant of OH+NO has been thoroughly studied in He bath gas, but there is a lack of data across atmospherically relevant temperatures and pressures in N₂ or Air. Furthermore, literature data, until recently, had completely neglected water vapor (H₂O), which can make up to 2