Criegee intermediates (R₁R₂COO) are understood to be critical intermediates in the ozonolysis of alkenes, but their high reactivity has traditionally made them very difficult to study directly. Although the smallest Criegee intermediates have now been generated in the laboratory using a diiodomethane photolysis scheme, numerous questions still remain about the product branching ratios of Criegee intermediates formed directly from ozonolysis. This talk will discuss our recent detection of the simplest Criegee intermediate, CH₂OO, in the ozonolysis of ethylene, using Fourier transform microwave spectroscopy and a modified pulsed nozzle. Nine other product species of the reaction were also detected, in abundances that qualitatively support the published mechanisms and rate constants.

The simplest Criegee intermediate CH₂OO, important in atmospheric chemistry, has recently been detected with infrared (IR) absorption in the reaction of CH₂I + O₂.Y.-T.  Su, Y.-H.  Huang, H. A.  Witek, and Y.-P.  Lee, Science 340, 174 (2013).e have recorded high-resolution infrared spectrum of CH₂OO with rotational lines partially resolved. In additional to derivation of some critical spectral parameters to confirm the previous assignments of ν₃ at 1434.1 cm⁻¹, ν₄ at 1285.7 cm⁻¹, ν₆ at 909.2 cm⁻¹, and ν₈ at 847.4 cm⁻¹, the high-resolution spectra enable us to assign with confidence the 2ν₉ at 1233.5 cm⁻¹ and ν₅ at 1213.0 cm⁻¹. Observed vibrational wavenumbers, relative intensities, and rotational structures agree well with those predicted by high-level quantum calculations. Some additional hot bands and combination bands are also observed. We also recorded the IR spectrum of ICH₂OO under high-pressure conditions. Observed IR intensities and vibrational wavenumbers of 1233.8 (ν₄), 1221 (ν₅), 1087 (ν₆), and 923 (ν₇) cm⁻¹ agree with those simulated according to theoretical predictions and those observed in solid p-H₂.Y.-F.  Lee and Y.-P.  Lee, Chem. Phys. DOI: 10.1080/00268976.2015.1012129he ν₄ band of ICH₂OO interferes with the 2ν₉ band of CH₂OO even at pressure as low as 100 Torr. With direct detection of both CH₂OO and ICH₂OO, we determined the pressure dependence of the yield of CH₂OO. The yield of CH₂OO near one atmosphere is greater than previous reports.

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Footnotes:

Y.-T.  Su, Y.-H.  Huang, H. A.  Witek, and Y.-P.  Lee, Science 340, 174 (2013).W Y.-F.  Lee and Y.-P.  Lee, Chem. Phys. DOI: 10.1080/00268976.2015.1012129T

The Criegee intermediates are carbonyl oxides that play critical roles in ozonolysis of alkenes in the atmosphere. Su et al. reported the mid-infrared spectrum of the simplest Criegee intermediate CH₂OO. Y.-T.  Su, Y.-H.  Huang, H. A.  Witek, Y.-P.  Lee, Science 340, 174 (2013).ethyl substitution of CH₂OO produces two conformers of CH₃CHOO and consequently complicates the infrared spectrum. We report the transient infrared spectrum of both syn- and anti-CH₃CHOO, produced from CH₃CHI + O₂ in a flow reactor, using a step-scan Fourier-transform spectrometer. Guided and supported by high-level full-dimensional quantum calculations, rotational contours of the four observed bands are simulated successfully and provide definitive identification of both conformers. Although nearly all observed bands of anti-CH₃CHOO overlapped with syn-CH₃CHOO, the Q-branch of ν₈ near 1090.6 cm⁻¹ is contributed solely by syn-CH₃CHOO, and that of ν₇ near 1280.8 cm⁻¹ is also dominated by syn-CH₃CHOO. Furthermore, anti-CH₃CHOO shows a reactivity greater than syn-CH₃CHOO toward NO/NO₂; at the later period of reaction, the spectrum can be simulated with only syn-CH₃CHOO. Without NO/NO₂, anti-CH₃CHOO also decays much faster than syn-CH₃CHOO. The direct infrared detection of syn- and anti-CH₃CHOO should prove useful for field measurements and laboratory investigations of the Criegee mechanism. Y.-T.  Su, Y.-H.  Huang, H. A.  Witek, Y.-P.  Lee, Science 340, 174 (2013).M

Moderate resolution cavity ring-down spectroscopy(CRDS) is used to obtain the Ã-~X electronic transition of the CH₂BrOO and CH₂ClOO radicals in the near-infrared region at room temperature. The CH₂BrOO radical was generated by 248nm excimer laser photolysis of a gas mixture of CH₂Br₂, O₂ and inert gas. The CH₂ClOO radical was generated similarly except for using CH₂ClI as the precursor. In both spectra, the first strong transition is located near 6800 cm⁻¹, and is assigned as the origin band. Several transitions are observed in the region between the origin and 9000 cm⁻¹. A strong vibrational transition is observed around 800 cm⁻¹ to the blue of the origin and attributed to the OO stretch which is characteristic of the peroxy radical spectra. Our analysis of the vibrational structure is conducted using frequencies and Franck-Condon factors based on electronic structure calculations. Rotational structure analyses with ab-initio calculated rotational constants and dipole moments show good agreement with the contour of the origin band. Numerous transitions around the origin band in the CH₂BrOO radical spectrum can be explained by excitation from low-lying torsional levels in the ~X state that are populated at room temperature.

In the past few years, the photolysis of CH₂I₂ in the presence of O₂ has received much attention. It has been shown to be an attractive method for producing the Criegee intermediate, CH₂O₂. Under certain conditions the reaction is also expected to produce the iodomethyl peroxy radical, CH₂IO₂. Interestingly both species are expected to have electronic transitions in the near infrared (NIR). The transition in CH₂O₂ would be analogous to the ã−X̃ singlet-triplet transition in O₃ and a NIR Ã−X̃ transition in well-known to be characteristic of peroxy radicals. Notwithstanding the above, NIR spectra have not been reported for either CH₂O₂ or CH₂IO₂. Based upon these considerations, we have performed the CH₂I₂ photolysis with O₂ in the optical cavity of our room temperature cavity ringdown spectrometer and have discovered a spectrum in the NIR. Our recorded spectrum stretches from a complex origin structure at ≈ 6800 cm⁻¹to beyond 9000 cm⁻¹. Aside from the origin its strongest feature is a similar, complex band ≈ 870 cm⁻¹to the blue of it, which is likely an O-O stretch vibrational transition, which is present in peroxy radicals but might also be expected for CH₂O₂. With the aid of high-level ab initio calculations (described in detail in the subsequent talk) we have undertaken the analysis of the spectrum. We find that a spectral analysis, including a number of hot bands arising from populated torsional levels, is consistent with the electronic structure calculations for the à and X̃ states of CH₂IO₂.

Criegee intermediates (R₁R₂COO or CIs) arise from ozonolysis of biogenic and anthropogenic alkenes, which is an important process in the atmosphere. Recent breakthroughs in producing them in the gas phase have resulted in a flurry of experimental and theoretical studies. Producing the simplest CI (CH₂OO) in the lab via photolysis of CH₂I₂ in the presence of O₂ yields both CH₂OO and CH₂IOO with pressure dependent branching. As discussed in the preceding talk, both species might be expected to have electronic transitions in the near IR (NIR). Here we discuss electronic structure calculations used to characterize the electronic states of both systems in the relevant energy range. Using explicitly-correlated multireference configuration interaction (MRCI-F12) and coupled-cluster (UCCSD(T)-F12b) calculations we were first able to exclude CH₂OO as the carrier of the observed NIR spectrum. Next, by computing frequencies and relaxed full torsional scans for the à and ~X states, we were able to aid in analysis and assignment of the NIR spectrum attributed to CH₂IOO.

The vibrational structures of the à ²A₁ and X̃ ²E states of t-butoxy were obtained in jet-cooled laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectroscopic measurements. The observed transitions are assigned based on vibrational frequencies calculated using Complete Active Space Self-Consistent Field (CASSCF) method and the predicted Franck-Condon factors. The spin-orbit (SO) splitting was measured to be 35(5) cm⁻¹ for the lowest vibrational level of the ground (X̃ ²E) state and increases with increasing vibrational quantum number of the CO stretch mode. Vibronic analysis of the DF spectra suggests that Jahn-Teller (JT)-active modes of the ground-state t-butoxy radical are similar to those of methoxy and would be the same if methyl groups were replaced by hydrogen atoms. Coupled-cluster calculations show that electron delocalization, introduced by the substitution of hydrogens with methyl groups, reduces the electronic contribution of the SO splittings by only around ten percent, and a calculation on the vibronic levels based on quasidiabatic model Hamiltonian clearly attributes the relatively small SO splitting of the X̃ ²E state of t-butoxy mainly to stronger reduction of orbital angular momentum by the JT-active modes when compared to methoxy. The rotational and fine structure of the LIF transition to the first CO stretch overtone level of the òA₁ state has been simulated using a spectroscopic model first proposed for methoxy, yielding an accurate determination of the rotational constants of both à and X̃ states.

In the series of the nitrosyl halides, XNO (where X = F, Cl, Br, I), the millimeter-wave spectrum of INO remains so far unknown. We report our investigation on the first high-resolution rotational spectroscopy of nitrosyl iodide, INO.
One of the motivation for this work comes from the growing need in developing a more complete understanding of atmospheric chemistry, especially halogen and nitrogen oxides chemistry that adversely impacts ozone levels. In the family of the nitrogen oxyhalides such as nitrosyl (XNO), nitryl (XNO), nitrite (XONO), and nitrate (XON0₂) halides, those with X = F, Cl, Br have been well studied, both theoretically and experimentally. However, relatively little is known about the iodine-containing analogues, although they also are of potential importance in tropospheric chemistry. In 1991, the Fourier-transform IR spectroscopic detection of INO, INO₂ and IONO₂ in the gas phase has been reportedI. Barnes, K. H. Becker and J. Starcke, J. Phys. Chem. 1991, 95, 9736-9740.
The INO molecule was generated by in situ mixing continuously I₂ and NO in a 50-cm long reaction glass tube whose outlet was connected to the absorption cell using a teflon tube. At the time of writing this abstract, 68 μ_a-type transitions (K_a = 0−10), all weak, have been successfully assigned. The hyperfine structures due to both I and N nuclei will also be presented.
S.B. and D.D. acknowledge support from the Laboratoire d’Excellence CaPPA (Chemical and Physical Properties of the Atmosphere) through contract ANR-10-LABX-005 of the Programme d’Investissement d’Avenir.

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Footnotes:

I. Barnes, K. H. Becker and J. Starcke, J. Phys. Chem. 1991, 95, 9736-9740..

It is well known that rate constants of certain reactions of alkoxy radicals, e.g., unimolecular dissociation (decomposition by C-C bond fission) and isomerization via 1,5 H-shift, are highly sensitive to the molecular structure. In the present and the next talks, we report dispersed fluorescence (DF) spectra of various alkoxy radicals obtained under supersonic jet-cooled conditions by pumping different vibronic bands of their B̃ ← X̃ laser induced fluorescence (LIF) excitation spectra.Wu, Q.; Liang, G.; Zu, L.; Fang, W. J. Phys. Chem A 2012, 116, 3156-3162.^,Lin, J.; Wu, Q.; Liang, G.; Zu, L.; Fang, W. RSC Adv. 2012, 2, 583−589.^, Liang, G.; Liu , C.; Hao, H.; Zu, L.; Fang, W. J. Phys. Chem. A 2013, 117, 13229- 13235.his talk focuses on the DF spectra of 2-methyl-1-propoxy (isobutoxy), 2-methyl-1-butoxy, and 3-methyl-1-butoxy (isopentoxy). In all cases, strong CO-stretch progressions were observed, as well as transitions to other vibrational levels, including low-frequency ones. Quantum chemical calculations were carried out to aid the assignment of the DF spectra. Franck-Condon factors were calculated using the ezSpectrum program.V. Mozhayskiy and A. I. Krylov, http://iopenshell.usc.edu/html:<hr /><h3>Footnotes: Wu, Q.; Liang, G.; Zu, L.; Fang, W. J. Phys. Chem A 2012, 116, 3156-3162.\end Lin, J.; Wu, Q.; Liang, G.; Zu, L.; Fang, W. RSC Adv. 2012, 2, 583−589. Liang, G.; Liu , C.; Hao, H.; Zu, L.; Fang, W. J. Phys. Chem. A 2013, 117, 13229- 13235.T V. Mozhayskiy and A. I. Krylov, http://iopenshell.usc.edu/

Chirped-Pulse Fourier-transform microwave spectroscopy has been applied in a uniform supersonic flow (Chirped-pulse/Uniform flow, CPUF) to study the 193 nm photodissociation of methyl isothiocyanate (MITC). Several products (CH₃NC, NCS, H₂CS, HCN and HNC) were identified via their pure rotational spectra. Observation of CH₃NC and NCS are consistent with previous studies of this system, however it is the first detection of H₂CS and HCN/HNC. Branching ratios were obtained from these data and will be discussed.

Vibrational structures of the nearly degenerate X̃ and à states of all four positional isomers of the methylcyclohexoxy (MCHO) radicals were studied by jet-cooled dispersed fluorescence (DF) spectroscopy, which unravels the effect of methyl substitution at different positions on the six-membered ring. Experimentally observed vibronic transitions in the DF spectra were assigned based on vibrational frequencies from quantum chemical calculations and predicted Franck-Condon factors that take into account the Duschinsky rotation. DF spectra of 2-, 3-, and 4-MCHO radicals are dominated by CO-stretch progressions or the progressions of CO-stretch modes in combination with the excited vibrational modes. DF spectra of two lowest-energy conformers of the tertiary 1-MCHO radical, chair-axial and chair equatorial, are significantly different from each other and from those of the other three positional isomers. Strong C-CH₃ stretch progressions as well as progressions of its combination bands with the CO stretch modes or the excited modes were observed. Such differences between the isomers and the conformers can be explained by variation of geometry and symmetry of the electronic states of cyclohexoxy upon methyl substitution at different positions. DF study of MCHO provides direct measurement of the energy separation between the à and X̃ states that are subject to the pseudo-Jahn-Teller effect.