OH Roaming and Beyond in the Unimolecular Decay of the Methyl-Ethyl-Substituted Criegee Intermediate: Observations and Predictions.

Alkene ozonolysis generates short-lived Criegee intermediates that are a significant source of hydroxyl (OH) radicals. This study demonstrates that roaming of the separating OH radicals can yield alternate hydroxycarbonyl products, thereby reducing the OH yield. Specifically, hydroxybutanone has been detected as a stable product arising from roaming in the unimolecular decay of the methyl-ethyl-substituted Criegee intermediate (MECI) under thermal flow cell conditions. The dynamical features of this novel multistage dissociation plus a roaming unimolecular decay process have also been examined with ab initio kinetics calculations. Experimentally, hydroxybutanone isomers are distinguished from the isomeric MECI by their higher ionization threshold and distinctive photoionization spectra. Moreover, the exponential rise of the hydroxybutanone kinetic time profile matches that for the unimolecular decay of MECI. A weaker methyl vinyl ketone (MVK) photoionization signal is also attributed to OH roaming. Complementary multireference electronic structure calculations have been utilized to map the unimolecular decay pathways for MECI, starting with 1,4 H atom transfer from a methyl or methylene group to the terminal oxygen, followed by roaming of the separating OH and butanonyl radicals in the long-range region of the potential. Roaming via reorientation and the addition of OH to the vinyl group of butanonyl is shown to yield hydroxybutanone, and subsequent C-O elongation and H-transfer can lead to MVK. A comprehensive theoretical kinetic analysis has been conducted to evaluate rate constants and branching yields (ca. 10-11%) for thermal unimolecular decay of MECI to conventional and roaming products under laboratory and atmospheric conditions, consistent with the estimated experimental yield (ca. 7%).

Bimolecular Reaction of Methyl-Ethyl-Substituted Criegee Intermediate with SO2.

Methyl-ethyl-substituted Criegee intermediate (MECI) is a four-carbon carbonyl oxide that is formed in the ozonolysis of some asymmetric alkenes. MECI is structurally similar to the isoprene-derived methyl vinyl ketone oxide (MVK-oxide) but lacks resonance stabilization, making it a promising candidate to help us unravel the effects of size, structure, and resonance stabilization that influence the reactivity of atmospherically important, highly functionalized Criegee intermediates. We present experimental and theoretical results from the first bimolecular study of MECI in its reaction with SO2, a reaction that shows significant sensitivity to the Criegee intermediate structure. Using multiplexed photoionization mass spectrometry, we obtain a rate coefficient of (1.3 ± 0.3) × 10-10 cm3 s-1 (95% confidence limits, 298 K, 10 Torr) and demonstrate the formation of SO3 under our experimental conditions. Through high-level theory, we explore the effect of Criegee intermediate structure on the minimum energy pathways for their reactions with SO2 and obtain modified Arrhenius fits to our predictions for the reaction of both syn and anti conformers of MECI with SO2 (ksyn = 4.42 × 1011 T-7.80exp(-1401/T) cm3 s-1 and kanti = 1.26 × 1011 T-7.55exp(-1397/T) cm3 s-1). Our experimental and theoretical rate coefficients (which are in reasonable agreement at 298 K) show that the reaction of MECI with SO2 is significantly faster than MVK-oxide + SO2, demonstrating the substantial effect of resonance stabilization on Criegee intermediate reactivity.

Vibrational Excitation Hindering an Ion-Molecule Reaction: The c-C_{3}H_{2}

Experiments within a cryogenic 22-pole ion trap have revealed an interesting reaction dynamic phenomenon, where rovibrational excitation of an ionic molecule slows down a reaction with a neutral partner. This is demonstrated for the low-temperature hydrogen abstraction reaction c-C_{3}H_{2}^{+}+H_{2}, where excitation of the ion into the ν_{7} antisymmetric C-H stretching mode decreased the reaction rate coefficient toward the products c-C_{3}H_{3}^{+}+H. Supported by high-level quantum-chemical calculations, this observation is explained by the reaction proceeding through a c-C_{3}H_{2}^{+}-H_{2} collision complex in the entrance channel, in which the hydrogen molecule is loosely bound to the hydrogen atom of the c-C_{3}H_{2}^{+} ion. This discovery enables high-resolution vibrational action spectroscopy for c-C_{3}H_{2}^{+} and other molecular ions with similar reaction pathways. Moreover, a detailed kinetic model relating the extent of the observed product depletion signal to the rate coefficients of inelastic collisions reveals that rotational relaxation of the vibrationally excited ions is significantly faster than the rovibrational relaxation, allowing for a large fraction of the ions to be vibrationally excited. This result provides fundamental insight into the mechanism for an important class of chemical reactions, and is capable of probing the inelastic collisional dynamics of molecular ions.

High-resolution double resonance action spectroscopy in ion traps: vibrational and rotational fingerprints of CH2NH2.

By applying various action spectroscopic techniques in a 4 K cryogenic ion trap instrument, protonated methanimine, CH2NH2+, has been investigated by high-resolution rovibrational and pure rotational spectroscopy for the first time. In total, 39 rovibrational transitions within the fundamental band of the ν2 symmetric C-H stretch were measured around 3026 cm-1, which were used to predict pure rotational transition frequencies of CH2NH2+ in the ground vibrational state. Based on these predictions, nine rotational transitions were observed between 109 and 283 GHz using a novel double resonance method, which significantly improved the sensitivity of the rotational measurements. This double resonance method consists of rotational excitation followed by vibrational excitation, which is finally detected as a dip in the number of CH2NH2+-He complexes formed in the 4 K He bath of the trap. The new measurements and the derived predictions of pure rotational transitions will enable the first radio-astronomical search for CH2NH2+.

Highly accurate experimentally determined energy levels of H3.

A sub-Doppler rovibrational spectroscopic survey of H3 + has been conducted which included 36 transitions in the ν2 ← 0 fundamental band, 15 transitions in the 2ν2 2←ν2 hot band, and 7 transitions in the 2ν2 2←0 overtone band, improving the uncertainties of most transitions by more than an order of magnitude to ∼4 MHz. Combination differences were used to determine relative energy levels and forbidden rotational transitions up to J = 6. A fit of the ground state to an effective Hamiltonian was used to connect ortho and para states, and to determine the absolute energy levels relative to the forbidden (0, 0) state. Ultimately, 62 rovibrational energy levels in the ground, ν2, and 2ν2 2 states were determined with ∼10 MHz uncertainty. Comparing the experimentally determined energy levels with ab initio calculations revealed an unexpected dependence of the residuals on the quantum number G.

Improving cavity-enhanced spectroscopy of molecular ions in the mid-infrared with up-conversion detection and Brewster-plate spoilers.

The performance of sensitive spectroscopic methods in the mid-IR is often limited by fringing due to parasitic etalons and the background noise in mid-infrared detectors. In particular, the technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS), which is capable of determining the frequencies of strong rovibrational transitions of molecular ions with sub-MHz uncertainty, needs improved sensitivity in order to probe weaker transitions. In this work, we have implemented up-conversion detection with NICE-OHVMS in the 3.2 - 3.9 µm region to enable the use of faster and more sensitive detectors which cover visible wavelengths. The higher bandwidth enabled detection at optimized heterodyne frequencies, which increased the overall signal from the H3+ cation by a factor of three and was able to resolve sub-Doppler features which had previously overlapped. Also, we demonstrate the effectiveness of Brewster-plate spoilers to remove fringes due to parasitic etalons in a cavity enhanced technique. Together, these improvements reduced the instrument's noise equivalent absorption to 5.9×10-11 cm-1 Hz-1/2, which represents a factor of 34 improvement in sensitivity compared to previous implementations of NICE-OHVMS. This work will enable extended high-precision spectroscopic surveys of H3+ and other important molecular ions.

Well-Defined Cobalt(I) Dihydrogen Catalyst: Experimental Evidence for a Co(I)/Co(III) Redox Process in Olefin Hydrogenation.

The synthesis of a cobalt dihydrogen Co(I)-(H2) complex prepared from a Co(I)-(N2) precursor supported by a monoanionic pincer bis(carbene) ligand, (Mes)CCC ((Mes)CCC = bis(mesityl-benzimidazol-2-ylidene)phenyl), is described. This species is capable of H2/D2 scrambling and hydrogenating alkenes at room temperature. Stoichiometric addition of HCl to the Co(I)-(N2) cleanly affords the Co(III) hydridochloride complex, which, upon the addition of Cp2ZrHCl, evolves hydrogen gas and regenerates the Co(I)-(N2) complex. Furthermore, the catalytic olefin hydrogenation activity of the Co(I) species was studied by using multinuclear and parahydrogen (p-H2) induced polarization (PHIP) transfer NMR studies to elucidate catalytically relevant intermediates, as well as to establish the role of the Co(I)-(H2) in the Co(I)/Co(III) redox cycle.

Communication: High precision sub-Doppler infrared spectroscopy of the HeH⁺ ion.

The hydrohelium cation, HeH(+), serves as an important benchmark for ab initio calculations that take into account non-adiabatic, relativistic, and quantum electrodynamic effects. Such calculations are capable of predicting molecular transitions to an accuracy of ~300 MHz or less. However, in order to continue to push the boundaries on these calculations, new measurements of these transitions are required. Here we measure seven rovibrational transitions in the fundamental vibrational band to a precision of ~1 MHz using the technique of Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy. These newly measured transitions are included in a fit to the rotation-vibration term values to derive refined spectroscopic constants in the v = 0 and v = 1 vibrational states, as well as to calculate rotation-vibration energy levels with high precision.

Chemical Kinetic Study of the Reaction of CH2OO with CH3O2.

Criegee intermediates play an important role in the oxidizing capacity of the Earth's troposphere. Although extensive studies have been conducted on Criegee intermediates in the past decade, their kinetics with radical species remain underexplored. We investigated the kinetics of the simplest Criegee intermediate, CH2OO, with the methyl peroxy radical, CH3O2, as a model system to explore the reactivities of Criegee intermediates with peroxy radicals. Using a multipass UV-Vis spectrometer coupled to a pulsed-laser photolysis flow reactor, CH2OO and CH3O2 were generated simultaneously from the photolysis of CH2I2/CH3I/O2/N2 mixtures with CH2OO measured directly near 340 nm. We determined a reaction rate coefficient kCH2OO+CH3O2 = (1.7 ± 0.5) × 10-11 cm3 s-1 at 294 K and 10 Torr, where the influence of iodine adducts is reduced. This rate coefficient is faster than previous theoretical predictions, highlighting the challenges in accurately describing the interaction between zwitterionic and biradical characteristics of Criegee intermediates.

Mid-infrared cross-comb spectroscopy.

Dual-comb spectroscopy has been proven beneficial in molecular characterization but remains challenging in the mid-infrared region due to difficulties in sources and efficient photodetection. Here we introduce cross-comb spectroscopy, in which a mid-infrared comb is upconverted via sum-frequency generation with a near-infrared comb of a shifted repetition rate and then interfered with a spectral extension of the near-infrared comb. We measure CO2 absorption around 4.25 µm with a 1-µm photodetector, exhibiting a 233-cm-1 instantaneous bandwidth, 28000 comb lines, a single-shot signal-to-noise ratio of 167 and a figure of merit of 2.4 × 106 Hz1/2. We show that cross-comb spectroscopy can have superior signal-to-noise ratio, sensitivity, dynamic range, and detection efficiency compared to other dual-comb-based methods and mitigate the limits of the excitation background and detector saturation. This approach offers an adaptable and powerful spectroscopic method outside the well-developed near-IR region and opens new avenues to high-performance frequency-comb-based sensing with wavelength flexibility.

A white cell based broadband transient UV-vis absorption spectroscopy with pulsed laser photolysis reactors for chemical kinetics under variable temperatures and pressures.

UV-vis spectroscopy is widely used for kinetic studies in physical chemistry, as species' absolute cross-sections are usually less sensitive to experimental conditions (i.e., temperature and pressure). Here, we present the design and characterization of a multipass UV-vis absorption spectroscopy white cell coupled to a pulsed-laser photolysis flow reactor. The glass reactor was designed to facilitate studies of gas phase chemical reactions over a range of conditions (239-293 K and 10-550 Torr). Purged windows mitigate contamination from chemical precursors and photolysis products. We report the measured impact of this purging on temperature uniformity and the absorption length and present some supporting flow calculations. The combined optical setup is unique and enables the photolysis laser to be coaligned with a well-defined absorption pathlength probe beam. This alignment leverages the use of one long-pass filter to increase the spectrum flatness and increase the light intensity vs other systems that use two dichroic mirrors. The probe beam is analyzed with a dual exit spectrograph, customized to split the light between an intensified CCD and photomultiplier tube, enabling simultaneous spectrum and single wavelength detection. This multipass system yields a pathlength of ∼450 cm and minimum observable concentrations of ∼3.7 × 1011 molecule cm-3 (assuming cross-sections ∼1.2 × 10-17 cm2). The temperature profile across the reaction region is ±2 K, defined by the worst-case temperature of 239 K, validated by measurements of the N2O4 equilibrium constant. Finally, the system is implemented to study the simplest Criegee intermediate, demonstrating the instrument performance and advantages of simultaneous spectrum and temporal profile measurements.

High-resolution infrared action spectroscopy of the fundamental vibrational band of CN.

Rotational-vibrational transitions of the fundamental vibrational modes of the 12C14N+ and 12C15N+ cations have been observed for the first time using a cryogenic ion trap apparatus with an action spectroscopy scheme. The lines P(3) to R(3) of 12C14N+ and R(1) to R(3) of 12C15N+ have been measured, limited by the trap temperature of approximately 4 K and the restricted tuning range of the infrared laser. Spectroscopic parameters are presented for both isotopologues, with band origins at 2000.7587(1) and 1970.321(1) cm-1, respectively, as well as an isotope independent fit combining the new and the literature data.