The reduced reactivity of resonance-stabilized hydrocarbon radicals allows for them to reach higher concentrations in harsh environments such as flames or planetary atmospheres. Reactions between them remain rapid and could explain the formation of aromatics in aliphatic flames and molecular growth in soot formation processes. Here, we have measured the Ã←~X bands of the resonance-stabilized 1- and 2-methylallyl radicals near 410 nm using cavity ringdown spectroscopy. Although the absorption cross sections are relatively small, the extended lifetimes of the excited states allows for resolved vibronic structure of both C₄H₇ isomers. The transition has both n←π and π^*← n character, which we analyze using quantum chemical calculations. We also discuss how these results can be employed in chemical kinetics experiments.

First high-resolution mid infrared rovibrational spectral data on jet cooled bromocarbene (HCBr) diradical are obtained using quantum shot noise limited laser absorption methods. The HCBr diradical is generated by coflowing bromoform (HCBr₃) in a mixture of Ne, He, and H₂ through a pulsed slit discharge beam source and is subsequently cooled down to approximately 40 K in a slit-jet supersonic expansion. Interestingly, optimization of the HCBr radical requires the presence of H₂, presumably due to decomposition of HCBr₃ by H atom chemistry in the discharge. Rotationally resolved absorption spectra of the CH stretch ν₁ = 1 ← 0 fundamental are analyzed, resulting in precise determination of ground and excited state rovibrational constants for both ⁷⁹Br and ⁸¹Br isotopologues. The results for the ground state are in good agreement with previous high resolution near IR electronic studies in the Sears groupJournal of Molecular Spectroscopy 202, 131–143 (2000) The ν₁ band origin is now unambiguously assigned at 2799 cm⁻¹, which agrees well with previous LIF/dispersed fluorescence studies of Reid and coworkerJ. Chem. Phys. 124, 134302 (2006)

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Footnotes:

Journal of Molecular Spectroscopy 202, 131–143 (2000). J. Chem. Phys. 124, 134302 (2006).

The addition reactions of iodine atom with propene in solid para-hydrogen (p-H₂) matrices were investigated with infrared (IR) absorption spectroscopy. Mixtures of propene and I₂ seeded in p-H₂ were co-deposited at 3.3 K for 7 to 8 hours, followed by irradiations with ultraviolet lights at various wavelengths to study the reaction of I atoms with propene. Quantum chemical calculations were carried out at the B3LYP/aug-cc-pVTZ-pp level in order to determine the relative energies, vibrational wavenumbers and IR intensities of 1-iodopropyl and 2-iodopropyl radicals. The transitions belong to I₂-propene complex, 1-iodopropyl radical, and the anti conformer of 1,2-diiodopropane were recorded and assigned. The assignments were based on expected reaction, the vibrational wavenumbers and IR intensities from theoretical calculations, and secondary photolysis behavior. The observation of 1-iodopropyl radical, the isomer with the least energy, indicates that the addition of iodine atom occurs at the terminal carbon atom. The role of the p-H₂ matrices, the difference between this reaction and the previously reported Cl + propene reaction will be discussed.

Methyl vinyl ketone oxide [MVKO, C₂H₃C(CH₃)OO], an important Criegee intermediate in ozonolysis of isoprene, was recently identified in laboratories with near infrared action spectrum (to produce OH) using photolysis of a gaseous mixture of 1,3-diiodo-but-2-ene [(CH₂I)HC=C(CH₃)I] and O₂, V. P.  Barber, S.  Pandit, A. M.  Green, N.  Trongsiriwat, P. J.  Walsh, S. J.  Klippenstein, M. I.  Lester, J. Am. Chem. Soc. 140, 10866 (2018).ut its mid-infrared spectrum and the detailed mechanism of its formation remains unexplored. We employed a step-scan Fourier-transform infrared spectrometer to investigate the reaction intermediates. Upon irradiation at 248 nm of gaseous (Z)-1,3-diiodo-but-2-ene, the (Z)-3-iodo-but-2-en-1-yl [C₂H₃C(CH₃)I] radical was observed, indicating the fission of the terminal allylic C-I bond, not the central vinylic C-I bond. This radical is characterized by infrared absorption bands at 1406, 1261, 1109, 1019, 925, and 873 cm⁻¹. Upon irradiation at 248 nm of a gaseous mixture of (Z)-1,3-diiodo-but-2-ene and O₂ at 35 Torr, the Criegee intermediate MVKO, characterized by infrared absorption bands at 1416, 1383, 1346, 1060, 987, 948, and 908/916 cm⁻¹, was observed. At pressure 236 Torr, the reaction adduct 3-iodo-but-1-en-1-yl-peroxy [C₂H₃CI(CH₃)OO] radical, characterized by infrared absorption bands at 1375, 1213, 1108, 1063, 986, and 885 cm⁻¹, was observed. These new spectra of C₂H₃C(CH₃)I, C₂H₃C(CH₃)OO, and C₂H₃CI(CH₃)OO provide valuable information for the understanding of the formation mechanism of the Criegee intermediate MVKO from the source reaction of photolysis of (CH₂I)HC=C(CH₃)I in O₂ in laboratories. If time permits, we will discuss the formation kinetics and self-reaction of MVKO using a quantum-cascade laser.

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Footnotes:

V. P.  Barber, S.  Pandit, A. M.  Green, N.  Trongsiriwat, P. J.  Walsh, S. J.  Klippenstein, M. I.  Lester, J. Am. Chem. Soc. 140, 10866 (2018).b

Glycolaldehyde (HOCH₂CHO) has been detected in the interstellar medium; it is considered to play an important role in the formation of complex organic molecules, especially higher sugars. We took the advantages of unique properties of para-hydrogen (p-H₂), which serves as a quantum-solid matrix host and a medium for efficient hydrogen-atom reaction, to investigate the reaction between HOCH₂CHO and H atoms. H atoms were generated on photolysis at 365 nm of a co-deposited mixture of HOCH₂CHO/p-H₂ and Cl₂ and subsequent IR irradiation for promoting the Cl + H₂ (ν = 1) → H + HCl reaction. Based on the orientation of O-H and C=O bonds, HOCH₂CHO has 4 conformers; cis-cis conformer is found to have the least energy among all due to the intramolecular H-bonding. In the reaction H + HOCH₂CHO at 3.2 K, 5 lines in the first group and 6 lines in the second group are assigned to HOCH₂CO and HOCHCHO radicals, products of the first H-abstraction from -CHO and -CH₂ groups, respectively. In addition, 2 lines were assigned to HOCHCO, a product of the second H-abstraction. IR irradiation employed during H-atoms generation was also responsible for the conformational conversion of HOCH₂CHO, cc → tt; which intricates the H + cc-HOCH₂CHO reaction to some extent. Since photolysis at 266 nm also promotes this conversion; we photolyzed the cc-HOCH₂CHO at 266 nm prior to 365 nm/IR irradiation for complete conversion to the tt-form to facilitate the H + tt-HOCH₂CHO reaction. Four lines in one group and 3 lines in the second group, those differ from their cc-form analogs, were assigned to the tt-HOCH₂CO and tt-HOCHCHO radicals, respectively; 2 lines in the third group were assigned to HOCHCO. Besides these assignments, a line at 2079.3 cm⁻¹was tentatively assigned to the OCHCO radical, a product of the third H-abstraction. The assignments of all these products agree satisfactorily with vibrational wavenumbers predicted with the B3LYP/aug-cc-pVTZ method. The PES of H + cc- and tt-HOCH₂CHO reactions reveal the feasibility of H-abstraction processes for HOCH₂CO, HOCHCHO, HOCHCO, and OCHCO formations after sequential H-abstractions from the CHO, CH₂, CH₂/CHO, and OH moieties, respectively.

Nitrogen-substituted polycyclic aromatic hydrocarbons (PANHs) are fundamental species important in biological molecules, astrochemistry, and combustion. They have been detected on meteorites with non-terrestrial isotopic abundances, potentially play a role in prebiotic chemistry, and have been observed in the atmosphere of Titan. They are suggested to be responsible for some of the unidentified infrared (UIR) emission bands in the interstellar medium and have been identified as intermediates in the combustion of biomass. Pyridine is a prototypical nitrogen-containing aromatic molecule, and recent studies suggest that the pyridyl radicals are key intermediates to the formation of PANHs. However, no high-resolution spectra or coupled-cluster calculations of the pyridyl radicals are available. To complement our search for the pyridyl radicals with rotational spectroscopy, we calculated the equilibrium geometries, relative energies, and spectroscopic parameters for the three pyridyl radicals at the CCSD(T) level of theory. In addition, anharmonic vibrational frequencies were evaluated using second-order vibrational perturbation theory. We will discuss the results of our calculations and our progress to measure the rotational spectra of the pyridyl radicals using Fourier transform microwave spectroscopy.

A wide variety of nitrogen-containing heterocycles have been detected on meteorites with non-terrestrial isotopic abundances, indicative of an interstellar origin. However, no N-heterocycles have been detected in space and their formation pathways are unclear. Experimental work has shown that N-heterocycles can likely form through barrierless radical-neutral gas-phase reactions, which are feasible in the low temperature environments of cold molecular clouds and the outer regions of protoplanetary disks. Astronomical searches and laboratory studies of potential precursors and depletion products are critical to determining if such mechanisms play a role in N-heterocycle formation. Pyrrolyl is both a photodissociation product of the N-heterocycle pyrrole () and a potential astrophysical precursor to larger N-heterocycles. Furthermore, pyrrolyl is a known intermediate in the combustion of biomass and was tentatively detected in Titan's atmosphere by the Cassini-Huygens mission. Here, the first treatment of 1-, 2-, and 3-pyrrolyl at the CCSD(T) level of theory and progress towards the first experimental detection in the cm-wave region will be discussed. Equilibrium geometries were optimized at CCSD(T)/cc-pwCVTZ and quadratic, cubic, and partial quartic force constants were evaluated at CCSD(T)/ANO0 in order to obtain harmonic and anharmonic vibrational frequencies. Additional parameters for vibrational and rotational spectroscopy were calculated at similar levels of theory. This work will enable astronomical searches, laboratory spectroscopy, and kinetics and dynamics studies.

The electronic spectra of the resonance-stabilized radical C₉H₉ isomers cis- and trans-meta-vinylbenzyl (MVB), generated in a supersonically cooled discharge of m-vinyltoluene, have been investigated using resonant two-colour two-photon ionization (R2C2PI) and laser-induced fluorescence. The D₀−D₁ band origins of the cis and trans conformers are at 19037 cm⁻¹ and 18939 cm⁻¹, respectively. Adiabatic ionization energies near 7.17 eV were determined for both conformers from two-colour ion-yield scans. Dispersed fluorescence (DF) was used to conclusively identify the cis conformer: ground-state cis-MVB eigenvalues calculated from a Fourier series fit of a computed (B3LYP/6-311G++(d,p) vinyl torsion potential are in excellent agreement with torsional transitions observed in the 19037 cm⁻¹ DF spectrum. Features arising from cis- or trans-MVB in the R2C2PI spectrum were distinguished by optical-optical holeburning spectroscopy and vibronic assignments were made with guidance from DFT and TDDFT calculations. There is a notable absence of mirror symmetry between excitation and emission spectra for several totally symmetric modes, whereby a′ fundamentals that are conspicuous in emission are nearly absent in excitation, and vice-versa. This effect is largely ascribed to interference between Franck-Condon and Herzberg-Teller contributions to the electronic transition moment, as the former is carried with a different sign in excitation vs. emission, while the latter is not. Rampant HT activity among a′ modes is attributed to the low symmetry (C_s) of the molecule, which permits intensity-borrowing from several relatively bright electronic states of A" symmetry.

Aromatic oxide free radicals play an important role in atmospheric and combustion chemistry, where they are involved in the formation of secondary organic aerosols and soot. The oxygen atom lone pairs can readily mix with the aromatic system giving rise to extensive electronic delocalization and influencing molecular reactivity. The oxygen atom also imparts a significant dipole moment to the radical system, which supports low-lying resonance and dipole bound states. We probed three cryogenically cooled aromatic oxide radicals (phenoxy, 1-naphthoxy, 2-naphthoxy) via Slow Electron Velocity Map Imaging (SEVI) to unravel their electronic and vibrational structure. We determined electron affinities and excited state energies for the three systems, and observed numerous active vibrational modes. Resonant photoexcitation to the dipole bound state was found to drastically alter the observed vibrational structure, and care was taken to avoid these resonances during direct photodetachment studies.

We are currently studying the B̃ ²E′ - X̃ ²A₂′ system of jet cooled NO₃ via laser induced fluorescence ( LIF ) spectroscopy. The dispersed fluorescence ( DF ) spectrum from the vibrationless level of the upper electronic state suggests the existence of two closely lying vibrational levels in the ν₁ fundamental region,  ∼  1050 cm⁻¹M. Fukushima and T. Ishiwata, ISMS2013, paper WJ03 (2013). Recently, Babin et al. reported the high resolution photo-electron ( PE ) spectrum of NO₃⁻ measured by a spectroscopic method, slow photo-electron velocity-map imaging of cryogenically cooled anion ( cryo-SEVI )M. Babin, J. A. DeVine, M. DeWitt, J. F. Stanton, and D. M. Neumark, J. Phys. Chem. Lett. 11, 395 (2019) but unfortunately, the cryo-SEVI PE spectrum does not resolve the two levels in the ν₁ fundamental region. In the report, it is also shown that the PE band intensities depend on the excess kinetic energy of PE by the photo-detachment light source. This dependence is also reported in their spectra measured by traditional PE spectroscopyA. Weaver, D. W. Arnold, S. E. Bradforth, and D. M. Neumark, J. Phys. Chem. 94, 1740 (1991) By combining results from these two methodologies, we have analyzed the excess energy dependence of the PE bands, and found that there are two components with different excess energy dependence in the ν₁ fundamental region. The two components can be attributed two vibrational levels, consistent with our DF results. The dependence of one of the two components corresponds to that of the vibrationless band ( vibrationally a₁′ ), while the other does not agree with that of the ν₄ fundamental ( e′ ),  ∼  350 cm⁻¹.

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Footnotes:

M. Fukushima and T. Ishiwata, ISMS2013, paper WJ03 (2013).. M. Babin, J. A. DeVine, M. DeWitt, J. F. Stanton, and D. M. Neumark, J. Phys. Chem. Lett. 11, 395 (2019), A. Weaver, D. W. Arnold, S. E. Bradforth, and D. M. Neumark, J. Phys. Chem. 94, 1740 (1991).

Calcium monohydride CaH is found in the Sun and other stars. Indeed, its A-X electronic spectrum has been a useful probe for stellar classification.For example, J.E. Gizis, Astron. J. 113, 806 (1997).e have been working on this molecule in the visible and ultraviolet regions. Using Laser Induced Fluorescence (LIF) we have identified many new vibrational levels of the A, B/B′, and 1²∆ states. Our primary interest has been the detailed investigation of the B/B′ state which has a double-minimum potential energy function. We have previously studied vibrational levels lying both below and above the potential energy barrier between the two wells.K. Watanabe, N. Yoneyama, K. Uchida, K. Kobayashi, F. Matsushima, Y. Moriwaki, S. C. Ross, Chem. Phys. Lett. 657, 1 (2016).^,K. Watanabe, I. Tani, K. Kobayashi, Y. Moriwaki, S. C. Ross,Chem. Phys. Lett. 710, 11 (2018).n that work, we were able to confirm the strong irregularity in thevibrational energy spacings that had been predicted by the quantumchemical study of Carlsund−Levin et al.C. Carlsund−Levin,N. Elander, A. Nunez, A. Scrinzi, Phys. Scripta 65, 306 (2002).e previously reported the first experimental observation of the direct, but forbidden, excitation 1^2 X^2.J. Furuta, K. Watanabe, I. Tani, K. Kobayashi, Y. Moriwaki, S. C. Ross, 74th International Symposium on Molecular Spectroscopy, TI06, (2019).his was done usingLIF. This excitation is possible due to interaction of levels of the 1^2state with nearby levels of states for which excitation from the ground state is not forbidden, so called "gateway" states. By studying the ensuing relaxation from the 1^2 K. Watanabe, I. Tani, K. Kobayashi, Y. Moriwaki, S. C. Ross,Chem. Phys. Lett. 710, 11 (2018).IC. Carlsund-Levin,N. Elander, A. Nunez, A. Scrinzi, Phys. Scripta 65, 306 (2002).WJ. Furuta, K. Watanabe, I. Tani, K. Kobayashi, Y. Moriwaki, S. C. Ross, 74th International Symposium on Molecular Spectroscopy, TI06, (2019).T

The copper hydroxide ion, CuOH+ , serves as the catalytic core in several recently developed water-splitting catalysts, and an understanding of its chemistry is critical to determining viable catalytic mechanisms. In spite of its importance, the electronic structure of this open-shell ion has remained ambiguous in the literature. The source of this ambiguity is demonstrated to be the propensity of this ion to exist between traditional Cu(I) and Cu(II) oxidation-state limits. The spin density of the radical is shown to delocalize between the metal center and surrounding ligands, and increasing hydration serves to exacerbate this behavior. Equation-of-motion coupled-cluster methods were needed for spectral simulations, as well as direct simulation of the role of the deuterium “tag” molecules that are used in modern predissociation spectroscopy experiments. This nominally benign tag molecule underwent direct complexation with the open-valence metal ion, thereby forming a species akin to known metal- complexes and strongly impacting the resulting spectrum. Thermal populations of this configuration and other more traditional non-covalently bound isomers led to considerable broadening of the spectral lineshapes. Therefore, at least for the hydrates, these ions serve as a cautionary tale for the spectroscopy community, wherein the role of the tag is far from benign.

A decade ago, the advent of broadband chirped-pulse Fourier-transform microwave spectrometers revolutionized rotational spectroscopy in the centimeter-wave region [1]. Commercial solution are available in the millimeter-wave region, and new molecular spectroscopy investigations can now be undertaken. Motivated by the prospect of enabling new interstellar detections, we have developed a new set-up associating a broadband chirped-pulse Fourier-transform millimeter-wave (W-band) spectrometer and a supersonic jet chamber. Radicals are produced by an electric discharge in the high pressure part of the jet and reactive species are directly probed by the radiation at temperatures as low as few Kelvin. Using different organic precursors such as acetonitrile (CH₃CN), methanol (CH₃OH) or allene (C₃H₄), we were able to study rich discharge mixtures. Among others, we were able to produce and detect the cyanomethyl radical (CH₂CN) [2], the methoxy radical (CH₃O) [3] and cyclopropenylidene (c−C₃H₂) [4] in the vibrational ground state and in several vibrational excited states. In the presentation, technical details of the set-up will be provided, together with the preliminary results we obtained.

[1] Brown, G. G et al., Review of Scientific Instruments, 79, 053103 (2008) [2] Saito, S. and Yamamoto, S., The Journal of Chemical Physics, 107, 1732 (1997) [3] Endo, Y. et al., The Journal of Chemical Physics, 81, 122-135 (1984) [4] Thaddeus, P. et al., The Astrophysical Journal, 299, L63-L66 (1985)

High-resolution anion photoelectron spectra of cryogenically cooled Si₃C⁻, Si₂C⁻₂, and SiC⁻₃ obtained using slow photoelectron velocity-map imaging (cryo-SEVI) are presented, providing insight into the geometries, energetics, and vibronic structure of the anionic and the neutral clusters. These spectra yield accurate vibrational frequencies for the neutral clusters. They also yield refined adiabatic detachment energies (ADEs) for the ground states of Si₃C⁻ and Si₂C⁻₂ of 1.5374(6) eV and 1.9019(4) eV, respectively, while the ADE of a low-lying isomer of SiC⁻₃ is found to be 1.9050(7) eV. The cryo-SEVI spectra show that the ground state of Si₂C⁻₂ is a distorted trapezoid, and represent the first confirmation of the distorted trapezoid structure of Si₂C⁻₂, the only low-lying isomer of this cluster with a permanent dipole moment. Additional transitions are observed from two low-lying anion isomers: a linear structure and a rhombus. The spectrum of SiC⁻₃, in combination with electronic structure calculations, suggests that the true ground state of SiC₃ is a ring structure with a transannular C–C bond, addressing a longstanding controversy surrounding this cluster. All three spectra exhibit Franck-Condon forbidden transitions; these are attributed to Herzberg-Teller coupling in Si₃C and SiC₃ and autodetachment from an excited electronic state of Si₂C⁻₂.