We performed time-resolved dual-comb spectroscopy near 3400 cm⁻¹upon flash photolysis of flowing mixtures of (COCl)₂ or Cl₂/CH₃OH/O₂/N₂/NO. The high-resolution transient absorption spectroscopy was demonstrated by utilizing an electro-optic dual-comb systemRef: Phys. Chem. Chem. Phys. 21, 18400, (2019)oupled with a Herriott flow cell. With this new approach, absorbance signals of several species in the flow reactor, such as HO₂, CH₃OH, H₂CO, and OH were able to be simultaneously monitored with adequate spectral and temporal resolutions. Based on great 3D spectral features, reaction kinetics of HO₂ with NO were also studied.

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Ref: Phys. Chem. Chem. Phys. 21, 18400, (2019)c

Studying the chemical inventory of the interstellar medium (ISM) is critical to developing new theories of molecular formation and evolution. Furthermore, the search for biologically relevant species and their precursors has been at the forefront of astrobiology and astrochemistry in recent years. As such, this work focuses on the dissociation products of methylamine (CH₃NH₂), a known precursor to the simplest amino acid, glycine (C₂H₅NO₂). It is likely that the radical products of cosmic-ray induced photodissociation of methylamine are important in prebiotic interstellar pathways as well as atmospheric models of planetary bodies such as Titan. Therefore, we are studying the radical species produced in a methylamine discharge as a guide for future studies of methylamine photodissociation. Our initial molecular targets are the radicals CH₂NH₂, CH₃NH, and CH₃N, for which no rotational spectroscopic information is available. We examined the structure of these radicals using high-level computational methods. We then compared the predictions of these calculations to the rotational spectra of species obtained using a high voltage discharge of methylamine in argon at the throat of a supersonic expansion. Here we will present the spectroscopic predictions and the initial experimental results and discuss the implications for astrochemistry and astrobiology.

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, 924, and 902 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 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, 1296, 1213, 1161, 1108, 1063, 986, 934, 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.

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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

Calcium monohydride CaH is an astronomical molecule identified in the Sun and other stars by using visible transitions. We have found many new vibrational levels of the B/B′ state using Laser Induced Fluorescence (LIF) from the visible to ultraviolet region. 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). In the analysis of these transitions, we fixed the molecular constants in the ground state to those analyzed for the A²Π, B²Σ⁺→ X²Σ⁺ in 12000 - 17000 cm⁻¹ region and the v=v′+1→ v′(v′=0−3) in X²Σ⁺ transitions by Shayesteh et al. A. Shayesteh, R. S. Ram, and P. F. Bernath, J. Mol. Spectrosc. 288, 46 (2013).^,A. Shayesteh, K. A. Walker, I. Gordon, D. R. T. Appadoo, and P. F. Bernath, J. Mol. Struct., 695 (2004).he pure rotational spectra in the ground state have been reported and hyperfine structure was analyzed. C. I. Frum, J. J. Oh, E. A. Cohen, H. M. Pickett, Astrophys. J. Lett. 408, L61 (1993). W. L. Barclay Jr., M. A. Anderson, L. M. Ziurys, Astrophys. J. Lett. 408, L65 (1993). However, the range was limited up to N = 2−1. the highest frequency was about 500 GHz. In this study, we will report our new measurement in the terahertz region. The spectra were taken by using tunable far−infrared spectrometer at the University of Toyama. Calcium monohydride was produced in a cell where Ca vapor was introduced by heating Ca at 750^C and DC discharge was applied under H_2 K. Watanabe, I. Tani, K. Kobayashi, Y. Moriwaki, S. C. Ross, Chem. Phys. Lett. 710, 11 (2018).A. Shayesteh, K. A. Walker, I. Gordon, D. R. T. Appadoo, and P. F. Bernath, J. Mol. Struct., 695 (2004).T C. I. Frum, J. J. Oh, E. A. Cohen, H. M. Pickett, Astrophys. J. Lett. 408, L61 (1993).

Indenyl and phenylpropargyl are the most stable isomers of C₉H₇ and have emerged as ubiquitous products in flames and hydrocarbon discharges. Along with ortho-ethynylbenzyl, they are the only C₉H₇ isomers to have been spectroscopically studied in detail, but there presumably exists a wide variety of other resonance-stabilized C₉H₇ radicals that have not yet been detected. We report the observation of para-ethynylbenzyl radical, first found in a discharge of para-vinyltoluene during an unsuccessful search for p-vinylbenzyl, and subsequently observed with high signal-to-noise ratio when p-ethynyltoluene was used as the precursor. Identification of the radical is secured by comparison of the observed (7.17 eV) and calculated (7.08-7.25 eV) adiabatic ionization energy, and a convincing assignment of the electronic spectrum can be made from calculated excited state frequencies. The molecule adopts C_2v symmetry in its ground and first excited states. In almost every case where a single, strong a₁ fundamental is predicted in excitation, there exist two or more bright states in the spectrum. We attribute this to Fermi-resonance, a consequence of pervasive near-degeneracies of overtones of non-totally symmetric modes with Franck-Condon-active a₁ modes. Some instances have been confirmed by dispersed fluorescence spectroscopy at the time of writing. In addition, in the same m/z = 115 R2C2PI spectrum, we find unmistakable evidence of 1-phenylpropargyl, as well as the previously undetected m-ethynylbenzyl radical. Examination of the precursor by NMR proves that both radicals are present at levels that cannot be accounted for by sample impurities unless computed transition strengths are too small by several orders of magnitude, suggesting rearrangement of all three radicals via an intermediate that remains unobserved.

Resonance-stabilized radical (RSR) isomers of C₉H₉ persist in complex energetic environments such as flames and plasmas derived from aromatic precursors. Of the myriad possible C₉H₉ RSRs, only 1-indanyl (the global minimum) and 1-phenylallyl have been conclusively identified spectroscopically. Electronic spectra near 600 nm, close to several DIBs, were recently reported by the group of Maier for C₉H₉ products of a heptadiyne discharge. They were tentatively ascribed to isomers of vinylbenzyl on the basis of computed transition energies and ionization potentials, the best agreement with the latter property (ca. 7.3 eV) provided by the meta isomer. To further examine this conjecture, we have undertaken surveys for the electronic spectra of o-,m-, and p-vinylbenzyl radicals in discharges of vinyltoluenes, using resonant two-color ionization and fluorescence spectroscopy. In a jet-cooled discharge of m-vinyltoluene, we have detected cis- and trans-m-vinylbenzyl radicals near 525 nm. We observe adiabatic ionization energies (ca. 7.15 eV) for both conformers that are comfortably bracketed by B3LYP (7.11 eV) and CBS-QB3 (7.22 eV) calculations, from which we conclude that the carrier of the 600 nm spectrum remains unidentified. Optical-optical holeburning spectroscopy has been used to untangle cis and trans features of m-vinylbenzyl. There is very little to distinguish the two conformers from calculations, thermochemically or spectroscopically: they are similarly stable, their ground and excited state equilibrium geometries are planar, and their electronic transition energies, AIEs, predicted rotational contours, and excited state vibrational frequencies are highly similar. The most significant point of difference is a large (for cis) and relatively small (for trans) increase in the vinyl torsion force constant upon excitation, strongly suggesting identification of the cis conformer from an origin dispersed fluorescence spectrum, acceptance of which identification allows several other ground-state assignments to fall into place. A considerable breakdown in mirror symmetry between excitation and emission spectra for a′ modes is tentatively attributed to interference between Franck-Condon and Herzberg-Teller contributions to the transition moment, the low symmetry of the molecule (C_s) placing little restriction on such an interaction.

Alkoxy radicals are vital in the process of oxidation and have been well studied spectroscopically. The simplest species, CH₃O, has a degenerate X̃²E ground electronic state, which has a near-UV transition to a non-degenerate electronic state, B̃²A₁. Larger alkoxy radicals are formed by substitution of the H atom(s) with the B̃-X̃ electronic transition shifting to the red as the size of the alkyl group increases. Most importantly, when the H atom(s) substitution is asymmetric, the degeneracy of the X̃ state is resolved into two non-degenerate electronic states, X̃ and Ã. Typically the spin-orbit-free energy separation, ∆E₀, between these two states, is small ( ≤ 1000 cm⁻¹ and even ≤ 100 cm⁻¹ in some cases). Historically, the approach to the analysis of spectra has been to treat the rotational structure in the X̃ and à states separately via a Hamiltonian including an asymmetric top rotational term and a spin-rotation interaction term. Recently LiuJ. Liu, J. Chem. Phys. 148, 124112 (16 pages) (2018).uggested that, as is done with the X̃²E state of methoxy, the structure of both the X̃ and à states, now separated by ∆E₀ and coupled by the spin-orbit interaction and the Coriolis interaction, should be better considered together. This “coupled two-state model” also allows semi-quantitative prediction of effective spin-rotation constants using molecular geometry and spin-orbit constants, which can be calculated with considerable accuracy. In the present work, we have simulated rotationally and fine-structure resolved laser-induced fluorescence (LIF) spectra of alkoxy radicals with the Liu model and fit the rotational constants, as well as the spin-orbit and Coriolis coupling parameters between the à and X̃ states. The Coriolis coupling constant (ζ_t) was held equal to the quenched electronic orbital angular momentum (ζ_e d) of the spin-orbit constant. For these fits the spin-rotation parameters are held zero. The fits have been carried out for isomers and conformers of alkoxy radicals with four or less carbon atoms for which high-resolution LIF spectra have been obtained. Dependence of fit values of molecular constants (ζ_t, ζ_e d, and ∆E₀) on the size and conformation of alkoxy radicals will be discussed.

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J. Liu, J. Chem. Phys. 148, 124112 (16 pages) (2018).s

Small silicon carbides are of particular interest to the astrochemical community given the appearance of SiC, SiC₂, Si₂C, and SiC₃ in the interstellar medium. Here, we report high resolution anion photoelectron spectra of cryogenically cooled SiC₃⁻, Si₂C₂⁻, and Si₃C⁻, providing new insight into the geometric and vibronic structure of the corresponding neutral radicals. Comparison to theory suggests the anionic ground states for all three species are ring structures, with the ground state of Si₂C₂⁻ being a distorted rhombic structure possessing a dipole moment. Detachment from these anions yield vibrational frequencies for the three neutral ring structures, with a number of Franck-Condon forbidden transitions being ascribed to Herzberg-Teller coupling to excited neutral states.

Chirality, the molecular characteristic of having non-superimposable mirror images, is a major component of life as we know it. Investigation of chiral molecules in interstellar space may provide insight into the origins of homochirality in biology. Propylene oxide (CH₃CHCH₂O) was the first chiral molecule to be detected in space. Reactions inolving it and its radical derivatives may provide a means for installing chiral centers in more complex molecules, especially in protostellar regions. We have calculated the equilibrium structures of four radical derivatives of propylene oxide at the CCSD(T)/cc-pwCVTZ level of theory along with spin-rotation interaction terms. In addition, we have explored the barrier to internal rotation of the methyl group in two of the radicals at the CCSD(T)/cc-pwCVDZ level. We will discuss the results of these calculations and searches for the pure rotational spectra of these radicals.

Butyl radicals (n-, s-, i-, and tert-butyl) are formed from the pyrolysis of nitrite precursors (1-pentyl nitrite, 2-methyl-butyl nitrite, 3-methyl-butyl nitrite, and 2,2-dimethyl-propyl nitrite, respectively). The radicals are doped into a beam of liquid helium droplets and probed with infrared action spectroscopy from 2700-3125 cm⁻¹, allowing for a low temperature measurement of the CH stretching region. The presence of anharmonic resonance polyads in the 2800-3000 cm⁻¹  region complicates assignments. To facilitate spectral assignment, the anharmonic resonances are modeled with two effective Hamiltonian approaches that explicitly couple CH stretch fundamentals to HCH bend overtones and combinations: a VPT2+K normal mode model based on CCSD(T) quartic force fields and a semi-empirical local mode model. Both methods have been previously applied to the n- and i-propyl radicals. P.R. Franke, D.P. Tabor, C.P. Moradi, G.E. Douberly, J. Agarwal, H.F. Schaefer, E.L. Sibert, Infrared laser spectroscopy of the n-propyl and i-propyl radicals: stretch-bend Fermi coupling in the alkyl CH stretch region, J. Chem. Phys. 145 (2016) 224304.xtension to the butyl radicals allows for examination of the transferability of the empirical local mode coupling parameters to all types of local alkyl radical environment.

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P.R. Franke, D.P. Tabor, C.P. Moradi, G.E. Douberly, J. Agarwal, H.F. Schaefer, E.L. Sibert, Infrared laser spectroscopy of the n-propyl and i-propyl radicals: stretch-bend Fermi coupling in the alkyl CH stretch region, J. Chem. Phys. 145 (2016) 224304.E