The closed shell species fulvenallene (C₇H₆) and the fulvenallenyl radical (C₇H₅) are produced via thermal decomposition of phthalide (C₈H₆O₂) in a continuous-wave SiC pyrolysis furnace. Prompt pick-up and solvation of these species in helium droplets allows for the measurement of well-resolved infrared spectra in the CH stretching region. VPT2+K simulations based on a hybrid CCSD(T) force field with quadratic (cubic and quartic) force constants computed using the ANO1 (ANO0) basis set are used to predict anharmonic frequencies for both species. The 3300 cm⁻¹region of the spectrum contains the acetylenic stretch of fulvenallenyl which serves as a sensitive marker for the extent of delocalization between the conjugated propargyl and cyclopentadienyl subunits of the radical. This delocalization is explored with spin density calculations at the B3LYP/aug-cc-pVTZ and ROHF-CCSD(T)/ANO1 levels of theory.

The reaction of hydrogen atoms (H) with isoprene (C₅H₈) in solid para-hydrogen (p-H₂) matrices at 3.2 K has been studied using infrared spectroscopy. The production of H atoms for reaction with C₅H₈ was essentially a three step process. First, mixtures of C₅H₈ and Cl₂ were co-deposited in p-H₂ at 3.2 K for several hours, then the matrix was irradiated with ultraviolet light at 365 nm to produce Cl atoms from the Cl₂, and finally the matrix was irradiated with infrared light to induce the reaction of the Cl atoms with p-H₂ to produce HCl and H atoms. Upon infrared irradiation, a series of new lines appeared in the infrared spectrum, with the strongest lines appearing at 776.0 and 766.7 cm⁻¹. To determine the grouping of lines to distinct chemical species, secondary photolysis was performed using a 365-nm light-emitting diode and a low-pressure mercury lamp in combination with filters. Based on the secondary photolysis, it was determined that the majority of the new lines belong to two distinct chemical species, designated as set X (3030.6, 1573.2, 1452.0, 1435.6, 1123.2, 1051.4, 982.7, 922.5, 792.5, 776.0, 699.2, 524.7, 469.0 cm⁻¹) and set Y (3110.1, 2972.0, 1564.4, 1471.1, 1430.2, 1379.7, 1376.2, 1335.4, 1233.0, 1205.4, 1050.1, 766.7, 570.0 cm⁻¹). The most likely reactions to occur under the low temperature conditions in solid p-H₂ are the addition of the H atom to the four alkene carbon atoms to produce the corresponding hydrogen atom addition radicals (HC₅H₈). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level for the four possible HC₅H₈ radicals in order to determine the relative energetics and the predicted vibrational spectra for each radical. The addition of H to each of the four carbons is exothermic, with relative energies of 0.0, 93.3, 77.0, and 8.4 kJ/mol for the addition to carbons 1 – 4, respectively. When the lines in set X and Y are compared to the scaled harmonic and anharmonic vibrational spectra, the best agreement for set X is with the radical produced by the addition to carbon 4 (1,2-dimethylallyl radical) and the best agreement for set Y is with the radical produced by addition to carbon 1 (1,1-dimethylallyl radical).

The 1-butyn-3-yl radical (C₄H₅) is thought to play a role in the formation of hydrocarbons in the interstellar medium and planetary atmospheres, but it is not well characterized. In this study, the 1-butyn-3-yl radical was formed by the pyrolysis of gas-phase 3-bromo-1-butyne at temperatures of 800-1200 K. Nascent radicals were isolated in an argon marix, followed by FTIR spectroscopy. Vibrational bands in the experimental spectra were matched to frequencies predicted by Gaussian 09. Pyrolysis of 3-methyl-1-butyne was also investigated as a possible pyrolytic precursor to the 1-butyn-3-yl radical under similar conditions. Evidence of 1-butyn-3-yl formation was observed, but other radicals may have formed as well.

The benzyl radical (C₆H₅CH₂) is a classic example of a relatively long lived, resonantly stabilized transient molecule. Its relative stability makes it a likely intermediate in the formation of polycyclic aromatic hydrocarbons and ultimately soot during the combustion of fossil fuels. Benzyl radical is also thought to be candidate for detection in interstellar molecular clouds. Benzyl radical is generated by seeding benzyl chloride in a rare gas He/Ne mixture through a pulsed slit discharge, with the radical formation process likely dominated by electron dissociative attachment. The radicals are subsequently cooled in a slit jet supersonic expansion to a 15K rotational temperature. Narrow band infrared radiation is produced through difference frequency generation of two single-mode visible lasers. High frequency stability (±11  MHz) is achieved through servo-locking techniques, and meticulous suppression of noise permits detection sensitivities approaching the quantum shot-noise limit. The slit jet has an inherent sub-Doppler resolution of 60 MHz. The present work reports the first ro-vibrationally resolved infrared spectra of antisymmetric (ν₃, B₂) and symmetric (ν₄, A₁) CH ring stretch modes in benzyl radical, with band origins (3073.2350 ±0.0005  cm⁻¹ and 3067.0576 ±0.0006  cm⁻¹, respectively) and rotational constants determined by least-squares fits to an asymmetric top Hamiltonian. Surprisingly, the benzyl spectrum shows little evidence of dark-state perturbations despite the relatively large number of atoms (N = 14) in this molecule. This is most likely due to the highly delocalized resonance structure of the benzyl radical, which generates a large barrier (∆E = 11.5  kcal/mol) for internal rotation of the methylene group, suppresses the rovibrational density of states, and makes the tunneling splittings too small to detect with the present sub-Doppler resolution. Particular effort is directed toward accurate determination of the ground state rotational constants, with a goal of assisting microwave search for benzyl radical in the interstellar medium.

We report the observation and analysis of spectra in part of the near-infrared spectrum of C₂H, originating in rotational levels in the ground and lowest two excited bending vibrational levels of the ground X̃ ²Σ⁺ state. In the analysis, we have combined present and previously reported high resolution spectroscopic data for the lower levels involved in the transitions to determine significantly improved molecular constants to describe the fine and hyperfine split rotational levels of the radical in the zero point, v₂=1 and the ²Σ⁺ component of v₂=2. Two of the upper state vibronic levels involved, a ²Π symmetry level at 6819.3 cm⁻¹ and a ²Σ⁺ one at 7527.1 cm⁻¹,  had not been previously observed. The data and analysis indicate the electronic wavefunction character changes with bending vibrational excitation in the ground state and provide avenues for future measurements of reactivity of the radical as a function of vibrational excitation.
Work at Brookhaven National Laboratory was carried out under Contract No. DE-SC0012704 with the U.S. Department of Energy, Office of Science, and supported by its Division of Chemical Sciences, Geosciences and Biosciences within the Office of Basic Energy Sciences.

r0pt Figure The ethynyl radial, C₂H, has a complex spectral structure due to vibronic coupling between the ground X̃ ²Σ⁺ and low-lying à²Π electronic states, and a Renner-Teller interaction within the Π state. A good understanding of the low-lying rovibrational structure has come from measurements, including slow electron velocity-map imaging of anion photoelectron spectraJ. Zhou et al. J. Chem. Phys. 127, 114313 (2007). and ab initio calculationsR. Tarroni and S. Carter, J. Chem. Phys. 119, 12878 (2003). that give wavefunction character. In this work, high-resolution photoelectron velocity-map imaging of C₂H⁻ and C₂D⁻ photodetachment (the 355 nm wavelength illustrated), provide a quantitative comparison over an extended energy range, to reveal unassigned structure, anomalous intensities, and illustrate the dramatic difference between isotopologues in the region of the A-state. These measurements, together with the measured photoelectron angular distributions, provide new insight into the non-adiabatic couplings of ethynyl.

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Footnotes:

J. Zhou et al. J. Chem. Phys. 127, 114313 (2007)., R. Tarroni and S. Carter, J. Chem. Phys. 119, 12878 (2003).,

In an effort to optically detect reactive intermediates implicated in mechanisms of C₇H₇ decomposition and formation, we have been interrogating radical products formed in supersonically cooled discharges of various C₇H₈ precursors. So far, two C₇H₇ isomers (neither of which is benzyl or tropyl) have been observed in the 470−455 nm region in resonant two-color two-photon ionization spectra. Both isomers were first observed in our laboratory in a toluene discharge, but they can be more efficiently produced from other precursors: one of them, for which we have measured an adiabatic ionization energy (AIE) of 6.92 eV, is highly conspicuous in a discharge of 1,6-heptadiyne; the other, with an AIE of 7.16 eV, is most efficiently generated from cycloheptatriene. Both species possess low frequency vibrational modes suggestive of acyclic structures, and because they absorb at similar wavelengths to the 1-vinylpropargyl radical, may also incorporate substituted propargyl chromophores. Quantum chemical calculations, additional chemical tests, and measurements of ground state vibrational frequencies by dispersed fluorescence are on-going to conclusively assign each spectrum to a particular carrier. The implications of our surprisingly facile discovery of two putatively unknown isomers will be discussed in the context of recent investigations of combustion and pyrolysis processes that begin or terminate with C₇H₇.

Bromomethyl radical (CH₂Br) has recently been used as a novel precursor for producing the simplest Criegee intermediates (CH₂OO). With the goal of spectroscopically investigating a Criegee intermediate, we have pursued high resolution characterization of the CH₂Br radical in our slit jet discharge spectrometer. The bromomethyl radical is generated by seeding CH₂Br₂ into a Ne/He/H₂ mixture in a pulsed slit discharge. The radical is produced through either electron dissociative attachment to form bromine anions or hydrogen abstraction of bromine, with subsequent cooling in a supersonic expansion to about 15 K. Infrared absorption in the CH₂ symmetric stretch vibrational band is fully resolved at high single-to-noise ratios for both the ⁷⁹Br and ⁸¹Br isotopologues . The sub-Doppler rotational structure is fitted to a rigid-rotor Hamiltonian with spin-rotation coupling, generating principal rotational constants and the spin-orbit coupling tensor for the vibrationally excited state. The results are consistent with a vibrationally averaged planar π-radical with unpaired electron spin density in a partially filled p_π-orbital on the central C atom. Relative band intensities in the symmetric and antisymmetric CH₂ stretch manifolds provide further elucidation of the "charge-sloshing" mechanism noted in CH₂F, CH₂Cl, and CH₂I radical species due to vibrationally mediated shifts in electron density along the carbon-halogen bond axis.

In our past SiCN investigationM. Fukushima and T. Ishiwata, J. Chem. Phys. 145, 124304 (2016). we found unknown bands with ¹Σ - ¹Σ rotational structure in the laser induced fluorescence ( LIF ) excitation spectrum of SiCN. From the rotational constants, the spectral species may possibly be attributed to SiOSi. Although the most stable geometry of the ground electronic state is reported to be cyclic structureS. J. Paukstis, et al., J. Chem. Phys. A 106, 8435 (2002). our CCSD(T) calculation with arg-cc-pCVTZ indicates the linear geometry, ¹Σ_g⁺, lying  ∼ 2,000 cm⁻¹ above it. The potential energy surface calculated is very strange, and it indicates a barrier between the two geometries,  ∼ 10,000 cm⁻¹ from the bottom. The dispersed fluorescence ( DF ) spectra from the single vibronic levels have fairly long progressions with very harmonic structure, but no hot-band structure. More precise computational works are underway, and we will discuss the assignment of the spectral species in this talk.

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Footnotes:

M. Fukushima and T. Ishiwata, J. Chem. Phys. 145, 124304 (2016)., S. J. Paukstis, et al., J. Chem. Phys. A 106, 8435 (2002).,

The nascent steps in the pyrolysis of the lignin components, salicylaldehyde (o-HOC₆H₄CHO) and catechol (o-HOC₆H₄OH), have been studied in a set of heated micro-reactors. The micro-reactors are small (roughly 1 mm ID x 3 cm long); transit times through the reactors are about 100 μsec. Temperatures in the micro-reactors can be as high as 1600 K and pressures are typically a few hundred Torr. The products of pyrolysis are identified by a combination of photoionization mass spectrometry and matrix isolation infrared spectroscopy. The main pathway by which salicylaldehyde decomposes is a concerted fragmentation: o-HOC₆H₄CHO (+ M) → . At temperatures above 1300 K, fulveneketene loses CO to yield a mixture of , , and . These alkynes decompose to a mixture of radicals ( and and H atoms. H-atom chain reactions convert salicylaldehyde to phenol: o- → . Catechol has similar chemistry to salicylaldehyde. Electrocyclic fragmentation produces water and fulveneketene: o- (+ M) → . These findings have implications for the pyrolysis of lignin itself.

r0pt Figure Peroxy radicals are key reaction intermediates formed during the oxidation of hydrocarbons in the atmosphere and in low-temperature combustion. Allyl group-containing peroxy radicals are particularly important because they are generated in large quantities by the OH-initiated oxidation of isoprene, the most abundant non-methane biogenic hydrocarbon. In this talk, room-temperature cavity ring-down (CRD) spectra of the à ← X̃ electronic transition of 1-, 2- and 3-methyl allyl peroxy radicals will be reported. Peroxy radicals were produced in 193 nm photolysis of selected methyl-substituted allyl chlorides, e.g., 1-chloro-2-butene, 3-chloro-2-methyl-1 propene, and 3-chloro-1-butene, in the presence of O₂. Vibronic structure of the experimentally observed spectra are simulated using calculated electronic transition frequencies, vibrational frequencies, and Franck-Condon factors. Spectroscopic detection and characterization of isoprene peroxy radicalsA. P. Teng, J. D. Crounse, and P. O. Wennberg, J. Am. Chem. Soc. 139, 5367 (2017).re underway. A. P. Teng, J. D. Crounse, and P. O. Wennberg, J. Am. Chem. Soc. 139, 5367 (2017).a

Peroxy radicals are important chemical reaction intermediates in low-temperature combustion systems as well as in the Earth’s troposphere; hence spectroscopic detection of peroxies and studies of their kinetics are essential to improving the efficiency of internal combustion engines and reducing air pollution. In this talk, we report the room-temperature cavity ring-down (CRD) spectra of the à ← X̃ electronic transition of the tetrahydropyranyl peroxy (THPOO) and tetrahydrofuranyl peroxy (THFOO) radicals. Both THP and THF are building blocks of lignocellulose-derived biofuels. THPOO and THFOO therefore play critical roles in the oxidation of biofuels.Chen, M. W. et al., Phys. Chem. Chem. Phys., 2018, DOI: 10.1039/c7cp08164b.^, Rotavera, B. et al., Proc. Combust. Inst., 2017, 36 (1), 597–606.^, Antonov, I. O. et al., J. Phys. Chem. A, 2016, 120 (33), 9823-9840.n the present experiment, they are produced via hydrogen abstraction of THP and THF by chlorine atoms followed by oxygen addition. Chlorine atoms are produced in 193 nm photolysis of oxalyl chloride (COCl)₂. The presence of oxygen in the ring defines 3 distinct positions on THP (α,β and γ) and 2 for THF (α and β), which leads to different regioisomers. Moreover, compared to chain alkyl peroxy radicals, cyclic ones possess significantly different conformational landscapes. Quantum chemical calculations have been performed and provide electronic transition frequencies, vibrational frequencies, Franck-Condon factors, as well as relative energies of isomers and conformers. Spectral simulation using these calculated result suggests that all isomers and most of the possible conformers contribute to the experimentally observed spectra. Also determined in the spectral simulation is the branching ratios of reactions that produce different regioisomers. The CRD technique has been used for lifetime measurements and investigation of the ring-opening mechanism. Comparison between the target molecules and the controls, namely, homocyclic peroxy radicals, will be briefly discussed.

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Footnotes:

Chen, M. W. et al., Phys. Chem. Chem. Phys., 2018, DOI: 10.1039/c7cp08164b.\end Rotavera, B. et al., Proc. Combust. Inst., 2017, 36 (1), 597–606. Antonov, I. O. et al., J. Phys. Chem. A, 2016, 120 (33), 9823-9840.I