The reaction of hydrogen atoms (H) with pyrrole (C₄H₅N) 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 C₄H₅N was essentially a three step process. First, mixtures of C₄H₅N 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, resulting from the products of the H + C₄H₅N reaction. To determine the grouping of lines to distinct chemical species, secondary photolysis was performed using 533-nm and 455-nm light-emitting diodes. Based on the secondary photolysis, it was determined that the majority of the new lines belong to two distinct chemical species, designated as set A (3491.0, 2754.4, 1412.7, 1260.4, 1042.8, 963.2, 922.1, 673.6 cm⁻¹) and set B (3468.3, 2784.9, 1470.6, 1449.5, 136.3, 1266.5, 1151.1, 1098.0, 960.6, 949.5, 924.0, 860.8, 574.2 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 nitrogen atom or the two carbon atoms of C₄H₅N to produce the corresponding hydrogen atom addition radicals (H-C₄H₅N). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level for the three possible H-C₄H₅N radicals in order to determine the relative energetics and the predicted vibrational spectra for each radical. The addition of the H atom to carbons 2 and 3 is predicted to be exothermic by 112.1 and 76.1 kJ/mol, respectively, while the addition of the H atom to the nitrogen is predicted to be endothermic by 67.8 kJ/mol. When the lines in set A and B are compared to the scaled harmonic and anharmonic vibrational spectra for all three possible radicals, the best agreement for set A is with the radical produced by the addition to carbon 3 (2,3-dihydropyrrol-2-yl radical) and the best agreement for set B is with the radical produced by addition to carbon 2 (2,3-dihydropyrrol-3-yl radical).

Photochemistry of nitrogen-based heterocyclic radicals plays a vital role in our understanding of the fundamental chemical processes in multiple fields including combustion, and atmospheric/troposphere chemistryPeeters, Z. et al., A&A 2005, 433, 583-590. Photochemical studies provide insights about the mechanistic pathways and the origin of various interstellar molecules. Radical and biradical species containing heteroatoms play a significant role as intermediates in photochemical processes. The photolysis and pyrolysis studies of one such heterocycle, pyrazine have been done using various spectroscopic and computational techniquesWilhelm, M. J. et al., J. Phys. Chem. A 2018, 122, 9001-9013. However, the photochemical investigations of the pyrazine radical have not been reported so far. In this work, we have explored the electronic structures of the radicals corresponding to all the possible diazines- pyrimidine, pyridazine, and pyrazine, at various levels of theories. Investigations for 3c-5e interactions between the two nitrogen lone pairs and the radical center have also been carried outSaraswat, M. et al., Phys. Chem. Chem. Phys. 2018, 20, 4386-4395. This has been coupled with an experimental study; the photochemical generation and characterization of the pyrazine radical using Matrix Isolation FT-Infrared spectroscopy. Isolated pyrazine radical photochemistry in solid nitrogen and argon matrices at 4 K using strong UV irradiation leads to the formation of numerous products via ring opening and fragmentation channels, which were identified by comparing the experimental spectrum with the computationally obtained spectrum.

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Peeters, Z. et al., A&A 2005, 433, 583-590.. Wilhelm, M. J. et al., J. Phys. Chem. A 2018, 122, 9001-9013.. Saraswat, M. et al., Phys. Chem. Chem. Phys. 2018, 20, 4386-4395..

l0pt Figure Combining laser ablation and pulsed discharge supersonic beam techniques, we investigated the laser-induced fluorescence (LIF) excitation spectra of jet-cooled WO molecule in the range of 18900 – 23500 cm⁻¹. Fig.1 (a) shows our experimental setup, where the gas phase WO molecules were produced by the reaction of O₂ molecules with the tungsten atoms ablated from a pure tungsten target. In this energy range, totally 63 bands were observed, and 60 bands were classified to 10 electronic transition progressions as shown in Fig.1 (b). Among them, 6 electronic transition progressions were previously reported by Ram et al. [1] using Fourier transform emission (FTE) spectroscopy, 4 electronic transition progressions [21.2]0⁺ – X0⁺, [21.5]0⁺ – X0⁺, [22.2]0⁺ – X0⁺, and [23.3]1 – X0⁺ were only identified in the LIF spectra. The symmetry of all upper electronic states were determined using the ducted selection rules in the known symmetry of the ground state. Among the four transition systems only observed in LIF spectra, three electronic states have the 0⁺ symmetry, which are [21.2]0⁺, [21.5]0⁺, and [22.2]0⁺states. Considering the E0⁺ and F0⁺ states, there are five electronic states having the symmetry of 0⁺ in this energy region. Ram et al. performed a high-level ab initio calculation of the lowest three configurations of WO molecule involving to the electron promotions of 3σ→1δ, 1δ→2π, and 3σ→2π, respectively, in which only two states have the 0⁺ symmetry in this energy region. Therefore, it is a great challenge for the theoretical calculation to comprehensively understand the electronic structure of WO molecule. References: [1] R. S. Ram, J. Liévin b, Peter F. Bernath, J. Mol. Spectrosc. 256, 216 –227 (2009).

The resonance-stabilized 1-vinylpropargyl radical (1vpr), which can adopt cis- and trans- conformations, is held to be a significant intermediate in hydrocarbon pyrolysis and has also been observed in the crossed-beam reaction of C₂ with propyne. We have cleanly generated 1vpr in a discharge of pent-1-ene-4-yne and measured the adiabatic ionization energy (AIE) of each conformer by two-colour ion-yield spectroscopy. Our work takes advantage of an earlier report by one of us of the 1vpr electronic origin bands near 460 nm and 470 nm. Extrapolation to zero-field yields AIEs of 7.823(1) eV and 7.894(1) eV for the trans- and cis- forms respectively, in superb agreement with a QCISD(T) calculation (Hansen et al., J. Phys. Chem. A 2006, 110, 4376-4388), and further supporting the identification of 1vpr in flames. As part of on-going work, we have also secured vibrational assignments for a number of levels in the ground and first excited states of both conformers, first by decomposing the mass-resolved electronic spectrum (as much as possible) into cis- and trans- contributions using hole-burning spectroscopy, and then by measuring SVLE spectra for sufficiently well-resolved bands. The apparent projection of a low-frequency excited state mode onto both a′ and a" levels in the ground state is taken as evidence of a Duschinsky rotation that mixes a′ and a" modes, and perhaps, therefore, of some reduction in planarity in the excited state, particularly for the cis isomer. The extent to which this interpretation is reflected by B3LYP and CASSCF calculations will be discussed.

We describe on-going efforts to diagnose the hydrocarbon carrier of an electronic band system observed near 459 nm. The spectral carrier can be produced in jet-cooled discharges of toluene and linear and cyclic heptatrienes, but is most efficiently generated from 1,6-heptadiyne. The spectrum is observed at m/z=91 by resonant two-colour ionization, has an origin at the same wavelength in Ne and Ar expansions, and almost certainly does not result from photofragmentation of heavier species. Several ground-state frequencies have been obtained from single-vibronic-level emission (SVLE) spectroscopy. Optical-optical hole-burning and two-colour ion-yield measurements reveal that the spectrum arises from a single isomer with an adiabatic ionization energy near 6.93 eV. On this basis, a large number of C₇H₇ radicals can be simply ruled out using relatively inexpensive electronic structure calculations. The resonance-stabilized, substituted allylic radicals 1,2,5,6-heptatetraen-4-yl and 2-ethynylcyclopentenyl have predicted (CBS-QB3) AIEs within  ∼  0.1 eV of experiment, but simulations of Franck-Condon activity in totally symmetric modes from CCSD and EOM-CCSD calculations disagree qualitatively with the experimental origin SVLE spectrum, for both isomers. The 1-ethynylcyclopentenyl radical, which can be viewed as hosting an embedded, strained 1-vinylpropargyl radical chromophore (absorbing near 462 nm), is under investigation at the time of writing.

Isoprene is the most prevalent non-methane volatile organic compound in the atmosphere, comprising about half of non-methane total biogenic emissions.Guenther, A. et al. J. Geophys. Res. 100, 8873–8892 (1995)xidation of isoprene by hydroxyl radicals results in important and diverse atmospheric chemistry, including the formation of secondary organic aerosols and other atmospheric radicals.Wennberg, P. O., et al. Chem. Rev. 118, 3337-3390 (2018)owever, the high reactivity of the hydroxyl-isoprene (OH-isoprene) complex with oxygen and water makes direct measurements difficult in situ. We utilize anion photoelectron spectroscopy as a “back-door” technique to examine vibrational and electronic structure of the OH-isoprene adduct. Spectral assignment is aided by quantum chemical calculations. Structural variations between anion and neutral species result in significant vibrational progressions. These progressions appear to resemble low electron binding energy features in the photoelectron spectrum, but additional higher electron binding energy features attributed to other species are also present. Potential explanations of these features will be explored.

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Guenther, A. et al. J. Geophys. Res. 100, 8873–8892 (1995)O Wennberg, P. O., et al. Chem. Rev. 118, 3337-3390 (2018)H

Ozonolysis of volatile organic compounds is one of the principle oxidative reactions in the atmosphere. Such reactions are known to produce hydroxyl radical, as well as secondary organic aerosols.Guenther, A. et al. J. Geophys. Res. 1995, 100, 8873–8892.soprene is the most abundant non-methane hydrocarbon in the troposphere, and is thought to be a nighttime source of hydroxyl radical in densely vegetated areas.Gutbrod, R. et al. J. Am. Chem. Soc. 1997, 119, 7330-7342.e “tag” unstable radical species with an electron in order to study their molecular identities and low-lying electronic structures via mass spectrometry and anion photoelectron spectroscopy. We will present photoelectron spectra of products of isoprene ozonolysis at 2.33 and 3.49 eV, along with quantum chemical calculations of several reaction intermediates that are formed via ozonolysis of isoprene.

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Footnotes:

Guenther, A. et al. J. Geophys. Res. 1995, 100, 8873–8892.I Gutbrod, R. et al. J. Am. Chem. Soc. 1997, 119, 7330-7342.W