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A Chemical Dynamics Study on the Gas-Phase Formation of Triplet and Singlet C5H2 Carbenes Chao Hea,1, Galiya R

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A Chemical Dynamics Study on the Gas-Phase Formation of Triplet and Singlet C5H2 Carbenes Chao Hea,1, Galiya R A chemical dynamics study on the gas-phase formation of triplet and singlet C5H2 carbenes Chao Hea,1, Galiya R. Galimovab,c,1, Yuheng Luoa,1, Long Zhaoa, André K. Eckhardtd, Rui Suna,2, Alexander M. Mebelb,2, and Ralf I. Kaisera,2 aDepartment of Chemistry, University of Hawai’i at Manoa, Honolulu, HI 96822; bDepartment of Chemistry and Biochemistry, Florida International University, Miami, FL 33199; cLaboratory of Combustion Physics and Chemistry, Samara National Research University, Samara 443086, Russia; and dInstitute of Organic Chemistry, Justus Liebig University, 35392 Giessen, Germany Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved October 14, 2020 (received for review September 15, 2020) Since the postulation of carbenes by Buchner (1903) and Staudinger helium–acetylene discharge (13) prior to its observation toward (1912) as electron-deficient transient species carrying a divalent car- TMC-1 (4). Cyclopropenylidene (1) ring opens to propargylene (3) bon atom, carbenes have emerged as key reactive intermediates in upon photoexcitation at 360 nm (3, 5). The triplet multiplicity of the organic synthesis and in molecular mass growth processes leading electronic ground state of the C2 symmetric propargylene (3), in eventually to carbonaceous nanostructures in the interstellar medium which each unpaired electron is formally located at each terminal and in combustion systems. Contemplating the short lifetimes of carbon atom, leading to an unconventional 1,3-diradical, was rec- these transient molecules and their tendency for dimerization, free ognized by Mebel et al., who computed the ground triplet 3B to lie − carbenes represent one of the foremost obscured classes of organic 53 to 61 kJ·mol 1 below the first excited singlet 1A′ state (14–16). reactive intermediates. Here, we afford an exceptional glance into the Recent reaction dynamics studies provided compelling evidence fundamentally unknown gas-phase chemistry of preparing two pro- that cyclopropenylidene (1) along with vinylidenecarbene (2) and/or — totype carbenes with distinct multiplicities triplet pentadiynylidene propargylene (3) can be easily formed in the bimolecular gas-phase (HCCCCCH) and singlet ethynylcyclopropenylidene (c-C5H2) carbene— 3 — reactions of ground-state carbon atoms [C( P)] with the vinyl radical via the elementary reaction of the simplest organic radical [C H (X2A′)] (17) and of methylidyne [CH(X2Π)] with acetylene methylidyne (CH)—with diacetylene (HCCCCH) under single-collision 2 3 [C H (X1Σ +)] (18). conditions. Our combination of crossed molecular beam data with 2 2 g Whereas a unified picture on the underlying chemistry and electronic structure calculations and quasi-classical trajectory chemical bonding of the cyclopropenylidene (1)–vinylidenecarbene simulations reveals fundamental reaction mechanisms and facil- 2 – 3 itates an intimate understanding of bond-breaking processes and ( ) propargylene ( ) system is beginning to emerge, gas-phase re- isomerization processes of highly reactive hydrocarbon interme- action dynamics studies sustaining a directed synthesis of distinct — 1 ′ 4 diates. The agreement between experimental chemical dynamics C5H2 isomers ethynylcyclopropenylidene (c-C3HCCH, X A , ), 1 5 studies under single-collision conditions and the outcome of tra- pentatetraenylidene (H2CCCCC, X A1, ), and pentadiynylidene 3Σ − 6 — jectory simulations discloses that molecular beam studies merged (HCCCCCH, X g , )(Fig.1) are remarkably lacking. Ethynylcy- 4 — 1 with dynamics simulations have advanced to such a level that clopropenylidene ( ) an ethynyl-substituted cyclopropenylidene ( ) polyatomic reactions with relevance to extreme astrochemical and combustion chemistry conditions can be elucidated at the molecular Significance level and expanded to higher-order homolog carbenes such as buta- diynylcyclopropenylidene and triplet heptatriynylidene, thus offering a Carbenes represent key reactive intermediates in molecular versatile strategy to explore the exotic chemistry of novel higher- mass growth processes leading to carbonaceous nano- order carbenes in the gas phase. structures in the interstellar medium and in combustion sys- tems. However, due to their short lifetimes and tendency for reaction dynamics | molecular beams | trajectory calculations | electronic dimerization, carbenes represent one of the foremost obscured structure theory | carbenes classes of reactive intermediates, with the preparation of car- benes in the gas phase remaining largely elusive. By merging ince the pioneering discovery of the structural isomers cyclo- molecular beams with electronic structure and quasi-classical 1 1 trajectory calculations and exploiting triplet pentadiynylidene Spropenylidene (c-C3H2,XA1, ), vinylidenecarbene (H2CCC, 1 3 (HCCCCCH) and singlet ethynylcyclopropenylidene (c-C H ) X A1, 2), and propargylene (HCCCH, X B, 3)(Fig.1)inthelab- 5 2 oratory and in extraterrestrial environments (1–9), the homologous carbene as benchmarks, we present a versatile protocol to unravel the dynamics leading to exotic carbenes under single- series of the isomers C3H2 (1 to 3)andC5H2 (4 to 6) fascinated the physical (organic) and computational chemistry communities from collision conditions. These elementary mechanisms are of sig- the fundamental points of view of electronic structure and chemical nificance to our understanding of the fundamental processes bonding (10–12)asprototypesofhighlyunsaturated (partially) 2π- leading to unsaturated organic transients in extreme, hydrocarbon- Hückel aromatics (1 and 4), singlet carbenes (2 and 5), and triplet rich environments. diradicals (3 and 6). Cyclopropenylidene (1)—a three membered C Author contributions: R.I.K. designed research; C.H., G.R.G., Y.L., L.Z., A.K.E., R.S., A.M.M., ring species of 2v symmetry depicting an aromatic character and a and R.I.K. performed research; C.H., G.R.G., Y.L., R.S., A.M.M., and R.I.K. analyzed data; carbene moiety at the apical carbon atom—represents the ther- and C.H., R.S., A.M.M., and R.I.K. wrote the paper. modynamically most stable C3H2 isomer. This molecule was first The authors declare no competing interest. detected by Reisenauer et al. in low-temperature argon matrices (3) This article is a PNAS Direct Submission. – prior to its identification in a helium acetylene gas discharge and Published under the PNAS license. toward the Taurus Molecular Clods (TMC-1) and the star-forming 1C.H., G.R.G., and Y.L. contributed equally to this work. C region Sagittarius B2 (Sgr B2) (1). The 2v symmetric vinyl- 2To whom correspondence may be addressed. Email: [email protected], mebela@ 1 idenecarbene (2)islinkedtovinylidene(H2CC; X A1) by formally fiu.edu, or [email protected]. adding a carbon atom to the carbene moiety and can be produced This article contains supporting information online at https://www.pnas.org/lookup/suppl/ via photolysis of propargylene (3)inanargonmatrixat10Kat doi:10.1073/pnas.2019257117/-/DCSupplemental. 313 nm (5). Gas-phase vinylidenecarbene (2) was identified in a First published November 16, 2020. 30142–30150 | PNAS | December 1, 2020 | vol. 117 | no. 48 www.pnas.org/cgi/doi/10.1073/pnas.2019257117 Downloaded by guest on October 2, 2021 Fig. 1. Molecular structures, electronic states, point groups, and the relative energies of C3H2 (1 through 3), C5H2 (4 through 6), and C7H2 (7 through 9) isomers. Energies are shown in kilojoules per mole. molecule with a Cs symmetry and a carbene moiety—signifies the through bimolecular reactions of the methylidyne radical (CH, 2 1 + CHEMISTRY thermodynamically most stable C5H2 isomer. Along with the C2v X Π) with diacetylene (HCCCCH; X Σg ) via resonantly stabi- symmetric cumulene carbene pentatetraenylidene (5), rotationally lized free 1- and 3-ethynylpropargyl radicals employing the crossed cold ethynylcyclopropenylidene (4) was detected in a helium– molecular beams method and merging these experimental studies acetylene gas discharge via microwave spectroscopy (8, 19–23). with electronic structure calculations and molecular dynamics Due to the absence of a permanent dipole moment, Maier et al. studies. The exploration of elementary reactions at the most fun- exploited electronic absorption spectroscopy to identify the linear damental, microscopic level conveys exclusive perceptions into the pentadiynylidene molecule (6), which is classified as a carbene- reaction mechanisms and underlying chemistry through which centered diradical with a triplet ground state, in a 5-K neon matrix highly reactive carbenes such as the singlet ethynylcyclopropenyli- (24, 25). McMahon and coworkers prepared pentadiynylidene (6) dene (4) and triplet pentadiynylidene (6) are formed without suc- in argon matrices at 10 K and solidified the assignment of a triplet cessive reactions in the gas phase. The coupling of experimental molecular structure via infrared, ultraviolet-visible, and electron dynamics experiments with computational studies and trajectory spin resonance spectra (26). Schwarz and coworkers employed simulations is very challenging for experiments and theory consid- negative ion chemical ionization mass spectroscopy in the gas ering the polyatomic character of five heavy atoms along with the phase to prepare pentatetraenylidene (5) and pentadiynylidene (6) high dimensionality of this surface. This system is also appealing (27). Very recently, thermally labile diazo precursors were from the physical organic chemistry viewpoint to benchmark the employed to prepare pentadiynylidene
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