Chemistry of Energetically Activated Cumulenes—From Allene (H₂CCCH₂) to Hexapentaene (H₂CCCCCCH₂)

Abstract

During the last decade, experimental and theoretical studies on the unimolecular decomposition of cumulenes (H(2)C(n)H(2)) from propadiene (H(2)CCCH(2)) to hexapentaene (H(2)CCCCCCH(2)) have received considerable attention due to the importance of these carbon-bearing molecules in combustion flames, chemical vapor deposition processes, atmospheric chemistry, and the chemistry of the interstellar medium. Cumulenes and their substituted counterparts also have significant technical potential as elements for molecular machines (nanomechanics), molecular wires (nano-electronics), nonlinear optics, and molecular sensors. In this review, we present a systematic overview of the stability, formation, and unimolecular decomposition of chemically, photo-chemically, and thermally activated small to medium-sized cumulenes in extreme environments. By concentrating on reactions under gas phase thermal conditions (pyrolysis) and on molecular beam experiments conducted under single-collision conditions (crossed beam and photodissociation studies), a comprehensive picture on the unimolecular decomposition dynamics of cumulenes transpires.

Reductive Aromatization/Dearomatization and Elimination Reactions to Access Conjugated Polycyclic Hydrocarbons, Heteroacenes, and Cumulenes

“…Cumulenes consist of linear sp‐hybridized carbon atoms in a contiguous sequence of double bonds (vs. polyynes composed of alternating single and triple bonds; Figure ). In addition to potentially serving as model compounds for carbyne, both theory and experiment predict a number of fascinating properties for these compounds which could give rise to applications in nanoelectronic or spintronic devices, molecular wires, nonlinear optics, and others . Carbo ‐mers are compounds where molecular symmetry is retained while one cumulenic unit (=C=C=) is inserted into each −C−C− single bond and one acetylenic unit (−C≡C−) is inserted into each −C=C− bond of a CPH .…”

Formation and Oxidation of Linear Carbon Chains and Their Role in the Wear of Carbon Materials

“…We use both even-numbered and odd-numbered clusters since, for any given structure of the anchoring sites, the parity of the carbon atoms plays a crucial role in the electronic structure of the chains [38,39] (e.g., on the orientation of the p orbitals of double C-C bonds). In principle, two isomeric forms of carbyne are possible.…”

A Perspective on Designing Chiral Organic Magnetic Molecules with Unusual Behavior in Magnetic Exchange Coupling

“…24 Cumulenes find major application in molecular machines (nanomechanics), molecular wires (nano-electronics), nonlinear optics, and molecular sensors due to their unique electronic structures. 25 It is found that among all conjugated oligomers, cumulene wires with odd number of carbon atoms show the highest conductance with metallic-like ballistic transport behavior. 26 1 2 3 1 2 3 4 5 1 2 3 4 5 6 7 1 2 3 1 2 3 4 5 1 2 3 4 5 6 7 1 2 3 1 2 3 4 5 1 2 3 4 5 6 7 Series 1 Series 2 -indicates the number of carbon atoms in coupler).…”