title: Reactive carbon-chain molecules: synthesis of 1-diazo-2,4-pentadiyne and spectroscopic characterization of triplet pentadiynylidene (H-C[triple bond]C-:C-C[triple bond]C-H).
abstract: 1-Diazo-2,4-pentadiyne (6a), along with both monodeuterio isotopomers 6b and 6c, has been synthesized via a route that proceeds through diacetylene, 2,4-pentadiynal, and 2,4-pentadiynal tosylhydrazone. Photolysis of diazo compounds 6a-c (lambda > 444 nm; Ar or N2, 10 K) generates triplet carbenes HC5H (1) and HC5D (1-d), which have been characterized by IR, EPR, and UV/vis spectroscopy. Although many resonance structures contribute to the resonance hybrid for this highly unsaturated carbon-chain molecule, experiment and theory reveal that the structure is best depicted in terms of the dominant resonance contributor of penta-1,4-diyn-3-ylidene (diethynylcarbene, H-C[triple bond]C-:C-C[triple bond]C-H). Theory predicts an axially symmetric (D(infinity h)) structure and a triplet electronic ground state for 1 (CCSD(T)/ANO). Experimental IR frequencies and isotope shifts are in good agreement with computed values. The triplet EPR spectrum of 1 (absolute value(D/hc) = 0.6157 cm(-1), absolute value(E/hc) = 0.0006 cm(-1)) is consistent with an axially symmetric structure, and the Curie law behavior confirms that the triplet state is the ground state. The electronic absorption spectrum of 1 exhibits a weak transition near 400 nm with extensive vibronic coupling. Chemical trapping of triplet HC5H (1) in an O2-doped matrix affords the carbonyl oxide 16 derived exclusively from attack at the central carbon.
authors.affiliation: Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706-1396, USA.
authors.forename: Nathan P
authors.initials: NP
authors.lastname: Bowling
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Halter
authors.forename: Jonathan A
authors.initials: JA
authors.lastname: Hodges
authors.forename: Randal A
authors.initials: RA
authors.lastname: Seburg
authors.forename: Phillip S
authors.initials: PS
authors.lastname: Thomas
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Simmons
authors.forename: John F
authors.initials: JF
authors.lastname: Stanton
authors.forename: Robert J
authors.initials: RJ
authors.lastname: McMahon