Reactive Carbon-Chain Molecules: Synthesis of 1-Diazo-2,4-pentadiyne and Spectroscopic Characterization of Triplet Pentadiynylidene (H−C⋮C−C̈−C⋮C−H)
作者:Nathan P. Bowling、Robert J. Halter、Jonathan A. Hodges、Randal A. Seburg、Phillip S. Thomas、Christopher S. Simmons、John F. Stanton、Robert J. McMahon
DOI:10.1021/ja058252t
日期:2006.3.1
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 N-2, 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 equivalent to C-C-C equivalent to C-H). Theory predicts an axially symmetric (D-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 (vertical bar D/hc vertical bar = 0.6157 cm(-1), vertical bar E/hc vertical bar = 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 O-2-cloped matrix affords the carbonyl oxide 16 derived exclusively from attack at the central carbon.