Electronic States of the Quasilinear Molecule Propargylene (HCCCH) from Negative Ion Photoelectron Spectroscopy

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• 作者：David L. Osborn ; Kristen M. Vogelhuber ; Scott W. Wren ; Elisa M. Miller ; Yu-Ju Lu ; Amanda S. Case ; Leonid Sheps ; Robert J. McMahon ; John F. Stanton ; Lawrence B. Harding ; Branko Ruscic ; W. Carl Lineberger
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：July 23, 2014
• 年：2014
• 卷：136
• 期：29
• 页码：10361-10372
• 全文大小：479K
• ISSN：1520-5126

文摘

We use gas-phase negative ion photoelectron spectroscopy to study the quasilinear carbene propargylene, HCCCH, and its isotopologue DCCCD. Photodetachment from HCCCH^{鈥?/sup> affords the X̃(3B) ground state of HCCCH and its 茫(1A), b̃ (1B), d̃(1A₂), and B̃(3A₂) excited states. Extended, negatively anharmonic vibrational progressions in the X̃(3B) ground state and the open-shell singlet b̃ (1B) state arise from the change in geometry between the anion and the neutral states and complicate the assignment of the origin peak. The geometry change arising from electron photodetachment results in excitation of the 谓₄ symmetric CCH bending mode, with a measured fundamental frequency of 363 卤 57 cm鈥? in the X̃(3B) state. Our calculated harmonic frequency for this mode is 359 cm鈥?. The Franck鈥揅ondon envelope of this progression cannot be reproduced within the harmonic approximation. The spectra of the 茫(1A), d̃(1A₂), and B̃(3A₂) states are each characterized by a short vibrational progression and a prominent origin peak, establishing that the geometries of the anion and these neutral states are similar. Through comparison of the HCCCH鈥?/sup> and DCCCD鈥?/sup> photoelectron spectra, we measure the electron affinity of HCCCH to be 1.156 卤 _0.0950.010 eV, with a singlet鈥搕riplet splitting between the X̃(3B) and the 茫(1A) states of 螖E_ST = 0.500 卤 _0.010.10 eV (11.5 卤 _0.22.3 kcal/mol). Experimental term energies of the higher excited states are T₀ [b̃(1B)] = 0.94 卤 _0.200.22eV, T₀ [d̃(1A₂)] = 3.30 卤 _0.020.10eV, T₀ [B̃(3A₂)] = 3.58 卤 _0.020.10eV. The photoelectron angular distributions show significant 蟺 character in all the frontier molecular orbitals, with additional 蟽 character in orbitals that create the X̃(3B) and b̃(1B) states upon electron detachment. These results are consistent with a quasilinear, nonplanar, doubly allylic structure of X̃(3B) HCCCH with both diradical and carbene character.}