Geometric structure of diacetylene cation in the X̃ 2Πg and Ã 2Πu electronic states

Abstract

The complete rs structure of the diacetylene cation in the X̃ 2Πg and à 2Πu electronic states is reported. To this end the rotationally resolved origin bands in the à 2Πu–X̃ 2Πg laser excitation spectra of H–13C 3/4 C–C 3/4 C–H+ and H–C 3/4 13C–13C 3/4 C–H+ have been measured and analyzed. From the determined rotational constants as well as those known for H–C 3/4 C–C 3/4 C–H+ and D–C 3/4 C–C 3/4 C–D+, all the bond lengths in the two electronic states have been inferred.

Rotationally Resolved Ã2Π –X̃2Π Electronic Transition of HC4D+

“…This yields an H-H distance in theX 2 g (Ã 2 u ) state of 592 (600) pm, which is in perfect agreement with the values derived from the previous isotopic studies: 591 (599) pm (4).…”

“…Some remaining lines, particularly in the warmer spectrum, are due to transitions originating from HC 4 H + and DC 4 D + as can be concluded from the line positions given in Ref. (4).…”

“…The diacetylene cation, HC 4 H + , is considered to be a reactive intermediate in plasma, combustion, and astrochemical reaction schemes and has been the topic of several spectroscopic studies (1)(2)(3)(4). The first assigned gas phase spectrum dates back to 1956 (1) when previously observed emission spectra (2) were identified as the origin band of theà 2 u ←X 2 g electronic transition.…”

“…The first assigned gas phase spectrum dates back to 1956 (1) when previously observed emission spectra (2) were identified as the origin band of theà 2 u ←X 2 g electronic transition. In subsequent emission studies the absolute rotational numbering was verified (3) and the bond lengths in theX 2 ground andà 2 electronically excited state were determined for the vibrational ground state upon isotopic substitution (4). The only main isotopic geometry not studied spectroscopically so far is the mixed diacetylene cation: H 12 C 4 D + .…”

Electronic Gas-Phase Spectrum of the Pentaacetylene Cation

“…The strong electronic transitions have 2 Π u -X 2 Π g symmetry and shift to the red with increasing number of carbon atoms. Rotationally resolved electronic spectra have been recorded previously in the gas phase for the even members with n ) 2 at 255.7 nm, 1 n ) 4 at 507.0 nm, 2,3 n ) 6 at 600.4 nm, 4,5 and n ) 8 at 707.1 nm. 6 Little is known on the next member in the series, the pentaacetylene cation HC 10 H + .…”