Theoretical equilibrium geometry, vibrational frequencies and the first electronic transition energy of HCC

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Abstract
Ab initio calculations with the 6–311 G** basis set and all single and double excitations in the CI treatment have been carried out to determine the structure of the HCC radical. The theoretical geometry of R(CC) = 1·209, r(CH) = 1·067 Å (or 1·205 and 1·063, respectively, if corrected for residual errors) is in excellent accordance with the experimental rotational constant. The calculated harmonic vibrational frequencies are v 1 ⋍ 3450, v 2 ⋍ 540 and v 3 ⋍ 2040 cm-1. An extremely low energy around 2000 cm-1 is obtained for the first electronic transition A 2II ← X 2∑. Results for the excited state are also given. The theoretical vibrational frequencies, with all possible errors taken into account, are inconsistent with the accepted interpretations of the few experimental results. It is shown, however, that a complete reinterpretation of the spectroscopic observations is possible, by which the present calculations fit with the recent gas phase infrared laser spectroscopic data but remain in definite contradiction with the infrared matrix results.