COMBINED EXPERIMENTAL AND THEORETICAL-STUDY OF THE C-H BOND STRENGTH AND THE GAS-PHASE ACIDITY OF TRIACETYLENE, C6H2, AND THE ELECTRON-AFFINITY OF THE C6H-CENTER-DOT RADICAL

The thermochemistry of triacetylene, C6H2, the corresponding neutral triacetylide radical, C,H-., and the triacetylide anion, C6H-, has been evaluated by combining mass spectrometric means with high-level ab initio MO calculations at the CCSD(T) level. Experimentally, the ion/molecule bracketing method has been applied to determine the gas phase acidity (Delta G(acid)) Of C6H2 as 347 +/- 3 kcal/mol. For this purpose, C6H- anions were reacted with various bases for which Delta G(acid) is known. The computations lead to a dissociation energy of 126.8 +/- 1.6 kcal/mol for a C-H bond of triacetylene and an electron affinity of 3.69 +/- 0.05 eV for the triacetylide radical C6H.. Combining the calculated figures in terms of a thermochemical cycle leads to a theoretical prediction of Delta G(acid) (298 K) = 346.7 +/- 2.8 kcal/mol; this is in excellent agreement with the experimental value. The implications of the present results for interstellar chemistry are discussed.