Gas-phase acidity and C-H bond energy of diacetylene

Threshold energies for the proton transfer reaction (CH-)-H-4 + C2H2 --> C4 H2 + C2H- and for competitive collision-induced dissociation of the proton- bound complex of diacetylide and acetaldehyde, [HC4 . . H . . CH2CHO](-), a re measured using guided ion beam tandem mass spectrometry. From the bimole cular proton transfer threshold, the gas-phase acidity of diacetylene is De lta(acid)H(0)(C4H2) greater than or equal to 1500 +/- 8 kJ/mol as a lower l imit. The competitive dissociation is modeled using RRKM theory, yielding D elta(acid)H(0)(C4H2) = 1508 +/- 12 kJ/mol. Using the literature electron af finity of C4H, the derived bond dissociation energy of diacetylene is D-0(H C4-H) = 539 +/- 12 kJ/mol. (C) 2000 Elsevier Science B.V.