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

Threshold energies for the proton transfer reaction C4H−+C2H2 → C4H2+C2H− and for competitive collision-induced dissociation of the proton-bound complex of diacetylide and acetaldehyde, [HC4··H··CH2CHO]−, are measured using guided ion beam tandem mass spectrometry. From the bimolecular proton transfer threshold, the gas-phase acidity of diacetylene is ΔacidH0(C4H2)≥1500±8 kJ/mol as a lower limit. The competitive dissociation is modeled using RRKM theory, yielding ΔacidH0(C4H2)=1508±12 kJ/mol. Using the literature electron affinity of C4H, the derived bond dissociation energy of diacetylene is D0(HC4–H)=539±12 kJ/mol.