Proton affinities of diacetylene, cyanoacetylene, and cyanogen

The proton affinity of cyanoacetylene HCCCN was determined by pulsed high pressure mass spectrometric equilibrium measurements as 180.1±1 kcal/mol. Ion cyclotron resonance (ICR) bracketing experiments yielded the proton affinity of diacetylene HCCCCH as 180±1 kcal/mol and of cyanogen NCCN as 161±2 kcal/mol. Ab initio calculations at the 6-31+G** level are used to evaluate the heats of formation of HCCCCH as 105 and of HCCCN as 87 kcal/mol. The ab initio results show also that protonation of diacetylene on a terminal carbon converts a destabilizing antibonding interaction between the triple bonds in the neutral molecule into an attractive interaction, leading to a significant shortening of the C2–C3 bond.