A theoretical and experimental study of the structures and stabilities of the [C5H3]+ cation

The structures and energies for 16 different [C5H3]+ isomers were geometry optimized from ab initio molecular orbital calculations. The global minimum on the [C5H3]+ surface at the MP3/6–31G* level was found to be the ethynyl cyclopropenylium cation (a), with a penta‐1,3‐diynylium structure (b) and a penta‐1,4‐diynylium structure (c) being 90 and 100 kJ mol−1 higher in energy, respectively. An isodesmic reaction was used to calculate a value of 1339 kJ mol−1 for the enthylpy of formation of b. Photoinization appearance energies were measured for the [C5H3]+ ions from penta‐1,3‐diyne, hexa‐2,4‐diyne and hexa‐1,5‐diyne as 11.66, 12.25 and 11.8 eV respectively The penta‐1,3‐diyne fragmentation was used to estimate an experimental enthalpy of formation for b of 1315 ± 2 kJ mol−1. There is some theoretical evidence that the penta‐1,3‐diyne fragmentation occurs without any significant reverse activation energy whereas both the hexa‐2,4‐diyne and hexa‐1,5‐diyne fragmentations involve excess energy.