A THEORETICAL AND EXPERIMENTAL-STUDY OF THE STRUCTURES AND STABILITIES OF THE [C5H3]+ CATION

The structures and energies for 16 different [C5H3]+ isomers were geom etry optimized from ab initio molecular orbital calculations. The glob al 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-diynyliu m structure (h) and a penta-1,4-diynylium structure (c) being 90 and 1 00 kJ mol-1 higher in energy, respectively. An isodesmic reaction was used to calculate a value of 1339 kJ mol-1 for the enthalpy of formati on of b. Photoionization appearance energies were measured for the [C5 H3]+ ions from penta-1,3-diyne, hexa-2,4-diyne and hexa-1,5-diyne as 1 1.66, 12.25 and 11.8 eV respectively. The penta-1,3-diyne fragmentatio n 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 activa tion energy whereas both the hexa-2,4-diyne and hexa-1,5-diyne fragmen tations involve excess energy.