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

✍ Scribed by Barbara M. Kompe; J. Barrie Peel; John C. Traeger

• Publisher: John Wiley and Sons
• Year: 1993
• Tongue: English
• Weight: 846 KB
• Volume: 28
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/oms.1210281042

✦ Synopsis

The structures and energies for 16 different [ C,H, 1 + isomers were geometry optimized from ab initio molecular orbital calculations. The global minimum on the [ C,H,] + surface at the MP3/6-31G* level was found to be the ethynyl cyclopropenylium cation (a), with a penta-1,Miynylium structure (b) and a penta-1,4-diynylium structure (c) being 90 and 100 kJ mol-' higher in energy, respectively. An isodesmic reaction was used to calculate a value of 1339 kJ mol-' for the enthalpy of formation of b. Photoionization appearance energies were measured for the [ C,H,] + ions from penta-l,fdiyne, hexa-2,4-diyne and hexa-1,Sdiyne 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 f 2 kJ mol-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,Sdiyne fragmentations involve excess energy.