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A theoretical and experimental study of the structures and stabilities of the [C 5 H 3 ] + cation
Author(s) -
Kompe Barbara M.,
Peel J. Barrie,
Traeger John C.
Publication year - 1993
Publication title -
organic mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 0030-493X
DOI - 10.1002/oms.1210281042
Subject(s) - hexa , isodesmic reaction , chemistry , enthalpy , fragmentation (computing) , molecular orbital , ab initio , ion , standard enthalpy of formation , potential energy surface , crystallography , computational chemistry , molecule , thermodynamics , physics , organic chemistry , computer science , operating system
The structures and energies for 16 different [C 5 H 3 ] + isomers were geometry optimized from ab initio molecular orbital calculations. The global minimum on the [C 5 H 3 ] + 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 [C 5 H 3 ] + 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.