AB-INITIO CALCULATIONS ON THE ISOMERIZATION OF C4H4 RADICAL CATIONS

Ab initio calculations at the MRCl/ROHF/6-31G* level on the possible isomerization pathways of C4H4 radical cations give a clear explanatio n of experimental results obtained from neutralization-reionization ma ss spectrometry and from photodissociation experiments. The observed p hoton-induced isomerization of vinyl acetylene radical cations to the methylene cyclopropene structure is in excellent agreement with the ca lculated results. It is suggested that the methylene cyclopropene radi cal cation is not a primary product of fragmentation of C6H6 precursor ions but is formed via a low-energy isomerization of ions having a no n-classical structure. This isomerization presumably takes place withi n the ion/molecule complex before the fragments go apart. (C) 1997 Els evier Science B.V.