HEPTENE RADICAL CATIONS - HOW WELL IS THE DOUBLE-BOND LOCALIZED

The fragmentation of radical cations of multiply labeled 1-, 2-, and 3 -heptenes is studied with different mass spectrometric techniques, all owing, together with published data, conclusions to be drawn about the time dependence of the fragmentations. No skeletal isomerization can be observed, but the position of the double bond seems to be shifted r ather rapidly, making the ion in the time average rather symmetric. Th is can be explained either by an equilibrium of different positions (w here the end positions have less weight) or some unknown intermediate structure(s). The important loss of neutral fragments from within the ions at longer time scales may speak in favor of the latter proposal. On the other hand, reactions such as allyl scission or McLafferty-type fragmentations take place before the double bond is displaced. The ad ditional energy gained by shifting the double bond from a terminal pos ition may explain the differences observed between 1- and 2 or 3-hepte nes even at longer time scales. In particular, it remains unexplained that even after a few milliseconds the non-fragmented molecular ions i n their ground state seem to retain some information about the initial localization of the double bond. (C) 1997 Elsevier Science B.V.