The gas phase ion chemistry of the acetyl cation and isomeric [C2H3O]+ ions. On the structure of the [C2H3O]+ daughter ions generated from the enol of acetone radical cation
Authors: Burgers Peter C. , Holmes John L. , Szulejko Jan E. , Mommers Alexander A. , Terlouw Johan K. 
Published: May 2005

Methods are described for the unequivocal identification of the acetyl, [CH3\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document} O] (a), 1‐hydroxyvinyl, [CH2\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}OH] (b), and oxiranyl, (d), cations. They involve the careful examination of metastable peak intensities and shapes and collision induced processes at very low, high and intermediate collision gas pressures. It will be shown that each [C2H3O]+ ion produces a unique metastable peak for the fragmentation [C2H3O]+ → [CH3]++CO, each appropriately relating to different [C2H3O]+ structures. [CH3\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}O] ions do not interconvert with any of the other [C2H3O]+ ions prior to loss of CO, but deuterium and 13C labelling experiments established that [CH2\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}OH] (b) rearranges via a 1,2‐H shift into energy‐rich leading to the loss of positional identity of the carbon atoms in ions (b). Fragmentation of b to [CH3]++CO has a high activation energy, c. 400 kJ mol−1. On the other hand, , generated at its threshold from a suitable precursor molecule, does not rearrange into [CH2\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}OH], but undergoes a slow isomerization into [CH3\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}O] via [CH2\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}HO]. Interpretation of results rests in part upon recent ab initio calculations.

The methods described in this paper permit the identification of reactions that have hitherto lain unsuspected: for example, many of the ionized molecules of type CH3COR examined in this work produce [CH2\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}OH] ions in addition to [CH3\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}O] showing that some enolization takes place prior to fragmentation. Furthermore, ionized ethanol generates a, b and d ions. We have also applied the methods for identification of daughter ions in systems of current interest. The loss of OH˙ from [CH3COOD]+˙ generates only [CH2\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}OD]. Elimination of CH3˙ from the enol of acetone radical cation most probably generates only [CH3\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}O] ions, confirming the earlier proposal for non‐ergodic behaviour of this system. We stress, however, that until all stable isomeric species (such as [CH3\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^{\rm + } $\end{document}C:]) have been experimentally identified, the hypothesis of incompletely randomized energy should be used with reserve.