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

Methods are described for the unequivocal identification of the acetyl, [CH₃\documentclass{article}\pagestyle{empty}$ \mathop {\rm C}\limits^{\rm + } $ O] (a), 1‐hydroxyvinyl, [CH₂\documentclass{article}\pagestyle{empty}$ \mathop {\rm C}\limits^{\rm + } $ 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}$ \mathop {\rm C}\limits^{\rm + } $ 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}$ \mathop {\rm C}\limits^{\rm + } $ 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}$ \mathop {\rm C}\limits^{\rm + } $ OH], but undergoes a slow isomerization into [CH₃\documentclass{article}\pagestyle{empty}$ \mathop {\rm C}\limits^{\rm + } $ O] via [CH₂\documentclass{article}\pagestyle{empty}$ \mathop {\rm C}\limits^{\rm + } $ 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}$ \mathop {\rm C}\limits^{\rm + } $ OH] ions in addition to [CH₃\documentclass{article}\pagestyle{empty}$ \mathop {\rm C}\limits^{\rm + } $ 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}$ \mathop {\rm C}\limits^{\rm + } $ OD]. Elimination of CH₃^˙ from the enol of acetone radical cation most probably generates only [CH₃\documentclass{article}\pagestyle{empty}$ \mathop {\rm C}\limits^{\rm + } $ 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}$ \mathop {\rm O}\limits^{\rm + } $ C:]) have been experimentally identified, the hypothesis of incompletely randomized energy should be used with reserve.