THE LONG-LIVED RADICAL CATIONS OF SIMPLE CARBON ESTERS ISOMERIZE TO THE LOWEST-ENERGY STRUCTURE

The structures of the low-energy, long-lived radical cations of simple gaseous carbon esters were investigated using bimolecular reactions in a dual-cell FT/ICR mass spectrometer. The molecular ions of dimethyl carbonate, methyl and ethyl formate, methyl and ethyl acetate, and methyl and ethyl propionate were generated by electron impact or thermoneutral charge exchange. Reference ions with the enol and the expected distonic structures (spatially separated odd spin and charge site) were formed by electron impact on suitable neutral precursors or by ion-molecule reactions using distonic reactant ions. Ionized gamma-butyrolactone and ionized delta-valerolactone were chosen as models for conventional ester radical cations. The experimental results demonstrate that the lactone ions indeed are stable toward isomerization. The neutral reagents cyclohexanone, triethylamine, and 2-propanol-d(8) proved to be useful when differentiating reference ions with different structures: (1) Cyclohexanone reacts by charge exchange with the conventional ester radical cations while enol and distonic ions react by proton transfer. (2) Triethylamine reacts with the distonic ions by proton transfer while charge exchange dominates for the enol ions and the conventional radical cations. (3) Conventional ester radical cations abstract a deuterium atom from 2-propanol-d(8), while distonic ions and enol ions react by H/D exchange. Some enol ions also abstract a deuterium atom from this reagent. Comparison of the reactions of the reference ions to those of the ester molecular ions demonstrates unambigously that the long-lived ionized carbonates and formates have a distonic structure, while the acetate and propionate radical cations have an enol structure.