Dt. Leeck et al., THE LONG-LIVED RADICAL CATIONS OF SIMPLE CARBON ESTERS ISOMERIZE TO THE LOWEST-ENERGY STRUCTURE, Journal of the American Chemical Society, 116(7), 1994, pp. 3028-3038
The structures of the low-energy, long-lived radical cations of simple
gaseous carbon esters were investigated using bimolecular reactions i
n a dual-cell FT/ICR mass spectrometer. The molecular ions of dimethyl
carbonate, methyl and ethyl formate, methyl and ethyl acetate, and me
thyl and ethyl propionate were generated by electron impact or thermon
eutral charge exchange. Reference ions with the enol and the expected
distonic structures (spatially separated odd spin and charge site) wer
e formed by electron impact on suitable neutral precursors or by ion-m
olecule reactions using distonic reactant ions. Ionized gamma-butyrola
ctone and ionized delta-valerolactone were chosen as models for conven
tional ester radical cations. The experimental results demonstrate tha
t 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 structure
s: (1) Cyclohexanone reacts by charge exchange with the conventional e
ster radical cations while enol and distonic ions react by proton tran
sfer. (2) Triethylamine reacts with the distonic ions by proton transf
er while charge exchange dominates for the enol ions and the conventio
nal radical cations. (3) Conventional ester radical cations abstract a
deuterium atom from 2-propanol-d(8), while distonic ions and enol ion
s 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 th
e long-lived ionized carbonates and formates have a distonic structure
, while the acetate and propionate radical cations have an enol struct
ure.