THE FRAGMENTATION-PATTERN OF 1,4-DIOXANE ION

Fragmentation of 1,4-dioxane ions at low internal energies was studied by photodissociation in the ion cyclotron resonance (ICR) ion trap. Branching ratios, rate-energy curves and kinetic parameters were assigned for the m/z 58, 57 and 45 fragment ions. M/z58 is formed through a slightly tight rate-limiting transition state (E0 = 1.46 eV, DELTAS1000K(dagger) = -1.4 e.u.) which probably corresponds to rupture of the dioxane ring. M/z45 is formed through a tight transition state (E0 = 1.4 2eV, DELTAS1000K(double dagger) = -10.9 eV) which presumably reflects a cyclic rearrangement pathway. The kinetic data for formation of 57 are almost equally well fitted by assuming competitive (RRKM) fragmentation from the parent ion, or assuming sequential fragmentation through an intermediate 58 ion. If the mechanism is solely competitive, the transition state is extraordinary loose (E0 = 1.87 eV, DELTAS1000K(double dagger) = 14.8 e.u.) which would be very surprising for this rearrangement dissociation. Accordingly we lean toward a sequential dissociation mechanism leading to the 57 product, and suggest further experimental testing of this possibility via structural studies of the 57 fragment.