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. B ranching ratios, rate-energy curves and kinetic parameters were assign ed 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, DELTAS100 0K(dagger) = -1.4 e.u.) which probably corresponds to rupture of the d ioxane ring. M/z45 is formed through a tight transition state (E0 = 1. 4 2eV, DELTAS1000K(double dagger) = -10.9 eV) which presumably reflect s a cyclic rearrangement pathway. The kinetic data for formation of 57 are almost equally well fitted by assuming competitive (RRKM) fragmen tation from the parent ion, or assuming sequential fragmentation throu gh an intermediate 58 ion. If the mechanism is solely competitive, the transition state is extraordinary loose (E0 = 1.87 eV, DELTAS1000K(do uble dagger) = 14.8 e.u.) which would be very surprising for this rear rangement dissociation. Accordingly we lean toward a sequential dissoc iation mechanism leading to the 57 product, and suggest further experi mental testing of this possibility via structural studies of the 57 fr agment.