GAS-PHASE ION CHEMISTRY OF DIMETHYL PEROXIDE WITH THE BARE TRANSITION-METAL CATIONS CR+, MN+, FE+, AND CO+

Unimolecular and collision-induced fragmentations of dimethyl peroxide complexes (CH3OOCH3)M(+), with M(+) = Cr+, Mn+, Fe+, and Co+, were ex amined by means of tandem mass spectrometry and compared to results fr om bimolecular gas-phase reactions of the corresponding ''bare'' metal cations with dimethyl peroxide as studied by Fourier transform ion cy clotron resonance (FT-ICR) experiments. Bismethoxide metal ions, (CH3O )(2)M(+), are generated in the initial reaction step by insertion of t he metal cation into the weak O-O bond of the peroxide. In the ion-bea m apparatus these intermediates undergo a variety of processes involvi ng beta-H shifts, intramolecular redox reactions, and radical losses, depending on the respective metal cations. In contrast, the insertion products (CH3O)(2)M(+) formed in the ion/molecule reactions of dimethy l peroxide with bare metal cations in the FT-ICR mass spectrometer dec ompose to complexes M(OCH3)(+) with concomitant loss of a methoxy radi cal, a process which is not observed in the unimolecular dissociation of metastable (CH3OOCH3)M(+) ions. The distinct differences between th e unimolecular chemistry of dimethyl peroxide/M(+) complexes in the io n-beam apparatus and the bimolecular reactions of M(+) with dimethyl p eroxide in the FT-ICR are explained in terms of internal energy effect s, which result from the method of ion formation. This analysis is fur ther supported by ligand-exchange reactions and collisional-activation experiments. In addition, analogies and differences of the gas-phase chemistry of dimethyl peroxide with M(+) as compared to metal-catalyze d decomposition of dialkyl peroxides in the condensed phase are discus sed.