A neutralization-reionization mass spectrometric study of alkyl hydroperoxide cation radicals and four distinguishable [C,H 3,O 2] + isomers

Abstract

The cation radicals of simple alkyl hydroperoxides ROOH + (R = CH 3, C 2H 5, iso-C 3H 7, tert-C 4H 9) have been studied by various mass spectrometric techniques, and neutralization-reionization (NR) methods in particular. Collisional activation (CA) of the beam of fast neutrals before reionization (NCR) and collision experiments with the mass-selected survivor ions (NR/CA) demonstrate that the peroxide OO bond remains intact in ionized methyl hydroperoxide. In the series of hydroperoxides, the abundances of the survivor ions decrease with α-substitution, which can be traced back to increasing contributions of [ R + OOH ] ion/dipole complexes to the parent ion beam. For the elucidation of ion structures, +NR − experiments are shown to be particularly helpful; in addition, they also provide insight into the bonding situations in hydroperoxides. Unimolecular loss of H' from metastable ions (MI) of CH 3OOH + leads to hydroperoxy methyl cations, CH 2OOH +, which are characterized by their MI/CA mass spectra. For comparison, four distinguishable [C,H 3,O 2] + isomers (CH 2OOH +, CH 3OO +, HC(OH) 2 , and H 3O +·CO) have been generated and examined by MI, CA, and NR experiments. With the exception of the proton-bound species H 3O +·CO, the corresponding neutral [C,H 3,O 2] . radicals exist as well and do not interconvert into each other. In addition, HOCH 2O . radicals can be probed by −NR + experiments with HOCH 2O − anions. Unimolecular and collision-induced decomposition of CH 2OOH + gives rise inter alia to a composite [C,H 2,O] + peak, which consists of a narrow gaussian and a broad, dish-topped component. The narrow component vanishes in the +NR + experiment.