Four isomeric [C, H3, O2]+ ions

The isomeric ions [HC(OH)2]+, carbonyl oxygen-protonated formic acid [H2C(O+)OH], carbon-protonated formic acid, the methylene hydroperoxy cation, [CH2OOH]+, and the methylperoxy cation, [CH3OO]+, have been characterised by their collisional activation mass spectra, [HC(OH)2]+ and [CH2OOH]+ were generated by dissociative ionisation whereas the other two ions were produced by collisionally induced charge reversal of the corresponding anions. The fragmentations of [HC(OH)2]+ to yield [HC+ O] and [H3O]+ proceed via a common rate-determining energy barrier.