J. J. Zwinselman

A field ionization kinetics and metastable ion study of the fragmentation of some pentenols

The mechanisms of methyl loss from the molecular ions of three isomeric pentenols, 2-methyl-3-buten-2-ol (I), 3-penten-2-ol (II), and 3-methyl-3-buten-2-ol (III),.and the structures of the resulting fragment ions have been investigated using isotopic labelling, metastable ion studies, and field ionization kinetics (FIK) studies. The molecular ion of I loses predominantly the methyl group in the α position with respect to the hydroxyl group and allylic to the double bond, to form the protonated methyl vinyl ketone ion, although a minor loss of a methyl group incorporating the three vinylic hydrogen atoms is observed for long-lived molecular ions. FIK studies of the fragmentations of CH3CH CHCH(OH)CD3+. (IIa) and CH2 C(CH3)CH(OH)CD3+. (IIIa) show, for both isomers, loss of both CD3 and CH3 at reaction times < 10−10.5 s, with loss of CH3 predominating at times between 10−10 and 10−9 s; loss of CD3 increases in importance again for long-lived ions fragmenting as metastable ions in the drift regions. The initial loss of CD3 at short times is shown to be the result of simple cleavage yielding the appropriate protonated aldehyde. Rapid [ 1,2 ] H-shifts yield the enol ion of the appropriate propyl methyl ketone which fragments by loss Of CH3 to give protonated vinyl methyl ketone. At longer times a further [1,4] shift of the hydroxylic hydrogen atom yields the propyl methyl ketone molecular ion which fragments by loss of CD3. Evidence for this complex series of hydrogen shifts is presented in terms of metastable ion intensities and kinetic energy releases in metastable ion fragmentation reactions.

A field ionization kinetic study of the methyl and ethylene elimination from ionised n-butanoic acid

Abstract A time-resolved study of the methyl and ethylene elimination from specifically 13 C-labelled n-butanoic acid ions is presented.

Ionization by proton abstraction in negative ion field desorption mass spectrometry

Abstract In negative ion field desorption mass spectrometry (M - H) − ions are generated from non-acidic compounds such as saccharides and nucleosides. These ions are not “preformed” in a sample solution but are generated by ion molecule reactions in the space charge region of the negatively charged sample layer. The chemistry of the ionization by proton abstraction is investigated and discussed.

Carbocation-like isomerisations at radical sites precede methyl losses from ionised butanoic acid

The low-energy losses of methyl from ionised butanoic acid and isomeric ions have been further characterised utilising isotopic labelling, field ionisation kinetics, and translational energy releases. Long lived butanoic acid ions lose methyl groups containing C-2, C-3, and C-4. The loss of methyl containing C-2 follows migration of the protonated carboxy group to the radical site of CH3ĊHCH2C(OH)2+. Methyl containing C-3 is lost following a 1,2-shift of the terminal methyl from C-3 to C-2. 1,2- and 1,4-hydrogen shifts take place in all of the pathways to methyl loss. The 1,2-shifts of methyl and hydrogen may be permitted by a reduced electron density on the α carbon atoms of the intermediate ions CH3CH2ĊHC(OH)2+ and (CH3)2ĊC(OH)2+. The isomerisations of ionised butanoic acid can be rationalised as a mixture of free-radical-like and carbocation-like reactions.