|Ph.D Student||Ben Ari Julius|
|Subject||Steric Effects in Mass Spectrometry|
Experiment and Theory
|Department||Department of Chemistry||Supervisor||Professor Emeritus Asher Mandelbaum|
Steric effects that influence the fragmentation of ions in the gas phase were explored in this work.
An enhanced elimination of methanol under isobutane CI conditions has been observed in several primary and secondary methyl ethers having a tertiary b-position (methine), as compared with those with b-methylene. This elimination is stereospecific in some stereoisomeric ethers. The enhanced elimination of alcohol was explained by involvement of a 1,2-hydride migration in the course of elimination. DFT calculations supported the proposed mechanism.
The study of alcohol elimination from diethers upon isobutane CI demonstrated that protonated 1,4-dialkoxybutanes lose alcohol via anchimerically assisted elimination mechanism. Isomeric mixed 1,4-dialkoxybutanes, substituted at position 2 with alkyl groups of variable bulkiness, exhibit preferential elimination of alcohol from position C-4 rather than from C-1, and this tendency increases with the size of the substituents. This steric effect is explained by a more hindered transition state involved in the anchimerically assisted elimination of alcohol from C-1 due to interaction of the substituent(s) at position 2 with the protonated alkoxy group at position 1.
An unusual charge-exchange reaction upon CI, resulting in cation radicals M+• in addition to the M+ ions, was observed in amines with bulky N-substituents, where the approach to the nitrogen atom is hindered for protonation and the energy of ionization is lowered.
The investigation of the collisionally induced fragmentation of three isomeric dehydrobenzenesulfonic acid anions leads to the conclusion that the 3- and 4-dehydrobenzenesulfonic acid anions do not isomerize to the more stable benzenesulfonate structure due to the high activation energy required for 1,2-proton migration.