Stereochemical effects enhanced by using selective “self-ionization” under electron ionization conditions in a quadrupole ion trap mass spectrometer

Abstract

From a previous study on the reactivity of the cis and trans (4-hydroxy-4-methyl-cyclohexyl)-benzoates (Mw = 234) under ammonia chemical ionization conditions in high pressure source, it was demonstrated that competitive decompositions, have specific orientations. In particular, highly stereospecific benzoic acid loss from the trans isomer takes place, suggesting that protonation by NH 4 + regioselectively occurred at the benzoate site rather than at the hydroxyl group. From the cis epimer elimination of water was only observed. This behavior could be explained by (1) possible proton transfer to both the functional groups and (2) decomposition occurring via water loss due to a higher rate constant compared with that of the benzoic acid loss (PA C 6H 5 COOH > PA H 2O ), which is not observed. These epimers are now studied under low pressure chemical ionization (CI) and electron impact (EI) conditions by using an ion trap mass spectrometer. Epimer differentiation from the stereospecific benzoic acid loss can be achieved independently of the gas-phase reagent used for chemical ionization, even if quasimolecular ions are entirely decomposed (i.e. in low pressure isobutane CI and methane CI). More spectacular differentiation is displayed in the ammonia CI mass spectra of epimers. Among the various quasimolecular species, the product [MNH 4–H 2O] + ions, termed substituted ions, usually containing a covalent C–NH 3 + bond are herein in fact a noncovalent form similar to the ammonia solvating protonated(4-hydroxy-4-methyl-cyclohexyl)-benzoate structure, as shown by deuterium labeling. Other product ions such as C 7H 11NH 3 + and {NH 3, C 6H 5COOH 2 +} are detected in contrast to those observed under high pressure source conditions. Collision induced dissociation spectra of the adduct MNH 4 + ions in addition to those of protonated molecules are investigated to obtain information about the location of ammonium (or proton) attachment. It appears for the trans isomer, a regioselective approach to the benzoate group, yielding [MH–C 6H 5COOH] +, takes place rather than attack at the OH site resulting into [MH–H 2O] +. This difference is at the origin of the observed stereospecific decomposition, which occurs via anchimeric assistance. Alternatively, these epimers give similar EI mass spectra, masking all the stereochemical effects, which reappear when using a residence time of 80 ms prior to apply the analytical scan. Under such conditions, ion–molecule reactions are enhanced, yielding formation of the epimeric MH + ions. These “ self-ionization” processes are induced through exothermical proton transfers from many EI fragment ions. Competitive decompositions of the product even-electron MH + species provide a direct cis/ trans differentiation. The diagnostic cleavages involved are comparable to those observed under low pressure CI conditions except that the loss of water occurs from both the precursors. The latter loss indicates that protonation is in competition and takes place at either of the basic sites, in contrast with that observed in chemical ionization. Furthermore, the exothermicity of proton transfer is preserved as internal energy of MH + which allows consecutive decompositions. At higher m/z ratio ranges, several adduct species are stereoselectively observed in the self-ionization mass spectrum of the trans epimer. These specific processes provide enhancement of the stereochemical cis/ trans differentiation.