Stereochemistry of organic ions in the gas phase: A review

Abstract

The stereochemistry of organic ions in the gas phase can be regarded from two different points of view: (i) stereoselectivity in ion formation and (ii) stereospecifity of ion fragmentations. Fast ionization by electron or photon impact shows little stereoselection. Differences in the ionization energies and cross sections between stereoisomers are generally small, save for a few exceptions. Proton or larger ion transfer, as employed in chemical ionization mass spectrometry, gives more possibilities for stereoselection. Bi- or polyfunctional molecules can capture the proton in a hydrogen-bond stabilized [M + H]+ ion, which is feasible only with a favourable spatial orientation of the chelating groups. Adduct ions [M + R]+ can also be formed stereoselectively. The use of a chiral ionizing medium adds a new dimension, since enantiomers can be distinguished, or even independently identified. The stereochemistry of even-electron cations in the gas-phase is most pronounced with polyfunctional species. The stereochemical behaviour is ruled by two reactivity principles, i.e. the geometry-dependent stabilization of [M + H]+ ions by chelation, and the anchimeric assistance by neighbouring groups in elimination of small molecules (water, ammonia, alcohols, acetic acid, etc.). The stereochemistry of odd-electron cations seems to be governed by three principles, i.e. the thermochemistry of decompositions proceeding with simple-bond cleavage, stereoelectronic effects on bond dissociations in the presence of a control orbital, and long-range interactions resulting in transfer of a hydrogen atom or a larger group. All these three reaction classes have limited areas of application. The stereochemistry of even-electron anions has been developing rapidly. The reactivity of gas-phase anions finds numerous analogies in their chemistry in solution, e.g. hydride transfer reactions and nucleophilic substitution. The applications of mass spectrometry to configurational assignment and structure elucidation remain restricted to selected classes of organic compounds.