Some newer aspects of mass spectrometric ortho effects.

While in the early days of organic mass spectrometry the ortho effect was preferentially used as a diagnostic tool for structure elucidation, the recent research activities have concentrated on mechanisms and related aspects. Among these the following were the most important topics: hydrogen transfer and especially the question of concerted or multistep reaction; the influence of the structure on the selectivity of hydrogen migration via higher-membered transition states; the apparent cleavage of carbon/heteroatom bonds induced by hydrogen transfer; the redox processes which formally correspond to photochemical transformations, but which are completely different according to the reaction mechanisms; the different types of cyclization reactions; and the field of exchange processes preceding or accompanying unimolecular decompositions of ions in the gas phase. If a prognosis concerning the future activities in the field of electron impact induced ortho effects may be risked, the following aspects will probably become more important in addition to the topics discussed in this review:(i)Kinetic studies in the time range of nanoseconds to elucidate the time dependence of the different ortho effect reactions;(ii)ion/molecule reactions with the scope to obtain more information concerning the reactivities of open and closed shell systems;(iii)comparative studies in the fields of mass spectrometry, photochemistry and electrochemistry. In principle all these studies may extend our knowledge concerning the inherent properties of isolated ionic species. [ABSTRACT FROM AUTHOR]