Half a century of scrambling in organic ions: Complete, incomplete, progressive and composite atom interchange

Positional exchange of atoms within the given frameworks of gaseous ions and positional exchange of the framework atoms themselves-generally addressed as atom scrambling-belong to the best-known but, in the same time, most irritating characterics of the reactions of organic ions in the gas phase. Half a century now after the phenomenon of atom scrambling has been uncovered, this article spans arbitrarily a number of examples from the manifold of scrambling reactions in organic mass spectrometry. Starting out from of the best-established systems (C7H8.+-->C7H7+ + H-.), to illustrate an ubiquitous but very complex case, various organic systems that undergo complete ("statistical") hydrogen scrambling are presented. This includes two chemically very different cases where 21 protons are completely interchanged within the lifetime of the ions. Some tutorial aspects are woven in here on the simple combinatorics which have to be applied for calculating the "statistical pattern" which is to be expected for the relative abundances of the isotopomeric fragment ions (and neutral fragments) formed upon fragmentation of an ion which has undergone complete equilibration of a given set of atoms over a given set of molecular sites. Besides the species undergoing complete scrambling, some incompletely scrambling ones are also discussed, including a case where random distribution is reached separately within two moities of the ions alongside with a slow exchange process between these moieties. Ionic species that undergo incomplete but progressive (i.e. lifetime-dependent) scrambling on way to the full equilibration ("randomization") of the partners are also discussed. Finally, returning to apparently simple even- and odd-electron carbocations, several cases of "fractional" scrambling behavior are demonstrated. Here, two or more fractions of the reacting ions exhibit distinct scrambling behavior, for example, one of them fragmenting specifically, another after slow exchange, and still another after complete hydrogen equilibration, but all of them contributing to the same "composite" pattern of isotopomeric fragment ions in the mass spectrum. Although the selection of examples presented here reflects a part of die author's personal fascination on these systems over the years, it is hoped that studies on the variety, beauty and complexity of scrambling phenomena in gaseous organic ions will remain alive for at least a fun century when ions to be studied are getting larger and larger. (Int J Mass Spectrom 213 (2002) 101-144) (C) 2002 Elsevier Science B.V.