TABLE OF CONTENTS The formation of new bonds between atoms in an ion leads to rearrangement. Several such processes have been covered in the sections above, as with hydrogen migration, which seems particularly facile. In some instances, mainly with odd-electron ions, the site from which a hydrogen is transferred is fairly specific. A particularly ubiquitous example is the six-centre transfer of hydrogen in carbonyl and related compounds (sometimes called the McLafferty rearrangement) as illustrated in schemes (10.26) and (10.27). In other cases, generally with even-electron ions, the reaction appears to be far less specific. The randomization of hydrogen in ions is widespread and can make the interpretation of deuterium labelling studies difficult or ambiguous.
Skeletal rearrangement is defined as the formation of new bonds between atoms other than hydrogen. The classification of these rearrangements is useful for the interpretation of mass spectra because known structural features giving rise to this phenomenon can be compared with the behaviour of compounds of unknown but postulated structure. Skeletal rearrangements in mass spectrometry are diverse but attempts have been made to classify them.
One of the earliest and most useful classifications was based on (i) the migration of alkyl or aryl groups and (ii) the nature of the neutral particle ejected after rearrangement (Brown and Djerassi, 1967). These compilations are useful and, for structural organic chemistry, the nature of the ejected species may be particularly informative. Typical neutral species eliminated following rearrangement include CO, CO 2, SO 2, CH 2
