Before delving into rearrangement chemistry, it should be emphasized
that in ion decomposition chemistry, one often has free radicals as
products. Therefore, one must consider the 'unpairing' of electrons
during the course of a reaction, and this should be indicated in the
mechanism diagram. To do this, we adapt McLafferty's convention of
using a double barbed arrow, 
, to indicate movement
of a pair of electrons (such as an entire bonding pair). On
the other hand, a single barbed arrow, such as
,
is used to indicate a single electron. {insert figure}