  Radical-ion-pair reactions were recently shown to manifest a host of
non-trivial quantum effects accounted for by quantum measurement theory. An
alternative approach purporting to describe the fundamental quantum dynamics of
spin-selective radical-ion pair reactions was introduced most recently,
bringing to three the competing theories, including the one traditionally used
in spin chemistry. We here consider entropy as a fundamental concept enabling a
comparison of the predictions of these theories against what is physically
acceptable on quite general grounds.
