Salts and cocrystals are multicomponent crystals that can be distinguished by the
location of the proton between an acid and a base. At the salt end of the spectrum proton transfer
is complete, and on the opposite end proton transfer is absent in cocrystals. However, for
acid-base complexes with similar pKa values, the extent of proton transfer in the solid state is
not predictable and a continuum exists between the two extremes. For these systems, both the
¢pKa value (pKa of base - pKa of acid) and the crystalline environment determine the extent of
proton transfer. A total of 20 complexes containing theophylline and guest molecules with ¢pKa
values less than 3 have been prepared, resulting in 13 cocrystals, five salts, and two complexes
with mixed ionization states based on IR spectroscopy and single-crystal diffraction data. We
propose modifications to the ¢pKa rule for selecting salt screen counterions that focus on the
discovery of solid forms with useful physical properties rather than an arbitrary cutoff value
for ¢pKa