A new co-crystal of theophylline and phthalic acid with 1:1 molar ratio has been prepared. It crystallises in the monoclinic
crystal system, space group P21/c, a=11.5258(9), b=10.1405(6), c=13.9066(12) Å, β=106.827(4)°. The structure of the co-crystal has been revealed by single crystal X-ray diffraction. An infinite
helical polymeric chain is formed by intermolecular hydrogen bonds of the two neutral constituents. The hydroxyl group and
carbonyl oxygen atom in one of the carboxyl groups of phthalic acid form hydrogen bonds to O6 and to N(7)H atoms of theophylline,
respectively, while the other carboxyl OH group of phthalic acid is in hydrogen bond to N9 atom of theophylline by very strong
intermolecular interactions proven by 1883 cm−1 centred peak in FTIR spectrum.
Thermal degradation of this new supramolecular compound is a two-step process in air. At first phthalic acid (47.4%) released
up to 230°C, meanwhile it loses water and transforms into phthalic anhydride. In EGA-MS spectra, the characteristic fragments
of water (m/z=17, 18) appear from about 180°C, while absorption bands of phthalic anhydride are shown in EGA-FTIR spectrum at about 210°C.
In the second step theophylline begins to sublime, melts at 276°C, and then evaporates up to 315°C with minute residues.