The crystal structure of phthalic acid

The space group is ei ther C2/c or Cc, and the absence of pyroand piezo-electric effects is consistent wi th the choice of C2/c. With 4 molecules in the uni t cell, the molecule mus t have an axis of symmet ry which coincides wi th a twofold axis. This is sufficient to exclude the possibility of any intramolecular hydrogen bonding. A Pa t te rson project ion on (010) gives the ti l t of the benzene ring to the (100) plane as approximate ly 45% By assuming a regular benzene ring of C-C distance 1.4 /~, approximate z coordinates for the a toms could be determined. Wi th the further aid of t t a rke r -Kaspe r inequalit ies an electron-densi ty project ion on (100) was obtained. The final project ion is shown in Fig. 1 (R ----