(3S,4R)-4-(4-Fluoro-phen-yl)-3-(hydroxy-meth-yl)piperidinium chloride.

The title compound, C(12)H(17)FNO(+)·Cl(-), is a degradation impurity of paroxetine hydro-chloride hemihydrate (PAXIL), an anti-depressant belonging to the group of drugs called selective serotonin reuptake inhibitors (SSRIs). Similar to the paroxetine hydro-chloride salt with protonation having taken place on the basic piperidine ring, the degradation impurity also exists as the hydro-chloride salt. The cyclic six-membered piperidinium ring adopts a chair conformation with the hydroxy-methyl and 4-fluoro-phenyl groups in the equatorial positions. The ions form a tape along the b axis through charge-assisted N(+)-H⋯Cl(-) hydrogen bonds; these tapes are connected by O-H⋯Cl(-) hydrogen bonds along the a axis.

Supramolecular synthon polymorphism in 2 : 1 co-crystal of 4-hydroxybenzoic acid and 2,3,5,6-tetramethylpyrazine.

Co-crystals of 4-hydroxybenzoic acid and 2,3,5,6-tetramethylpyrazine (2 : 1) exhibit the first supramolecular synthon polymorphism in a co-crystal; metastable anti-hierarchic polymorph I converts to stable hierarchic form II.