## Abstract

Remarkable aspects of the crystallization of 3,4-dihydroxybenzoic acid (protocatechuic acid, PCA) from water as described by R. W. Wood (Proc. R. Soc. Lond. A 197, 283-294 (1949)) are confirmed. This compound crystallizes as the monohydrate in four different polymorphic phases: three are triclinic (oblique needles, needles and rhombs) and belong to one subgroup, while the fourth, monoclinic, phase constitutes a separate subgroup. It is probable that the triclinic rhombs are the stable phase at 25 $^\circ$C, with the other phases monotropically related to it. Crystal data for the triclinic needles are a = 9.926(9), b = 9.532(9), c = 8.131(8) $\overset{\circ}{\mathrm A}$, $\alpha$ = 100.8(1), $\beta$ = 90.7(1), $\gamma$ = 102.4(1)$^\circ$, Z = 4, space group P1; and for the triclinic rhombs a = 12.693(9), b = 8.011(6), c = 8.121(6)$\overset{\circ}{\mathrm A}$, $\alpha$ = 72.2(1), $\beta$ = 108.5(1), $\gamma$ = 103.2(1)$^\circ$, Z = 4, space group P1. Both crystals can be described in terms of a unit cell containing eight formula units, with dimensions a $\approx$ 12.7, b $\approx$ 9.5, c $\approx$ 13.0 $\overset{\circ}{\mathrm A}, \alpha \approx 88$, $\beta \approx$ 101, $\gamma \approx$ 107$^\circ$; the space group for the triclinic needles is B$\overline{1}$ and for the triclinic rhombs A$\overline{1}$. Crystal structure analyses (four-circle diffractometer, MoK$\alpha$) of these two phases (triclinic needles, 1287 reflexions used in the final refinement cycle, R$_F$ = 11.1%; triclinic rhombs, 1761 reflexions, R$_F$ = 6.9%) show that both contain essentially similar layer pairs of PCA and H$_2$O molecules, with hydrogen bonding both within each layer and, apparently, between them; however, the stacking of the layer pairs in the two phases differs. The crystals of the oblique needles are so small and unstable to stress that crystal structure analysis was not possible. The crystal structure of the monoclinic needles (a = 12.32(1), b = 3.64(1), c = 17.60(2) $\overset{\circ}{\mathrm A}, \beta$ = 107.7(2)$^\circ$, Z = 4, space group P2$_1$/n) was determined (CuK$\alpha$, 787 reflexions, R$_F$ = 6.8%); the overall molecular arrangement differs from that in the triclinic phases in an absence of PCA$\cdot$H$_2$O layers although there are distinct resemblances between the hydrogen bonding schemes. The phase transition 'oblique needles $\rightarrow$ triclinic needles' is 'single crystal to single crystal' and is a topotaxic transition. The phase transition 'triclinic needles $\rightarrow$ triclinic rhombs' is 'single crystal to oriented polycrystal' and is described as partially topotaxic, there being preservation of layer arrangement but not of complete three-dimensional orientation.

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