Temperature-dependent solid-state phase transition with twinning in the crystal structure of 4-methoxyanilinium chloride

Abstract

At room temperature, the title salt, C7H10NO+·Cl−, is orthorhombic, space group Pbca with Z′ = 1, as previously reported [Zhao (2009). Acta Cryst. E65, o2378]. Between 250 and 200 K, there is a solid-state phase transition to a twinned monoclinic P21/c structure with Z′ = 2. We report the high temperature structure at 250 K and the low-temperature structure at 100 K. In the low-temperature structure, the –NH3 hydrogen atoms are ordered and this group has a different orientation in each independent molecule, in keeping with optimizing N—H...Cl hydrogen bonding, some of which are bifurcated: these hydrogen bonds have N...Cl distances in the range 3.1201 (8)–3.4047 (8) Å. In the single cation of the high-temperature structure, the NH hydrogen atoms are disordered into the average of the two low-temperature positions and the N...Cl hydrogen bond distances are in the range 3.1570 (15)–3.3323 (18) Å. At both temperatures, the methoxy group is nearly coplanar with the rest of the molecule, with the C—C—O—C torsion angles being −7.0 (2)° at 250 K and −6.94 (12) and −9.35 (12)° at 100 K. In the extended orthorhombic structure, (001) hydrogen-bonded sheets occur; in the monoclinic structure, the sheets propagate in the (010) plane.

Keywords

• crystal structure
• 4-alkoxyacetanilides
• 4-alkoxyanilinium salts
• nonsteroidal analgesics
• mechanisms of toxicity