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

The room temperature (298 K) structure of the title salt has been redetermined with disordered H atoms for the –NH3 group. Additionally, a twinned monoclinic structure has been identified with lower symmetry at low temperature (100 K) in which the H atoms of the –NH3 groups are ordered to optimize N—H⋯Cl hydrogen bonding.


Structural commentary
At room temperature (298 K), I crystallizes in the orthorhombic space group Pbca with one formula unit in the asymmetric unit.The cell dimensions are a = 8.8778 ( 5), b = 8.4660 (5), c = 21.7236(11) A ˚and V = 1632.73(16) A ˚3. Cooling the sample causes a solid-state phase transition to a twinned structure with lower symmetry and two formula units in the asymmetric unit.At 250 K, the structure is still orthorhombic, but at 200 K, the space group is monoclinic P2 1 /c with a = 8.3772 ( 11 The crystal structure at 250 K, shown in Fig. 1, closely aligns with the published structure (Zhao, 2009) except that we find disorder in the NH hydrogen atoms, while Zhao treated them as ordered.Using Zhao's intensity data, we do see evidence of a second orientation of the NH 3 group, and we also see it from our crystal when warmed to 298 K.The methoxy group in I is nearly coplanar with the rest of the molecule, with the torsion angle C7-O1-C4-C3 = À 7.0 (2) � .
The asymmetric unit of the 100 K structure is shown in Fig. 2. The major difference between the two independent molecules is the conformation of the -NH 3 group, in which one molecule has one set of the disordered positions in the 250 K structure and the second has the other.As in the 250 K structure, the methoxy groups are twisted only slightly out of the planes of the aromatic rings, with C7-O1-C4-C3 and C14-O2-C11-C10 torsion angles of À 6.94 (12) and À 9.35 (12) � , respectively.

Supramolecular features
In both structures, the intermolecular interactions are predominantly N-H� � �Cl hydrogen bonds, as listed in Tables 1 and  2 and illustrated in Figs. 3 and 4. The N� � �Cl separations are in the range 3.1201 (8)-3.4047(8) A ˚in the monoclinic 100 K

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Figure 1 The asymmetric unit of the 250 K structure of I, with 50% displacement ellipsoids and hydrogen bonds indicated by blue dashed lines.

Figure 2
The asymmetric unit of the 100 K structure of I, with 50% displacement ellipsoids and hydrogen bonds indicated by blue dashed lines.

Table 2
Hydrogen-bond geometry (A ˚, � ) for I at 100 K. structure and 3.1570 (15)-3.3323(18) A ˚in the orthorhombic 250 K structure.In Fig. 4, it can be seen that for each NH 3 group, two of the H atoms are involved in direct hydrogen bonds and the third in a bifurcated N-H� � �(Cl,Cl) bond with two acceptors.The graph set (Etter et al., 1990) patterns are centrosymmetric R 2 4 (8) rings and R 2 2 (4) rings.At 250 K, (001) sheets arise in the extended structure and at 100 K similar sheets propagate in the (010) plane, due to the change in unitcell settings.

Database survey
A review of the literature revealed that the room temperature (298 K) structure of 4-methoxyanilinium chloride was previously reported (Zhao, 2009; Cambridge Structural Database refcode CUCTUQ).Similarly, the structure of 4-ethoxyanilinium chloride at 100 K has been documented (Fu, 2010;Hines et al., 2023).However, these studies did not provide information on phase transitions and twinning.

Synthesis and crystallization
A saturated solution of the title compound, C 7 H 10 ClNO (CAS 20265-97-8 from AmBeed, Arlington Heights, IL, USA) in boiling water was allowed to pass through a short column of activated charcoal.The resulting colorless solution (eluent) was left to cool to room temperature and evaporate slowly in the dark.Pink laths of I, prepared through this process, were suitable for X-ray diffraction studies.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.For the structure at 250 K, all the H atoms were located in difference maps and those on carbon were relocated to geometrically idealized positions with C-H = 0.94 A ˚and U iso (H) = 1.2U eq (C) for the aromatic C atoms and C-H = 0.97 A ˚and U iso (H) = 1.5U eq (C) for the methyl group.The N-bound H atoms were idealized as six halfpopulated sites at 60 � torsional intervals with N-H = 0.90 A ånd U iso (H) = 1.5U eq (N) and the torsion angle was refined.For the structure at 100 K, the H atoms were handled similarly, except that C-H distances were fixed at 0.95 A ˚for aromatic C atoms and 0.98 A ˚for the methyl group, and the H atoms on N were ordered with their positions individually refined.The twin law for the monoclinic structure is (1 0 0, 0 À 1 0, 0 0 À 1) and the BASF parameter refined to 0.4484 (6).(Bruker, 2016), SHELXT2018/2 (Sheldrick, 2015), SHELXL2018/1 (Sheldrick, 2015), Mercury (Macrae et al., 2020), and publCIF (Westrip, 2010).

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Figure 3
Figure 3Hydrogen bonding in the 250 K structure of I.

Figure 4
Figure 4Hydrogen bonding in the 100 K structure of I.

Table 1
Hydrogen-bond geometry (A ˚, � ) for I at 250 K.

Table 3
Experimental details.
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