Phases II and IV of 1,3,5-trichloro-2,4,6-trimethylbenzene: Ab initio crystal structure determination by high‐resolution powder diffraction

Competing stacking modes in crystals of trihalogeno-trimethyl-benzene: Theory meets experiment

“…The planar bc layers are stacked in an antiparallel manner along the axis a. This triclinic arrangement below room temperature was confirmed later by XRD [17] and ND experiments [18]. The Figures 1 and 2 give two projections of the common structure.…”

“…The Figures 1 and 2 give two projections of the common structure. Figure 1 is a projection on the ab plane and Figure 2 the projection on the bc plane in the case of the TIM at 293 K. In fact, the case of TCM is a little more complicated: from X-ray [12,18] and calorimetric measurements [19,20] it has been established that below 300 K, TCM exhibits two different triclinic crystal phases differing only by translations of the molecules in the bc plane while there is a "sluggish' transition around 160 K. To summarize, at temperatures below 160 K, TCM, TBM and TIM, are rigorously isostructural, irrespective of the nature of the methyl group (CH 3 or CD 3 ) as shown in Table 1. However, no experiments have been done above room temperature and in consequence only partial information was available to draw general conclusions regarding the structures of TXM species in the solid state.…”

Rotationally disordered phase of 1,3-dibromo-5-iodo-2,4,6-trimethylbenzene at 293 K

“…For molecules in different layers, the shortest intermolecular contact distances, compared with those between molecules within the same layer, are significantly shortened for C Me Á Á ÁC Me contacts, almost unchanged for C Me Á Á Áhalogen contacts and significantly increased for halogen-halogen contacts. The same structural topology is encountered at room temperature in TBM (Meinnel et al, 2000) and TIM (Boudjada et al, 2001), and also in TCM below 160 K (lowest temperature phase, or phase IV; Hernandez et al, 2006).…”

“…This novel feature has been characterized by diffractometry thanks to the idealized (instantaneous) C 2v molecular symmetry of the isolated molecule (ignoring H atoms), which is time-averaged towards an apparent idealized D 3h symmetry by means of the diffraction probe. The same kind of rotational disorder, although suspected at room temperature in TCM (Tazi et al, 1995;Hernandez et al, 2006), TBM and TIM, has not been directly proved so far, due to the ternary symmetry of these latter molecules.…”

“…Another crystal packing is observed, too, at room temperature, especially if the molecules display a threefold axis of symmetry: they are stacked into a triclinic unit cell, space group P1 (Z = 2), derived from the hexamethylbenzene structure type (Hamilton et al, 1969;Le Maguè res et al, 2001). This is the case for the trihalogenomesitylene series (1,3,5-trihalogeno-2,4,6-trimethylbenzene; Table 1), of which only trichloromesitylene (TCM) (Tazi et al, 1995;Hernandez et al, 2006), tribromomesitylene (TBM) (Meinnel et al, 2000;Bosch & Barnes, 2002) and triiodomesitylene (TIM) (Boudjada et al, 2001(Boudjada et al, , 2002Bosch & Barnes, 2002) have been characterized so far. The methyl groups in these compounds experience relatively small hindering potentials and interesting tunnelling properties have been reported (Meinnel et al, 1992(Meinnel et al, , 1995(Meinnel et al, , 2000Boudjada et al, 2002).…”