Single-crystal X-ray and neutron powder diffraction investigation of the phase transition in tetrachlorobenzene.

The polymorphic phase transition of 1,2,4,5-tetrachlorobenzene (TCB) has been investigated using neutron powder diffraction and single-crystal X-ray diffraction. The diffraction experiments show a reversible phase change that occurs as a function of temperature with no apparent loss of sample quality on transition between the two phases. Neutron powder diffraction gives detailed information on the molecular structural changes and lattice parameters from 2 K to room temperature. The structure of the low-temperature form has been elucidated for the first time using single-crystal X-ray diffraction. Comparison of the alpha and beta structures show that they are both based on the same sheet motif, with the differences between the two being very subtle, except in terms of crystal symmetry. Detailed analysis of the structures revealed the changes required for inter-conversion. A computational polymorph search showed that these two sheet structures are more thermodynamically stable than alternative herringbone-type structures.

Correspondence between molecular functionality and crystal structures. Supramolecular chemistry of a family of homologated aminophenols.

The crystal structures and packing features of a family of 13 aminophenols, or supraminols, are analyzed and correlated. These compounds are divided into three groups: (a) compounds 1-5 with methylene spacers of the general type HO-C6H4-(CH2)n-C6H4-NH2 (n = 1 to 5) and both OH and NH2 in a para position; (b) compounds 1a, 2a, 2b, 2c, and 3a in which one or more of the methylene linkers in 1 to 3 are exchanged with an S-atom; and (c) compounds 2d, 1b, and 6a prepared with considerations of crystal engineering and where the crystal structures may be anticipated on the basis of structures 1-5,1a, 2a, 2b, 2c, and 3a. These 13 aminols can be described in terms of three major supramolecular synthons based on hydrogen bonding between OH and NH2 groups: the tetrameric loop or square motif, the infinite N(H)O chain, and the beta-As sheet. These three synthons are not completely independent of each other but interrelate, with the structures tending toward the more stable beta-As sheet in some cases. Compounds 1-5 show an alternation in melting points, and compounds with n = even exhibit systematically higher melting points compared to those with n = odd. The alternating melting points are reflected in, and explained by, the alternation in the crystal structures. The n = odd structures tend toward the beta-As sheet as n increases and can be considered as a variable series whereas for n = even, the beta-As sheet is invariably formed constituting a fixed series. Substitution of a methylene group by an isosteric S-atom may causes a change in the crystal structure. These observations are rationalized in terms of geometrical and chemical effects of the functional groups. This study shows that even for compounds with complex crystal structures the packing may be reasonably anticipated provided a sufficient number of examples are available.