The (13)C CPMAS spectrum is presented for the polymorph of oxybuprocaine hydrochloride which is stable at room temperature, i.e. Mod. II degrees . It shows crystallographic splittings arising from the fact that there are two molecules, with substantially different conformations, in the asymmetric unit. An INADEQUATE two-dimensional experiment was used to link signals for the same independent molecule. The chemical shifts are discussed in relation to the crystal structure. Of the four ethyl groups attached to NH(+) nitrogens, one gives rise to unusually low chemical shifts, very different from those of the other three ethyl groups. This is attributed empirically to gamma-gauche conformational effects, as is confirmed by shielding computations. These considerations allow (13)C signals to be assigned to specific carbons in the two crystallographically inequivalent molecules in the crystal structure. Indeed, information about the conformations is inherent in the NMR spectrum, which thus provides data of crystallographic significance. A (13)C/(1)H HETCOR experiment enabled resolution to be obtained in the (1)H dimension and allowed (1)H and (13)C signals for the same independent molecule to be linked.

The most stable is not necessarily the most easy to obtain! Two new stable crystal forms of the 120 year old compound sym‐trinitrobenzene are reported. These elusive forms have been obtained by the use of an additive, trisindane, that can act as a mimic in the crystallization experiment (see scheme).

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Too difficult to predict? The second polymorph of the title compound, is presented. A comparison of hydrogen‐bond patterns and lattice energies suggests that the previously obtained form (see picture, left) is a kinetic polymorph, while the second polymorph (right) is most likely to represent the more thermodynamically stable form. The new polymorph, however, has two symmetry‐independent molecules, which rendered it beyond the limit of previous structural predictions.

The hexagonal layer structure of host atoms in chloro, bromo, and iodo derivatives of 2,4,6-tris(4-halophenoxy)-1,3,5-triazine, X-POT, is stabilized by a cyclic and cooperative halogen trimer (X···X) synthon. The X···X distance is ∼3.5 Å in isostructural channel inclusion adducts of Cl-POT and Br-POT, whereas I···I is ∼3.8 Å in cage structures of I-POT with aromatic and hydrophobic guests. X-ray crystal structures of I-POT with mesitylene, collidine, tribromomesitylene, triiodomesitylene, hexachlorobenzene, hexafluorobenzene, 1-methylnaphthalene, CH2Cl2, CH2Br2, and CH2I2 guests in R3̄ space group are reported. Host molecules are fully ordered in these isostructural clathrates, whereas guest atoms are disordered except for C6Cl6 and C6F6. The guest molecule resides in a penta-decker sandwich surrounded by double layers of iodo trimer and triazine ring. Supramolecular isomerism from channel to cage framework and the persistent crystallization of trigonal X-POT molecules in high-symmetry host networks (space groups P63/m, P63, R3̄) are discussed in crystal engineering terms:  halogen trimer synthon, C 3 i -Piedfort unit, weak C−H···O/N interactions, changes in size/polarizability of halogen atom, and CSDSymmetry statistics. Br-POT crystallizes in a channel or cage lattice depending on the guest species. Guest release from the cage framework occurs at a higher temperature compared to the channel structure in thermal gravimetric analysis, suggesting applications in organic zeolites. This study illustrates several aspects of crystal engineering from the understanding of intermolecular interactions to the design of crystal structures and their utility as functional solids.

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X-ray structure determinations of two different single crystals of octamethylferrocenium tetrafluoroborate (OMFc(+)BF(4)(-)) revealed conformational polymorphism with ligand twist angles of 180 degrees and 108 degrees , respectively. Their concomitant occurrence could be explained by the small lattice energy difference of 3.2 kJ mol(-1). Temperature-dependent Moessbauer spectroscopy of (57)Fe-labeled OMFc(+)BF(4)(-) over the range 90 < T < 370 K did not show the anomalous sudden increase in the motion of the metal atom as observed in neutral OMFc. Broadened absorption curves characteristic of relaxation spectra were obtained with an isomer shift of 0.466(6) mm s(-1) at 90 K. The temperature dependence of the isomer shift corresponded to an effective vibrating mass of 79 +/- 10 Da and, in conjunction with the temperature dependence of the recoil-free fraction, to a Moessbauer lattice temperature of 89 K. The spin relaxation rate could be better described by an Orbach rather than a Raman process. At 400 K, a reversible solid-solid transition to a plastic crystalline mesophase was noted.