Low-density preference of the ambient and high-pressure polymorphs of DL-menthol

Lower-density polymorphs of DL-menthol were nucleated and crystallized in their high-pressure stability regions. Up to 0.30 GPa, the triclinic DL-menthol polymorph α, which is stable at atmospheric pressure, is less dense than a new β polymorph, which becomes stable above 0.40 GPa, but is less dense than the α polymorph at this pressure. The compression of polymorph α to at least 3.37 GPa is monotonic, with no signs of phase transitions. However, recrystallizations of DL-menthol above 0.40 GPa yield the β polymorph, which is less compressible and becomes less dense than α-DL-menthol. At 0.10 MPa, the melting point of the β polymorph is 14°C, much lower compared with those of α-DL-menthol (42-43°C) and L-menthol (36-38°C). The structures of both DL-menthol polymorphs α and β are very similar with respect to the lattice dimensions, the aggregation of OH...O molecules bonded into Ci symmetric chains, the presence of three symmetry-independent molecules (Z' = 3), their sequence ABCC'B'A', the disorder of the hydroxyl protons and the parallel arrangement of the chains. However, the different symmetries relating the chains constitute a high kinetic barrier for the solid-solid transition between polymorphs α and β, hence their crystallizations below or above 0.40 GPa, respectively, are required. In the structure of polymorph α, the directional OH...O bonds are shorter and the voids are larger compared with those in polymorph β, which leads to the reverse density relation of the polymorphs in their stability regions. This low-density preference reduces the Gibbs free-energy difference between the polymorphs: when polymorph α is compressed to above 0.40 GPa, the work component pΔV counteracts the transition to the less dense polymorph β, and on reducing the pressure of polymorph β to below 0.40 GPa, its transition to the less dense polymorph α is also hampered by the work contribution.

Lack of Cooperativity in the Triangular X3 Halogen-Bonded Synthon?

We have investigated 44 crystal structures, found in the Cambridge Structural Database, containing the X₃ synthon (where X = Cl, Br, I) in order to verify whether three type II halogen-halogen contacts forming the synthon exhibit cooperativity. A hypothesis that this triangular halogen-bonded motif is stabilized by cooperative effects is postulated on the basis of structural data. However, theoretical investigations of simplified model systems in which the X₃ motif is present demonstrate that weak synergy occurs only in the case of the I₃ motif. In the present paper we computationally investigate crystal structures in which the X₃ synthon is present, including halomesitylene structures, that are usually described as being additionally stabilized by a synergic interaction. Our computations find no cooperativity for halomesitylene trimers containing the X₃ motif. Only in the case of two other structures containing the I₃ synthon a very weak or weak synergy, i.e. the cooperative effect being stronger than -0.40 kcal mol^-1, is found. The crystal structure of iodoform has the most pronounced cooperativity of all investigated systems, amounting to about 10% of the total interaction energy.

Revisiting van der Waals Radii: From Comprehensive Structural Analysis to Knowledge-Based Classification of Interatomic Contacts

Weak noncovalent interactions are responsible for structure and properties of almost all supramolecular systems, such as nucleic acids, enzymes, and pharmaceutical crystals. However, the analysis of their significance and structural role is not straightforward and commonly requires model studies. Herein, we describe an efficient and universal approach for the analysis of noncovalent interactions and determination of van der Waals radii using the line-of-sight (LoS) concept. The LoS allows to unambiguously identify and classify the "direct" interatomic contacts in complex molecular systems. This approach not only provides an improved theoretical base to molecular "sizes" but also enables the quantitative analysis of specificity, anisotropy, and steric effects of intermolecular interactions.

Melting point, molecular symmetry and aggregation of tetrachlorobenzene isomers: the role of halogen bonding

Tetrachlorobenzenes represent one of the best known, but not yet fully understood, group of isomers of the structure-melting point relationship. The differences in melting temperatures of these structurally related compounds were rationalized in terms of the hierarchy and nature of formed noncovalent interactions, and the molecular aggregation that is influenced by molecular symmetry. The highest melting point is associated with the highly symmetric 1,2,4,5-tetrachlorobenzene isomer. The structures of less symmetrical 1,2,3,4-tetrachlorobenzene and 1,2,3,5-tetrachlorobenzene, determined at 270 and 90 K, show a distinct pattern of halogen bonds, characterized by the different numbers and types of interactions. The evolution of Cl...Cl/H distances with temperature indicates the attractive character of intermolecular interactions and their importance to the structural and thermodynamic parameters of isomeric compounds. The favoured Cl...Cl halogen bonds were found to play a decisive role in differentiating the melting temperatures of tetrachlorobenzene isomers. It was also found that, besides the molecular symmetry and ability to form specific intermolecular interactions, both the type and the distribution of interactions are the important factors responsible for the melting behaviour of the studied isomers. The observed preferences, in tetrachlorobenzenes, for the formation of specific noncovalent interactions correspond to the distribution of calculated partial atomic charges and to the magnitudes of electrostatic potential on the molecular surfaces as well as correlate with the enthalpy of melting parameters.