No abstract

The quest for concepts of isostructurality in organic crystals has been long and mostly based on geometric data, even with the development of modern software. This field of study is of great interest to the pharmaceutical industry and for the prediction of crystal structures. Despite this, there is still no methodology that provides broad quantitative and comparable similarity data between two complete crystalline structures. The present study demonstrated that the similarity between two crystalline structures could be estimated from the similarity between the two “supramolecular clusters”. Quantitative indexes for similarity comparisons of crystal structures were shown using nine 5-aryl-1-(1,1-dimethylethyl)-1 H -pyrazoles as a model. This proposal includes the quantitative data of a geometric parameter ( I D ), a contact area parameter ( I C ), and an energetic parameter ( I G ). The proposed indexes exhibited good perspective regarding the similarity data and distinct regions of similarity. The range of similarity was set at I X ≥ 0.80, 0.80 > I X > 0.60, and I X ≤ 0.60 (X = D, C, or G). Indexes with a value near 1.0 indicate systems with isostructural, isocontact, and isoenergetic behavior. The results indicated that supramolecular structures with high similarity must have high values for all three indexes ( I D , I C , and I G ).

Polymorphs have been widely studied since different crystalline phases of the same compound may have distinct properties. Macromolecules are an area of major study with just a few polymorphs reported. This investigation presents the first case of polymorphism in a [2]rotaxane molecule. This rotaxane contains a succinamide station bearing nonsymmetric stoppers and a tetralactam macrocycle. A different polymorphic phase is achieved by varying the crystallization solvent. These polymorphs presented relevant differences at the molecular level and were classified as conformational polymorphs. At the supramolecular level, polymorph I is −21 kcal mol −1 more stable than polymorph II. Crystallization mechanisms to assess the stages of the crystallization process were proposed. Similar stabilization of the first nuclei formed was observed to be responsible for the possibility of both forms to coexist. The large differences observed between the polymorphs were not significantly reflected in the types and contribution of the intermolecular interactions, in which a high resemblance was observed.

A series of seven N -phenylamides [R–C(O)NHPh, in which R: CH 3 , C(CH 3 ) 3 , Ph, CF 3 , CCl 3 , CBr 3 , and H] were used as models in this study. Molecular packing and intermolecular interactions were evaluated by theoretical calculations, solution NMR, and quantum theory of atoms in molecules analyses. Crystallization mechanisms were proposed based on the energetic and topological parameters using the supramolecular cluster as demarcation. Concentration-dependent 1 H NMR experiments corroborated the proposed interactions between molecules. For all compounds (except for R: H, which initially formed tetramers), layers (two-dimensional) or chains (one-dimensional) were formed in the first stage of the proposed crystallization mechanisms. The presence of strong intermolecular NH···O=C interactions promoted the first stages. The study in solution provided different values of association constant ( K ass ) governed by the hydrogen bond NH···O=C, showing that the stronger interactions are directly influenced by the substituent steric hindrance. A correlation between K ass(NH···O=C) from the solution and the NH···O=C interaction energy in the crystal showed a good trend.

The first Quantum Theory of Atoms in Molecules (QTAIM) analysis of [2]­rotaxanes was used in combination with quantum mechanical calculations and variable temperature NMR experiments. The study shows all the intramolecular interactions of tetraalkylsuccinamide[2]­rotaxanes with different templates. The threads have different stoppers [R1R2NC­(O)-CH2CH2-C­(O)­NR2R1, in which R2/R1 = CH2 cy-Hex/CH2Ph, i-Bu, Bu, and Pr]. The different threads used allowed us verify that the contact area between the submolecular components (C Mcy···Thr) is closely correlated with the interaction energy (G Mcy···Thr) in the [2]­rotaxanes studied. Furthermore, the QTAIM data and quantum mechanical calculations confirmed that, in all of the compounds, the hydrogen bonds are responsible for most of the energy from the intramolecular interactions that follow the C–H···π and H···H interactions, independent of the thread used. In the liquid state, using NMR 1H some intramolecular interactions were observed, which is in agreement with the data obtained in the solid state, thus making possible a comparison between the energy data obtained via the quantum mechanical calculations and the molecular movements of the [2]­rotaxanes in solution. Consequently, a new way of understanding the intramolecular interactions in [2]­rotaxanes and the influence they have on the movement of molecular machines is presented.