This study proposes an approach with the necessary steps to answer some questions: (i) how to build a proper demarcation for the study of salts since there are two ionic entities? (ii) would it be ...possible to build a proposal for a crystallization mechanism for salts, similar to the one for uncharged organic compounds? (iii) considering that destabilizing interactions between ionic salt species are expected, how to consider this in a proposal for a crystallization mechanism? Thus, a series of ammonium salts containing halide anions as models were used in this study. These models allow the modulation of the organic part of the salt, allowing the evaluation of different patterns of electrostatic potential dispersion in the ammonium cation. It was possible to propose a step-by-step approach to determine the stabilization energy and contact area data from the crystal structures and be able to propose crystallization mechanisms for these kinds of structures. Moreover, a new parameter was presented to assess the cluster energy efficiency (CEE), allowing the proposal of an efficiency hierarchy and a comparison with uncharged organic molecules. As such, ammonium salts with large alkyl groups lead to a
salt-without-charge-like supramolecular behavior
; otherwise, the absence of carbon chains in nitrogen and/or the presence of N-H leads to a
salt-with-charge-like supramolecular behavior
. The electrostatic complementarity was explored
via
molecular electrostatic potential maps. Finally, it was possible to build the supramolecular clusters and propose crystallization mechanisms for all 27 compounds, corroborating the proposed demarcation and helping to understand the crystalline structures of the studied ammonium salts.
A series of ammonium salts was used to propose a step-by-step approach to obtain the stabilization energy and contact area data of supramolecular clusters in charged molecules. A new parameter was presented to assess the cluster energy efficiency.
The quest for understanding crystal structures using supramolecular cluster demarcation has been applied to various uncharged compounds, and, more recently, it has also been applied to charged ...compounds in ammonium salts. Given this context, this study sought to expand this approach for intermediate compounds, between compounds with and without localized charges. The selected structures were mesoionic compounds, which have delocalized charges. This study raises some questions: do mesoionic compounds have intermediate characteristics between salts and uncharged molecules? Or are they similar to one of these models? A molecular and supramolecular investigation of mesoionic models was carried out to answer these questions. The study was based on demarcating the supramolecular cluster, enabling us to propose the crystallization mechanisms of twenty-three mesoionic compounds, in which stabilizing and destabilizing energies were observed. In addition, the Cluster Energy Efficiency (CEE) parameter was applied, allowing us to quantitatively evaluate the similarity between compounds containing both stabilizing and destabilizing energies in the crystalline lattice. The CEE data revealed that most mesoionic compounds have CCE = 1000, a characteristic comportment of uncharged compounds. In addition, the compound 2,3-diphenyl-1,3,4-thiadiazolium-5-thiolate (
18
) was synthesized and characterized to better understand the molecular and supramolecular behavior. Concentration-dependent NMR and LC-MS/MS experiments reveal the first aggregates in solution in the crystallization process of compound
18
. The crystallization mechanisms evidenced six different crystallization patterns, and molecular electrostatic potential (MEP) allowed us to evaluate the different patterns of electrostatic potential dispersion of all compounds.
The quest for understanding crystal structures using supramolecular cluster demarcation has been applied to various uncharged compounds, and, more recently, to charged compounds. So, what would be the supramolecular behavior of mesoionic compounds?
The present study discusses the crystalline packing formation of several 2rotaxanes with Leigh‐type tetralactam macrocycle bearing different threads. The presence of solvent molecules in some ...structures are also addressed to shed some light on this matter. Additionally, the degree of similarity between supramolecular structures of rotaxanes was discussed using similarity indices. For this, new descriptors and crystallization mechanisms, which were proposed in terms of contact area and stabilization energy, were carried out to evaluate the rotaxane molecules. It was possible to observe similar general stages of crystallization dominated by the formation of 1D‐blocks and, in fewer cases, by dimers in the first stage of nucleation. The preference for the formation of 1D nuclei resides in the large contact area and complementarity involved in the large set of interactions between the rotaxanes at the earliest stages of crystallization. In this context, it was possible to propose when solvent molecules are trapped between the rotaxanes during crystal formation. Therefore, a unique example of a rotaxane whose topology favored the entrapment of water molecules between rotaxanes during the first stage of the crystallization process is presented. Crystallization mechanisms showed to be a valuable asset in the supramolecular investigation of rotaxanes in the crystalline state.
The current investigation provides insights on how the crystallization process, evaluated in terms of contact area and stabilization energy, can occur in different rotaxanes molecules bearing Leigh‐type tetralactam macrocycle.
A systematic investigation to assess the degree of similarity between polymorphs was carried out. A similarity indices (IX) approach was applied in ten series of polymorphs with different ...characteristics and number of molecules in the asymmetric unit. Geometric (ID), contact area (IC), and stabilization energy (IG) parameters were used. It was possible to situate each comparison in different regions of similarity within the polymorphism phenomenon and determine the boundaries between quasi-isostructural polymorphs and polymorphs of low similarity. The multiparameter IDCG index was used as a robust tool to determine the total similarity within the polymorphism phenomenon. The highest contribution of the stabilization energy parameter (45%) toward the final value of similarity (IDCG) was observed, followed by the contact area index (32%). The geometric index contributed approximately 23% to the final value of IDCG. This information reinforces the importance of the contact area and stabilization energy in assessing the degree of similarity between crystalline structures. A new descriptor (IQ) based on the comparison of the energetic contribution of intermolecular interaction types present in each crystal structure is presented. IQ can be a versatile tool and applicable even for systems that do not share any similarity.
During synthesis and purification, compounds are exposed to various solvents. These solvents can, at times, assemble in the solid state. This association can arise from intermolecular interactions, ...allowing solvent molecules to bind with chemical substances and resulting in a unique crystalline structure known as a solvate. Nevertheless, such an association with solvents can substantially alter the properties of the compounds. In the crystalline solid state, the occurrence of solvates is recurrent among mechanically interlocked molecules (MIMs), including rotaxanes; however, solvates are frequently considered incidental or inconsequential. In this context, we sought to investigate the role and relevance of solvent molecules in the crystallization process of rotaxanes by systematically analyzing 24 models, employing the supramolecular cluster as a demarcation tool and using tetralactam-macrocycle based 2rotaxanes. In general, the most frequently occurring 2rotaxane solvates are with chloroform and water molecules (∼66%). The occurrence of solvates does not follow any rule or pattern, implying that similar structures can and do contain different solvates in varying proportions. By the crystallization mechanisms proposed, it was feasible to determine at which stage of the crystallization process the solvates are formed, and the energetic and topological contributions of the solvent molecules during crystallization were assessed. The most rotaxanes “trap” solvent molecules during crystallization, and interestingly, in certain instances, these solvent molecules are essential for crystallization, contributing up to 40% of the total stabilization energy of the supramolecular cluster.
Carboxamide groups are interesting models of study in crystal engineering as they can be hydrogen-bond acceptors and donors. In this sense, organic molecules containing carboxamide groups could act ...as acceptors for water molecules, enabling hydrate formation. To study amide-based hydrate formation, a series of crystal structures of pyridine-2,6-dicarboxamides (carboxamide groups are −C(O)-NH-R, where R = (1) CH2CHPh2, (3) CH2CH2Ph, (4) CH2Ph, (5) Ph-4-F, (6) Ph-4-Cl, (7) Ph-4-Br, and (8) naphth-1-yl) were chosen. A similar compound containing benzene 1,3-dicarboxamide substituted (2) was also included. Compounds 1 and 3–8 presented a structure with folded molecular conformation, where the amide chains showed the two NH groups positioned between both chains. Compound 2 had a linear molecular structure, where the amide chains were distant from each other. Compounds 1 and 3 presented hydrate phases (1:1), with 1 showing three polymorphic phases. Compounds 2 and 4–8 had anhydrous crystalline phases. Crystallization mechanisms of compounds 1–8 were proposed using the supramolecular cluster as demarcation and corroborated with the concentration-dependent 1H NMR experiments. Our findings enabled us to propose the stages in which each polymorph was formed, and the proposed crystallization mechanisms presented as first-stage stacking molecules for 1I-1II and 4–8 (one-dimensional nuclei), two-dimensional growth for 2, and dimer formation for 1III and 3. QTAIM analysis was used for a more detailed evaluation of intermolecular interaction contributions. The steric hindrance caused by the aryl groups is the possible reason for the greatest difficulty in forming hydrates for compounds 4–8. The water molecules in 1I-1III and 3 occupy the cavity formed by the organic molecules since the early crystallization stage. In this sense, it was possible to contribute insights regarding the influences between molecular conformation and the formation of hydrate structures.
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•Subtle modification on station of rotaxane considerably interferes with the rotational energy barrier in solution.•The rotational movement of fumaramide-based 2rotaxans is over 14% ...more energetic than succinamide-based 2rotaxanes.•The intercomponent stabilization energy at the crystalline state does not follow the same trend as the solution state.•More flexible station allows more energetic intercomponent interactions in solid state.
Due to their intrinsic topological features, most artificial molecular devices are based on rotaxane molecules. One of the most important characteristics of this class of compounds is the likelihood of performing molecular movements, which are directly correlated with the intercomponent interactions present in these molecules. The tetralactam macrocycle is quite versatile and used in numerous molecular threads, which have stations that work as fundamental parts in the formation, molecular stabilization, and molecular movements performed. Inspired by how these stations may influence molecular movements (e.g., rotational ones), we sought to study two diamides commonly used as rotaxane stations: fumaramide and succinamide. In this context, we explored the change of station in six pairs of 2rotaxanes. Studies were carried out in solution through NMR experiments with temperature variation techniques. In the crystalline solid state, the quantum theory of atoms in molecules analysis was performed through quantum mechanics calculations using the density functional theory. The rotational movement found for fumaramide-based 2rotaxanes is, on average, over 14.91 kcal mol−1. In comparison, succinamide-based 2rotaxanes have values of 12.78 kcal mol−1 (a 14% difference). In solution, fumadamide-based rotaxanes better retain the macrocycle due to the rigidity of the double bond, making its rotation more difficult and consequently leading to higher rotational energy barriers. In the crystalline solid state, the succinamide station, which has greater structural freedom due to the simple carbon–carbon bond, promotes more interactions, having a greater “complementary adjustment” and a higher stabilization energy. Lastly, our findings demonstrated how intercomponent interactions present in the studied stations can be changed to be used as a “lubricant” in solution for these molecular machine prototypes.
•New pentacoordinated semicarbazone copper complex was reported.•Factors that lead to the formation of a pentacoordinated complex instead of the expected hexacoordinated complex were ...analyzed.•Absence of the Cu-O bond in the pentacoordinated complex leads to redistribution of the energies in the remaining Cu-ligand bonds.•The metal-ligand energies change as a function of the length of each Cu-ligand bond.•Supramolecular similarity index ID was calculated and an isostructural behavior was observed.
The structural characterization of a new pentacoordinated semicarbazone copper complex, Cu(N,N',O-HSCPy)(N,N'-HSCPy)ClO42 (1) is reported. The structure of 1 was compared to the hexacoordinated complex, Cu(N,N',O-HSCPy)2ClO42⋅H2O (2), which was already published, to shed some light on the formation of both forms. Energetic data of Cu-ligand bonds showed that the absence of a Cu-O bond in cation 1 does not significantly change the total stabilization energy in the pentacoordinate cation relative to the hexacoordinate cation (less than 4%). This is because the Cu-ligand bonds have their energies redistributed into the five existing bonds of 1. Additionally, we extended the study revisiting analogous HSCPy complexes with other coordinated metals (M), with general formula M(N,N',O-HSCPy)2X2⋅solvent, where M = Ni (3), Co (4), Zn (5), Fe (6), and Mn (7); X = ClO4, and solvent = H2O (3-6) or EtOH (7); and complex Cu(N,N',O-HSCPy)2ClO4I3 (8) was included in this series. In general, all additional complexes showed similar behavior to that of the Cu-hexacoordinated complex (2). In this sense, was possible to observe that the energies of the bonds M-O and M-N have strong linear correlation in the three groups: M-O3′//M-O3, M-N1′//M-N1, and M-N2′//M-N2, for complexes 2-7 (r = 0.9766; N = 18). Also, a strong correlation between stabilization energy data of metal-ligand and their respective bond lengths (r =0.9393; N = 47) was observed in all studied complexes (1-8). The supramolecular geometric similarity index ID was calculated comparing 2 with 1, 3-8. It was observed a high geometric similarity between 2 and 3-6 (ID < 0.95), indicating an isostructural behavior. The comparison between 2 and complexes 1,8 indicated values in the intermediate region (ID = 0.75) and close to a border region with an intermediate-to-high region of similarity (ID = 0.83), respectively. The similarity comparison between 2 and 7 had a value of 0.77, in the intermediate region of similarity, showing that the change of a water molecule for an ethanol molecule considerably affected the crystal packing.
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Machine learning (ML) techniques are enjoying rapidly increasing adoption. However, designing and implementing the systems that support ML models in real-world deployments remains a significant ...obstacle, in large part due to the radically different development and deployment profile of modern ML methods, and the range of practical concerns that come with broader adoption. We propose to foster a new systems machine learning research community at the intersection of the traditional systems and ML communities, focused on topics such as hardware systems for ML, software systems for ML, and ML optimized for metrics beyond predictive accuracy. To do this, we describe a new conference, MLSys, that explicitly targets research at the intersection of systems and machine learning with a program committee split evenly between experts in systems and ML, and an explicit focus on topics at the intersection of the two.
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![[2]Rotaxane Solvates: Insig...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/1528-7483/small "[2]Rotaxane Solvates: Insights into the Presence of Solvents in the Crystal Lattice")
