Discriminating and understanding molecular crystal polymorphism

Polymorph discrimination for a molecular crystal has long been a challenging task, which, nonetheless, is a major concern in the pharmaceutical industry. In this work, we have investigated polymorph discrimination on three different molecular crystals, tetrolic acid, oxalic acid, and oxalyl dihydrazide, covering both packing polymorphism and conformational polymorphism. To gain more understanding, we have performed energy decomposition analysis based on many-body expansion, and have compared the results from the XO-PBC method, that is, the eXtended ONIOM method (XO) with the periodic boundary condition (PBC), with those from some commonly used dispersion corrected density functional theory (DFT-D) methods. It is shown here that, with the XYG3 doubly hybrid functional chosen as the target high level to capture the intra- and short-range intermolecular interactions, and the periodic PBE as the basic low level to take long range interactions into account, the XO-PBC(XYG3:PBE) method not only obtains the correct experimental stability orderings, but also predicts reasonable polymorph energy ranges for all three cases. Our results have demonstrated the usefulness of the present theoretical methods, in particular XO-PBC, while highlighted the importance of a better treatment of different kinds of interactions to be beneficial to polymorph control.

Controlled Self-assembly of Gold(I) Complexes by Multiple Kinetic Aggregation States with Nonlinear Optical and Waveguide Properties

Introduction of multiple kinetic aggregation states (Aggs) into the self-assembly pathway could bring complexity and flexibility to the self-assemblies, which is difficult to realize due to the delicate equilibria established among different Aggs bonded by weak noncovalent interactions. Here, we describe a series of chiral and achiral d¹⁰ Au^I bis(N-heterocyclic carbene, NHC) complexes, and the achiral complex could undergo self-assembly with multiple kinetic Aggs. Generation of multiple kinetic Aggs was realized by applying chiral or achiral seeds exhibiting large differences in elongation temperatures for their respective cooperative self-assembly processes. We further showed that the chiral Au^I self-assemblies having non-centrosymmetric packing forms exhibit nonlinear optical response of second harmonic generation (SHG), while the SHG signal is absent in the achiral analogue. The crystalline achiral Au^I self-assemblies could function as optical waveguides with strong emission polarization.

Theoretical Study of Macrocyclic Host Molecules: From Supramolecular Recognition to Self-Assembly

Supramolecular chemistry focuses on molecular recognition and self-assembly of various building blocks through weak non-covalent interactions, including anion-π, hydrogen bond (HB), hydrophobic interactions, van der Waals (vdW) interactions, etc, which...

The design of anion-π interactions and hydrogen bonds for the recognition of chloride, bromide and nitrate anions

The role of anions in several biochemical processes has given rise to enormous interest in the identification/exploration of compounds with the potential ability to recognize anions. Here, an anthracene-squaramide conjugated compound, O2C4[NH(C14H10)][(NH(C6H6)], has been modified through the substitutions (i) H → F and (ii) H → OH at the anthracene and benzene rings to improve the capabilities of these structures for recognizing chloride, bromide, and nitrate anions. Through an energy decomposition analysis method, the recognition of the anions is chiefly identified as a non-covalent process. H → F substitutions at the benzene ring and, principally, the anthracene ring favor anion recognition, since H → F substitutions create a π-acid region in the aromatic ring, as indicated based on the molecular electrostatic potential surfaces. Similarly, H → OH substitutions also improve the recognition of anions, which is related to the establishment of partly covalent chemical bonds of the form O-H(Cl-, Br- and O-), which are verified based on the quantitative analysis of the maximum and minimum values of the molecular electrostatic potential surfaces and the quantum theory of atoms in molecules method. The presence of large electron density has a key role in the recognition of Cl- anions, and the more favorable electrostatic interactions between the anthracene structure and Br- anions, relative to NO3- anions, mean that receptorBr- interactions are more attractive than receptorNO3- ones. These data can contribute to the design of structures with the relevant abilities to interact with anions.

Exploring Anion-π Interactions and Their Applications in Supramolecular Chemistry

Noncovalent bond interactions provide primary driving forces for supramolecular processes ranging from molecular recognition to self-assembly of sophisticated abiotic and biological machineries. While hydrogen bonding and π-π interactions are arguably textbook concepts playing indispensable parts in various scientific disciplines, noncovalent anion-π interactions have been emerging as attractive forces between π systems and negatively charged species for just about two decades. At the beginning of this century, three research groups reported independently their computational studies on the interactions between anions and aromatic compounds, proposing attractive anion-π interactions. Since π systems such as aromatic rings are traditionally noted as electron rich entities, anions and π systems would be repulsive to each other if there are any interactions. In stark contrast to the acknowledged cation-π interactions, the seemingly counterintuitive noncovalent anion-π bindings invoked great interest in the following years. Although a plethora of calculations had been published, the lack of experimental evidence cast doubt on the existence of anion-π interactions between anions and charge-neutral aromatic systems.During the same time when anion-π interactions were coined, we were studying the chemistry of novel macrocyclic compounds, namely, heteracalixaromatics, and their applications in supramolecular chemistry. It has been shown that heteracalixaromatics are powerful and versatile macrocyclic hosts to bind various guest species forming interesting assembled structures and organometallic complexes. Being a member of heteracalixaromatics, tetraoxacalix[2]arene[2]triaizne adopts a 1,3-alternate conformational structure yielding a V-shaped cavity or cleft formed by two electron-deficient triazine rings. Advantageously, the macrocycle is able to self-tune the cavity sizes by altering the degrees of conjugation between the bridging oxygen atoms with their bonded aromatic rings in response to the guest species in present, rendering it an ideal tool to explore anion-π interactions. We initiated our study on anion-π interactions using tetraoxacalix[2]arene[2]triazine as a molecular tool with the primary aim to clarify experimentally the uncertainty of whether exclusive anion-π interactions exist between anions and charge-neutral aromatic rings. We provided for the first time concrete evidence substantiating the formation of typical anion-π interaction between the anions and 1,3,5-triazine ring and demonstrated subsequently the generality and binding motifs of anion-π interactions. We have then extended our study to anion-π interaction-directed or -driven anion recognition and selective sensing, transmembrane anion transport, molecular self-assembly, and stimuli-responsive aggregation systems. A number of new generation macrocycles and cages constructed from electron-deficient tetrazine and benzenetriimide segments have also been developed in the meantime, advancing the study of anion-π interactions. This Account summarizes our endeavors to explore nascent anion-π interactions and their applications in supramolecular chemistry. We hope this Account will inspire scientists from various disciplines to explore all aspects of the nascent yet fruitful research area of anion-π interactions.

XO-PBC: An Accurate and Efficient Method for Molecular Crystals

In this work, we propose the XO-PBC method, which combines the eXtended ONIOM method (XO) with the periodic boundary condition (PBC) for the description of molecular crystals. XO-PBC tries to embed a finite cluster cut out from the solid into the periodic environment, making it feasible to employ advanced molecular quantum chemistry methods, which are usually prohibitively expensive for direct PBC calculations. In particular, XO-PBC utilizes the results from force calculations to design the scheme to fragment the molecule when crystals are made of large molecules and to select cluster model systems automatically consisting of dimer up to tetramer interactions for embedding. By applying an appropriate theory to each model, a satisfactory accuracy for the system under study is ensured, while a high efficiency is achieved with massively parallel computing by distributing model systems onto different processors. A comparison of the XO-PBC calculations with the conventional direct PBC calculations at the B3LYP level demonstrates its accuracy at substantially low cost for the description of molecular crystals. The usefulness of the XO-PBC method is further exemplified, showing that XO-PBC is able to predict the lattice energies of various types of molecular crystals within chemical accuracy (<4 kJ/mol) when the doubly hybrid density functional XYG3 is used as the target high level and the periodic PBE as the basic low level. The XO-PBC method provides a general protocol that brings the great predictive power of advanced electronic structure methods from molecular systems to the extended solids.

Diversity and uniformity in anion-π complexes of thiocyanate with aromatic, olefinic and quinoidal π-acceptors

Despite the progress in the study of anion-π interactions, there are still inconsistencies in the use of this term and the experimental data about factors affecting the strength of such bonding are limited. To shed light on these issues, we explored supramolecular associations between NCS^- anions and a series of aromatic, olefinic or quinoidal π-acceptors. Combined experimental and computational studies revealed that all these complexes were formed by an attraction of the anion to the face of the π-system, and the arrangements of thiocyanate followed the areas of the most positive potentials on the surfaces of the π-acceptors. The stabilities of the complexes increased with the π-acceptor strength (reflected by their reduction potentials), and were essentially independent of the magnitudes of the maximum electrostatic potentials on their surfaces. The complexes showed intense absorption bands in the UV-Vis range, and the energies of these bands were correlated with the difference of the redox potentials of the anions and π-acceptors. Such features, as well as results of atoms-in-molecules and non-covalent index analyses suggested that besides electrostatics, molecular orbital interactions play a substantial role in the formation of these complexes. The unified trends in variations of the characteristics of the complexes between thiocyanate and a variety of π-acceptors point to their common nature. To embrace diversity and uniformity of the anion-π associates, we suggest (following the halogen bond's definition) that anion-π bonding occurs when there is evidence of a net attraction between the anions and the face of the electrophilic π-system.

An accurate single descriptor for ion-π interactions

Non-covalent interactions between ions and π systems play an important role in molecular recognition, catalysis and biology. To guide the screen and design for artificial hosts, catalysts and drug delivery, understanding the physical nature of ion-π complexes via descriptors is indispensable. However, even with multiple descriptors that contain the leading term of electrostatic and polarized interactions, the quantitative description for the binding energies (BEs) of ion-π complexes is still lacking because of the intrinsic shortcomings of the commonly used descriptors. Here, we have shown that the impartment of orbital details into the electrostatic energy (coined as OEE) makes an excellent single descriptor for BEs of not only spherical, but also multiply-shaped, ion-π systems, highlighting the importance of an accurate description of the electrostatic interactions. Our results have further demonstrated that OEEs from a low-level method could be calibrated to BEs from a high-level method, offering a powerful practical strategy for an accurate prediction of a set of ion-π interactions.

Synthesis and anion binding properties of phthalimide-containing corona[6]arenes

Functionalized O6-corona[3]arene[3]tetrazines were synthesized efficiently and conveniently by means of a macrocyclic condensation reaction between N-functionalized 3,6-dihydroxyphthalimides and 3,6-dichlorotetrazine under mild conditions in a one-pot reaction manner. The novel macrocycles exist as a mixture of rapidly interconvertible conformers in solution while in the solid state they adopt the conformation in which three phthalimide units are cis,trans-orientated. Acting as electron-deficient macrocyclic hosts, the synthesized O6-corona[3]arene[3]tetrazines self-regulated conformational structures to complex anions in the gas phase and in the solid state owing to the anion-π noncovalent interactions between anions and the tetrazine rings.

Tritopic ion-pair receptors based on anion–π interactions for selective CaX2 binding

Selective ion-pair binding of CaX₂ (X = Br⁻ and I⁻) was realized by a tritopic receptor incorporating two homoditopic anion–π binding sites and a pentaethylene glycol moiety.

Accurate potential energy surfaces for hydrogen abstraction reactions: A benchmark study on the XYG3 doubly hybrid density functional

The potential energy surface (PES) for the H + CH₄ system has been constructed with the recently developed XYG3 doubly hybrid functional, while those with the standard B3LYP hybrid functional, and the Møller-Plesset perturbation theory up to the second order (MP2) are also presented for comparison. Quantum dynamics studies demonstrated that satisfactory results on the reaction probabilities and the rate coefficients can be obtained on top of the XYG3-PES, as compared to the results based on the highly accurate, yet expensive, CCSD(T)-PES (Li et al., J. Chem. Phys. 2015, 142, 204302). Further investigation suggested that the XYG3 functional is useful in providing accurate rate coefficients for some larger systems involving H atom abstractions. © 2017 Wiley Periodicals, Inc.

Synthesis and conformational structure of hydrazo-bridged homo calix[2]pyridine[2]triazines

Presented in this paper are the practical synthesis and conformational structure of hydrazo-bridged homo calix[4]arenes.

How the electron-deficient cavity of heterocalixarenes recognizes anions: insights from computation

The nature of bridging heteroatoms in a heterocalixarene structure has a crucial role in anion recognition.

When a nanoparticle meets a superhalogen: a case study with C60 fullerene

The ability of a selected nanoparticle to form stable systems with superhalogens (i.e. AlF4, AlCl4, MgF3, MgCl3, LiF2, LiCl2, and LiI2) is examined on the basis of theoretical considerations supported by ab initio calculations. It is demonstrated that the C60 fullerene molecule should form stable and strongly bound (C60)˙(+)(superhalogen)(-) radical cation salts when combined with an appropriately chosen superhalogen radical (acting as an oxidizing agent). The conclusion is supported by providing: (i) the structural deformation of superhalogens and C60 nanoparticles upon ionization, (ii) predicted charge flow between the fullerene and each superhalogen (which allows estimating the amount of electron density withdrawn from the C60 molecule during the ionization process), (iii) the localization of the spin density distribution, and (iv) the interaction energies for the compounds obtained both at the B3LYP/6-31+G(d) level and at the B3LYP-D3/6-31+G(d) level. Solvent effects have been considered in the present study by means of the polarizable continuum model. It is found that the stability of C60/superhalogen species can be improved in solvents. We believe that the results provided in this contribution may likely be of prospective relevance in the future studies on the issue of binding and removal of this potentially risky nanoparticle.

Beyond energies: geometry predictions with the XYG3 type of doubly hybrid density functionals

The scaled mean absolute deviations (s-MADs) of the optimized geometric parameters for covalent bondings (the CCse set), nonbonded interactions (the S22G30 set) and the transition state structures (the TSG36 set), with Tot referring to the averaged s-MAD for general performances.