Relative substituent position on the strength of π-π stacking interactions

It was observed that the relative position of the arene substituents has a profound influence on the strength of π-π stacking in the 9-benzyl-substituted triptycene system. A new series of model compounds (3a-i) capable of revealing quantitatively π-π stacking interactions was studied. This series of compounds (3a-i) has an ortho-substituted methyl group in one of the two interacting arenes and the syn/anti ratios were determined and compared to a series previously studied compounds (4a-i) that have a para methyl group on the corresponding arene. A greater than 50% increase in the strength of π-π stacking interactions was observed with the methyl group in the ortho position comparing to that in the para position. No difference in π-π stacking interactions was observed when the other aromatic ring was a pentafluorobenzoate group.     

Interplay between halogen bonds and π-π stacking interactions: CSD search and theoretical study

According to our survey of the Cambridge Structural Database (CSD), a great number of crystal structures, in which halogen bonds and aromatic stacking interactions are present and play an important role in crystal packing, have been extracted. In this work, ab initio calculations at the MP2 level of theory were performed to investigate the mutual influence between halogen bonds and π-π stacking interactions. Different energetic effects are observed in the studied complexes where the two kinds of noncovalent interactions coexist, which can be rationalized by the direction of charge transfer for the two interactions. These effects have been analyzed in detail in terms of the structural, energetic, and charge transfer properties of the complexes. In addition, the quantum theory of atoms in molecules (QTAIM) was also employed to characterize the interactions and to examine the strengthening or weakening of the interactions, depending on the variations of electron density on the bond and cage critical points. Finally, certain crystal structures retrieved from the CSD have been selected to provide experimental evidence of the combination of the two interactions.     

Metastability of π-π stacking between the closed-shell ions of like charges

Planar cations or anions can form stacks in crystals or solutions,where the surrounding or environment plays a decisive role as demonstrated in previous studies.However,it remains unclear whether these counterintuitive interactions possess any inherent stability or are thoroughly repulsive if the constraint of environment is removed.In this work,we explored the inherent stability of π-π stacking between closed-shell ions of like charges with prototypes derived from experimental studies.The inherent metastability was identified by the characteristic local minima and the transition states preventing their dissociation and verified by ab initio molecular dynamics(AIMD)simulations.The nature of involved interactions was deciphered with the energy decomposition approach based on the block-localized wavefunction method(BLW-ED).Like the conventional neutral π-π stacking interactions,electron correlation is the most attractive energy component.But it is overturned by the Coulombic repulsion between net charges for all modes of dimerization,resulting in the overall repulsive inter-cation or anion in-teractions.Contributions from van der Waals interactions were also observed in the reduced density gradient analysis.The origin of the metastability was elucidated by examining the contributions of individual physical factors to the well-depths.The inherent metastability originates from the electron correlation,which dramatically increases due to the enhanced overlap between ions from a transition state to its corresponding minimum.     

Noncovalent functionalization of graphene via π-hole···π and σ-hole···π interactions

Physisorption of bromopentafluorobenzene (C₆F₅Br) on graphene can occur due to the unique σ-hole and π-hole characters of C₆F₅Br and the rich π-electrons region of graphene, leading to the formation of three types of π-hole···π and σ-hole···π interactions. The π-hole···π interactions are even stronger than the σ-hole···π interactions. The property of graphene was significantly affected by such physisorption.

     

Strong π-π stacking interactions led to the mis-assignment of dimer emissions to the monomers of 1-acetylpyrene

Understanding relationships between molecular structures and fluorescent properties is critical to enable rational deployment of fluorophores. 1-Acetyl pyrene is an important pyrene-derivative, used extensively as an environment-sensitive probe. In the past, the fluorescence of 1-acetylpyrene was considered to be polarity-sensitive with a large positive solvatochromism, and its monomer emissions were believed in the range of 410-470 nm. In this paper, our experimental and theoretical studies showed that the monomer fluorescence of 1-acetylpyrene is centered at ～390 nm,which is similar to that of pyrene dyes and not polarity-sensitive. Previously observed \"monomer emission\" has been re-assigned to that of dimers, which represent the dominant existence form of 1-acetylpyrene in the solution phase,as a result of strong intermolecular π-π stacking interactions.     

The X<Superscript>−</Superscript>···benzohydrazide complexes: the interplay between anion-π and H-bond interactions

The compounds containing the benzohydrazide (BH) nucleus have a variety of biological activities because of various noncovalent intermolecular interactions. The interplay between anion-π and H-bond interactions, which can affect the activity of compounds, has been investigated in ten substituted BH exposed to the chloride ion using the quantum mechanical calculations. The total interaction energy is separated into the anion-π (ΔE _Aπ) and H-bond (ΔE _HB) contributions where both interactions are presented in the complexes. The electron-withdrawing substituents (EWSs) increase |ΔE _Aπ| and decrease |ΔE _HB|, while reversed changes are observed with the electron-donating substituents (EDSs). In addition, the total binding energy (ΔE) becomes more/less negative in the presence of EWSs/EDSs. The synergetic effects of mentioned interactions and substituent effects have also been investigated using the atoms in molecules (AIM), natural bond orbital (NBO) and molecular electrostatic potential (MEP) analyses. A good correlation is found between the energy data and the Hammett constants, the minimum of electrostatic potential (V _min) and the results of population analyses.     

Some peculiarities of π—π interactions between tetraphenylporphine metal complexes and aromatic solvents

The — complexes of metal tetraphenylporphinates with benzene, toluene, and xylenes were characterized by means of thermogravimetry. The ability of metalloporphyrins to form — complexes with certain -donor molecules depends largely on the -acceptor capacity of the macroheterocycle, and on the peculiarities of the metal—porphyrin coordinative linkage. Stoichiometry, energy parameters, and thermal stability of the - complexes of metalloporphyrins with various aromatic ligands are determined to a great extent by the molecular structure of solvents.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp.850–853, May, 1993.     

Molecular recognition at the active site of factor Xa: cation-π interactions, stacking on planar peptide surfaces, and replacement of structural water

Factor Xa, a serine protease from the blood coagulation cascade, is an ideal enzyme for molecular recognition studies, as its active site is highly shape-persistent and features distinct, concave sub-pockets. We developed a family of non-peptidic, small-molecule inhibitors with a central tricyclic core orienting a neutral heterocyclic substituent into the S1 pocket and a quaternary ammonium ion into the aromatic box in the S4 pocket. The substituents were systematically varied to investigate cation-π interactions in the S4 pocket, optimal heterocyclic stacking on the flat peptide walls lining the S1 pocket, and potential water replacements in both the S1 and the S4 pockets. Structure-activity relationships were established to reveal and quantify contributions to the binding free enthalpy, resulting from single-atom replacements or positional changes in the ligands. A series of high-affinity ligands with inhibitory constants down to K(i)=2 nM were obtained and their proposed binding geometries confirmed by X-ray co-crystal structures of protein-ligand complexes.     

Strong phenyl-perfluorophenyl π-π stacking and C-H?F-C hydrogen bonding interactions in the crystals of the corresponding aromatic aldimines

The X-ray diffraction analysis of two co-crystals, 1·2 (aldimines 1 and 2) and 3·4 (aldimines 3 and 4), reveals that there are strong phenyl-perfluorophenyl π-π stacking and intermolecular hydrogen bonding interactions. The new perfluoroaryl-aryl face-to-face interaction of the crystalline aldimines provides a design motif for a new class of self-assembling system.     

Conformational changes of α-lactalbumin adsorbed at oil-water interfaces: interplay between protein structure and emulsion stability

The conformation and structural dimensions of α-lactalbumin (α-La) both in solution and adsorbed at oil-water interfaces of emulsions were investigated using synchrotron radiation circular dichroism (SRCD) spectroscopy, front-face tryptophan fluorescence (FFTF) spectroscopy, and dual polarization interferometry (DPI). The near-UV SRCD and the FFTF results demonstrated that the hydrophobic environment of the aromatic residues located in the hydrophobic core of native α-La was significantly altered upon adsorption, indicating the unfolding of the hydrophobic core of α-La upon adsorption. The far-UV SRCD results showed that adsorption of α-La at oil-water interfaces created a new non-native secondary structure that was more stable to thermally induced conformational changes. Specifically, the α-helical conformation increased from 29.9% in solution to 45.8% at the tricaprylin-water interface and to 58.5% at the hexadecane-water interface. However, the β-sheet structure decreased from 18.0% in solution to less than 10% at both oil-water interfaces. The DPI study showed that adsorption of α-La to a hydrophobic C18-water surface caused a change in the dimensions of α-La from the native globule-like shape (2.5-3.7 nm) to a compact/dense layer approximately 1.1 nm thick. Analysis of the colloidal stability of α-La stabilized emulsions showed that these emulsions were physically stable against droplet flocculation at elevated temperatures both in the absence and in the presence of 120 mM NaCl. In the absence of salt, the thermal stability of emulsions was due to the strong electrostatic repulsion provided by the adsorbed α-La layer, which was formed after the adsorption and structural rearrangement. In the presence of salt, although the electrostatic repulsion was reduced via electrostatic screening, heating did not induce strong and permanent droplet flocculation. The thermal stability of α-La stabilized emulsions in the presence of salt is a combined effect of the electrostatic repulsion and the lack of covalent disulfide interchange reactions. This study reports new information on the secondary and tertiary structural changes of α-La upon adsorption to oil-water interfaces. It also presents new results on the physical stability of α-La stabilized emulsions during heating and at moderate ionic strength (120 mM NaCl). The results broaden our understanding of the factors controlling protein structural change at emulsion interfaces and how this affects emulsion stability.     

Complexation of monosubstituted benzenes and methanes by cyclotetrachromotropylene in aqueous solution: Substituent effects on π-π and Ch-π interactions

The stability constantsK of 11 complexes formed in aqueous solution between several monosubstituted benzenes (C₆H₅X) and methanes (CH₃X) as guests and cyclotetrachromotropylene as host were determined by proton NMR spectroscopy. Variations ofK with the substituent X are attributed to the electronic effect of X and the presence of C–H or aromatic bonds, if any, interacting with the host bonds.     

Coexistence of two conformational isomeric chains in a zinc(II) phosphonate induced by π···π stacking interactions

Hydrothermal reaction of zinc acetate with diethyl [(phenylsulfonyl)methyl]phosphonate as well as 1,10-phenanthroline (phen) afforded a novel zinc(II) phosphonate with the formula of [Zn₄(PhSO₂CH₂PO₃)₄(phen)₂(H₂O)₂]·2H₂O. Such compound features two conformational isomeric 1D chains which are regulated by two different π···π stacking interactions. In addition, it exhibits broad blue fluorescent emission band at 387 nm.     

Combination of π-π stacking and electrostatic repulsion between carboxylic carbon nanoparticles and fluorescent oligonucleotides for rapid and sensitive detection of thrombin

On the basis of simultaneous electrostatic repulsion and π-π stacking interactions of carboxylic carbon nanoparticles (cCNPs) with single-stranded DNA, a noncovalent assembly of cCNPs and fluorescent aptamers is reported for rapid, sensitive, and selective detection of thrombin.     

Cation···π interactions: QTAIM and NBO studies on the interaction of alkali metal cations with heteroaromatic rings

The cation···π interactions of alkali metal cations (Li⁺, Na⁺, and K⁺) with five-membered heteroaromatic rings [furan(C₄H₄O), thiophene(C₄H₄S), pyrrole(C₄H₅N)] were examined by high level ab initio calculations, to investigate the different roles of C₄H₄O, C₄H₄S, and C₄H₅N as the electron donor, the influential factors that affect these interactions, the nature of this kind of cation···π interaction, and to determine topological and energetical properties to characterize these interactions. The sulfur atom in C₄H₄S plays a certain role in the cation···π interactions except the C–C π bond, which is different from C₄H₄O and C₄H₅N. The size of cation and the character of heteroaromatic ring are two influential factors that affect the cation···π interactions. The studied cation···π interactions can be classified as “closed-shell” and noncovalent interactions. The electron density and its Laplacian at the bond critical points and ring critical points generated upon complexation are useful measurements for the strength of cation···π interactions.     

Characteristics of X-H···π interactions: ab initio and QTAIM studies

The MP2/6-311++G(d,p) calculations were performed on several hydrogen-bonded systems. Different complexes were taken into account to analyze various types of hydrogen bonds, possessing different types of proton donors and proton acceptors as well as characterized by the broad range of the interaction energy. The Quantum Theory of Atoms in Molecules is applied. The results of the hybrid variational-perturbational approach are discussed. The unique properties of hydrogen bonds, where π-electrons act as the proton acceptor (X-H···π), are analyzed, and these interactions are compared with the other types of hydrogen bonds, mainly with C-H···Y interactions. It is shown that for X-H···π systems the ellipticity at the bond critical point of the proton···acceptor interaction is much greater than for the other types of hydrogen bonds. However, both X-H···π and C-H···Y interactions are characterized by the dominance of the dispersive energy.     

Comparable charge transport property based on S…S interactions with that of π-π stacking in a bis-fused tetrathiafulvalene compound

Compact molecular packing with short π-π stacking and large π-overlap in organic semiconductors is desirable for efficient charge transport and high carrier mobility.Thus charge transport anisotropy along different directions is commonly observed in organic semiconductors.Interestingly,in this article,we found that comparable charge transport property were achieved based on the single crystals of a bis-fused tetrathiafulvalene derivative(EM-TTP) compound along two interaction directions,that is,the multiple strong S…S intermolecular interactions and the π-π stacking direction,with the measured electrical conductivity and hole mobility of 0.4 S cm~(-1),0.94 cm~2 V~(-1) s~(-1) and 0.2 S cm~(-1),0.65 cm2 V~(-1) s~(-1),respectively.This finding provides us a new molecular design concept for developing novel organic semiconductors with isotropic charge transport property through the synergistic effect of multiple intermolecular interactions(such as S…S interactions) and π-π stacking.