Selective stabilization of self-assembled hydrogen-bonded molecular capsules through π-π interactions

Subtle noncovalent forces such as π-π interactions play an import role in the folding of biological macromolecules such as DNA and proteins. We describe here a system where such interactions on the outside of a molecular capsule trigger a selective change of its structure as a self-assembled receptor.     

Diisocyanoarene-linked pentacarbonylvanadate(I-) ions as building blocks in a supramolecular charge-transfer framework assembled through noncovalent π-π and contact ion interactions

Three synthetic routes to the unusual supramolecular complex ([Cp(2)Co](2)[{(OC)(5)V}(2)(μ-1,4-CNC(6)Me(4)NC)])(∞), which was crystallographically characterized, are presented. The dianion [{(OC)(5)V}(2)(μ-1,4-CNC(6)Me(4)NC)](2-) constitutes the first subvalent organometallics featuring a diisocyanoarene linker.     

Inclusion of dynamic σ-π polarization in π-electronab initio calculations

Summary A method is presented, whereby dynamic - polarization, i.e. the correlation effect expressed by simultaneous (-*, -*) excitations, can be approximately included in a multi-reference configuration interaction (MRCI) or multi-configurational self-consistent field (MC-SCF) calculation, without need to explicitly correlate the sigma orbitals. The method, which we call the capacitance matrix method, is based on the use of conventional one-electron integrals, from which a polarization potential (SPP) contribution is computed and added to the one- and two-electron Hamiltonian. In the present form, the method requires one parameter for each type of atom, and one for each type of bond. These parameters were adjusted to reproduce the dynamic - polarization energy, computed by restricted multi-reference CI calculations, of a number of states of different hydrocarbons, and the agreement was within a few percent. Using the same parameters in CAS (Complete Active Space) SCF calculations of various states of benzene gives excitation energies, when SPP is included, which is comparable to those obtained by much more elaborate MRCI calculations.     

Solvent Dependency of Catalyst-Substrate Aggregation Through π-π Stacking in Photoredox Catalysis

Assemblies of photoredox catalysts and their target substrates prior to photoexcitation is a phenomenon naïvely overlooked by the majority of synthetic chemists, but can have profound influences on reactivity and selectivity in photocatalytic reactions. In this study, we determine the aggregation states of triarylamine radical cationic photocatalysts with various target arene substrates in different solvents by specifically parameterized polarizable molecular dynamics simulations. A π-stacking interaction previously implicated by more expensive, less-representative quantum calculations is confirmed. Critically, this study presents new insights on: i) the ability of solvents (MeCN vs DMF) to make or break a photocatalytic reaction by promoting (MeCN) or demoting (DMF) its catalyst-substrate assemblies, which is a determining factor for reactivity, ii) the average “lifetimes” of assemblies in solution from a dynamic simulation. We find that both in the ground state and the photoexcited state, the cationic radical assemblies remain intact for periods often higher than 60 ps, rendering them ideally suitable to undergo intra-assembly electron transfer reactions upon photoexcitation. Such aspects have not addressed by previous studies on synthetic photocatalytic reactions involving non-covalent assemblies.     

Folding of a supramolecular framework based on a tetrametallic clip driven by π-π interactions

Supramolecular extended frameworks based on a rotational molecular clip and having the same "primary structure" can exhibit either an unfolded or a folded framework. This process is driven by intramolecular π-π interactions, resulting in the formation of an infinite π-stacked column within the folded supramolecular framework.     

Energetics of n-π* and π-π* charge-transfer complexes formed between amine donors and chloranil as π-electron acceptor

The heats of formation ofn- ^* and- ^* charge-transfer interactions have been computed from the charge-transfer spectra of molecular complexes formed in the pyridinechloranil and aniline-chloranil systems.

---

Zusammenfassung Anhand der Charge-Transfer-Spektren von Molekülkomplexen aus Pyridin-Chloranil und Anilin-Chloranil wurde die Wärme für die Bildung vonn- ^* und -^* Charge-Transfer-Wechselwirkungen berechnet.

---

     

Π delocalisation in 1,3-butadiene: a nonempirical molecular orbital study

A value of −0.33 eV or −7.6 kcal mol⁻¹ has been obtained for the vertical delocalisation energy of trans-1,3-butadiene from a nonempirical molecular orbital calculation on the π system. The result agrees well enough with ab initio calculations to suggest that a simplified approach need not be semiempirical. In a basis of orthogonalised atomic orbitals the central bond order is found to be 0.295 (Hückel value 0.447) for the delocalised structure and 0.125 for the localised (Hückel value zero). Core resonance integrals between neighbouring atoms, the analogues of Hückel's β, have theoretical values of −3.9 and −3.2 eV compared with −3.6 eV in benzene. Received: 11 May 1999 / Accepted: 22 July 1999 / Published online: 2 November 1999     

Synthesis and spectroscopic studies on charge-transfer molecular complexes formed in the reaction of imidazole and 1-benzylimidazole with σ- and π-acceptors

The spectrophotometric characteristics of the solid charge-transfer molecular complexes (CT) formed in the reaction of the electron donors imidazole (IML) and 1-benzylimidazole (BIML) with the σ-acceptor iodine and π-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoethylene (TCNE) and 2,3,5,6-tetrachloro-1,4-benzoquinone (CHL) have been studied in chloroform at 25 °C. These were investigated through electronic and infrared spectra as well as elemental analysis. The results show that the formed solid CT-complexes have the formulas [(IML)2 I]I3, [(IML)(DDQ)], [(IML)2(TCNE)5] and [(IML)(CHL)] for imidazole and [(BIML) I]I3, [(BIML)(DDQ)2], [(BIML)(TCNE)2] and [(BIML)(CHL)2] for 1-benzylimidazole in full agreement with the known reaction stoichiometries in solution as well as the elemental measurements. The formation constant KCT, molar extinction coefficient ?CT, free energy change ΔG0, CT energy ECT and ionization potential Ip have been calculated for the CT-complexes [(IML)2 I]I3, [(IML)(DDQ)], [(IML)(CHL)], [(BIML) I]I3, [(BIML)(DDQ)2], [(BIML)(TCNE)2] and [(BIML)(CHL)2].     

Is a molecular orbital measurable by means of tomographic imaging?

Interpretation of experiments involving use of vacuum ultraviolet radiation to effect ionization of N₂ in terms of measurements of a molecular orbital is erroneous.     

Physicochemical properties of π-π complexes of zinctetraphenylporphyrin with aromatic molecules

Features of the solvation of zinctetraphenylporphyrin (ZnTPP) in benzene, toluene,ortho-, meta-, andpara-xylenes were studied by a thermogravimetric method. The temperature ranges of the stability and the compositions of the corresponding specific - complexes were determined from the results of the thermogravimetric investigation of the crystallosolvates of the metalloporphyrin with the solvent molecules, and the energy characteristics of the intermolecular metalloporphyrin—solvent interactions were calculated.Institute of the Chemistry of Nonaqueous Solutions, Russian Academy of Sciences, 153018 Ivanovo. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1545–1548, July, 1992.     

Importance of CH/π hydrogen bonds in recognition of the core motif in proline-recognition domains: an ab initio fragment molecular orbital study

We examined CH/π hydrogen bonds in protein/ligand complexes involving at least one proline residue using the ab initio fragment molecular orbital (FMO) method and the program CHPI. FMO calculations were carried out at the Hartree–Fock (HF)/6‐31G*, HF/6‐31G**, second‐order Møller–Plesset perturbation (MP2)/6‐31G*, and MP2/6‐31G** levels for three Src homology 3 (SH3) domains and five proline‐recognition domains (PRDs) complexed with their corresponding ligand peptides. PRDs use a conserved set of aromatic residues to recognize proline‐rich sequences of specific ligands. Many CH/π hydrogen bonds were identified in these complexes. CH/π hydrogen bonds occurred, in particular, in the central part of the proline‐rich motifs. Our results suggest that CH/π hydrogen bonds are important in the recognition of SH3 and PRDs by their ligand peptides and play a vital role in the signal transduction system. Combined use of the FMO method and CHPI analysis is a valuable tool for the study of protein/protein and protein/ligand interactions and may be useful in rational drug design. © 2011 Wiley Periodicals, Inc. J Comput Chem 2011     

pH-responsive assembly and disassembly of a supramolecular cryptand-based pseudorotaxane driven by π-π stacking interaction

Driven by π-π stacking interaction, a supramolecular cryptand-based [2]pseudorotaxane was formed and its formation was demonstrated to be pH-responsive.     

The use of a molecular balance derived from 5,5′-bipyrazole to calculate π−π stacking interactions

Theoretical calculations have been carried out on 3,3′-dimethyl-1,1′-diphenyl-5,5′-bi-1H-pyrazole to evaluate its usefulness as a molecular balance to measure π−π stacking. After removing the methyl groups (1,1′-diphenyl-5,5′-bi-1H-pyrazole), the N-phenyl groups have been replaced by a series of aromatic rings, and the energy difference between the syn and anti forms is discussed in terms of π−π stacking and dipole-dipole interactions.     

Degradation processes in hydrogen–air fuel cell as a function of the operating conditions and composition of membrane–electrode assemblies

The optimal composition of membrane–electrode assemblies and operating conditions of hydrogen–air fuel cells, which provide a high efficiency and stability of catalytically active cathode layers and the fuel cell as a whole are determined for commercial monoplatinum electrocatalysts on the highly dispersed carbon support containing 60–70 wt % Pt. The degradation processes in the Pt/C catalysts are studied by a complex of electrochemical methods and the methods of structural analysis.     

Reversible and irreversible higher-order cycloaddition reactions of polyolefins with a multiple-bonded heavier group 13 alkene analogue: contrasting the behavior of systems with π-π, π-π*, and π-n+ frontier molecular orbital symmetry

The heavier group 13 element alkene analogue, digallene Ar(iPr(4))GaGaAr(iPr(4)) (1) [Ar(iPr(4)) = C(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr(2))(2)], has been shown to react readily in [n + 2] (n = 6, 4, 2 + 2) cycloaddition reactions with norbornadiene and quadricyclane, 1,3,5,7-cyclooctatetraene, 1,3-cyclopentadiene, and 1,3,5-cycloheptatriene to afford the heavier element deltacyclane species Ar(iPr(4))Ga(C(7)H(8))GaAr(iPr(4)) (2), pseudoinverse sandwiches Ar(iPr(4))Ga(C(8)H(8))GaAr(iPr(4)) (3, 3(iso)), and polycyclic compounds Ar(iPr(4))Ga(C(5)H(6))GaAr(iPr(4)) (4) and Ar(iPr(4))Ga(C(7)H(8))GaAr(iPr(4)) (5, 5(iso)), respectively, under ambient conditions. These reactions are facile and may be contrasted with other all-carbon versions, which require transition-metal catalysis or forcing conditions (temperature, pressure), or with the reactions of the corresponding heavier group 14 species Ar(iPr(4))EEAr(iPr(4)) (E = Ge, Sn), which give very different product structures. We discuss several mechanistic possibilities, including radical- and non-radical-mediated cyclization pathways. These mechanisms are consistent with the improved energetic accessibility of the LUMO of the heavier group 13 element multiple bond in comparison with that of a simple alkene or alkyne. We show that the calculated frontier molecular orbitals (FMOs) of Ar(iPr(4))GaGaAr(iPr(4)) are of π-π symmetry, allowing this molecule to engage in a wider range of reactions than permitted by the usual π-π* FMOs of C-C π bonds or the π-n(+) FMOs of heavier group 14 alkyne analogues.     

Synthesis of 1,3-diphenyl-6-alkyl/aryl-substituted fulvene chromophores: observation of π-π interactions in a 6-pyrene-substituted 1,3-diphenylfulvene

The synthesis, structural, and electronic properties of nine 1,3-diphenyl-6-alkyl/aryl substituted pentafulvenes were studied. Pyrene ring π-π interactions were revealed from analysis of the experimental crystal packing of 1,3-diphenyl-6-(1-pyrene)fulvene and supporting DFT calculations. Photophysical properties derived from UV-vis and fluorescence emission measurements demonstrated tunable and low HOMO-LUMO band gaps for the series. The presented results point to a model synthetic approach for incorporation of extended π systems and donor-π-acceptor groups for fulvene-based electronic materials.     

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.