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The dissolution problem is the main obstacle for the value-added modification and depolymerization of industrial lignin. Here, a green urea aqueous solution for complete dissolution of various lignin is presented and the dissolution mechanism is analyzed by AFM, DLS and NMR. The results show that the molecular interaction of lignin decreases from 32.3 mN/m in pure water to 11.3 mN/m in urea aqueous solution. The immobility of 1 H NMR spectra and the shift of 17 O NMR spectra of urea in different lignin/urea solutions indicate that the oxygen of carbonyl in urea and the hydrogen of hydroxyl in lignin form new hydrogen bonds and break the original hydrogen bonds among lignin molecules. The shift of 1 H NMR spectra of lignin and the decrease of interactions in model compound polystyrene indicate that urea also breaks the π-π interactions between aromatic rings of lignin. Lignin dissolved in urea aqueous has good antioxidant activity and it can scavenge at least 63% free radicals in 16 min. [ABSTRACT FROM AUTHOR]

A sensitive microextraction method based on a new poly(methacrylic acid-ethylene glycol dimethacrylate- N-vinylcarbazole) monolithic capillary column, coupled with gas chromatography and electron capture detection, was established for the determination of three benzodiazepines (estazolam, alprazolam, and triazolam) in urine and beer samples. Owing to the abundant π electrons and polar surface of N-vinylcarbazole, N-vinylcarbazole-incorporated monolith showed a higher extraction performance than neat poly(methacrylic acid-ethylene glycol dimethacrylate) because of the enhanced π-π stacking interactions derived from the π-electron-rich benzene groups from N-vinylcarbazole. The monolith exhibited a homogeneous and continuous structure, good permeability, and a long lifetime. Factors affecting the extraction such as solution pH, salt concentration, sample volume, desorption solvent, and desorption volume were investigated. Under the optimized conditions, limits of detection of 0.011-0.026 ng/mL were obtained. The one-column and column-to-column precision values were ≤7.2 and ≤9.8%, respectively. The real samples were first diluted with deionized water and then treated by the monolith microextraction before gas chromatography analysis. The recoveries were 81.4-93.3 and 83.3-94.7% for the spiked samples, with relative standard deviations of 4.1-8.1 and 3.8-8.5%, respectively. This method provides an accurate, simple, and sensitive detection platform for drug analysis. [ABSTRACT FROM AUTHOR]

The vibrational Raman spectra of several series of aromatic and quinoidal compounds have been analyzed considering the downshifts and upshifts of the frequencies of the relevant Raman bands as a function of the number of repeating units. Oligothiophenes, oligophenylene-vinylenes, and oligoperylenes (oligophenyls) derivatives are studied in a common context. These shifts are taken as spectroscopic fingerprints of the changes in π-conjugation. For a given family, aromatic and quinoidal oligomers have been studied together, and according to their Raman frequency shifts located in the two-well BLA-energy curve of their ground electronic state as a function of the bond-length-alternation pattern (BLA). The connection among BLA values, π-conjugation, and Raman frequencies is taken here as the basis of the study. These Raman shifts/BLA changes have been related to important electronic properties of these one-dimensional linear π-electron delocalized systems such as quinoidal (polyene) and aromatic characters. [ABSTRACT FROM AUTHOR]

We describe herein a palladium-catalyzed tandem oxidative arylation/olefination reaction of aromatic tethered alkenes/alkynes for the synthesis of dihydrobenzofurans and 2 H-chromene derivatives. This reaction features a 1,2-difunctionalization of C−C π-bond with two C−H bonds using O2 as terminal oxidant at room temperature. The products obtained are valuable synthons and important scaffolds in biological agents and natural products. [ABSTRACT FROM AUTHOR]

A boryl-substituted diphosphene was synthesized through the nucleophilic borylation of PCl3 with a borylzinc reagent, followed by a reduction with Mg. A combined analysis of the resulting diboryldiphosphene by single-crystal X-ray diffraction, DFT calculations, and UV/Vis spectroscopy revealed a σ-electron-donating effect for the boryl substituent that was slightly weaker than that of the 2,4,6-tri- tert-butylphenyl (Mes*) ligand. The reaction of this diboryldiphosphene with nBuLi afforded a boryl-substituted phosphinophosphide that was, in comparison with the thermally unstable Mes*-substituted diaryldiphosphene, stabilized by a π-electron-accepting effect of the boryl substituent. [ABSTRACT FROM AUTHOR]

The bimolecular and termolecular complexes involving POCl and XCN/CH (X = F, Cl, Br) were designed to form the π-hole pnicogen bonds and σ-hole halogen bonds, to compare the two types of interactions and investigate the mutual influences between them. POCl was used as simultaneous π-hole and σ-hole donor; it can interact with electron donor to form π-hole pnicogen bond and σ-hole halogen bond. The π-hole interactions are stronger than the σ-hole interactions, in both the bimolecular and the termolecular complexes. Comparing the mutual effects of the π-hole interactions and σ-hole interactions, the π-hole interaction has a greater influence on the σ-hole interaction than vice versa. With the addition of σ-hole halogen bond, the V value outside the π-hole region of POCl becomes decreasingly positive, resulting in a weaker π-hole interaction. With the addition of π-hole pnicogen bond, the V value outside the σ-hole region of POCl becomes small, also resulting in a weaker σ-hole interaction. The π-hole pnicogen bond and σ-hole halogen bond weaken each other, i.e., there is a negative cooperative effect in the termolecular complexes. [ABSTRACT FROM AUTHOR]

In the present work, the influence of substituents on cooperativity between CH··· π and N···H hydrogen bonds is theoretically investigated in the T-shaped configuration of X-benzene⊥(FH···pyrazine···HF) complexes. The calculations are performed at MP2(FC)/6-311++G(d,p) level of theory. The results indicate that the absolute value of binding energy increases as the substituent changes from strong electron acceptor to strong electron donor. Moreover, cooperative and synergistic energies are computed in the complexes. The energetic data reveal a direct correlation between the electron-donating power of substituents and favorable influences of CH··· π and N···H hydrogen bonds on each other. The results of AIM and NBO analyses show that the N···H hydrogen bond is strengthened by going from electron acceptor substituents to electron donor ones. It is also found that Hammett constants can be used to predict cooperative and synergistic effects between CH··· π and N···H hydrogen bonds. [ABSTRACT FROM AUTHOR]

Ab initio MP2/aug-cc-pVDZ calculations were performed to investigate mutual effect between π-π stacking and halogen bond interactions in X-ben||pyr···Cl-F complexes (X = CN, F, Cl, Br, CH, OH and H where || and ··· denote π-π stacking and halogen bonds). The results indicate the cooperativity of π-π stacking and halogen bonds in these complexes. This effect was discussed in terms of the energetic, geometrical parameters and charge-transfer properties of the complexes. To explore on the two-bonded spin-spin coupling constant J(N-F) across N···Cl-F halogen bond in X-ben||pyr···Cl-F complexes, NMR calculations were performed at PBE0/aug-cc-pVDZ levels of theory. To get more insight into the physical nature of the binding energies, Symmetry Adapted Perturbation Theory calculations were carried out. Energy decomposition indicates that the percentage of the electrostatic term in the halogen bonding system constitutes approximately half of the total attractive binding energies, while the percentage of the dispersion term in the π-π stacking complexes constitutes approximately half of the attractive binding energies. In addition, atoms in molecules, natural bond orbital and molecular electrostatic potential were also used to probe the π-π stacking interactions and halogen bonding strengths. [ABSTRACT FROM AUTHOR]

The intrinsic bond strength of C2 in its 1Σg+ ground state is determined from its stretching force constant utilizing MR-CISD+Q(8,8), MR-AQCC(8,8), and single-determinant coupled cluster calculations with triple and quadruple excitations. By referencing the CC stretching force constant to its local counterparts of ethane, ethylene, and acetylene, an intrinsic bond strength half way between that of a double bond and a triple bond is obtained. Diabatic MR-CISD+Q results do not change this. Confinement of C2 and suitable reference molecules in a noble gas cage leads to compression, polarization, and charge transfer effects, which are quantified by the local CC stretching force constants and differences of correlated electron densities. These results are in line with two π bonds and a partial σ bond. Bond orders and bond dissociation energies of small hydrocarbons do not support quadruple bonding in C2. [ABSTRACT FROM AUTHOR]

Ever since Lewis depicted the triple bond for acetylene, triple bonding has been considered as the highest limit of multiple bonding for main elements. Here we show that C2 is bonded by a quadruple bond that can be distinctly characterized by valence-bond (VB) calculations. We demonstrate that the quadruply-bonded structure determines the key observables of the molecule, and accounts by itself for about 90 % of the molecule's bond dissociation energy, and for its bond lengths and its force constant. The quadruply-bonded structure is made of two strong π bonds, one strong σ bond and a weaker fourth σ-type bond, the bond strength of which is estimated as 17-21 kcal mol−1. Alternative VB structures with double bonds; either two π bonds or one π bond and one σ bond lie at 129.5 and 106.1 kcal mol−1, respectively, above the quadruply-bonded structure, and they collapse to the latter structure given freedom to improve their double bonding by dative σ bonding. The usefulness of the quadruply-bonded model is underscored by 'predicting' the properties of the 3Σu+ state. C2’s very high reactivity is rooted in its fourth weak bond. Thus, carbon and first-row main elements are open to quadruple bonding! [ABSTRACT FROM AUTHOR]

ABSTRACT Pyrene end-functionalized, telechelic poly(dimethyl siloxane) (PDMS) materials were synthesized and their response to different thermal stimuli was evaluated. The incorporation of pyrene end groups introduces strong π-π interactions that facilitated a broad range of thermally responsive properties, in some circumstances forming pyrene nanocrystals that serve as physical crosslinks leading to elastic materials. By synthesizing different chain lengths, samples exhibiting a 7 orders of magnitude change in storage modulus in response to thermal stimuli were produced by modifying only the end-groups (0.6 wt % of all polymer segments). Repeated thermal cycling during rheological experiments revealed that π-π interaction and crystallization/melting kinetics of pyrene chain-ends plays a key role in their thermal responsiveness. The properties of these materials were tuned by adding free pyrene, neat PDMS, or graphene oxide (GO) nanoparticles, making them attractive for many applications (e.g., tunable damping materials, heat/light sensors, conductive gels, or light repositionable adhesives). For example, nanocomposites containing 1 wt % GO caused the melting temperature for pyrene crystal domains to more than double, and even induced pyrene end-group crystallization in samples that did not exhibit crystals in neat form. It is hypothesized that these features originate from π-π interactions between pyrene ends and GO surfaces. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 159-168 [ABSTRACT FROM AUTHOR]

A chemically non-innocent pyrrole-based trianionic (ONO)3- pincer ligand within [(pyr-ONO)TiCl(thf)2] (2) can access the dianionic [(3H-pyr-ONO)TiCl2(thf)] (1-THF) and monoanionic [(3H,4H-pyr-ONO)TiCl2(OEt2)][B{3,5-(CF3)2C6H3}4] (3-Et2O) states through remote protonation of the pyrrole γ-C π-bonds. The homoleptic [(3H-pyr-ONO)2Zr] (4) was synthesized and characterized by X-ray diffraction and NMR spectroscopy in solution. The protonation of 4 by [H(OEt2)2][B{C6H3(CF3)2}4] yields [(3H,4H-pyr-ONO)(3H-pyr-ONO)Zr][B{3,5-(CF3)2C6H3}4] (5), thus demonstrating the storage of three protons. [ABSTRACT FROM AUTHOR]

Chemically bound structures of several benzene molecules are studied both analytically and numerically. The bonds in these structures are established by replacing the intra π-bonds of C 6 rings by the inter σ-bonds. The rearrangements of different π-bonds may take place differently which causes distortions of C 6 rings. The reason of the distortions is the pseudo-Jahn–Teller effect (PJTE). In the case of two chemically bound benzene molecules in a hollow position (with direct on-top stacking), called as bi-benzene, three different distorted states in addition to the undistorted one are found to exist. PJTE leads also to a chemical bonding of three and more benzene molecules stacked directly on-top of each other. These bound structures do not exist for undistorted molecules. All bound states, although being metastable are separated from the unbound ones by energy barriers ≳2 eV. It is found that the height of the barrier for symmetric bi-benzene is essentially affected by the Jahn–Teller effect. [ABSTRACT FROM AUTHOR]

Coordination assembly of dimeric fluorenone ketyl (pinacolate, C26H16O22−) with boron subphthalocyanine (BSubPc) was obtained in the complex with fullerene, {(BSubPc)2(C26H16O2)}·C60, featuring an effective π–π interaction between concave-shaped BSubPc macrocycle and spherical C60 molecules. [ABSTRACT FROM AUTHOR]

Some of our group recently demonstrated combining theory and a statistical survey of the Cambridge Structural Database (CSD) that the interaction between the π-holes of nitro groups and electron-rich atoms is somewhat directional (Chem. Commun., 2015, 51, 1491–1493). In this communication, we present a joint experimental and theoretical study on a novel cadmium(ii) complex, [Cd(C7H3N2O6)2(C12H8N2)2]·H2O·DMF (1) [where C7H4N2O6 = 3,5-dinitrobenzoic acid and C12H8N2 = 1,10-phenanthroline] to unravel the important role of this π-hole interaction. [ABSTRACT FROM AUTHOR]

Coordination assembly of dimeric fluorenone ketyl (pinacolate, C26H16O22−) with boron subphthalocyanine (BSubPc) was obtained in the complex with fullerene, {(BSubPc)2(C26H16O2)}·C60, featuring an effective π–π interaction between concave-shaped BSubPc macrocycle and spherical C60 molecules. [ABSTRACT FROM AUTHOR]

A 2,2′-bipyridine-appended ferrocene rotor, 1,1′-di(5-yl-ethynyl-2,2′-bipyridine)ferrocene, can be switched from a folded/stacked (syn) conformation to an extended/unstacked (anti) conformation by the addition of [Cu(CH3CN)4](PF6) and 6,6′-dimesityl-2,2′-bipyridine. This extension and contraction process was completely reversible and could be triggered either chemically or electrochemically. [ABSTRACT FROM AUTHOR]

The introduction of a triazole-based ligand to the zeolitic imidazolate framework-7 via a postsynthetic ligand exchange strategy not only maintains its framework, high stability and morphology, but also provides extra uncoordinated nitrogen atoms to improve the π–π and Lewis acid–base interactions. [ABSTRACT FROM AUTHOR]

Biochars from nitrogen-rich biomaterials (i.e., α-amylase, chitin and zein) were produced at different temperatures (i.e. 170, 250, 350 and 450 °C) and characterized, and their sorption for phenanthrene, naphthalene and 1-naphthol was investigated. The organic carbon content normalized-sorption coefficient ( Koc) of the tested compounds by biochars increased with increasing charring temperature, attributed to the reduction of O-containing polar moieties especially the O-alkyl components, and the newly created aromatic carbon domains. The N-heterocyclic ring structure formed during charring process may enhance π–π interactions between aromatics and the aromatic components in the resulting biochars. However, π–π interactions did not dominate sorption of aromatics by N-rich biochars. Sorption of the tested compounds by N-rich biochars was predominantly controlled by the hydrophobic interactions between these chemicals and the aromatic components in biochars. Both N- and O-containing polar moieties at the biochar surfaces negatively affected their sorption for aromatics. [ABSTRACT FROM AUTHOR]

Elemental boron is electron-deficient and cannot form graphene-like structures. Instead, triangular boron lattices with hexagonal vacancies have been predicted to be stable. A recent experimental and computational study showed that the B36 cluster has a planar C6v structure with a central hexagonal hole, providing the first experimental evidence for the viability of atom-thin boron sheets with hexagonal vacancies, dubbed borophene. Here we report a boron cluster with a double-hexagonal vacancy as a new and more flexible structural motif for borophene. Photoelectron spectrum of B35− displays a simple pattern with certain similarity to that of B36-. Global minimum searches find that both B35− and B35 possess planar hexagonal structures, similar to that of B36, except a missing interior B atom that creates a doublehexagonal vacancy. The closed-shell B35− is found to exhibit triple n aromaticity with 11 delocalized π bonds, analogous to benzo(g,h,i)perylene (C22H12). The B35 cluster can be used to build atom-thin boron sheets with various hexagonal hole densities, providing further experimental evidence for the viability of borophene. [ABSTRACT FROM AUTHOR]

Trans—cis photoisomerization in an azo compound containing azobenzene chromophores and long alkyl chains leads to a photoinduced crystal-melt transition (PCMT). X-ray structure analysis of this crystal clarifies the characteristic coexistence of the structurally ordered chromophores through their π• • •π interactions and disordered alkyl chains around room temperature. These structural features reveal that the PCMT starts near the surface of the crystal and propagates into the depth, sacrificing the π• • •π interactions. A temporal change of the powder X-ray diffraction pattern under light irradiation and a two-component phase diagram allow qualitative analysis of the PCMT and the following reconstructive crystallization of the cis isomer as a function of product accumulation. This is the first structural characterization of a compound showing the PCMT, overcoming the low periodicity that makes X-ray crystal structure analysis difficult. [ABSTRACT FROM AUTHOR]

Organic nanowires are important building blocks for nanoscopic organic electronic devices. In order to ensure efficient charge transport through such nanowires, it is important to understand in detail the molecular parameters that guide self-assembly of π-conjugated molecules into one-dimensional stacks with optimal constructive π–π overlap. Here, we investigated the subtle relationship between molecular structure and supramolecular arrangement of the chromophores in self-assembled nanowires prepared from perylene bisimides with oligopeptide-polymer side chains. We observed a “two-fold” odd–even effect in circular dichroism spectra of these derivatives, depending on both the number of l-alanine units in the oligopeptide segments and length of the alkylene spacer between chromophore and oligopeptide substituents. Our results indicate that there is a complex interplay between the translation of molecular chirality into supramolecular helicity and the molecules’ inherent propensity for well-defined one-dimensional aggregation into β-sheet-like superstructures in the presence of a central chromophore. Strong excitonic coupling as expressed by the appearance of hypsochromically and bathochromically shifted UV–vis absorptions and strong CD signals was systematically observed for molecules with an odd number of l-alanines in the side chains. The latter derivatives gave rise to nanowires with a significantly higher electron mobility. Our results, hence, provide an important design rule for self-assembled organic nanowires. [ABSTRACT FROM AUTHOR]

Herein, we report our latest experimental investigations of halide-π interactions in solution. We base this research on the thermodynamic characterization of a series of 1:1 complexes formed between halides (Cl-, Br-, and I-) and several α,α-isomers of "two-wall" calix[4]pyrrole receptors bearing two six-membered aromatic rings in opposed meso positions. The installed aromatic systems feature a broad range of electron density as indicated by the calculated values for their electrostatic surface potentials at the center of the rings. We show that a correlation exists between the electronic nature of the aromatic walls and the thermodynamic stability of the X-receptor complexes. We give evidence for the existence of both repulsive and attractive interactions between π systems and halide anions in solution (between 1 and -1 kcal/mol). We dissect the measured free energies of binding for chloride and bromide with the receptor series into their enthalpic and entropic thermodynamic quantities. In acetonitrile solution, the binding enthalpy values remain almost constant throughout the receptor series, and the differences in free energies are provoked exclusively by changes in the entropic term of the binding processes. Most likely, this unexpected behavior is owed to strong solvation effects that make up important components of the measured magnitudes for the enthalpies and entropies of binding. The use of chloroform, a much less polar solvent, limits the impact of solvation effects revealing the expected existence of a parallel trend between free energies and enthalpies of binding. This result indicates that halide-π interactions in organic solvents are mainly driven by enthalpy. However, the typical paradigm of enthalpy-entropy compensation is still not observed in this less polar solvent. [ABSTRACT FROM AUTHOR]

A bottom-up approach was employed to prepare novel Self-Assembled Monolayers (SAMs) in which a naphthyl moiety acted as a “π-binder” for unfunctionalised meso-tetraferrocenylporphyrin (H2TFcP). Four naphthalene derivatives with an appropriate functional group were synthesized and SAMs were prepared both on gold and ITO surfaces. Mixed H2TFcP–naphthalene films were thoroughly characterized using UV-Vis, XPS and electrochemical techniques. In particular, angle-dependent XPS experiments revealed an almost perpendicular orientation of the porphyrin on surfaces, suggesting that an intercalation occurred among naphthalene units. A large amount of porphyrin was deposited on both the surfaces (in the order of 10−10 mol × cm−2), comparable to that afforded by more conventional covalent linkages. However, significant differences in homogeneity between SAMs on gold and ITO resulted in a diverse electrochemical behaviour. The electrochemical activity of the oxidised porphyrin was restored by prolonged exposure of the modified gold electrode (tens of seconds) to a negative potential, whereas no response was detected after the same treatment on ITO. This novel approach provides a general and versatile strategy to bind meso-substituted porphyrins on a pre-formed monolayer without the necessity for further functionalisations. [ABSTRACT FROM AUTHOR]

We use first-principles density functional theory (DFT) and time-dependent DFT (TDDFT) calculations to investigate the structural, energetic and bonding aspects, and optical properties of two analogues of RNA nucleosides. The results are compared against the findings obtained for the natural and both modified nucleosides, as well as available experimental data. We find that the modified nucleosides form the hydrogen bonded (H-bonded) Watson–Crick (WC) base pairing with a similar H-bonding energy to that obtained for the natural nucleosides. These modified nucleosides can also form WC base pairing with their natural complementary nucleosides. We also focus on the optical absorption properties of these modified nucleosides and their WC H-bonded base pairs in gas phase as well as in implicit water solvent. Our simulated results show that the low energy peaks in the absorption spectra mainly arise because of the π–π* electronic transition for both the modified nucleosides. The calculated absorption and emission spectra in the presence of solvent follow the same trend as that found experimentally. Our results demonstrate that the modified nucleosides show significantly different electronic and optical properties, although the basic structural and bonding aspects remain the same. They also give microscopic details of the experimentally observed photophysical properties of the modified nucleosides. [ABSTRACT FROM AUTHOR]

Pendent groups or atoms attached to phosphorus atoms control the latters' behaviour in a variety of ways. Empirical quantitative determination of the extent of back-donation from a variety of metal centres to P-donor ligands is remarkably simple and can be accomplished quickly by comparison with readily available C–O stretching frequencies in Ni(CO)3L complexes coupled with pKa data that have been available for many years. However, these estimates are not the whole story and a fourth effect, in addition to σ-donation, π back-donation, and steric repulsions, can make a major contribution to metal–P bonding. This effect should be taken into account before any theoretical analysis of metal–P bonding is attempted; otherwise a profound disconnect between empirical evidence and theoretical descriptions of the bonding (i.e. between fact and “fiction”?) will hinder progress towards a better understanding of the behaviour of a wide variety of organometallic complexes. [ABSTRACT FROM AUTHOR]

Hydroxytriphenylmethane dyes are organic compounds whose properties suggest a potential use in nonlinear optics. They are also widely used in chemical analysis. The most commonly used is C.I. Mordant Blue 29. However, in the literature there are no complete spectroscopic characteristics of the dye and there is a lack of detailed information on the electrical properties of C.I. Mordant Blue 29 dye. The optimum geometry of C.I. Mordant Blue 29 was determined by minimizing the energy with respect to all geometrical parameters without imposing molecular symmetry constraints. The UV-VIS spectra of C.I. Mordant Blue 29 and its charged species were computed with time-dependent density functional theory (TDDFT) by employing the B3LYP functional together with the 6-31++G(d,p) basis set at optimized geometries. The calculations of UV-VIS spectra also considered the presence of a solvent in terms of the polarized continuum model (PCM). The calculated spectra were compared with those obtained experimentally. The dipole moment (μ), polarizability (α), and first hyperpolarizability (β) of C.I. Mordant Blue 29 were calculated using the time-dependent Hartree–Fock method, and the electron correlation contribution to the electric properties was introduced by DFT using the functionals B3LYP, PBE0, and HCTH. Calculations were performed using several basis sets from 6-31G to 6-311++G(d,p). [ABSTRACT FROM AUTHOR]

A series of homologous push–pull compounds, in which identical donor (a dimethylamino) and acceptor (a malonate ester) functionalities endcap crescent PPV fragments, exhibit striking differences in their supramolecular recognition of cations acting as Lewis acids. The shorter conjugated compound (one phenyl ring) coordinates a wide variety of lanthanide cations (Eu3+, Yb3+ and Er3+) in MeCN solutions to the 1,3-dicarbonyl acceptor end, resulting in an overall supramolecular polarization of the system (red shift of the intramolecular charge-transfer ICT band). With the “hard” cation Sc3+, recognition switches to the tertiary amine donor end, turning the conjugated system from D–π–A to A–π–A, and resulting in a blue shift of the ICT band upon complexation. Interestingly, increasing the conjugation by means of the insertion of sequential p-phenylenevinylene units into the ligand results in coordination to the donor end regardless of cation “hardness” (Sc3+, Eu3+ and Er3+), suggesting a relative change in the nucleophilicity of the two coordinating ends when increasing the length of the conjugated π-bridge. Such a hypothesis is supported by quantum chemical calculations on the ligands and subsequent atomic charges determination using two independent approaches (QTAIM and CHelpG). The characterization of the thermodynamic stabilities and the dimensionalities of the ligand–cation complexes in solution reveals striking differences from case to case, yet increasing affinities (from log Kav = 2.5 to log Kav = 4.9) are recorded with the increase of the π-conjugated bridge. [ABSTRACT FROM AUTHOR]

In 1996 J. Derek Woollins reported “π-facial” interactions between the chloride anion and the seven-membered aromatic [S4N3]+ and inorganic ring (Polyhedron, 1996, 15, 4603–4605). In spite of this manuscript has not been highlighted before (only 5 citations so far), it can be considered as a pioneering work in the field of noncovalent interactions involving anions, since unprecedented anion–π contacts were described as “π-facial” interactions. In this manuscript several complexes of [S4N3]+ with anions were optimized at the BP86-D3/def2-TZVPD level of theory and compared to X-ray structures retrieved from the CSD. We have evaluated the interaction energies between the chloride and nitrate anions and the [S4N3]+ ring. The anion–π complexes are energetically favourable and the complexation has been characterized by means of Bader's theory of “atoms in molecules”. Moreover, the π-orbitals of the [S4N3]+ ring have been computed and the aromaticity of the 10-π electron seven membered ring was analysed. [ABSTRACT FROM AUTHOR]

f Orbital bonding in actinide and lanthanide complexes is critical to their behavior in a variety of areas from separations to magnetic properties. Octahedral f1 hexahalide complexes have been extensively used to study f orbital bonding due to their simple electronic structure and extensive spectroscopic characterization. The recent expansion of this family to include alkyl, alkoxide, amide, and ketimide ligands presents the opportunity to extend this study to a wider variety of ligands. To better understand f orbital bonding in these complexes, the existing molecular orbital (MO) model was refined to include the effect of covalency on spin orbit coupling in addition to its effect on orbital angular momentum (orbital reduction). The new MO model as well as the existing MO model and the crystal field (CF) model were applied to the octahedral f1 complexes to determine the covalency and strengths of the σ and π bonds formed by the f orbitals. When covalency is significant, MO models more precisely determined the strengths of the bonds derived from the f orbitals; however, when covalency was small, the CF model was better than either MO model. The covalency determined using the new MO model is in better agreement with both experiment and theory than that predicted by the existing MO model. The results emphasize the role played by the orbital energy in determining the strength and covalency of bonds formed by the f orbitals. [ABSTRACT FROM AUTHOR]

Derivatives of the self-complementary 2-guanidiniocarbonyl pyrrole 5-carboxylate zwitterion (1) (previously reported by us to dimerize to 1·1 with an aggregation constant of ca. >1010 M-1 in DMSO) aggregate in a diverse manner depending on, e.g., variation of concentration or its protonation state. The mode of aggregation was analyzed by spectroscopic (NMR, UV) and microscopic (A.FM, SEM, HIM, and TEM) methods. Aggregation of dimers of these zwitterions to higher supramolecular structures was achieved by introduction of sec-arnide substituents at the 3-position, i.e., at the rearward periphery of the parent binding motif: A butyl amide substituent as in 2b enables the discoid dimers to thrther aggregate into one-dimensional (rod-like) stacks. Quantitative UV dilution studies showed that this aggregation is strongly cooperative following a nucleation elongation mechanism. The amide hydrogen seems to be essential for this rod-like aggregation, as neither 1 nor a corresponding tert-amide congener 2a form comparable structures. Therefore, a hydrogen bond-assisted π-π-interaction of the dimeric zwitterions is suggested to promote this aggregation mode, which is further affected by the nature of the amide substituent (e.g., steric demand), enabling the formation of bundles of strands or even two-dimensional sheets. By exploiting the zwitterionic nature of the aggregating discoid dimers, a reversible pH switch was realized: dimerization of all compounds is suppressed by protonation of the carboxylate moiety, converting the zwitterions into typical cationic amphiphiles. Accordingly, typical nanostructures like vesicles, tubes, and flat sheets are formed reversibly under acidic conditions, which reassemble into the original rod-like aggregates upon readjustment to neutral pH. [ABSTRACT FROM AUTHOR]

Herein we disclose the results of our investigations regarding the interactions between the biologically relevant nitrate oxoanion and several "two-wall" aryl-extended calix[4]pyrroles. There exists a clear relationship between the electronic nature of the aromatic walls of the calix[4]pyrroles and the stability of the nitrateccalix[4]pyrrole complex. This suggests that NO3--π interactions have an important electrostatic component. We provide energetic estimates for the interaction of nitrate with several phenyl derivatives. Additionally, we report solid-state evidence for a preferred binding geometry of the nitrate anion included in the calix[4]pyrroles. Finally, the "two-wall" aryl-extended calix[4]- pyrroles show excellent activity in ion transport through lipid-based lamellar membranes. Notably the best anion transporters are highly selective for transport of nitrate over other anions. [ABSTRACT FROM AUTHOR]

Two new metallaphosphacarboranes have been synthesised from [7, 8- nido-CPB9H11]- by deprotonation followed by treatment with a metal fragment. Starting with [HNMe3][7, 8- nido-CPB9H11] deprotonation and reaction with a source of {(indenyl)Co}2+ affords the η-bonded cobaltaphosphacarborane 3-(η-C9H7)-3,1,2- closo-CoCPB9H10 ( 1) the first 'half-sandwich' 3,1,2-metallaphosphacarborane. The indenyl ligand conformation in 1 ( cisoid between the indenyl bridgehead carbon atoms and cage heteroatoms, with the cage carbon atom lying below the indenyl C-C bond) reveals that the structural trans effect in a phosphacarborane ligand is weakest for carbon and therefore varies in the order B > P > C. Starting with [HNC5H11][7, 8- nido-CPB9H11] deprotonation and reaction with half an equivalent of [Ru( p-cymene)Cl2]2 affords the σ-bonded ruthenaphosphacarborane 8-{Ru( p-cymene)Cl(C5H11N)}-7, 8- nido-CPB9H11 ( 2), the first example of a molecule with a phosphacarborane σ-bonded to a metal to be crystallographically characterised. [ABSTRACT FROM AUTHOR]

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene 3b and triazacoronene 4a were elucidated. The photophysical properties for neutral compounds 1–4 were investigated using steady-state UV–vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S1–T1 energy gap (ΔES-T) to be 0.35 eV (1a), 0.26 eV (2a), and 0.36 eV (4a). Similar to the case of previously reported hexapyrrolohexaazacoronene 1 (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene p-systems were examined using UV-vis-NIR absorption, ESR, and NMR spectroscopies for the chemically generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin multiplicities. For HPHAC 1 and pentaazacoronene 2, the overall aromaticity predicted for closed-shell dications 12+ and 22+ was primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for tetraazacoronene 32+ and triazacoronene 42+ with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons (PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized. [ABSTRACT FROM AUTHOR]

Simultaneous atomic force microscopy (AFM)/scanning tunnelling microscopy (STM) is a powerful combination of two complementary techniques that enables high-resolution measurement of surface physical and chemical properties. Although blind characterization of the chemical structure of a surface or probe tip is extremely challenging, recent experimental reports suggest that measurement of the tip bonding symmetry is possible when it interacts with a well-defined sample, such as an adsorbed molecule. In order to gain understanding into the relationship between the symmetry observed in the experiments and the actual tip structure, we explore computationally the mechanism through which the tip apex electronic density and angular stiffness is revealed in AFM images. Our study is based on idealized 1-atom models of the surface, which allow us to isolate the effects of probe size and reactivity. Our results show that image symmetry is directly related to tip symmetry and that the contrast diminishes with larger or more reactive sample atoms. We also find that that the mechanics of the probe-sample interaction are not trivial even for the most idealized models, and that the electronic density distribution around the apex atom is also more complex than what can be inferred from the experimental images. [ABSTRACT FROM AUTHOR]

Arynes generated directly from alkyne building blocks in the presence of silver catalysts effectively activate primary, secondary, and tertiary alkane C-H bonds. This C-H insertion requires only a catalytic amount of silver complex and modest heating compared to harsh conditions and extra promoters including directing groups, oxidants, and bases in typical transition-metal-based C-H bond functionalizations. Preliminary mechanistic studies suggest that the C-H bond-breaking and new bond-forming events take place in a concerted manner, rendering a formal 1,2-addition of C-H bond across the p-bond of arynes. [ABSTRACT FROM AUTHOR]

A π-conjugated nanosheet comprising planar nickel bis(dithiolene) complexes was synthesized by a bottom-up method. A liquid-liquid interfacial reaction using benzenehexathiol in the organic phase and nickel(II) acetate in the aqueous phase produced a semiconducting bulk material with a thickness of several micrometers. Powder X-ray diffraction analysis revealed that the crystalline portion of the bulk material comprised a staggered stack of nanosheets. A single-layer nanosheet was successfully realized using a gas-liquid interfacial reaction. Atomic force microscopy and scanning tunneling microscopy confirmed that the π-conjugated nanosheet was single-layered. Modulation of the oxidation state of the nanosheet was possible using chemical redox reactions. [ABSTRACT FROM AUTHOR]

Development of a simple and efficient methodology to control the placement, spacing, and alignment of single-walled carbon nanotubes (SWCNTs) is essential for nanotechnology device application. Building on the growing understanding that the strong π-π interaction between the bases of single-stranded DNA (ssDNA) and CNTs is sufficient not only to drive CNT solubility in water but also to stabilize individual nanotubes against clustering in aqueous solution, a new motif for functionalizing DNA origami (DO) with CNTs is demonstrated. CNTs solubilized via wrapping with ssDNA react with DO constructs displaying linear arrays of ssDNA, leading to immobilization of the CNTs onto the DO scaffold. This study demonstrates the immobilization of ssDNA-wrapped CNTs at specific positions on single DO constructs. Furthermore, multiple DO constructs assembled into extended one-dimensional arrays have been used to successfully align pairs of CNTs exceeding 500 nm in length in a parallel orientation. This result provides a simplified, alternative approach to immobilization of CNTs with programmed spacing and orientation. [ABSTRACT FROM AUTHOR]

In the present paper, we ascertain two novel findings on chiral-index-selective binding/separating of single-walled carbon nanotubes (SWNTs) with a nonaromatic polymer, poly(dialkylsilane) (PSi). PSi is a typical σ-conjugated polymer, composed of alkyl side chains attached to the silicon (Si)-catenated main chain. First, PSi’s with linear alkyl side chains showed significant diameter-selective wrapping for SWNTs with ca. 0.9 nm in diameter, resulting in the selective separation of (7,6) and (9,4) SWNTs. Its driving force was demonstrated to be cooperative CH−π interactions among the alkyl side chains of PSi’s and the curved graphene of SWNTs. Second, the dynamic wrapping behavior of PSi’s onto SWNTs was elucidated with time-resolved UV spectroscopy. Highly anisotropic UV absorption of PSi along the Si main chain was utilized as a “chromophoric indicator” to monitor the global/local conformations, which enabled us to track kinetic structural changes of PSi’s on SWNTs. Consequently, we concluded that upon wrapping, flexible/helical PSi with an average dihedral angle (φ) of 145° and Kuhn’s segment length (λ–1) of 2.6 nm interconverted to the more stiffer/planar conformation with 170° and λ–1 of 7.4 nm. Furthermore, through kinetic analyses of the time-course UV spectra, we discovered the fact that PSi’s involve three distinct structural changes during wrapping. That is, (i) the very fast adsorption of several segments within dead time of mixing (<30 ms), following (ii) the gradual adsorption of loosely wrapped segments with the half-maximum values (τ1) of 31.4 ms, and (iii) the slow rearrangement of the entire chains with τ2 of 123.1 ms, coupling with elongation of the segment lengths. The present results may be useful for rational design of polymers toward chiral-index-selective binding/separating of desired (n,m) SWNTs. [ABSTRACT FROM AUTHOR]

We report a comparative study of the synthesis and the structural and electronic properties of hexagonal boron nitride (h-BN) and graphene layers grown on the Ir( 111) surface. By performing photoernission (PE) experiments time-dependently and supplementing them by low-energy electron diffraction (LEED) data, we demonstrate the differences and similarities of their formation, and we show the temperature regimes which allow these materials to be created with high crystalline quality. It has been demonstrated that synthesis at low temperatures leads to the formation of disordered domains of h-BN as well as graphene flakes on the metallic surface. The LEED patterns of such samples reveal a complex structure which indeed can be easily interpreted by using the model presented here. The angle-resolved PE study of h-BN/Ir(111) demonstrates the presence of replicated electronic states, which are quite similar to those detected and widely discussed for the graphenelIr(111) system. On the basis of these results and the corresponding model calculations, we argue that the appearance of these replicated bands is mainly caused by the geometrical corrugation of the system. [ABSTRACT FROM AUTHOR]

"Synergistic effect" is prevalent in natural metalloenzymes in activating small molecules, and the success has inspired the development of artificial catalysts capable of unprecedented organic transformations. In this work, we found that the attractive π–π interaction between organic additives (as electron-donors) and the perfluorinated arenes (as electron acceptors) is effective in gold hydride catalyzed activation of C-F bonds, specifically hydrodefluorination (HDF) of perfluoroarenes catalyzed by the Sadighi's gold hydrides [(NHC)AuH] (NHC = N-heterocyclic carbene). Although a weak interaction between [(NHC)AuH] and perfluoroarenes was observed from 1H NMR and UV-vis spectroscopies, low reactivity of [(NHC)AuH] toward HDF was found. In contrast, in the presence of p-N,N-dimethylaminopyridine (DMAP), the HDF of perfluoroarenes with silanes can be efficiently catalyzed by [(NHC)AuH], resulting in mainly the para-hydrodefluorinated products with up to 90% yield and 9 turnovers. The yield of the reaction increases with the more electron-withdrawing groups and degree of fluorination on the arenes, and the HDF reaction also tolerates different function groups (such as formyl, alkynyl, ketone, ester, and carboxylate groups), without reduction or hydrogenation of these function groups. To reveal the role of DMAP in the reactions, the possible π–π interaction between DMAP and perfluoroarenes was suggested by UV-vis spectral titrations, 1H NMR spectroscopic studies, and DFT calculations. Moreover, 1H and 19F-NMR studies show that this π–π interaction promotes hydrogen transfer from [(NHC)AuH] to pyridyl N atom, resulting in C-F bond cleavage. The interpretation of π–π interaction assisted C-F activation is supported by the reduced activation barriers in the presence of DMAP (31.6 kcal/mol) than that in the absence of DMAP (40.8 kcal/mol) for this reaction. An analysis of the charge distribution and transition state geometries indicate that this HDF process is controlled by the π–π interaction between DMAP and perfluoroarenes, accompanied with the changes of partial atomic charges. [ABSTRACT FROM AUTHOR]

Spectroscopic investigations have been performed for the oxidized forms of two quadruple-decker phthalocyanine complexes in order to clarify the electronic structures of multiply stacked π-systems. Up to three-electron-oxidized species were isolated by using phenoxathiin hexachloroantimonate as the oxidant. As the oxidations proceed, the Q-bands in the visible region shift bathochromically along with the clear isosbestic points. The one- and three-electron-oxidized species exhibited typical π-radical signals in the ESR spectra, while the neutral and two-electron oxidized species gave no indication of the presence of π-radicals. The electronic transitions observed for the oxidized species reach even into the so-called fingerprint region in IR spectroscopy (1000 cm-1). With the aid of theoretical calculations, these bands can be assigned to the π-π* transitions. Our results provide new insights into π-electronic systems having exceptionally small MO energy gaps. [ABSTRACT FROM AUTHOR]

Dihydrogenated boron clusters, H2Bn- (n = 7-12), were produced and characterized using photoelectron spectroscopy and computational chemistry to have ladderlike structures terminated by a hydrogen atom on each end. The two rows of boron atoms in the dihydrides are bonded by delocalized three-, four-, or five-center σ and π bonds. The π bonding patterns in these boron nanoladders bear similarities to those in conjugated alkenes: H2B7-, H2B8, and H2B9-, each with two π bonds, are similar to butadiene, while H2B102-, H2B11-, and H2B12, each with three π bonds, are analogous to 1,3,5-hexatriene. The boron cluster dihydrides can thus be considered as polyene analogues, or "polyboroenes". Long polyboroenes with conjugated π bonds (analogous to polyacetylenes), which may form a new class of molecular wires, should exist. [ABSTRACT FROM AUTHOR]

Two donor-acceptor copolymers, P1 and P2, containing the novel donor component benzo[2,1-b:3,4-b′:5,6-c″]trithiophene were synthesized. Both polymers show small π-stacking distances (0.35 nm for P1 and 0.37 nm for P2) due to the use of the disklike-shaped donor unit. However, they exhibit remarkable differences in supramolecular organization, film microstructure, and transistor performance. Indeed, P1 reveals a distinct supramolecular organization in the bulk in comparison to conventional conjugated polymers, including P2. Interestingly, no charge carrier transport was observed for P1 in field-effect transistors, while P2 exhibited a hole mobility of up to 0.04 cm2 V-1 s-1. This variation in device behavior is attributed to the evidently different degree of curvature in the polymer backbone induced by the introduction of two additional thiophene units in P2. [ABSTRACT FROM AUTHOR]

We report on the formation of single poly(N-vinylcarbazole) (PVCz) chains in one-dimensional channels of [La(1,3,5-benzenetrisbenzoate)]n, where the side carbazolyl groups of the confined PVCz are effectively p-stacked. This ideal conformation of PVCz chains in the coordination nanochannels contributed to a drastic increase in hole mobility, which was 5 orders of magnitude higher than that in the bulk state. It is also noteworthy that PVCz isolated from the nanchannels still had a high hole mobility. [ABSTRACT FROM AUTHOR]

Perylene 3,4:9,10-tetracarboxylic acid bisimide (PBI) was functionalized with ditopic cyanuric acid to organize it into complex columnar architectures through the formation of hydrogen-bonded supermacrocycles (rosette) by complexing with ditopic melamines possessing solubilizing alkoxyphenyl substituents. The aggregation study in solution using UV-vis and NMR spectroscopies showed the formation of extended aggregates through hydrogen-bonding and π-π stacking interactions. The cylindrical fibrillar nanostructures were visualized by microscopic techniques (AFM, TEM), and the formation of lyotropic mesophase was confirmed by polarized optical microscopy and SEM. X-ray diffraction study revealed that a well-defined hexagonal columnar (Colh) structure was formed by solution-casting of fibrillar assemblies. All of these results are consistent with the formation of hydrogen-bonded PBI rosettes that spontaneously organize into the Colh structure. Upon heating the Colh structure in the bulk state, a structural transition to a highly ordered lamellar (Lam) structure was observed by variable-temperature X-ray diffraction, differential scanning calorimetry, and AFM studies. IR study showed that the rearrangement of the hydrogen-bonding motifs occurs during the structural transition. These results suggest that such a striking structural transition is aided by the reorganization in the lowest level of self-organization, i.e., the rearrangement of hydrogen-bonded motifs from rosette to linear tape. A remarkable increase in the transient photoconductivity was observed by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements upon converting the Colh structure to the Lam structure. Transient absorption spectroscopy revealed that electron transfer from electron-donating alkoxyphenyl groups of melamine components to electron-deficient PBI moieties takes place, resulting in a higher probability of charge carrier generation in the Lam structure compared to the Colh structure. [ABSTRACT FROM AUTHOR]

A supramolecular polymer blend, formed via π-π interactions between a π-electron rich pyrenyl end-capped oligomer and a chain-folding oligomer containing pairs of π-electron poor naphthalene-diimide (NDI) units, has been reinforced with cellulose nanocrystals (CNCs) to afford a healable nanocomposite material. Nanocomposites with varying weight percentage of CNCs (from 1.25 to 20.0 wt %) within the healable supramolecular polymeric matrix have been prepared via solvent casting followed by compression molding, and their mechanical properties and healing behavior have been evaluated. It is found that homogeneously dispersed films can be formed with CNCs at less than 10 wt %. Above 10 wt % CNC heterogeneous nanocomposites were obtained. All the nanocomposites formed could be rehealed upon exposure to elevated temperatures although, for the homogeneous films, it was found that the healing rate was reduced with increasing CNC content. The best combination of healing efficiency and mechanical properties was obtained with the 7.5 wt % CNC nanocomposite which exhibited a tensile modulus enhanced by as much as a factor of 20 over the matrix material alone and could be fully rehealed at 85 °C within 30 min. Thus it is demonstrated that supramolecular nanocomposites can afford greatly enhanced mechanical properties relative to the unreinforced polymer, while still allowing efficient thermal healing. [ABSTRACT FROM AUTHOR]

Two series of oligorotaxanes R and R′ that contain -CH2NH2+CH2- recognition sites in their dumbbell components have been synthesized employing template-directed protocols. [24]Crown-8 rings self-assemble by a clipping strategy around each and every recognition site using equimolar amounts of 2,6-pyridinedicarboxaldehyde and tetraethyleneglycol bis(2-aminophenyl) ether to efficiently provide up to a [20]rotaxane. In the R series, the -NH2+- recognition sites are separated by trismethylene bridges, whereas in the R′ series the spacers are p-phenylene linkers. The underpinning idea here is that in the former series, the recognition sites are strategically positioned 3.5 Å apart from one another so as to facilitate efficient [π⋯π] stacking between the aromatic residues in contiguous rings in the rotaxanes and consequently, a discrete rigid and rod-like conformation is realized; these noncovalent interactions are absent in the latter series rendering them conformationally flexible/nondiscrete. Although in the R′ series, the [3]-, [4]-, [8]-, and [12]rotaxanes were isolated after reaction times of <5-30 min in yields of 72-85%, in the R series, the [3]-, [4]-, [5]-, [8]-, [12]-, [16]-, and [20]rotaxanes were isolated in <5 min to 14 h in 88-98% yields. It follows that while in the R′ series the higher order oligorotaxanes are formed in lower yields more rapidly, in the R series, the higher order oligorotaxanes are formed in higher yields more slowly. In the R series, the high percentage yields are sustained throughout, despite the fact that up to 39 components are participating in the template-directed self-assembly process. Simple arithmetic reveals that the conversion efficiency for each imine bond formation peaks at 99.9% in the R series and 99.3% in the R′ series. This maintenance of reaction efficiency in the R series can be ascribed to positive cooperativity, that is, when one ring is formed it aids and abets the formation of subsequent rings presumably because of stabilizing extended [π⋯π] stacking interactions between the arene units. Experiments have been performed wherein the dumbbell is starved of the macrocyclic components, and up to five times more of the fully saturated rotaxane is formed than is predicted based on a purely statistical outcome, providing a clear indication that positive cooperativity is operative. Moreover, it would appear that as the R series is traversed from the [3]- to the [4]- to the [5]rotaxane, the cooperativity becomes increasingly positive. This kind of cooperative behavior is not observed for the analogous oligorotaxanes in the R′ series. The conventional bevy of analytical techniques (e.g., HR-MS (ESI) and both 1H and 13C NMR spectroscopy) help establish the fact that all the oligorotaxanes are pure and monodisperse. Evidence of efficient [π⋯π] stacking between contiguous arene units in the rings in the R series is revealed by 1H NMR spectroscopy. Ion-mobility mass spectrometry performed on the R and R′ series yielded the collisional cross sections (CCSs), confirming the rigidity of the R oligorotaxanes and the flexibility of the R′ ones. The extended [π⋯π] stacking interactions are found to be present in the solid-state structures of the [3]- and [4]rotaxanes in the R series and also on the basis of molecular mechanics calculations performed on the entire series of oligomers. The collective data presented herein supports our original design in that the extended [π⋯π] stacking between contiguous arene units in the rings of the R series of oligorotaxanes facilitate an essentially rigid rod-like conformation with evidence that positive cooperativity improves the efficiency of their formation. This situation stands in sharp contrast to the conformationally flexible R′ series where the oligorotaxanes form with no cooperativity. [ABSTRACT FROM AUTHOR]

Although weak interactions play subtle but important roles in dictating protein structures, their experimental detection is nontrivial. From NOE experiments we provide direct evidence for the presence of CH···π interaction, operational between the Cα-H of the first Pro and the aromatic (Aro) side chain of Xaa, in a peptide series with the general sequence Ac-Pro-Pro-Xaa-NH2. Indirect evidence of CH···π interaction is provided from ring current-induced upfield displacement of Pro(1) Cα-H chemical shifts and restriction of side-chain (χ1) rotation of Xaa. A consequence of this interaction is the enhanced stability of the Pro-cisPro conformer in Ac-Pro-Pro-Xaa-NH2 when Xaa is aromatic. The free energies associated with trans to cis transformation of the Pro-Pro moiety are 0.35, 0.59, 0.64, and 0.82 kcal/mol when Xaa is Tyr, Trp, Phe, and His (pH of 8.4), respectively. In comparison, the corresponding free energy is ~1.55 kcal/mol when Xaa is nonaromatic. The observed population of Pro-cisPro-His and the pH-induced perturbation of electron density of the His side chain were correlated, providing further evidence for a direct role of CH· · ·π interaction in modulating the stability of Pro-cisPro population in Ac-Pro-Pro-Aro-NH2. Our study establishes Pro-Pro-Aro to be a new sequence motif that can stabilize Pro-cisPro peptide bonds. This study not only identifies a new structurally biased sequence motif but also directly demonstrates the role played by CH· · ·π interactions in subtly altering conformational preferences of three-residue peptide sequences with implications on the role played by cis-peptide bonds in unfolded proteins. [ABSTRACT FROM AUTHOR]