Bond Polarizability Research Articles

A Borane Lewis Acid in the Secondary Coordination Sphere of a Ni(II) Imido Imparts Distinct C-H Activation Selectivity.

Two borane-functionalized bidentate phosphine ligands that vary in tether length have been prepared to examine cooperative metal-substrate interactions. Ni(0) complexes react with aryl azides at low temperatures to form structurally unusual κ2-(N,N)-N3Ar adducts. Warming these adducts affords products of N2 extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN3) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp2 C-H bonds of arenes, even in the presence of benzylic C-H sites. Combined experimental and computational mechanistic studies demonstrate that the unique reactivity is a consequence of Lewis-acid-induced polarization of the Ni-NR bond, potentially providing a synthetic strategy for chemoselective reaction engineering.

Electronegativity at the Shock Front

AbstractIn this work, a scale for pressure‐adapted atomic electronegativity is used to make general predictions of bond polarity in H‐, C‐, N‐ and O‐based compounds experiencing shock conditions. The qualitative picture that emerges is one of increasing polarity of several bonds common in energetic materials. The general predictions made are compared to, and found to support, claims of ionic decomposition routes in compressed nitromethane and nitrate esters at high pressure. Changing electronegativity is also suggested as a factor driving the ionic disproportionation of various molecular phases with compression. Calculations using the eXtreme‐Pressure Polarizable Continuum Model (XP‐PCM) predict increasing energy differences between ground and excited states in non‐bonded H, C, N, and O atoms as a function of pressure. This data enables for a discussion on the reliability of electronegativity‐based rationales at more extreme thermodynamic conditions.

Cobalt-Carbon Bonding in a Salen-Supported Cobalt(IV) Alkyl Complex Postulated in Oxidative MHAT Catalysis.

The catalytic hydrofunctionalization of alkenes through radical-polar crossover metal hydrogen atom transfer (MHAT) offers a mild pathway for the introduction of functional groups in sterically congested environments. For M = Co, this reaction is often proposed to proceed through secondary alkylcobalt(IV) intermediates, which have not been characterized unambiguously. Here, we characterize a metastable (salen)Co(isopropyl) cation, which is capable of forming C-O bonds with alcohols as proposed in the catalytic reaction. Electron nuclear double resonance (ENDOR) spectroscopy of this formally cobalt(IV) species establishes the presence of the cobalt-carbon bond, and accompanying DFT calculations indicate that the unpaired electron is localized on the cobalt center. Both experimental and computational studies show that the cobalt(IV)-carbon bond is stronger than the analogous bond in its cobalt(III) analogue, which is opposite of the usual oxidation state trend of bond energies. This phenomenon is attributable to an inverted ligand field that gives the bond Coδ--Cδ+ character and explains its electrophilic reactivity at the alkyl group. The inverted Co-C bond polarity also stabilizes the formally cobalt(IV) alkyl complex so that it is accessible at unusually low potentials. Even another cobalt(III) complex, [(salen)CoIII]+, is capable of oxidizing (salen)CoIII(iPr) to the formally cobalt(IV) state. These results give insight into the electronic structure, energetics, and reactivity of a key reactive intermediate in oxidative MHAT catalysis.

Single Atom Catalysts for Selective Methane Oxidation to Oxygenates

Direct conversion of methane (CH4) to C1-2 liquid oxygenates is a captivating approach to lock carbons in transportable value-added chemicals, while reducing global warming. Existing approaches utilizing the transformation of CH4 to liquid fuel via tandemized steam methane reforming and the Fischer-Tropsch synthesis are energy and capital intensive. Chemocatalytic partial oxidation of methane remains challenging due to the negligible electron affinity, poor C-H bond polarizability, and high activation energy barrier. Transition-metal and stoichiometric catalysts utilizing harsh oxidants and reaction conditions perform poorly with randomized product distribution. Paradoxically, the catalysts which are active enough to break C-H also promote overoxidation, resulting in CO2 generation and reduced carbon balance. Developing catalysts which can break C-H bonds of methane to selectively make useful chemicals at mild conditions is vital to commercialization. Single atom catalysts (SACs) with specifically coordinated metal centers on active support have displayed intrigued reactivity and selectivity for methane oxidation. SACs can significantly reduce the activation energy due to induced electrostatic polarization of the C-H bond to facilitate the accelerated reaction rate at the low reaction temperature. The distinct metal-support interaction can stabilize the intermediate and prevent the overoxidation of the reaction products. The present review accounts for recent progress in the field of SACs for the selective oxidation of CH4 to C1-2 oxygenates. The chemical nature of catalytic sites, effects of metal-support interaction, and stabilization of intermediate species on catalysts to minimize overoxidation are thoroughly discussed with a forward-looking perspective to improve the catalytic performance.

QSAR, ADMET In Silico Pharmacokinetics, Molecular Docking and Molecular Dynamics Studies of Novel Bicyclo (Aryl Methyl) Benzamides as Potent GlyT1 Inhibitors for the Treatment of Schizophrenia.

Forty-four bicyclo ((aryl) methyl) benzamides, acting as glycine transporter type 1 (GlyT1) inhibitors, are developed using molecular modeling techniques. QSAR models generated by multiple linear and non-linear regressions affirm that the biological inhibitory activity against the schizophrenia disease is strongly and significantly correlated with physicochemical, geometrical and topological descriptors, in particular: Hydrogen bond donor, polarizability, surface tension, stretch and torsion energies and topological diameter. According to in silico ADMET properties, the most active ligands (L6, L9, L30, L31 and L37) are the molecules having the highest probability of penetrating the central nervous system (CNS), but the molecule 32 has the highest probability of being absorbed by the gastrointestinal tract. Molecular docking results indicate that Tyr124, Phe43, Phe325, Asp46, Phe319 and Val120 amino acids are the active sites of the dopamine transporter (DAT) membrane protein, in which the most active ligands can inhibit the glycine transporter type 1 (GlyT1). The results of molecular dynamics (MD) simulation revealed that all five inhibitors remained stable in the active sites of the DAT protein during 100 ns, demonstrating their promising role as candidate drugs for the treatment of schizophrenia.

The low dielectric constant hyperbranched polycarbosilane derived resins with spacing groups

AbstractThe route via the cross‐linking of hyperbranched prepolymers has potential advantage to construct low dielectric constant (low‐k) resins owing to the enhanced molecular free volume in hyperbranched structure. However, it is still a challenge to prepare hyperbranched resins with good film‐forming property and low‐k. In this paper, two hyperbranched polycarbosilanes with reactive benzocyclobutene groups were synthesized via hydrosilylation reaction to avoid the generation of SiO bonds for enabling the low polarity of chemical bonds. The spacing groups including phenyl or ethylene were incorporated into the hyperbranched structures, and the effect of the spacing groups on the physicochemical properties of hyperbranched polycarbosilane derived resins were investigated. The phenyl groups were found to effectively decrease the dielectric constant (k), while endowing the resins with good film forming ability and thermostability. The UV/thermally cured phenyl group resin owing to dual crosslinked structure, the patterns would not be deformed significantly during thermally cured process. Both hyperbranched polycarbosilane derived resins could be potential photoresists.

Radical‐Mediated Selective Functionalization of Unactivated Primary C—H Bonds

AbstractRadical‐mediated sp3 C—H functionalization has found broad applications in synthesis and late‐stage derivatization of complex molecules. The site‐selectivity of a given reaction is mainly dictated by the bond strength, polarity and steric accessibility of diverse C—H bonds as well as the characters of hydrogen atom acceptors. Unactivated sp3 C—H bonds usually follow the relative reactivity sequence of tertiary > secondary > primary, which is even recognized as the innate preferences of certain substrates. The development of innovative methods capable of overriding the “inherent” site‐selectivity is exceedingly attractive. Although it has been demonstrated that secondary C—H bonds could be selectively converted in the presence of weaker tertiary C—H bonds by tuning the steric and electronic properties of hydrogen atom acceptors, the selective activation of primary C—H bonds remains a daunting challenge. Herein, we highlight the recently emerging attempts towards this goal by interrogating the steric effect of hydrogen atom acceptors.

Nonlinear optical response of 9,10-bis(phenylethynyl)anthracene mediated by electron donating and electron withdrawing substituents: A density functional theory approach

Herein, we report linear polarizibilty (α) and hyperpolarizibility (β) of anthracyl-based organic materials containing electron acceptor (NO2) and electron donor (NH2) moieties. Six compounds (a–f) were designed by structural modification of a reported chromophore 9,10-bis(phenylethynyl)anthracene (R). Ground state geometries, HOMO-LUMO energies polarizability, first hyperpolarizability and natural bond orbital (NBO) analyses of all compounds (R and a–f) were studied through density functional theory. FMO's results showed that the energy gaps of designed molecules a–f (1.791–2.215eV) were much lower than parent molecule R (2.606 eV). UV Visible analysis of designed compounds showed a red shift in the absorption spectrum as compared with reference molecule (R). The highest λmax was observed for compound c (816.7 nm). NLO analysis and hyper polarizibility values computed for compounds a-f were much higher than reference compound R (0.023 a.u.). The observed red shifts in the absorption spectra indicated the decrease in excitation energy, an important factor in enhancing NLO response. Overall, compound c showed larger αtotal and βtotal values of 575.9 a.u. (LC-BLYP/6–31++G(d,p), and 58156.8 a.u. (LC-BLYP/6–31++G(d,p), respectively. The findings of our study provide a theoretical platform for useful conversion of centrosymmetric compounds to non-centrosymmetric materials for enhancing the NLO behavior through structural modifications for modern technological applications.

Ultrafast Vibrational Response of Activated C–D Bonds in a Chloroform–Platinum(II) Complex

The vibrational response of the activated C–D bond in the chloroform complex [Pt(C6H5)2(btz-N,N′)·CDCl3, where btz = 2,2′-bi-5,6-dihydro-4H-1,3-thiazine] is studied by linear and nonlinear two-dimensional infrared (2D-IR) spectroscopy. The change of the C–D stretching vibration of metal-coordinated CDCl3 relative to the free solvent molecule serves as a measure of the non-classical Pt···D–C interaction strength. The stretching absorption band of the activated C–D bond displays a red shift of 119 cm–1 relative to uncoordinated CDCl3, a strong broadening, and an 8-fold enhancement of spectrally integrated absorption. The infrared (IR) absorption and 2D-IR line shapes are governed by spectral diffusion on 200 fs and 2 ps time scales, induced by the fluctuating solvent CDCl3. The enhanced vibrational absorption and coupling to solvent forces are assigned to the enhanced electric polarizability of the activated C–D bond. Density functional theory calculations show a significant increase of C–D bond polarizability of CDCl3 upon coordination to the 16 valence electron Pt(II) complex.

Solute–Solvent Interactions in Hydrophilic Interaction Liquid Chromatography: Characterization of the Retention in a Silica Column by the Abraham Linear Free Energy Relationship Model

The Abraham linear free energy relationship model has been used to characterize a hydrophilic interaction liquid chromatography (HILIC) silica column with acetonitrile/water and methanol/water mobile phases. Analysis by the model for acetonitrile/water mobile phases points to solute volume and hydrogen bond basicity as the main properties affecting retention, whereas solute hydrogen bond acidity, dipolarity and polarizability practically do not affect it. Formation of a cavity is easier in acetonitrile-rich mobile phases than in the aqueous stationary phase, and hence increase of solute volume decreases retention. Conversely, hydrogen bond acidity is stronger in the aqueous stationary phase than in the acetonitrile-rich mobile phase and thus an increase of solute hydrogen bond basicity increases retention. Results are similar for methanol/water mobile phases with the difference that solute hydrogen bond acidity is significant too. Increase in hydrogen bond acidity of the solute decreases retention showing that methanol mobile phases must be better hydrogen bond acceptors than acetonitrile ones, and even than water-rich stationary phases. The results are like the ones obtained in zwitterionic HILIC columns bonded to silica or polymer supports for acetonitrile/water mobile phases, but different for solute hydrogen bond acidity for a polymer bonded zwitterionic column with methanol/water mobile phases, indicating that bonding support plays an important role in HILIC retention. Comparison to RPLC characterized systems confirms the complementarity of HILIC systems to RPLC ones because the main properties affecting retention are the same but with reversed coefficients. The least retained solutes in RPLC are the most retained in HILIC.

Scaling and Confinement in Ultrathin Chalcogenide Films as Exemplified by GeTe

Chalcogenides such as GeTe, PbTe, Sb2 Te3 , and Bi2 Se3 are characterized by an unconventional combination of properties enabling a plethora of applications ranging from thermo-electrics to phase change materials, topological insulators, and photonic switches. Chalcogenides possess pronounced optical absorption, relatively low effective masses, reasonably high electron mobilities, soft bonds, large bond polarizabilities, and low thermal conductivities. These remarkable characteristics are linked to an unconventional bonding mechanism characterized by a competition between electron delocalization and electron localization. Confinement, that is, the reduction of the sample dimension as realized in thin films should alter this competition and modify chemical bonds and the resulting properties. Here, pronounced changes of optical and vibrational properties are demonstrated for crystalline films of GeTe, while amorphous films of GeTe show no similar thickness dependence. For crystalline films, this thickness dependence persists up to remarkably large thicknesses above 15nm. X-ray diffraction and accompanying simulations employing density functional theory relate these changes to thickness dependent structural (Peierls) distortions, due to an increased electron localization between adjacent atoms upon reducing the film thickness. A thickness dependence and hence potential to modify film properties for all chalcogenide films with a similar bonding mechanism is expected.

Selectively Altering the Reactivity of Transient Organic Radical Ions via Their Solvation Environment.

Photoexcitation of the charge transfer band of electron donor-acceptor complexes composed of toluene and 1,2,4,5-tetracyanobenzene yields organic radical ion pairs whose ultrafast reactive dynamics are determined by equilibrium solvent properties. A comparative study of ultrafast reaction rates in a series of alkane alcohols identified their dependence on the local polarizability and hydrogen bond donating/accepting character of the solvent. Because of the rapid and efficient equilibration of these radical ion pairs into solvent-separated species, simple modifications to bulk conditions can be used as a means to selectively alter their decay rates. Selectively altering distinct stages in this photochemical cycle via cosolutes or additives is a valuable step toward understanding and controlling the reactivity of organic radical ions in complex environments.

Understanding Hydrogenation Chemistry at MgB2 Reactive Edges from Ab Initio Molecular Dynamics.

Solid-state hydrogen storage materials often operate via transient, multistep chemical reactions at complex interfaces that are difficult to capture. Here, we use direct ab initio molecular dynamics simulations at accelerated temperatures and hydrogen pressures to probe the hydrogenation chemistry of the candidate material MgB2 without a priori assumption of reaction pathways. Focusing on highly reactive (101̅0) edge planes where initial hydrogen attack is likely to occur, we track mechanistic steps toward the formation of hydrogen-saturated BH4- units and key chemical intermediates, involving H2 dissociation, generation of functionalities and molecular complexes containing BH2 and BH3 motifs, and B-B bond breaking. The genesis of higher-order boron clustering is also observed. Different charge states and chemical environments at the B-rich and Mg-rich edge planes are found to produce different chemical pathways and preferred speciation, with implications for overall hydrogenation kinetics. The reaction processes rely on B-H bond polarization and fluctuations between ionic and covalent character, which are critically enabled by the presence of Mg2+ cations in the nearby interphase region. Our results provide guidance for devising kinetic improvement strategies for MgB2-based hydrogen storage materials, while also providing a template for exploring chemical pathways in other solid-state energy storage reactions.

Photoelectron Transfer Mediated by the Interfacial Electron Effects for Boosting Visible-Light-Driven CO2 Reduction

Imine-linked covalent organic frameworks (COFs) are popular candidates for photocatalytic CO2 reduction, but high polarization of the imine bond is less efficient for π-electron delocalization between the linked building units, leading to low intramolecular electron transfer and poor photocatalytic efficiency. Herein, we present a structural and electronic engineering strategy through integrating the imine-linked COF consisting of Zn–porphyrin and Co–bipyridyl units with cadmium sulfide (CdS) nanowires to form a CdS@COF core–shell structure. The experimental and theoretical results have validated that CdS serves as the electron transfer channel through the interfacial electron effects, which induces photoelectron transfer from Zn–porphyrin to CdS and subsequent injection into Co–bipyridyl units for CO2 reduction. The as-prepared CdS@COF generates 4057 μmol g–1 CO in 8 h under visible-light irradiation, which is considerably higher than those of its neat CdS and imine-linked COF counterparts. This work provides protocols to tackle intramolecular charge transfer across polar linkages between photosensitizers and active sites for solar-to-chemical energy conversion.

Electronic structure and bonding in N-hetero cyclic carbene stabilized naked group 16 atoms – In-plane hyperconjugation and bond polarity

Bonding analysis of the exocyclic NHCCE (E = O, S and Se) bond in lesser explored five-membered N-heterocyclic carbene (NHC) stabilized naked Group 16 atoms, known as imidazolin-2-chalcogenones, using different theoretical methods reveal that imidazolin-2-one and imidazolin-2-thione can be best represented as 1,2-dipolar species with NHCC+↔O¯/S¯ electron sharing σ- and NHCC+←O¯/S¯ donor–acceptor π⊥ and π‖ hyperconjugative interactions, whereas the bonding situation in imidazolin-2-selone can be depicted as NHCC → Se σ- and NHCC ← Se π⊥-type donor–acceptor and NHCC ← Se π‖ hyperconjugative interactions between Se and carbene moiety. The in-plane hyperconjugation is stronger for polar CE bonds and lighter Group 16 elements.

Permeation of vegetable oils and slippery properties of extrusion coated paperboard

AbstractDue to the ability of oils to penetrate into the structure of polymers, it is possible to create slippery surfaces with low water droplet sliding angles based on polymer coatings. Polylactic acid, polybutylene succinate and low‐density polyethylene extrusion coatings were used as such surfaces in this study. The process of penetration of vegetable oils into the polymers was studied with the subsequent investigation of slippery behaviour of such system. Cottonseed oil and castor oil showed promising properties as lubricants. For oil infused polymer surfaces the water droplet sliding angles were significantly lower than for the untreated extrusion coating. The penetration of the permeant into the upper layers of the coating was confirmed by alteration of the static water contact angle, sliding angle and also by the cross section images. LDPE extrusion coating infused with castor oil showed the lowest water sliding angle − 8°. Biopolymers, being a main focus in this study, exhibited also promising results, PLA reached sliding angle of 14° with both olive and castor oil. Dependence of polymer permeability on various factors is rather complex and the wide range of oils used in this work helps to interpret the barrier performance of the studied materials. Degree of bond saturation, viscosity and polarity were found to influence mostly on the oil permeation through the studied polymers.

Selective sensing of heavy metal ions via fluorescence quenching of femtosecond-laser-synthesized 2D nanoparticles

A femtosecond laser is used to break and exfoliate two dimensional (2D) flakes of nitrogen-doped graphene oxide (NGO) or molybdenum disulfide (MoS2) dispersed in acetonitrile or a 1:1 ethanol-water mixture and convert them into nanoparticles (NPs) in 70–100 min. The chemically active NPs bond with atoms from the laser-dissociated acetonitrile or ethanol and water molecules to form 2D NPs functionalized with varying quantities of nitrogen, oxygen, hydrogen, and carbon groups. Sensitive and selective detection of heavy metal ions (HMI) Hg2+, As3+, Pb2+, and Cd2+ is demonstrated with the solvent-functionalized NGO and MoS2 NPs. Binding between the HMI and functional groups leads to quenching of the NPs’ photoluminescence. Limits of detection of 3 nm (Hg2+), 3.8 nm (As3+), 1.8 nm (Pb2+), and 13 nm (Cd2+) are demonstrated with the solvent-functionalized NGO in acetonitrile, NGO in acetonitrile, MoS2 in acetonitrile, and NGO in ethanol-water, respectively. The selectivity is attributed to varying bond polarities associated with the different combinations of NPs and functional groups, which electrostatically attract HMI with different affinities. The functional groups responsible for the selective sensing in the different NP solutions are identified using Fourier transform infrared spectroscopy. This rapid, one-pot, room-temperature process can be used to tailor 2D materials for targeted chemical sensing.

Reversible Capture and Release of a Ligand Mediated by a Long-Range Relayed Polarity Switch in a Urea Oligomer.

Ethylene-bridged oligoureas characterized by a continuous, switchable chain of hydrogen bonds and carrying a binding site (an N,N′-disubstituted urea) for a hydrogen-bond-accepting ligand (a phosphine oxide) were synthesized. These oligomers show stronger ligand binding when the binding site is located at the hydrogen-bond-donating terminus than when the same binding site is at the hydrogen-bond-accepting terminus. An acidic group at the terminus remote from the binding site allows hydrogen bond polarity, and hence ligand binding ability, to be controlled remotely by a deprotonation/reprotonation cycle. Addition of base induces a remote conformational change that is relayed through up to five urea linkages, reducing the ability of the binding site to retain an intermolecular association to its ligand, which is consequently released into solution. Reprotonation returns the polarity of the oligomer to its original directionality, restoring the function of the remote binding site, which consequently recaptures the ligand. This is the first example of a synthetic molecular structure that relays intermolecular binding information, and these “dynamic foldamer” structures are prototypes of components for chemical systems capable of controlling chemical function from a distance.

Site-Selective C(sp3)–H Functionalizations Mediated by Hydrogen Atom Transfer Reactions via α-Amino/α-Amido Radicals

AbstractAmines and amides, as N-containing compounds, are ubiquitous in pharmaceutically-active scaffolds, natural products, agrochemicals, and peptides. Amides in nature bear a key responsibility for imparting three-dimensional structure, such as in proteins. Structural modifications to amines and amides, especially at their positions α to N, bring about profound changes in biological activity oftentimes leading to more desirable pharmacological profiles of small drug molecules. A number of recent developments in synthetic methodology for the functionalizations of amines and amides omit the need of their directing groups or pre-functionalizations, achieving direct activation of the otherwise relatively benign C(sp3)–H bonds α to N. Among these, hydrogen atom transfer (HAT) has proven a very powerful platform for the selective activation of amines and amides to their α-amino and α-amido radicals, which can then be employed to furnish C–C and C–X (X = heteroatom) bonds. The abilities to both form these radicals and control their reactivity in a site-selective manner is of utmost importance for such chemistries to witness applications in late-stage functionalization. Therefore, this review captures contemporary HAT strategies to realize chemo- and regioselective amine and amide α-C(sp3)–H functionalization, based on bond strengths, bond polarities, reversible HAT equilibria, traceless electrostatic-directing auxiliaries, and steric effects of in situ-generated HAT agents.1 Introduction2 Functionalizations of Amines3 Functionalizations of Carbamates4 Functionalizations of Amides5 Conclusion

An Access to Benzo[a]fluorenes, Benzo[b]fluorenes, and Indenes Triggered by Simple Lewis Acid

This report illustrates BF3·OEt2 promoted intramolecular cascade cycloaromatization of 1,7-ynones toward synthesizing structurally diverse benzofluorene scaffolds. Remarkably, the present protocol promotes the formation of two consecutive C-C bonds intramolecularly and undergoes aromatization under mild reaction conditions to afford the tetracyclic benzo[a]fluorene frameworks. Besides, the formation of indenes was observed when 1-bromo-2-iodoarenes are relatively more electron-rich when compared with the one originating from the terminal arylacetylenes, under controlled conditions, wherein triple bond polarity has been just reversed due to the change of electronic effects exerted by the strong +M group of 1-bromo-2-iodoarenes, which is in conjugation to the connected triple bond. The same concept to generate indenes has also been extended by using aliphatic alkyne tethered ynones. Further, it was noticed that 1,7-ynones bearing the more electron-rich 1-bromo-2-iodoarenes than the arene ring arriving from the terminal arylacetylenes lead to benzo[b]fluorenes, under thermodynamic conditions, instead of delivering the benzo[a]fluorenes. In addition, this method features metal-free conditions, easily accessible starting materials, operational simplicity, gram-scale synthesis, and a wide range of substrate scopes.