Bond Shift Research Articles

Regularities and Anomalies in Neon Matrix Shifts of Hydrogen-Bonded O-H Stretching Fundamentals.

O-H bond stretching vibrations in hydrogen-bonded complexes embedded into cryogenic neon matrices are subtly downshifted from cold gas phase reference wavenumbers. To the extent that this shift is systematic, it enables neon matrices as more universally applicable spectroscopic benchmark environments for quantum chemical predictions. Outliers are indicative of either an assignment problem in one of the two cryogenic experiments or they reveal interesting dynamics or structural effects on the complexes as a function of the environment. We compile 6 literature-known pairs of experimental data in jet and neon matrix expansions and realize a 6-fold expansion of that number through targeted matrix isolation and/or slit jet expansion spectroscopy presented in this work. In many cases, the neon matrix shift is less than the uncertainty of the currently best-performing blind quantum chemical predictions for the gas phase, but in specific cases, it may exceed the currently achievable theoretical accuracy. Some evidence for a positive correlation of the matrix shift with the hydrogen bond shift is found, similar to observations for helium nanodroplets. Outliers in particular for water acting as a donor are discussed, and in a few cases they call for a future reinvestigation. Substantial improvement in the correlation of the matrix shift with the hydrogen bond shift is achieved for ketone monohydrates by removing a vibrational resonance. New insights into nitrile hydration isomerism are obtained, and the linear OH stretching spectrum of the jet-cooled ammonia-water complex is presented for the first time. Vibrational spectroscopy in weakly perturbing solid rare gas quantum matrices for the benchmarking of gas phase theory and future explicit theoretical treatments of the quantum matrix environment to better understand the outliers are both encouraged.

Chromium Complexes with Benzanellated N-Heterocyclic Phosphenium Ligands-Synthesis, Reactivity and Application in Catalytic CO2 Reduction.

A chromium complex carrying two benzanellated N-heterocyclic phosphenium (bzNHP) ligands was prepared by a salt metathesis approach. Spectroscopic studies suggest that the anellation enhances the π-acceptor ability of the NHP-units, which is confirmed by the facile electrochemical reduction of the complex to a spectroscopically characterized radical anion. Co-photolysis with H2 allowed extensive conversion into a σ-H2-complex, which shows a diverse reactivity towards donors and isomerizes under H-H bond fission and shift of a hydride to a P-ligand. The product carrying phosphenium, phosphine and hydride ligands was also synthesized independently and reacts reversibly with CO and MeCN to yield bis-phosphine complexes under concomitant Cr-to-P-shift of a hydride. In contrast, CO2 was not only bound but reduced to give an isolable formato complex, which reacted with ammonia borane under partial recovery of the metal hydride and production of formate. Further elaboration of the reactions of the chromium complexes with CO2 and NH3BH3 allowed to demonstrate the feasibility of a Cr-catalyzed transfer hydrogenation of CO2 to methanol. The various complexes described were characterized spectroscopically and in several cases by XRD studies. Further insights in reactivity patterns were provided through (spectro)electrochemical studies and DFT calculations.

Syntheses and reactivities of strained fused-ring metallaaromatics containing planar eleven-carbon chains

Carbolong complexes are one of the primary types of metallaaromatics, and they include metallapentalynes and metallapentalenes. A series of 7C-10C and 12C-carbolong complexes with planar ligand skeletons respectively containing 7-10 and 12 carbon atoms in their backbones, have been previously reported. Herein, two classes of strained substances, metallabenzyne-fused metallapentalenes and metallabenzene-fused metallapentalynes, were prepared, both representing 11C-carbolong complexes with a planar carbon-chain ligand. Furthermore, the former type is also the carbolong derivatives containing a metallabenzyne skeleton, another primary metallaaromatic framework. Metallabenzyne-fused metallapentalenes show versatile reactivities, and the most interesting one is the metal carbyne bond shift from a 6-membered to a more strained 5-membered ring, affording the above-mentioned metallabenzene-fused metallapentalyne. This work makes carbolong chemistry more complete, and provides a method to achieve metallabenzynes, which is anticipated to concurrently advance the development of these two types of metallaaromatics.

Light-triggered transformation of stilbene supramolecular polymers: thermodynamic versus kinetic control.

Light irradiation of stilbene supramolecular polymers produces [2+2] cycloadducts in the kinetically trapped state, which convert to the thermodynamically favorable state upon thermal annealing due to the shift of hydrogen bonds from intra- to inter-complexation modes.

Impact of swift heavy ion irradiation on as-grown gallium nitride epilayers by MOCVD technique

The as-grown GaN by Metalorganic Chemical Vapor Deposition was utilized to study the irradiation effects on GaN epilayers. The stopping range of ions in the matter is studied to choose the energy range for irradiation. The grown epilayers were irradiated with light and heavy ions of O7+ and Ag12+ by varying ion fluence. The X-ray Diffraction confirmed the wurtzite crystal structure for pristine and all the irradiated GaN epilayers. Atomic force microscopy showed high surface roughness for heavy ion irradiation when compared with light ion. Micro-Raman analysis was carried out through the Lorentz line fitting and calculated the carrier concentration and phonon lifetime. In light and heavy ion irradiation, the carrier concentration enhances with defect formations. X-ray Photoelectron Spectroscopy analysis showed that light ion irradiation caused oxygen bonding, while heavy ion irradiation led to the Ga–Ga bond shift, revealing film degradation.

Installing Quaternary Germanium Centers in Sila-Diamondoid Cores via Skeletal Isomerization.

This manuscript describes skeletal isomerization strategies to install one to four quaternary germanium atoms in the sila-adamantane core, in a cluster analogy to precision germanium doping in silicon-germanium alloys. The first strategy embodies an inorganic variant of single-atom skeletal editing, where we use a sila-Wagner-Meerwein bond shift cascade to exchange a peripheral Ge atom with a core Si atom. We can install up to four Ge atoms at the quaternary diamondoid centers based on controlling the SixGey stoichiometry of our precursor. We find that bridgehead Ge centers can be selectively functionalized over bridgehead Si centers in SiGe adamantanes; we use this chemistry in conjunction with scanning tunneling microscopy break-junction (STM-BJ) measurements to show that Si8Ge2 adamantane wires give a 60% increase in single-molecule conductance compared with Si10 adamantanes. These studies describe the first quantum transport measurements in sila-diamondoid structures, and demonstrate how main-chain Ge doping can be used to increase electronic transmission in sila-diamondoid-based molecular wires.

A first-principles study of interstitial oxygen on the strength/ductility of α-Ti: Crystal structures, elastic properties and electronic structures

Interstitial oxygen (O) is one of the most critical impurity elements for titanium (Ti) and its alloys. This study adopts the first-principles method based on the density function theory (DFT) to calculate the lattice constants, elastic properties and electronic structures of the pure Ti and Ti-O(O) structures with different O contents. Results show that the c/a lattice parameter increases with the increase of O content and reaches the maximum value of 1.642 when O content is 20 at%. In addition, the bulk modulus (B), shear modulus (G) and G/B of the Ti-O(O) structures gradually increase with the increase of O content, and the corresponding ductility becomes worse. Furthermore, interstitial O atom makes a uniform transformation of the metal bond shift toward a more covalent direction, which strongly influences the ductility of Ti-O(O) structures. Furthermore, the interstitial O leads to the great difference in the chemical bond behavior of the crystals in different directions, which shows anisotropy on the micro level. The aim of this study is to reveal the effect of interstitial O on the mechanical properties of Ti alloys, and also to provide reference for reasonable control of O content of Ti alloys in industrial production.

Asymmetric [5+1] Annulation via C–H Activation/1,4-Rh Migration/Double Bond Shift Using a Transformable Pyridazine Directing Group

N-Heterocycle-assisted C-H activation/annulation reactions have provided new concepts for the construction and transformation of azacycles. In this work, we disclose a [5+1] annulation reaction using a novel transformable pyridazine directing group (DG). The DG-transformable reaction mode led to the construction of a new heterocyclic ring accompanied by transformation of the original pyridazine directing group via a C-H activation/1,4-Rh migration/double bond shift pathway, affording the skeleton of pyridazino[6,1-b]quinazolines with a good substrate scope under mild conditions. Diverse fused cyclic compounds can be achieved by derivatization of the product. The asymmetric synthesis of the skeleton was also realized to afford the enantiomeric products with good stereoselectivity.

The crystalline structure transition and hydrogen bonds shift determining enhanced enzymatic digestibility of cellulose treated by ultrasonication

Global energy issue raised the necessity to develop second-generation biofuels, and biorefinery of cellulosic biomass becomes a promising solution. Various pretreatments were used to overcome the cellulose nature of recalcitrance and improve the enzymatic digestibility, but the lack of mechanism understanding hindered the development of efficient and cost-effective technologies of cellulose utilization. Using structure-based analysis, we demonstrate that the improved hydrolysis efficiency caused by ultrasonication was ascribed to the changed cellulose properties rather than the increased dissolubility. Further, isothermal titration calorimetry (ITC) analysis suggested that enzymatic digestion of cellulose is an entropically favored reaction driven by hydrophobic forces other than an enthalpically favored reaction. The changes in cellulose properties and thermodynamic paramenters due to ultrasonication accounted for the improved accessibility. Ultrasonication-treated cellulose showed porous, rough and disordered morphology, accompanying with the loss of crystalline structure. Despite the unaffected unit cell structure, ultrasonication expanded the crystalline lattice by increasing grain sizes and average cross-sectional area, resulting in the transformation from cellulose I to cellulose II, with the decreased crystallinity, better hydrophilicity and increased enzymatic bioaccessibility. Furthermore, FTIR combined with two-dimensional correlation spectroscopy (2D-CoS) verified that the sequential shift of hydroxyl group and intramolecular/intermolecular hydrogen bonds, the functional groups governing cellulose crystal structure and stability, accounted for the ultrasonication-induced transition of cellulose crystalline structure. This study provides a comprehensive picture of cellulose structure and property response caused by mechanistic treatments and will open up avenues to develop novel pretreatments for efficient utilization.

PhysikerversusOrganikerViews of Reaction “Mechanism”: How Natural Resonance Theory Bridges the Gap

Traditional physical chemistry conceptions of reaction mechanism are formulated in terms of stationary points of an Arrhenius-style “energy profile” that differs sharply (in purpose and form) from the corresponding Robinson-style “arrow-pushing” mechanistic conceptions of organic chemistry. We show here how these diverse “mechanistic” conceptions can be reconciled in a unified computational protocol based on a natural resonance theoretic (NRT) description of successive bond shifts between reactant and product bonding patterns. For pedagogical purposes, we employ a model SN2 halide exchange reaction described at a routine level of density functional theory, but the outlined NRT protocol involves no intrinsic dependence on theory level, reaction order, or perceived “elementary” character of the reaction. The NRT-based characterization of electronic bond-shifts provides a rigorous criterion for judging the correctness of a proposed arrow-pushing mechanism, while also adding rich details of the multiple electronic “transitions” that may accompany a chemical transformation along the reaction pathway, even if the associated energy profile is barrierless or marked by a single maximal “transition state” feature.

Highly efficient Al-Ti gel as a coagulant for surface water treatment: Insights into the hydrolysate transformation and coagulation mechanism

In view of the insufficient coagulation efficiency of traditional inorganic coagulants, a series of Al-Ti gels with different Ti/triethanolamine (TEA), Ti/H2O, and Ti/Al molar ratios were prepared by sol-gel process in this study. Fourier transform infrared (FTIR) spectra of the Al-Ti gels preliminarily confirmed the interaction between Al and Ti by detecting the appearance of the Al-O-Ti bond. The peak shift of the chemical bonds in X-ray photoelectron spectra (XPS) and the transformation of the hydrolysate species in the Al-Ti gels were analyzed to further explore the interaction mechanism between Al and Ti. It was found that moderate TEA could inhibit the hydrolysis of Ti precursors by taking up the coordination sites of H2O to form a CO-Ti bond. Density functional theory (DFT) calculation results showed that Ti could be incorporated into the framework of aluminum hydrolysates to form an Al-O-Ti bond, and [Al2Ti2(OH)x(TEA)y(H2O)8-x-y]14-x was the most possible copolymerization hydrolysate. Based on the above research results, the most efficient Al-Ti gel was selected and applied to the actual lake water treatment. The highest UV254 removal efficiency with the addition of Al-Ti gel was > 60%, nearly 25% higher than that of Ti gel. The hydrolysates of Al-Ti gel, such as TiO(OH)2(am), Al(OH)3(am), and [Al2Ti2(OH)x(TEA)y(H2O)8-x-y]14-x, could remove organic matters through the incorporation of charge neutralization, adsorption, complexation, and sweeping effects. These results provide a new idea for studying the interaction mechanism between Al and Ti in composite coagulants, and have theoretical guiding significance to actual water treatment.

A Closer Look at the Isomerization of 5-Androstene-3,17-Dione to 4-Androstene-3,17-Dione in Ketosteroid Isomerase

We report a comprehensive investigation of the 5-androstene-3,17-dione to 4-androstene-3,17-dione isomerization in Ketosteroid Isomerase, gas phase and aqueous solution, applying as tools the Unified Reaction Valley Approach (URVA) and the Local Vibrational Mode Analysis. Conformational changes of the steroid rings are monitored via Cremer-Pople puckering coordinates. URVA identifies simultaneous breakage of the C[Formula: see text]–H bond and O–H bond formation with the catalytic acid, leading to an intermediate with the acid positioned over ring [Formula: see text] as the major chemical events of the first reaction step. Via a barrier-less shift, a second intermediate is formed with the acid being positioned over ring [Formula: see text]. Then, according to URVA, breakage of the intermediate O–H bond, the formation of the new C[Formula: see text]–H bond accompanied by a double bond shift in rings [Formula: see text] and [Formula: see text] forms the major chemical events of the second reaction step, which is facilitated by favorable ring puckering. Reactions in protein and gas phase have comparable activation enthalpies, whereas the barrier in aqueous solution is higher, confirming that the major task of the enzyme pocket is to shield the migrating hydrogen atom and the catalyzing acid from interactions with solvent molecules diluting the catalytic power. We do not find exceptional H-bonding with Asp99 and Tyr14, excluding their catalytic activity. There is no strong hydrogen bonding in the TS, which could account for lowering the activation barrier. Our study provides a clear picture of the isomerization process, which will also inspire similar investigations of other important enzymatic reactions.

Catalytic 1,3‐H Atom Shift of a Terminal Benzylic Alkyne by Iron and Alkali Metal Silylamides – Switching between Allene and Internal Alkyne

Herein the examination of the transformation of alkynes by low-coordinate iron silylamides is presented. An anionic linear iron(I) silylamides ([Fe(N{Dipp}SiR3)2]−; Dipp=2,6-di-iso-propylphenyl, R=Me) acts as precatalyst for the cyclotrimerization of diphenyl acetylene but is unable to transform internal alkynes with aliphatic substituents or terminal alkynes accordingly. For a benzylic, terminal alkyne, however, the 1,3-H-shift to the internal alkyne is observed and proceeds via phenyl allene. Using 10 mol% of the iron complex, the terminal alkene is selectively transformed to phenyl allene within minutes, and further fully transformed to the internal alkyne within 24 h. The transformation is retraced on a stoichiometric level and leads to the isolation of a π-alkyne complex with a shifted triple bond. Further, the anionic, trigonal iron(II) silylamide [Fe(NR2)3]− also mediates the catalytic conversion of the terminal alkyne but is restricted to allene formation. Overall, a deprotonation/reprotonation mechanism is assumed for these transformations. This was ultimately proven by using potassium hexamethyldisilazanide KNR2, which is an even more active catalyst for the complete triple bond shift.

New Insight into Ultrasonication Pretreatment Prompting Enzymatic Digestibility of Cellulose: Functioning on the Crystalline Structure Transition and Hydrogen Bonds Shift

New Insight into Ultrasonication Pretreatment Prompting Enzymatic Digestibility of Cellulose: Functioning on the Crystalline Structure Transition and Hydrogen Bonds Shift

New Insight into Ultrasonication Pretreatment Prompting Enzymatic Digestibility of Cellulose: Functioning on the Crystalline Structure Transition and Hydrogen Bonds Shift

New Insight into Ultrasonication Pretreatment Prompting Enzymatic Digestibility of Cellulose: Functioning on the Crystalline Structure Transition and Hydrogen Bonds Shift

Chemical F/J‐Interconversion in the Prostaglandin Family: From Cloprostenol to Its Δ12‐J2 and 15‐Deoxy‐Δ12,14‐J2 Derivatives

AbstractRacemic cloprostenol methyl ester was converted via cyclic 9,11‐phenylboronate esters to 9α,11α‐dihydroxy‐15α‐siloxy (OTBDPS, OTBDMS) derivatives. TES‐Morf selectively reacts with last compounds to exclusively give 11‐TES derivative. In the course of further transformations, the J‐type PG was obtained. The optimal conditions for the 13,14‐double bond shift in the latter were determined and ▵12 derivatives were obtained. 15‐TBDMS derivative PGJ2 was converted to the target ω‐aryloxy derivatives ▵12‐PGJ2, its 13Z isomer, and ▵12,14‐PGJ2 by hydrolysis of the TBDMS protective group. The anticancer activity of some of the synthesized compounds was studied.

Efficient synthesis of 3-arylbutadiene sulfones using the Heck–Matsuda reaction

An efficient and practical method for a scalable synthesis of 3-arylbutadiene sulfones deals with the ligand-free Heck–Matsuda reaction of sulfolene with aryldiazonium tetrafluoroborates followed by triethylamine-promoted double bond shift.

New insight of Ca element refining grain structure in commercial purity Mg based on the first-principle calculation

ABSTRACT The first-principle calculation was used to understand new insight of Ca element refining grain structure in commercial purity Mg. Negative adsorption energy, strong ionic bond and negative shift of partial density of states verify that Ca element is readily adsorbed on the surface of MgO. Combined with analysis of interfacial bonding between Mg2Ca and α-Mg, new insight of Mg2Ca layer formed via Ca adsorption efficiently nucleating α-Mg was proposed. Adsorption of other alkaline earth elements (Be, Sr or Ba) and the crystallographic feature of Be, Mg17Sr2 and Mg17Ba2 along with the experimental results further consolidate new insight of Ca element refining grain structure in commercial purity Mg.

NBO/NRT Two-State Theory of Bond-Shift Spectral Excitation.

We show that natural bond orbital (NBO) and natural resonance theory (NRT) analysis methods provide both optimized Lewis-structural bonding descriptors for ground-state electronic properties as well as suitable building blocks for idealized “diabatic” two-state models of the associated spectroscopic excitations. Specifically, in the framework of single-determinant Hartree-Fock or density functional methods for a resonance-stabilized molecule or supramolecular complex, we employ NBO/NRT descriptors of the ground-state determinant to develop a qualitative picture of the associated charge-transfer excitation that dominates the valence region of the electronic spectrum. We illustrate the procedure for the elementary bond shifts of SN2-type halide exchange reaction as well as the more complex bond shifts in a series of conjugated cyanine dyes. In each case, we show how NBO-based descriptors of resonance-type 3-center, 4-electron (3c/4e) interactions provide simple estimates of spectroscopic excitation energy, bond orders, and other vibronic details of the excited-state PES that anticipate important features of the full multi-configuration description. The deep 3c/4e connections to measurable spectral properties also provide evidence for NBO-based estimates of ground-state donor-acceptor stabilization energies (sometimes criticized as “too large” compared to alternative analysis methods) that are also found to be of proper magnitude to provide useful estimates of excitation energies and structure-dependent spectral shifts.

Surfactant-modified three-dimensional layered double hydroxide for the removal of methyl orange and rhodamine B: Extended investigations in binary dye systems

Dyes pollution have raised great attention due to its fatal harm on aquatic ecosystem and human health. Generally, multiple dyes (anionic dyes, cationic dyes) present in real wastewater systems. In this work, methyl orange (MO) as anionic dye and rhodamine B (RhB) as cationic dye were chosen as typical dyes to investigate the removal behavior with surfactant-modified three-dimensional MgAl layered double hydroxide (S3D-LDH) via macroscopic and microscopic analyses. Adsorption isotherms revealed that the maximum adsorption capacity of MO and RhB could reach 380.2 and 49.6 ​mg·g−1, respectively. The removal process between S3D-LDH and ionic dyes was identified to be a chemical reaction via adsorption kinetics. XRD and MIR demonstrated a decrease of d-spacing value and a red shift of the stretching vibration of lattice water and hydroxyl group in the MO removal and increased d-spacing and a blue shift of water with hydroxyl group in the RhB removal. X-ray photoelectron spectroscopy (XPS) revealed that the RSO3 peak emerged after MO adsorption and the negative bond shift of unbound sulfur of S 2p after RhB adsorption. All investigations revealed that MO adsorbed by S3D-LDH via anion exchange and hydrogen bonding whereas surface adsorption was deemed as the primary pathway for RhB. Furthermore, the MO and RhB adsorption capacity by S3D-LDH was both enhanced in binary component systems. S3D-LDH was demonstrated as a potential broad-spectrum adsorbent for the treatment of dyes wastewater.