OC Bond Research Articles

Refractory Dissolved Organic Matter has Similar Chemical Characteristics but Different Radiocarbon Signatures With Depth in the Marine Water Column

AbstractThe >5,000‐year radiocarbon age (14C‐age) of much of the 630 ± 30 Pg C oceanic dissolved organic carbon (DOC) reservoir remains an enigma in the marine carbon cycle. The fact that DOC is significantly older than dissolved inorganic carbon at every depth in the ocean forms the basis of our current framing of the marine DOC cycle, where some component persists over multiple cycles of ocean mixing. As a result, 14C‐depleted, aged DOC is hypothesized to be present as a uniform reservoir with a constant 14C signature and concentration throughout the water column. However, key requirements of this model, including direct observations of DOC with similar 14C signatures in the surface and deep ocean, have never been met. Despite decades of research, the distribution of Δ14C values in marine DOC remains a mystery. Here, we applied a thermal fractionation method to compare operationally defined refractory DOC (RDOC) from different depths in the North Pacific Ocean. We found that RDOC shares chemical characteristics (as recorded by OC bond strength) throughout the water column but does not share the same 14C signature. Our results support one part of the current paradigm—that RDOC is comprised of structurally related components throughout the ocean that form a “background” reservoir. However, in contrast to the current paradigm, our results are consistent with a vertical concentration gradient and a vertical and inter‐ocean Δ14C gradient for RDOC. The observed Δ14C gradient is compatible with the potential addition of pre‐aged DOC to the upper ocean.

Biochemical composite material using corncob powder as a carrier material for ureolytic bacteria in soil cadmium immobilization

Corncob powder possessing its superiority in environmental sustainability and cost, was approved with strong capability of being a replacement of biochar in facilitating the microbial carbonate precipitation process. In this study, the ureolytic bacterial strain Bacillus sp. WA isolated from a pre-acquired metal contaminated soil in Guiyu, China, was showed to be well attached on the surfaces of corncob powder, indicating the carrier's role as a durable shelter for bacterial cells. The efficient immobilization helped develop biochemical composite material (BCM) and proven to function better the calcite precipitation. Afterwards, the mechanism and multi-directional benefits of BCM in edaphic cadmium remediation were examined through pot experiment and compared with corncob powder/bacterial strain/nutrient media as control groups. Integrated lab-scale analyses emphasized the advantages of BCM by the maximum soil urease activity (up to 3.440 U/mg and increased by 214% in 28 days), maximal bacterial propagation (most abundant population in fluorescence microscopy), richest surface functional group (most remarkable OC bond and CO bond in FTIR result), notable calcite precipitation (clear calcite crystals on the surface of BCM compared to control group under SEM-EDS), and highest Cd immobilization rate (exchangeable Cd decreased by 68.54%), among all treatments. The pH and electroconductivity measurements additionally led to the mechanism of corncob powder and NBU promoting pre-existed ureolytic bacteria in soil, which demonstrated the added value of corncob to be fine carbon source and residence shelter for soil microorganism, revealing its potential in developing agricultural materials.

Dialkyl Ether Formation at High-Valent Nickel.

In this article, we investigated the I2-promoted cyclic dialkyl ether formation from 6-membered oxanickelacycles originally reported by Hillhouse. A detailed mechanistic investigation based on spectroscopic and crystallographic analysis revealed that a putative reductive elimination to forge C(sp3)–OC(sp3) using I2 might not be operative. We isolated a paramagnetic bimetallic NiIII intermediate featuring a unique Ni2(OR)2 (OR = alkoxide) diamond-like core complemented by a μ-iodo bridge between the two Ni centers, which remains stable at low temperatures, thus permitting its characterization by NMR, EPR, X-ray, and HRMS. At higher temperatures (>−10 °C), such bimetallic intermediate thermally decomposes to afford large amounts of elimination products together with iodoalkanols. Observation of the latter suggests that a C(sp3)–I bond reductive elimination occurs preferentially to any other challenging C–O bond reductive elimination. Formation of cyclized THF rings is then believed to occur through cyclization of an alcohol/alkoxide to the recently forged C(sp3)–I bond. The results of this article indicate that the use of F+ oxidants permits the challenging C(sp3)–OC(sp3) bond formation at a high-valent nickel center to proceed in good yields while minimizing deleterious elimination reactions. Preliminary investigations suggest the involvement of a high-valent bimetallic NiIII intermediate which rapidly extrudes the C–O bond product at remarkably low temperatures. The new set of conditions permitted the elusive synthesis of diethyl ether through reductive elimination, a remarkable feature currently beyond the scope of Ni.

Application of the CLAYFF and the DREIDING Force Fields for Modeling of Alkylated Quartz Surfaces.

To extend applicability and to overcome limitations of combining rules for nonbond potential parameters, in this study, CLAYFF and DREIDING force fields are coupled at the level of atomic site charges to model quartz surfaces with chemisorpt hydrocarbons. Density functional theory and Bader charge analysis are applied to calculate charges of atoms of the OC bond connecting a quartz crystal and an alkyl group. The study demonstrates that the hydrogen atom of the quartz surface hydroxyl group can be removed and its charge can be redistributed among the oxygen and carbon atoms of the OC bond in a manner consistent with the results calculated at the density functional level of theory. Augmented with modified charges of the OC bond, force fields can then be applied to a practical problem of evaluation of the contact angle of a water droplet on alkylated quartz surfaces in a carbon dioxide environment, which is relevant for carbon geo-sequestration and in a broader context of oil and gas recovery. Alkylated quartz surfaces have been shown to be extremely hydrophobic even when the surface density of hydroxyl groups is close to the highest naturally observed density of 6.2 OH groups per square nanometer.

Structural Stability in Dimer and Tetramer Clusters of l-Alanine in the Gas Phase and the Feasibility of Peptide Bond Formation.

Stability in low-energy structures of the dimer and tetramer clusters of l-alanine in the gas phase is studied by accurate quantum chemical computations at the DLPNO2013-CCSD(T) level. It is found that the dispersion interaction energies in the dimer (-0.3 to -0.6 kcal/mol) and in the tetramer (-1.3 to -2.5 kcal/mol) have a small role in the stability of the clusters as compared to the hydrogen bond (HB) energies -4.1 to -14.2 and -32.2 to -40.1 kcal/mol, respectively. The HB energy in the alanine cluster is obtained from the binding energy (BE) of DLPNO2013-CCSD(T)//B2PLYP/def2-TZVP by subtracting the dispersion interaction energy. Local HB energies deduced from the dimer structures are found to be suitable to estimate total HB energies in similar environments. The BEs of OH···NH and OH···OC bonds are -9.5 and -7.1 kcal/mol, respectively. This suggests that the higher clusters are formed through OH···NH bonds as they confer more stability. Analysis of bonding in the tetramer shows that the low-energy tetramer and higher clusters are formed through the OH···NH mode of hydrogen bonding, unlike the dimer which is formed through the OH···OC bond. Feasibility of the amino acid cluster to function as a precursor for polypeptide formation is examined because the orientation of the OH···NH mode of hydrogen bonding is suitable for chemical condensation. The propensity of forming coiled structures in higher clusters and thus in the polypeptides is examined based on the conformational stability in the tetramer of alanine.

Excited-state dynamics of oxazole: A combined electronic structure calculations and dynamic simulations study

In the present work, the combined electronic structure calculations and surface hopping simulations have been performed to investigate the excited-state decay of the parent oxazole in the gas phase. Our calculations show that the S2 state decay of oxazole is an ultrafast process characterized by the ring-opening and ring-closure of the five-membered oxazole ring, in which the triplet contribution is minor. The ring-opening involves the OC bond cleavage affording the nitrile ylide and airine intermediates, while the ring-closure gives rise to a bicyclic species through a 25 bond formation. The azirine and bicyclic intermediates in the S0 state are very likely involved in the phototranspositions of oxazoles. This is different from the previous mechanism in which these intermediates in the T1 state have been proposed for these phototranspositions.

Experimental evidence for competitive N[sbnd]O and O[sbnd]C bond homolysis in gas-phase alkoxyamines

The extensive use of alkoxyamines in controlled radical polymerisation and polymer stabilisation is based on rapid cycling between the alkoxyamine (R1R2NOR3) and a stable nitroxyl radical (R1R2NO•) via homolysis of the labile OC bond. Competing homolysis of the alkoxyamine NO bond has been predicted to occur for some substituents leading to production of aminyl and alkoxyl radicals. This intrinsic competition between the OC and NO bond homolysis processes has to this point been difficult to probe experimentally. Herein we examine the effect of local molecular structure on the competition between NO and OC bond cleavage in the gas phase by variable energy tandem mass spectrometry in a triple quadrupole mass spectrometer. A suite of cyclic alkoxyamines with remote carboxylic acid moieties (HOOCR1R2NOR3) were synthesised and subjected to negative ion electrospray ionisation to yield [M−H]− anions where the charge is remote from the alkoxyamine moiety. Collision-induced dissociation of these anions yield product ions resulting, almost exclusively, from homolysis of OC and/or NO bonds. The relative efficacy of NO and OC bond homolysis was examined for alkoxyamines incorporating different R3 substituents by varying the potential difference applied to the collision cell, and comparing dissociation thresholds of each product ion channel. For most R3 substituents, product ions from homolysis of the OC bond are observed and product ions resulting from cleavage of the NO bond are minor or absent. A limited number of examples were encountered however, where NO homolysis is a competitive dissociation pathway because the OC bond is stabilised by adjacent heteroatom(s) (e.g. R3=CH2F). The dissociation threshold energies were compared for different alkoxyamine substituents (R3) and the relative ordering of these experimentally determined energies is shown to correlate with the bond dissociation free energies, calculated by ab initio methods. Understanding the structure-dependent relationship between these rival processes will assist in the design and selection of alkoxyamine motifs that selectively promote the desirable OC homolysis pathway.

Silicon α-Effect: A Systematic Experimental and Computational Study of the Hydrolysis of Cα- and Cγ-Functionalized Alkoxytriorganylsilanes of the Formula Type ROSiMe2(CH2)nX (R = Me, Et;n= 1, 3; X = Functional Group)

To understand the silicon α-effect in terms of an enhanced reactivity of the Si–OC bond of α-silanes of the formula type ROSiMe2CH2X compared to analogous γ-silanes ROSiMe2(CH2)3X (R = Me, Et; X = functional group), a systematic experimental and computational study of the kinetics and mechanisms of hydrolysis of such compounds was performed. For this purpose, a series of suitable model compounds was synthesized and studied for their hydrolysis kinetics in CD3CN/D2O under basic and acidic conditions, using 1H NMR spectroscopy as the analytical tool. To get more information about the reaction mechanisms, the experimental investigations were complemented by computational studies. These investigations demonstrated that the silicon α-effect cannot be rationalized in terms of a special single effect. The reactivities observed rather result from a summation of different components, such as electronic and steric effects, pD dependence, and hydrogen bonds between the functional group (or even protonated functional...

Chelation and Stereodynamic Equilibria in Neutral Hypercoordinate Organosilicon Complexes of 1-Hydroxy-2-pyridinone

A series of neutral organosilicon compounds, R3Si(OPO) (R = Me (1), Et (2), Ph (3)), cis-R2Si(OPO)2 (R = Me (4), Et (5), iPr (6), tBu (7), Ph (9)), (CH2)3Si(OPO)2 (8), and cis-R2Si(OPO)Cl (R = Me (10), Et (11)) (OPO = 1-oxo-2-pyridinone) have been prepared and fully characterized. X-ray crystallographic analyses show 1 to be tetracoordinate, 3, 7, and 10 to be pentacoordinate, and 4–6, 8, and 9 to be hexacoordinate. In the hexacoordinate structures, a mixture of diastereomers is observed in the form of C/N site disorder in each OPO ligand. Variable-temperature 13C and 29Si NMR studies indicate reversible Si←OC bond dissociation occurring in all pentacoordinate and hexacoordinate complexes to a varying degree with greater tendency toward dissociation in hydrogen-bonding donor solvents. Significant weakening of the dative Si←OC bond in 3 is observed in the cocrystallized adduct solvate 3·Ph3SiOH·1/2C5H12, providing structural evidence for the decrease in coordination number of the OPO ligand by hydrogen-bonding donors. In the hexacoordinate complexes, increasing steric bulk of ancillary ligands also was found to promote dissociation. 1H and 13C VT-NMR studies of 4, 6, 8, and 9 indicate stereoisomerization equilibria concurrent with Si←OC bond dissociation proposed to occur through trigonal-bipyramidal intermediates.

Synthesis, Structural, Spectroscopic, Electrochemical, Magnetic, and Catalytic Properties of the Trinuclear MnIII Triplesalen Complex [(talen){Mn(OAc)}3] Exhibiting Three Salen‐Subunits in a β‐cis‐Conformation

AbstractReaction of the triplesalen ligand H6talen with three equivalents Mn(OAc)2·4H2O in MeOH results in the formation of a brown solid which upon recrystallization from CH3CN provides the trinuclear complex [(talen){Mn(OAc)}3]·7CH3CN as evidenced by single‐crystal X‐ray diffraction. The triple tetradentate ligand (talen)6– coordinates to three MnIII ions in the rare β‐cis‐conformation of the salen‐like ligand compartments with the central oxygen donor (Oc) being rotated out of the plane. This results in a longer Mn–Oc bond length of 2.00 Å compared to the mean Mn–Ot bond lengths of the terminal phenolates at 1.86 Å. The six‐coordination is saturated by bidentate OAc– ligands. The electronic absorption spectrum measured in MeOH appears to be almost identical to all other complexes already studied possessing a {(talen)MnIII3}3+ subunit (in the trans‐conformation). The spectra measured in CH2Cl2 and CH3CN exhibit significant variations of the absorption features in the CT region above 20000 cm–1 and a low‐energy shift of the d‐d transitions from a shoulder around 18000 cm–1 in CH3OH to maxima around 13000 cm–1 in CH2Cl2 and CH3CN. This indicates a physical dissolution of [(talen){Mn(OAc)}3] in CH2Cl2 and CH3CN solutions without major structural rearrangements, while in MeOH solution a structural rearrangement to the preferred trans‐conformation of the salen‐like coordination compartments occurs loosing the bidentate coordination mode of the OAc– ligands. Electrochemical measurements reveal unresolved irreversible processes in the range 0.9–1.4 V vs. Fc+/Fc corresponding to oxidations of the MnIII‐phenolate units, while irreversible reductive waves in the range –0.7–(–1.2) V vs. Fc+/Fc correspond to MnIII to MnII reductions. The analysis of the magnetic data reveals a weaker antiferromagnetic interaction of J = –0.067 cm–1 and a stronger zero‐field splitting of D = –5.57 cm–1 in comparison to the complexes with {(talen)MnIII3}3+ subunits in the trans‐conformation consistent with the longer Mn–Oc distances and the asymmetric coordination environment, respectively. The complex [(talen){Mn(OAc)}3] catalyzes the epoxidation of 1, 2‐dihydronaphthalene with iodosylbenzene with complete conversion at room temperature.

Effect of nano Ni–Fe alloy addition on flash pyrolysis of poly(ethylene glycol)

Flash pyrolysis of p-PEG (pure PEG) and PEG/NiFe (mixture of PEG and nano Ni–Fe alloy) was carried out in argon atmosphere, aiming to study the effect of nano Ni–Fe alloy on the behavior and mechanism of PEG pyrolysis. A special combinational system was designed to perform pyrolysis experiment and collect pyrolysis products, while GC/MS was employed to identify the products. The experimental results showed that nano Ni–Fe alloy changed not only the relative contents but also the series of PEG pyrolysis products. It was interesting that all of 15 possible series of PEG pyrolysis products in theory were detected from p-PEG and/or PEG/NiFe pyrolysis in the present work. Six product series disappeared and two new product series were detected after adding nano Ni–Fe alloy. Based on quantitative analysis of the end groups of products, the effect of nano Ni–Fe alloy on PEG pyrolysis process as well as the mechanism was discussed. Ni and Fe played quite different roles in the process of PEG pyrolysis. Fe made end group OCH2CH3, OCHCH2, OH and OCH3 transfer to OCH2CHO by inducing hydrogen transfer and OC bond cleavage; Ni made OCHCH2 transfer to OCH2CH3 by inducing hydrogenation.

Surface analysis of poly(ethylene naphthalate) (PEN) films treated at atmospheric pressure using diffuse coplanar surface barrier discharge in air and in nitrogen

Surface modification of poly(ethylene naphthalate) (PEN) films by Diffuse Coplanar Surface Barrier Discharge (DCSBD) in ambient air and nitrogen at atmospheric pressure was studied at short plasma exposure times from 1 s up to 10 s. Chemical changes in the surface composition after the plasma treatments were studied using a high resolution XPS (X-ray Photoelectron Spectroscopy). The water contact angle was reduced from an initial value of 79° to values of 31° and 20° for the ambient air and nitrogen plasma treatments. After 3 days of storage in ambient air the contact angles were still about 50° and 40°, respectively. The polar component of surface energy was changed by the aging significantly less than the total surface energy. Despite the marked increase in the surface energy, the plasma treatment had no significant influence on the surface morphology.The surface modification in air plasma resulted in significant reduction the OC bond formation on the treated surface. The concentration of oxygen on PEN surface was increased from an initial value of 22 at.% up to 45 at.% already after 5 s air plasma exposure, and this oxygen concentration remained constant for longer plasma treatments. A small presence of nitrogen under 1.5 at.% from the air was also found after the treatments. The nitrogen plasma modified surface composition only by implanted nitrogen up to 5 at.% when the ratio of CO and C–O was not significantly changed. But some new bonds are also expected to be formed in a very low number only.

DOS Calculation Analysis of New Transparent Conductor Mg(OH)2-C

In this paper, the new transparent conductive material, Mg(OH)2-C is analyzed by an ab initio quantum chemical calculation based on the density of states (DOS) with DV-Xcalculation. In this study the calculation of DOS for Mg(OH)2 by DV-Xcalculation with Mg(OH)2 cluster model was carried out first to assure the validity of our calculation. Next, it was hypothesized that Mg(OH)2-C is modeled with MgOHOC cluster as well as the Mg(OH)2 cluster. The expected behavior of the MgOHOC DOS well agrees with the experimental results for Mg(OH)2-C, and the hypothesized MgOHOC model well explained the characteristics of the Mg(OH)2-C. Moreover, replacing a part of the H atoms with C atoms in some of nonconductive hydroxide materials yields conductivity as well as our MgOHOC, because the reason of the MgOHOC conductivity appearance is the one by the replacements from the OH bond to the OC bond and the characteristics of Mg atom doesn't especially take the part of the conductivity appearance. (doi:10.2320/matertrans.M2011046)

Germanium Bonding to AL2O3

Germanium has been bonded to both single crystal Al2O3 (sapphire) as well as fine grain Al2O3. A germanium to sapphire bonding energy of 3 J/m2 has been measured after a 200 oC bond anneal. Micro voids formed between the germanium/sapphire interface can be removed by employing an interfacial layer of silicon dioxide on either surface. Patterning the sapphire into a grid pattern prior to bonding creates an escape path for trapped gas or moisture allowing micro void free direct bonding to be achieved. Modifying the surface of the fine grain Al2O3 surface with a polycrystalline silicon deposition and polish creates a surface, having an rms roughness of 1.5nm(measured over a 250µm square area), suitable for bonding. Techniques employed in the germanium sapphire bonding can then be used in the bonding of fine grain Al2O3 to germanium.

Hydrogen‐transfer reaction in nitroxide mediated polymerization of methyl methacrylate: 2,2‐Diphenyl‐3‐phenylimino‐2,3‐dihydroindol‐1‐yloxyl nitroxide (DPAIO) vs. TEMPO

AbstractIn a recent article, we have showed that the nitroxide mediated polymerization of methyl methacrylate was possible up to 80% conversion for reasonable masses Mn = 60,000 g mol−1 when 2,2‐diphenyl‐3‐phenylimino‐2,3‐dihydroindol‐1‐yloxyl nitroxide (DPAIO) was used as control agent. We have claimed that the success of this experiment relied on the absence of H‐transfer reaction both in the alkoxyamine and between alkyl and nitroxyl radical. In this article, the decomposition of 4‐nitrophenyl 2‐(2,2,6,6‐tetramethylpiperidine‐1‐yloxy)‐2‐methylpropionate (1a) and 4‐nitrophenyl 2‐(2,2‐diphenyl‐3‐phenylimino‐2,3‐dihydroindol‐1‐yloxy)‐2‐methylpropanoate (2a) has been studied by 1H NMR in the presence and in the absence (persistent radical effect condition) of scavenger (thiophenol PhSH). At temperature lower than the one used for polymerization, fast and quantitative H‐transfer reaction was observed for 1a whereas no H‐transfer reaction was observed for 2a. The scavenging technique proved for the first time that the H‐transfer was an intermolecular process for 1a. However, the slow side‐reaction of NOC bond homolysis, which did not impede the control of the polymerization but may exert a detrimental effect on the livingness, was observed and quantified for 2a. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6828–6842, 2008

Solvation Structure of Li+ in Concentrated LiPF6−Propylene Carbonate Solutions

Time-of-flight neutron diffraction measurements were carried out for 6Li/7Li isotopically substituted 10 mol % LiPF6-propylene carbonate-d6 (PC-d6) solutions, in order to obtain structural information on the first solvation shell of Li+. Structural parameters concerning the nearest neighbor Li+...PC and Li+...PF6- interactions were determined through least-squares fitting analysis of the observed difference function, DeltaLi(Q). It has been revealed that the first solvation shell of Li+ consists in average of 4.5(1) PC molecules with an intermolecular Li+...O(PC) distance of 2.04(1) A. The angle Li+...O=C bond angle has been determined to be 138(2) degrees.

Reactions of Monomeric [1,2,4-(Me3C)3C5H2]2CeH and CO with or without H2: An Experimental and Computational Study

Addition of CO to [1,2,4-(Me3C)3C5H2]2CeH,Cp'2 CeH, in toluene yields the cis-(Cp'2Ce)2(mu-OCHCHO), in which the cis-enediolate group bridges the two metallocene fragments. The cis-enediolate quantitatively isomerizes intramolecularly to the trans-enediolate in C6D6 at 100 degrees C over 7 months. When the solvent is pentane, Cp'2Ce(OCH2)CeCp'2 forms, in which the oxomethylene group or the formaldehyde dianion bridges the two metallocene fragments. The cis-enediolate is suggested to form by insertion of CO into the Ce-C bond of Cp'2Ce(OCH2)CeCp'2, generating Cp'2CeOCH2COCeCp'2. The stereochemistry of the cis-enediolate is determined by a 1,2-hydrogen shift in the OCH2CO fragment that has the OC(H2) bond anti-periplanar relative to the carbene lone pair. The bridging oxomethylene complex reacts with H2, but not with CH4, to give Cp'2CeOMe, which is also the product of the reaction between Cp'2CeH and a mixture of CO and H2. The oxomethylene complex reacts with CO to give the cis-enediolate complex. DFT calculations on C5H5 model metallocenes show that the reaction of Cp2CeH with CO and H2 to give Cp2CeOMe is exoergic by 50 kcal mol-1. The net reaction proceeds by a series of elementary reactions that occur after the formyl complex, Cp2Ce(eta2-CHO), is formed by further reaction with H2. The key point that emerges from the calculated potential energy surface is the bifunctional nature of the metal formyl in which the carbon atom behaves as a donor and acceptor. Replacing H2 by CH4 increases the activation energy by 17 kcal mol-1.

Effects of ionization on N-glycylglycine peptide: Influence of intramolecular hydrogen bonds

The ionization effects on 28 conformations of N-glycylglycine are analyzed by means of the hybrid B3LYP and the hybrid meta-MPWB1K density functionals and by single-point calculations at the CCSD(T) level of theory. The most favorable process observed corresponds to the ionization of the only neutral conformation that presents a OH...NH2 intramolecular hydrogen bond, which leads to CO2 elimination after a spontaneous proton transfer from -COOH to NH2. The remaining neutral structures evolve to 20 different conformations of N-glycylglycine radical cation, which lie about 25-40 kcal/mol higher than the decarboxylated [NH3CH2CONHCH2]+*...[CO2] complex. Structural changes induced by ionization depend on the intramolecular hydrogen bonds of the initial conformation, since they determine the nature of the electron hole formed. In most cases, ionization takes place at the terminal -NH2 and -CO of the amide bond, which produces a strengthening of the peptide bond and the formation of new -NH2...OC(amide) and -NH2...OCOH hydrogen bonds. However, if -NH2 and -CO(amide) simultaneously act as proton acceptor in the neutral conformation, ionization is mainly localized at the carboxylic group, which produces a strengthening of the -COOH...OC(amide) bond. Both functionals lead to similar trends and compare well with CCSD(T) results except for a few cases for which B3LYP provides a too delocalized picture of the electron hole and consequently leads to artificial geometry reorganization.

Theoretical investigation of 1,1′-bi-2-naphthol isomerization

The mechanism of the isomerization of 1,1′-bi-2-naphthol (BINOL) has been investigated by means of density functional theory (DFT). There exist three isomers for BINOL; each of them has two enantiomers. The relative stabilities to the most stable isomer (referred to as I1) are 0.18eV (I2) and 0.33eV (I3). Two pathways were found for the isomerization: OH rotations around each of the OC bond and ring–ring torsion around the C–C inter-ring bond. The first one is far less energetic than the second one. For the ring–ring torsion, two pathways are possible, the cis and the trans rotation around the C–C inter-ring bond. Except for the isomer I3, the enantiomer may or may not be reached, depending on the starting isomer for I1 or I2. The most probable isomerization/enantiomerization pathway from I1 to I3 is presented.

Analysis of the NMR Spin−Spin Coupling Mechanism Across a H−Bond: Nature of the H-Bond in Proteins

The NMR spin−spin coupling constants (SSCCs) across the H-bond in proteins are sensitive to the electronic structure of the H-bonded system, i.e., the N−H···OC group in proteins. The spin−spin coupling mechanism across the H-bond involves a strong electric field effect, steric exchange interactions, and some weak covalent effects (transfer of electronic charge). The electric field effect is reflected by one-orbital contributions to the SSCC and can be tested with the help of probe charges. A negative charge opposite to the N−H bond leads to increased polarization of the N−H bond, a larger contact density at the N nucleus, and a stronger FC coupling mechanism for those SSCCs involving the N nucleus. Similarly, a positive charge opposite to the OC bond, distorts the O density into the direction of the external charge and in this way decreases the spin density at the O nucleus. All SSCCs across the H-bond depend primarily on the electric field effect and two-orbital steric exchange interactions. The lone pai...