An ab initio study of 15N-11B spin-spin coupling constants for borazine and selected derivatives

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been performed to investigate substituent effects on coupling constants for borazine and selected substituted borazines. For molecules in which F atoms are not bonded to adjacent atoms in the ring, F substitution increases the one-bond (11)B-(15)N coupling constants involving the atom at which substitution occurs but leaves the remaining one-bond B-N coupling constants essentially unchanged. For these molecules, the magnitudes of one-bond B-N coupling constants are only slightly dependent on the number of F atoms present. Fluorine substitution at adjacent B and N atoms in the borazine ring further increases the one-bond B-N coupling constant involving the substituted atoms and has the same effect on the other one-bond coupling constants as observed for corresponding molecules in which substitution occurs at alternate sites. In contrast to the effect of F substitution, substitution of Li at either N or B decreases one-bond B-N coupling constants relative to borazine. The effects of F and Li substitution on one-bond B-N coupling constants for borazine are similar to F and Li substitution effects on (13)C-(13)C coupling constants for benzene.     

Effect of the choice of the pressure coupling method on the spontaneous aggregation of DPPC molecules

The self-assembly of DPPC molecules starting from a random, solution-like configuration in the presence of water molecules is described in the present MD simulation study. Simulations were performed with either anisotropic or isotropic pressure coupling. Use of anisotropic pressure coupling led to the formation of a bilayer/bilayerlike aggregate; the features of the bilayer are in agreement with those reported from earlier simulation and experimental studies. In contrast, simulating the same system with isotropic pressure coupling led to the formation of a cylindrical micelle/lamellar structure with a large water hole. The formation of micelles seems unrealistic since diacylphosphatidylcholines having hydrocarbon tails with nine or more carbon atoms have been shown to form only bilayers. Simulations were also performed with preformed bilayerlike configurations with either anisotropic or isotropic pressure coupling. The bilayer characteristics deduced from simulations using anisotropic pressure coupling are in better agreement with those reported from earlier experimental and simulation studies. Thus, the choice of the pressure coupling method has a significant effect on the spontaneous aggregation of DPPC molecules but makes relatively lesser effect if the bilayer has formed already.     

具有光控功能的两种氮氧自由基体系磁偶合作用的理论研究

采用密度泛函结合对称性破损态(DFT-BS)方法, 通过计算具有光控磁性分子开关功能的二氮氧自由基二芳基乙烯化合物的磁偶合常数, 合理解释了其分子结构发生开环和闭环变化时, 分子磁性发生的改变．同时设计了二氮氧自由基二甲基二氢化芘分子光控开环和闭环模型, 并用同样的方法计算了模型分子的磁偶合常数, 发现这些模型分子的磁行为类似于二氮氧自由基二芳基乙烯化合物, 有可能也具有光控磁性分子开关功能．     

Modeling of the molar mass distribution of six-arm star-branched poly-epsilon-caprolactam using size-exclusion chromatography

This paper presents a modeling-based approach to the prediction of the molar mass distribution of the various species in a star-branched polycondensation mixture. The interpretation of experimental SEC data of the mixture of linear, cyclic and star-branched molecules is not straightforward, because of the different sizes of those molecules (having the same molecular mass). Therefore we have opted to use SEC analysis with only a concentration detector and fit the experimental data to the theoretical mass distribution, corrected for the volume of the various molecules. This allows the relative fraction and the distribution of the various species in the mixture (linear, cyclic and star-branched) to be determined. To demonstrate this, the six-arm star-branched poly-epsilon-caprolactam based on the six-functional coupling molecule, hexa(6-caproic acid) melamine has been analyzed. Five polymer mixtures with different initial concentration of coupling molecule have been synthesized. As the initial concentration of coupling molecule increased, we found that the weight fraction of star-branched molecules increased, while the weight fraction of linear and cyclic molecules decreased. We also found that the weight-average molar mass and the arm length decrease as the initial fraction of the coupling molecule increases.     

Iterative oxime bond chemistry leads to protease inhibitors

[structure: see text] In the present paper, we have looked at iterative coupling as a strategy to form new druglike molecules. We have developed an iterative coupling chemistry based on oxime bond formation between hydroxyaromatic aldehyde building blocks to form linear oxime oligomers. The strategy is validated by the discovery of micromolar protease inhibitors.     

Supramolecular reactor in an aqueous environment: aromatic cross Suzuki coupling reaction at room temperature

[reaction: see text] We have investigated supramolecular reactors for the Suzuki coupling reactions of aryl halides with phenyl boronic acids by using self-assembly of amphiphilic rod-coil molecules in aqueous solution at room temperature. All the rod-coil molecules synthesized in this work showed to self-assemble into discrete micelles consisting of aromatic rod bundles encapsulated by hydrophilic poly(ethylene oxide) coils. We present a comparative study of rod-coil molecules' efficiency as supramolecular reactors for Suzuki coupling reaction. The closed-packed aromatic bundles play an efficient role in supramolecular reactors for the coupling reactions at room temperature. The supramolecular reactor based on hexa-p-phenylene confers unprecedented activity, allowing reactions to be performed at very low catalyst levels, without conventional heating or microwave.     

Solvent effects on one-bond B-Li coupling constants in boryllithium compounds

EOM-CCSD 11B-7Li coupling constants and B chemical shifts have been computed for Li-diazaborole and its complexes with one H2O or FLi molecule. B-Li coupling constants for a model compound H(2)BLi and its complexes with up to 4 H2O or FLi molecules have also been obtained in an attempt to resolve discrepancies between the computed values of these properties for isolated Li-diazaborole and experimentally determined values for boryllithium in a THF solution. The presence of solvent molecules increases the ion-pair character of the B-Li bond, with the result that 1J(B-Li) decreases systematically as the basicity and the number of solvent molecules increases. In the presence of even a single solvent molecule, the boron chemical shift for Li-diazaborole increases, and approaches the experimental value. The computed results emphasize the role of the solvent in determining these NMR properties.     

Synthesis of C3-Symmetric star-shaped molecules containing 1,3-azoles via hetero-aryl Heck coupling

We have demonstrated a useful synthetic strategy to assemble star-shaped C₃-symmetric molecules containing 1,3-azole moieties at the periphery. To generate these C₃-symmetric heterocycles, we have employed the Pd/Cu-based coupling reactions. To this end, we have used benzoxazole, benzothiazole, and benzimidazole as coupling partners to generate the corresponding hetero-aryl Heck coupling products.     

Simultaneous photon absorption as a probe of molecular interaction and hydrogen-bond cooperativity in liquids

We have investigated the simultaneous absorption of near-infrared photons by pairs of neighboring molecules in liquid methanol. Simultaneous absorption by two OH-stretching modes is found to occur at an energy higher than the sum of the two absorbing modes. This frequency shift arises from interaction between the modes, and its value has been used to determine the average coupling between neighboring methanol molecules. We find a rms coupling strength of 46+/-1 cm(-1), larger than can be explained from a transition-dipole coupling mechanism, suggesting that hydrogen-bond mediated interactions also contribute to the coupling. The most important aspect of simultaneous vibrational absorption is that it allows for a quantitative investigation of hydrogen-bond cooperativity. We derive the extent to which the hydrogen-bond strengths of neighboring molecules are correlated by comparing the line shape of the absorption band caused by simultaneous absorption with that of the fundamental transition. Surprisingly, neighboring hydrogen bonds in methanol are found to be strongly correlated, and from the data we obtain an estimate for the hydrogen-bond correlation coefficient of 0.69+/-0.12.     

The application of thin-layer gel filtration chromatography in the rapid assessment of protein-protein coupling reactions.

The use of enzymes as markers when covalently coupled to various antigens or antibodies has wide application in medical science. A variety of bifunctional reagents has been used to produced the conjugates, and there have been some attempts to investigate the conditions of reaction necessary to obtain good coupling, whilst preserving the biological function of the molecules. The present study describes the influence of coupling conditions on the coupling of a alpha-amylase and albumin by a range of commercially available bifunctional coupling reagents, and the application of thin-layer gel filtration chromatography for the rapid qualitative/semi-quantitative assessment of coupling reactions.     

Theoretical Design of High－spin Organic Molecules with Heterocycles as Ferromagnetic Coupling Units

IntroductionThe design and the syntheses of organicmolecules with very high- spin ground states havebeen a topic of great interest[1— 5] .One of rationalapproaches to designing high- spin molecules,which has been proposed and studied by severalgroups[6,7] ,consists in conceptually dividing themolecules into two components,i.e.,a spin- con-taining( SC) fragment which provides the unpairedelectron and a ferromagnetic coupling ( FC) unitwhich is connected with radical centers ferromag-netically…     

Theoretical studies on the coupling interactions in H2SO4···HOO˙···(H2O)n (n = 0-2) clusters: toward understanding the role of water molecules in the uptake of HOO˙ radical by sulfuric acid aerosols

A detailed knowledge of coupling interactions among sulfuric acid (H(2)SO(4)), the hydroperoxyl radical (HOO˙), and water molecules (H(2)O) is crucial for the better understanding of the uptake of HOO˙ radicals by sulfuric acid aerosols at different atmospheric humidities. In the present study, the equilibrium structures, binding energies, equilibrium distributions, and the nature of the coupling interactions in H(2)SO(4)···HOO˙···(H(2)O)(n) (n = 0-2) clusters have been systematically investigated at the B3LYP/6-311++G(3df,3pd) level of theory in combination with the atoms in molecules (AIM) theory, natural bond orbital (NBO) method, energy decomposition analyses, and ab initio molecular dynamics. Two binary, five ternary, and twelve tetramer clusters possessing multiple intermolecular H-bonds have been located on their potential energy surfaces. Two different modes for water molecules have been observed to influence the coupling interactions between H(2)SO(4) and HOO˙ through the formations of intermolecular H-bonds with or without breaking the original intermolecular H-bonds in the binary H(2)SO(4)···HOO˙ cluster. It was found that the introduction of one or two water molecules can efficiently enhance the interactions between H(2)SO(4) and HOO˙, implying the positive role of water molecules in the uptake of the HOO˙ radical by sulfuric acid aerosols. Additionally, the coupling interaction modes of the most stable clusters under study have been verified by the ab initio molecular dynamics.     

HFI and DFT study of the bonding in complexes of halogen and interhalogen diatomics with Lewis base

We have analyzed by means of DFT calculations with use of the pseudo-potential the nuclear quadrupole coupling constants of a range of XYB complexes (n aσ type in Mulliken notation) formed between diatomic interhalogen molecules XY and Lewis bases B. The geometrical parameters, rotational and halogen nuclear quadrupole coupling constants obtained by these calculations substantially corresponded to the data of microwave spectroscopy in the gas phase. An analysis of the quality of the calculations that employ the pseudo-potential and the expanded basis set for the halogen compounds was carried out. The ZORA model is shown to be a viable alternative to the computationally demanding BH and HLYP model for the calculation of halogen and nitrogen coupling constants in molecules. In addition, the ZORA model, in contrast to the pseudo-potential model, leads to realistic values of iodine nuclear quadrupole coupling constants. From electron partitioning analyses and Klopman's approach it follows that for the IClB complexes the electrostatic bonding is predominant relative to covalent bonding.     

Cascade Cross‐Coupling of Dienes: Photoredox and Nickel Dual Catalysis

Chemical transformations based on cascade reactions have the potential to simplify the preparation of diverse and architecturally complex molecules dramatically. Herein, we disclose an unprecedented and efficient method for the cross‐coupling of radical precursors, dienes, and electrophilic coupling partners via a photoredox‐ and nickel‐enabled cascade cross‐coupling process. The cascade reaction furnishes a diverse array of saturated carbo‐ and heterocyclic scaffolds, thus providing access to a quick gain in C?C bond saturation.     

Spin-spin coupling and substituent and halogen isotope shift effects in the fluorine NMR spectra of fluorinated cyclopropanes and cyclopropyl ethers

The (19)F NMR spectra of a series of fluorinated cyclopropanes, most of which contain chlorine or bromine on the ring, have been observed and analyzed. A scheme has been developed to assign the resonances and the molecular stereochemistry, based on substituent effects, comparison of related molecules, and isotope shifts induced by the halogens. Replacement of fluorine by chlorine shifts cis fluorine resonances to lower field, and bromine has an even greater downfield influence. However, the shift effect of bromine compared to chlorine on gem or trans fluorines is variable. The magnitude of the isotope shifts is found to be regularly related to the geometrical relationship of the halogen to the observed fluorine and thus makes a significant contribution to the fluorine assignments. The three-bond spin-spin coupling constants between fluorine atoms in cyclopropanes display unusual behavior and are not helpful for assignment of the fluorine resonances. The signs of the coupling constants have been investigated by spin-tickling experiments, and the previously developed relation of the coupling constant sign to its temperature dependence has been found to be violated for some molecules.     

Rotational spectra, structures, hyperfine constants, and the nature of the bonding of KrCuF and KrCuCl

Rotational spectra of KrCuF and KrCuCl have been measured in the frequency range 8-18 GHz, using a pulsed jet cavity Fourier transform microwave spectrometer. The molecules were prepared by ablating Cu metal with a pulsed Nd:YAG laser (1064 nm) and allowing the plasma to react with appropriate precursors (Kr plus SF(6) or Cl(2)) contained in the backing gas of the jet (Ar or Kr). Rotational constants, internuclear distances, vibration frequencies, and (83)Kr, Cu, and Cl nuclear quadrupole coupling constants have all been evaluated. The Kr-Cu bonds are short and the complexes are rigid. The (83)Kr coupling constant of KrCuF is large (128.8 MHz). The Cu nuclear quadrupole coupling constants differ radically from those of uncomplexed CuF and CuCl molecules. The results are supported by those of ab initio calculations, which have also yielded Mulliken populations, MOLDEN plots of valence molecular orbitals and Laplace concentrations, and electron localization functions. The results are consistent with those reported earlier for other noble gas-noble metal halide complexes. The results have been used to assess the nature of the bonding in the complexes and have produced good evidence for weak noble gas-noble metal chemical bonding.     

Inclusion of hydrogen p orbitals in the semiempirical calculation of NMR parameters. I. Influence on the FPT INDO spin–spin coupling constants

Hydrogen 2p orbitals have been introduced into the basis set to calculate the Fermi contact term of spin–spin coupling constants using the FPT INDO method. Different coupling constants show different sensitivity to these hydrogen polarization functions. Some improvements are found for molecules containing N or F. Calculations of proton-proton geminal coupling constants give more negative results than those of FPT INDO , yielding a better agreement with experimental values. The π-transmission mechanism is notably exaggerated.     

A computational study of 2J(HH)(gem) indirect spin-spin coupling constants in simple hydrides of the second and third periods

Several theoretical methods have been used to compute (2)J(HH) in neutral, anionic and cationic HXH hydrides, X being the 14 nuclei from Li to Cl (28 molecules). Since the calculations also provide (1)J(XH) spin-spin coupling constants (SSCC), these have also been analyzed. The best results were obtained using Second-order polarization propagator approximation (SOPPA)/sadJ. The geminal coupling constants appear to be dependent on the electronegativity of the X-atom.     

Structural and spectroscopic study of the excited electronic states of silver dihalides by quantum chemical methods

The structural and electronic properties of the excited electronic states of AgX(2) (X = F, Cl, Br, and I), have been calculated, taking electron correlation and spin-orbit coupling into account and employing improved relativistic-effective-core potentials for silver and the halogen atoms. The relative ordering of the excited states of these molecules has been discussed via molecular-orbital arguments. The spin-orbit splittings of three degenerate electronic states ((2)Pi(g), (2)Pi(u), and (2)Delta(g)) have been calculated and the spin-orbit induced inter-state (Sigma - Pi) coupling has been discussed. The composition of the spin-orbit eigenstates is analyzed in terms of scalar-relativistic electronic states. Finally, a theoretical prediction of the photodetachment bands of the title molecules has been accomplished.