Stereoselective oxazaborolidine–borane reduction of biphenyl alkyl diketones–lignin models: enantiopure dehydrodiapocynol derivatives

Asymmetric reduction of two conformationally flexible biphenyl alkyl diketones 9 and 10 with (R)-oxazaborolidine 3-borane system was successfully carried out and the corresponding biphenyl alcohols 11 and 12 were obtained in high yield and e.e. with predominance of the homochiral (S,S) dicarbinols. The absolute configuration of diastereopure dehydrodiapocynol derivative (S,S)-14 was assigned by crystallographic analysis which confirms the known stereochemical course of CBS-catalysed reduction of ketones and gives useful information on spatial arrangement.     

Effects of bubble size and approach velocity on bubble–particle interaction – a theoretical study

A theoretical analysis of the atomic force microscopy (AFM) approach–retract dynamic interaction between an air bubble and a hydrophilic silica plane was carried out based on the well-established Stokes–Reynolds–Young–Laplace model. An air bubble with different radii attached to the end of a cantilever approached the silica surface with different approach velocities in a 10^?3?M KCl solution. Results showed that with increasing approach velocity (0.1, 1, and 10?µm/s), the repulsive force, flattened area of the film, and hydrodynamic suction force between the 100-µm bubble and the silica plane increased. The film continued thinning at the initial stages of bubble retraction because of the attractive hydrodynamic pressure. When the bubble size decreased, the influence of hydrodynamic pressure was less evident. The final film thickness before bubble retraction was similar to the theoretical equilibrium thickness when the Laplace pressure was equal to the disjoining pressure.     

Advances in theoretical study on transition-metal-catalyzed C−H activation

Transition-metal-catalyzed C–H activation represents one of most attractive research fields in modern organic chemistry while theoretical studies have become a popular and effective tool for elucidating mechanism nowadays. The recent achievements in the cross field of the two orientations are reviewed in this article. The first part introduced the advances in theoretical study on transition-metal-catalyzed C–H activation. The elegant work reported mainly in and after 2013, classified according to the mechanisms of C–H activation, were covered. The second part provided an analysis on the distribution of quantum-chemical methods, solvation models and basis sets in the collected theoretical studies.     

Structure–activity relationship of nitrosating agents in the nitrosation reactions of ammonia: a theoretical study

Nitrosating agents (NAs) stimulate the formation of powerful mutagenic and carcinogenic N-nitrosamines. As a model reaction, the nitrosation of ammonia by a series of NAs, i.e., HONO, N₂O₃, N₂O₄, ONOOH, ONCl, ONBr, ONSCN, ONOCH₃, ONOC₂H₅, ONNC, ONOCl, and ONOSOCH₃, was investigated at the CBS-QB3 level of theory. A structure–activity relationship of NAs in the nitrosation reactions of ammonia was established. The results indicate that the nitrosating reactivity of NAs (ON–X) has definite relationship with the heterolytic bond dissociation energies of ON–X and H–X, Mulliken charges of N and X atoms as well as number of the atoms in the ring of the transition state. In light of the established structure–reactivity relationship of NAs, ONNC was found to be a potential powerful nitrosating species.     

Mixed micellization of anionic–nonionic surfactants in aqueous media: a physicochemical study with theoretical consideration

Mixed micellization of binary and ternary mixtures of anionic and nonionic surfactants, such as lithium dodecyl sulfate, polyoxyethylene(23)laurylether, and polyoxyethylene-tert-octylphenylether, is studied in aqueous solution using tensiometric, conductometric, and spectrophotometric methods. Although tensiometry and conductometry complement each other closely, the spectroscopic critical micellar concentration (cmc) is far from agreement with tensiometric study. Several parameters, e.g., cmc, degree of counterion binding, free energies of micellization, and interfacial adsorption, have been evaluated. Established theories of Clint, Rosen, Rubingh, Motomura, Georgiev, Maeda, and Blankschtein were applied to evaluate the mole fraction of different components in the self-aggregated phase, the interaction parameter, free energy contributions, and expected cmc.     

JR 400–NaAOT interaction: a detailed thermodynamic study of polymer–surfactant interaction bearing opposite charges

The present study entails interaction between the cationic polymer N,N-dimethylhydroxyethyl cellulose (JR 400) and the double-tailed anionic surfactant Na-bis-2-ethyhexylsulphosuccinate (NaAOT). This oppositely charged polymer and surfactant are expected to cause coacervation and precipitation; hence, we have observed formation of thick solution similar to diluted gel at [JR 400]?~?0.01 and 0.10 %?w/v in aqueous solution. Viscometry, conductometry, tensiometry, and microcalorimetry techniques are used to monitor the interaction process. The results are explained in the light of both intrachain and interchain linking by way of NaAOT reverse micelle formation. Adsorption of NaAOT monomers onto the charged side chains of the polymer shields interchain electrostatic repulsion, leading to the formation of hydrophobic microdomains and microscopic heterogeneity in the solution. The morphologies of the domains depend on the level of addition of NaAOT in the system. The different stages of physiochemical changes that arise in solution have been identified by the use of different techniques, and correlations of the results have been attempted in terms of pragmatic models.     

Simultaneous interactions of pyridine with substituted benzene ring and H–F in X-ben⊥pyr···H–F complexes: a cooperative study

The cooperative effects between T-shape stacking and hydrogen bond interactions in X-ben⊥pyr···H–F complexes were investigated in this work. The results indicate that the electron-withdrawing/donating substituents decrease/increase the magnitude of the binding energies compared to the unsubstituted X-ben⊥pyr···H–F (X = H) complex. The cooperative effects have been studied while using the atoms in molecules (AIM) and natural bond orbital (NBO) methods, allowing us to evaluate the interplay between T-shape stacking and hydrogen bond interactions. There are good relationships among binding energies, Hammett constants, geometrical parameters, and the results of AIM and NBO analysis in X-ben⊥pyr···H–F complexes.     

The intricacies of the stacking interaction in a pyrrole–pyrrole system

Extensive calculations of potential energy surfaces for parallel-displaced configurations of pyrrole–pyrrole systems have been carried out by the use of a dispersion-corrected density functional. System geometries associated with the energy minima have been found. The minimum interaction energy has been calculated as ?5.38 kcal/mol. However, bonding boundaries appeared to be relatively broad, and stacking interactions can be binding even for ring centroid distances larger than 6 Å. Though the contribution of the correlation energy to intermolecular interaction in pyrrole dimers appeared to be relatively small (around 1.6 smaller than it is in a benzene–benzene system), this system’s minimum interaction energy is lower than those calculated for benzene–benzene, benzene–pyridine and even pyridine–pyridine configurations. The calculation of the charges and energy decomposition analysis revealed that the specific charge distribution in a pyrrole molecule and its relatively high polarization are the significant source of the intermolecular interaction in pyrrole dimer systems.     

Precision studies in supramolecular chemistry: a 1H NMR study of hydroxymethoxyacetophenone/β-cyclodextrin complexes

The association constants for the interactions of 2-hydroxy-4-methoxyacetophenone, 2-hydroxy-5-methoxyacetophenone, 2-hydroxy-6-methoxyacetophenone, 3-hydroxy-4-methoxyacetophenone and 4-hydroxy-3-methoxyacetophenone with β-cyclodextrin in water were measured by (1)H NMR and by isothermal titration calorimetry. Very good agreement was obtained between the different methods. The errors associated with the NMR method for measuring mM binding affinities were estimated to be 10-30%, and by isothermal titration calorimetry, 10-20%. Rotating frame nuclear Overhauser effect spectroscopy studies show that the solution phase host-guest complexes formed by β-cyclodextrin with these hydroxymethoxyacetophenone derivatives are not structurally well defined but that the hydroxymethoxyacetophenone derivatives are mostly associated with the narrow primary hydroxyl rim.     

Does back-bonding involve bonding orbitals in boryl complexes? A theoretical DFT study

Theoretical calculations at the DFT (B3LYP) level have been undertaken on tris- and bis(boryl) complexes. Two model d(6) complexes [Rh(PH(3))(3)(BX(2))(3) and Rh(PH(3))(4)(BX(2))(2)(+), X = OH and H] have been studied. In the model tris(boryl) complex (X = OH) we find a fac structure as a minimum, in accordance with the experimental data. The mer geometries are found to be higher in energy. Analysis of the energetic ordering in mer isomers shows that back-bonding in these complexes involves a bonding Rh-B orbital (and not a d-block orbital as usual). This surprising behavior is rationalized through a qualitative MO analysis and quantitative NBO analysis. Results on the bis(boryl) complex confirm the preceding analysis. Full optimization of unsubstituted (X = H) complexes leads to structures in which the BH(2) moieties are coupled. In the optimal geometry of the bis(boryl) complex, the B(2)H(4) ligand resembles the transition state of the C(2v)-->D(2d) interconversion of the isolated B(2)H(4) species. In the tris(boryl) complex, we find a B(3)H(6) ligand in which the B(3) atoms define an isosceles triangle with one hydrogen bridging the shorter B-B bond.     

Intermolecular complexes of [B6H6]2− with nH2 (n = 1-8) molecules: a theoretical study

Structural Chemistry - Ab initio calculations were used to analyze the interactions among [B6H6]2? dianion with H2 molecules at the MP2/aug-cc-pVTZ computational level. The vibrational...     

The lithium–thiophene interaction: a critical study using highly correlated electronic structure approaches of quantum chemistry

The fundamental multicentric interaction of a lithium atom with a single thiophene ring is addressed. A systematic study of the interaction energy (IE) and geometry for the Li–T charge-transfer complex is done at the MP2 and CCSD(T) levels using increasingly large basis sets up to aug-cc-pVQZ (AVQZ). Basis set superposition errors (BSSE) are evaluated and shown to have a major impact on the value of the IE. The Fixed-Node Diffusion Monte Carlo (FN-DMC) method is used as an alternative basis-set-free approach to obtain what is likely to be the most accurate estimate of the IE obtained so far. While counterpoise-corrected MP2/AVQZ and CCSD(T)/AVTZ interaction energies are found to be ?3.8 and ?7.5 kcal/mol, the FN-DMC method yields +1.3 ± 1.7 kcal/mol. The slow convergence of the ab initio IE (and some key structural parameters) with respect to basis set quality and the discrepancy with the FN-DMC result is discussed. A visualization of the electron pairing using the electron pair localization function (EPLF) for the Li-doped versus undoped thiophene is also presented.     

Ferromagnetic interaction in iron(II)–bis-Schiff base complexes

The preparation and magnetic properties of three Fe(II)–bis-Schiff base complexes, [Fe₂(L1)2(4,4′-bpy)] · MeOH (1), [Fe(L2)(EtOH)] (2) and [Fe(L3)(MeOH)] (3) (L1 = N,N′-bis(2-hydroxy-1-naphthaldehyde)-1,2-phenylenediimine; L2 = N,N′-bis(salicylidene)-1,2-phenylenediamine; L3 = N,N′-bis(5-Cl-salicylidene)-1,2-phenylenediamine; 4,4′-bpy = 4,4′-bipyridine) are reported. X-ray single crystal structure analyses for 1–3 reveal that 1 shows a dinuclear Fe(II)–bis-Schiff base complex bridged by 4,4′-bpy, while 2 and 3 show mononuclear structures. Molecular packing of 2 shows a uniform one-dimensional chain structure through hydrogen bonds and Fe?π interaction and that of 3 indicates significant π–π interaction to form a dimmer structure. The χT–T plots of 1–3 show all ferromagnetic interaction at low temperature. The origin of the ferromagnetic interaction observed in 2 is tentatively ascribed to the dimer formation through Fe?π interaction at low temperature.     

First example of the formation of a Si-C bond from an intramolecular Si-H?H-C diyhydrogen interaction in a metallacarborane: A theoretical study

The recently reported crystal structure of [NMe₄][1-SiMe₂H-3,3′-Co(1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)] shows short contacts between the Si-H proton acceptor group and the C_c-H proton donor moiety in the dicarbollide ligand. These short contacts were studied within the framework of the Quantum Theory of Atoms in Molecules (QTAIM) at different levels of DFT theory (B3LYP/6-311(d,p) and BP86/TZ2P(+)) that shows the existence of a bifurcated Si-H?H-C_c dihydrogen bond. This paper presents the study of an experimental uncommon Si-H group playing as proton acceptor bond in a dihydrogen bond where hydrides like M-H (M, as metal transition), B-H or Al-H usually perform this role. Furthermore, this paper accounts with a new simple method to estimate bonding energies for closed-shell intramolecular interactions in the scheme of Voronoi charge population analysis and Coulomb′s Law.     

Intramolecular CO ⋯ H interaction in Arene(tricarbonyl)-chromium complexes

Unlike benzene(tricarbonyl)chromium which displays two carbonyl stretching vibrations bands in the IR spectrum, analogous tricarbonylchromium complexes of the general formula (C₆H₅ZMe)Cr(CO)₃ [Z = O, CH(OH), N(Pr), CH=CH] are characterized by three carbonyl bands, one of which is displaced to the low-frequency region. The appearance of that band was rationalized in terms of intramolecular interaction between hydrogen atoms in the substituent on the benzene ring and carbonyl groups.     

Inclusion complexes of sulphanilamide drugs and β-cyclodextrin: a theoretical approach

PM3 theoretical methodology was used to access and compare the relative stability of inclusion complexes formed by sulphadiazene, sulphisomidine, sulphamethazine and sulphanilamide with β-cyclodextrin (β-CD). The study predicted that (i) the heterocyclic ring is encapsulated in the hydrophobic part and aniline ring is present in the hydrophilic part of the β-CD cavity and (ii) intermolecular hydrogen bonds were formed between host and guest molecules. The negative free energy and enthalpy changes indicated that all the four inclusion complexation processes were spontaneous and enthalpy driven process. HOMO and LUMO orbital investigation confirmed that the stability increased in the inclusion complexes and also proved no significant change in the electronic structure of the guest and host molecules after complexation.     

The Diels–Alder reactions of para-benzoquinone nitrogen-derivatives: an experimental and theoretical study

An experimental and theoretical study of the comparative reactivity and selectivity of the Diels–Alder reactions of para-benzoquinones and three nitrogen derivatives have been performed. The mono-oximes derivatives do not react under the tested reaction conditions, whereas the tosylated mono-oximes react slowly. However, the mono N-tosyl imines show excellent reactivity, and superior to the parent para-benzoquinones. DFT calculations support these experimental results.     

The structure of μ-oxo dimer of aluminium(III) porphyrin: a theoretical study

The gas-phase molecular structure of μ-oxo dimer of aluminium(III) porphyrin, (AlP)₂O, has been studied for the first time by density functional theory calculations using the B3LYP and M06 functionals and triple-ζ valence basis sets. The molecule has two conformers with equilibrium structures of D _4d and D _4h symmetries with parallel macrocycles and aluminium-oxygen distances of 1.680–1.684 Å (M06/cc-pVTZ). The aluminium atom lies out of the plane of the four central nitrogen atoms and forms a square-based pyramid with them, with the following parameters (M06/cc-pVTZ): r(Al–N) = 2.030–2.031 Å, r(N···N) = 2.803–2.804 Å (the side of the pyramid base), z(Al)–z(N) = 0.434–0.446 Å (the height of the pyramid).     

The Ti2H2 molecule: terminal or bridging hydrogens ?

Density functional calculations carried out for Ti₂H₂ show that several structures are thermodynamically stable with respect to Ti₂+H₂. The ground state of Ti₂H₂ is found to be ³B₁ and it involves two hydrogen bridges in a nonplanar C _{2 v} arrangement. The ¹A₁ state and other triplet states of the bridging structure are only a few kilocalories per mole above the ground state. The ¹Σ _g ⁺ state, which corresponds to the Ti analogue of acetylene, is energetically less favored than the bridging structure but it is still bound relative to Ti₂+H₂. Results are also presented for the Ti₂H molecule. Both bridging and terminal isomers are found to be very stable due to the formation of strong covalent Ti-H bonds. A comparison of the calculated harmonic vibrational frequencies and IR intensities of Ti₂H₂ and Ti₂H isomers with the spectra from matrix isolation IR studies on Ti _x /H₂ indicates that these molecules may have been produced in low-temperature reactions. Received: 15 September 1999 / Accepted: 7 December 1999 / Published online: 19 April 2000