Burkholderia cepacia lipase is an excellent enzyme for the enantioselective hydrolysis of β-heteroaryl-β-amino esters

The enantioselective (E >200) lipase PS-catalysed hydrolysis of β-heteroaryl-β-amino esters is described. The reactions were performed with H₂O (0.5 equiv) in either diisopropyl ether or tert-butyl methyl ether at 25 °C. The resulting β-heteroaryl-substituted β-amino acid enantiomers were formed in high enantiomeric excess (ee  97%) and in good yield (40%).     

Study of sol–gel derived di-ureasils doped with zinc triflate

Zinc triflate (Zn(CF₃SO₃)₂)-doped sol–gel derived di-urea cross-linked POE/siloxane ormolytes (designated as di-ureasils) with ∞>n1 (where the salt content is expressed as n, the molar ratio of oxyethylene moieties to Zn²⁺ ions) were investigated. The hybrids with n5 are entirely amorphous; those with n>10 are thermally stable up to approximately 305 °C. The siliceous network of representative samples (n=200 and 10) is essentially composed of (SiO)₃Si(CH₂)-environments and is thus highly branched. The distance between the structural units in samples with 200n10 and n7 is 4.2 and 4.3 Å, respectively. The estimated interdomain distance is 11 and 13 Å for xerogels with 200n20 and n10, respectively. At n=1 a crystalline POE/Zn(CF₃SO₃)₂ complex of unknown stoichiometry is formed. The conductivity maxima are located at n=60 () and n=20 () at 30 and 100 °C, respectively.     

Anisotropic exchange interactions in dinuclear systems (Ln = Dy, Tm, Yb): Magnetometry and Dual Mode X-band Electron Paramagnetic Resonance spectroscopic study

A recently developed experimental and theoretical procedure is used in order to calculate the magnitude and anisotropy of interaction between a lanthanide and a 3d-metal ion. The general formula of the molecular compounds is [Ln(H₂O)₃(dmf)₄(μ-CN)Fe–(CN)₅] · nH₂O where 1  n  1,5 and dmf = N,N′-dimethylformamide, abbreviated as [LnFe] from now on. The main parts of this procedure are (a) the evaluation of the effective g-parameters of the lanthanide ion with the help of EPR measurements. (b) The use of dual mode EPR spectroscopy to define the anisotropic exchange interactions with the help of an anisotropic Hamiltonian model. (c) Use of the same magnetic model to fit magnetization and susceptibility data in order to verify the EPR findings.It was possible to define some trends concerning the exchange components of the [DyFe] dimer according to which the antiferromagnetic isotropic exchange constant is smaller than 4 cm⁻¹ and the anisotropic components are [D_exc, E_exc] = [6(1), 0.0] cm⁻¹. Also for the case of [TmFe] and [YbFe] dimers the antiferromagnetic isotropic exchange constant is smaller than 0.3 cm⁻¹ while the anisotropic components are [D_exc, E_exc] = [12.0, 0.0] cm⁻¹ and [D_exc, E_exc] = [0.4(1), 0.0] cm⁻¹, respectively.     

An efficient new enzymatic method for the preparation of β-aryl-β-amino acid enantiomers

An efficient synthesis of β-aryl-β-amino acid enantiomers has been developed via the lipase-catalysed enantioselective hydrolysis of the corresponding racemic ethyl esters in an organic solvent. High enantioselectivities (E >100) were observed when the lipase PS-catalysed reactions were performed with H₂O (0.5 equiv) in diisopropyl ether at 45 °C. The products could be easily separated and were obtained in good yields of 40%.     

Electroreduction of anions on chemically etched and electrochemically polished Bi(1 1 1) electrode

Electroreduction kinetics of to anions at chemically etched (CHE) and electrochemically polished (EP) Bi(1 1 1) electrodes has been studied using rotating disc electrode method. The surface nanostructure of CHE Bi(1 1 1) and EP Bi(1 1 1) electrodes has been studied by in situ STM and the very different values of root mean squared roughness (Rms) have been obtained (1000 times higher for CHE Bi(1 1 1) (Rms  143 nm) than for EP Bi(1 1 1) (Rms  0.145 nm)). The influence of the nanoroughness of CHE Bi(1 1 1) on the current density, heterogeneous reaction rate constant and corrected Tafel plots (cTp) has been demonstrated. For CHE Bi(1 1 1) the more pronounced inhibition of electroreduction reaction at moderate negative surface charge density has been observed in comparison with EP Bi(1 1 1), caused by the differences in surface charge density and also in diffuse layer ψ₀ potential drop values at crystallographically different homogeneous regions (planes) exposed at the surface of the macroheterogeneous polycrystalline CHE Bi(1 1 1) surface. The very low apparent transfer coefficient α_app obtained indicates the nearly activationless charge transfer mechanism for electroreduction at the CHE Bi(1 1 1) electrode similarly to EP Bi(1 1 1). However, α_app only very weakly depends on Rms for the Bi electrodes at high negative surface charge densities where the values of ψ₀ potential are nearly equal for different planes at fixed electrode potential. At very high negative surface charge densities the cationic catalysis through the adsorbed ion pairs is possible.     

A bonding study of cyclopentene (c-C5H8) adsorption on Ni(1 1 1) surface

The adsorption of cyclopentene (c-C₅H₈) on Ni(1 1 1) was studied using DFT and semiempirical calculations. Preferred site and geometry calculations were carried out considering a Ni(1 1 1) surface and a unit cell of 64-atoms. The tetrahedral threefold hollow position was identified as the most favorable site, with a surface-molecule minimum distance of 1.83 Å. A bending structure is adopted when the molecule is adsorbed where the carbon atoms of the double bond are closer to the surface forming an angle of 160° among non-equivalents carbon atoms. The metal surface was represented by a two-dimensional slab with an overlayer of c-C₅H₈/Ni of 1/9 ratio. We also computed the density of states (DOS) and the crystal orbital overlap populations (COOP) corresponding to CC, CNi, CH, and NiNi bonds. We found that both NiNi bonds interacting with the ring, and the CC bond are weakened after adsorption, this last bond is linked significantly to the surface. The hydrogen atoms belonging to the saturated carbon atoms also participate in the adsorbate–surface bonding. The main interactions include the 4s, 3p_z and 5d_z² bands of nickel and 2p_z bands of the carbon atoms of the double bond.     

Electrochemical wettability control on conductive TiO2 nanotube surfaces modified with a ferrocene redox system

This work reports on an electrochemical system which allows the control of surface wettability properties by voltage induced changes in contact angle (Θ) of ΔΘ  50°. For this we used conductive TiO₂ nanotubular layers that were modified with ferrocene coupled to the TiO₂ surface via triethoxysilane. To enhance the hydrophobic character of the nanotubular TiO₂ surface, also mixed organic monolayers namely perfluorotriethoxysilane, were explored. Formation of the ferrocene and mixed organic monolayer was confirmed by X-ray-photoelectron-spectroscopy (XPS). Contact angle combined with electrochemical measurements show that ferrocene in these monolayers can successfully be switched from Fe²⁺ to Fe³⁺ and that this change in the redox state considerably alters the wetting properties. Using a conductive nanotube substrate allows us to amplify this change by a factor of more than 10, and thus this surface can be used to trigger significant wetting alterations.     

The dielectric response of a colloidal spheroid

In this article, we present a theory for the dielectric behavior of a colloidal spheroid, based on an improved version of a previously published analytical theory [C. Chassagne, D. Bedeaux, G.J.M. Koper, Physica A 317 (2003) 321–344]. The theory gives the dipolar coefficient of a dielectric spheroid in an electrolyte solution subjected to an oscillating electric field. In the special case of the sphere, this theory is shown to agree rather satisfactorily with the numerical solutions obtained by a code based on DeLacey and White's [E.H.B. DeLacey, L.R. White, J. Chem. Soc. Faraday Trans. 2 77 (1981) 2007] for all zeta potentials, frequencies and κa1 where κ is the inverse of the Debye length and a is the radius of the sphere. Using the form of the analytical solution for a sphere we were able to derive a formula for the dipolar coefficient of a spheroid for all zeta potentials, frequencies and κa1. The expression we find is simpler and has a more general validity than the analytical expression proposed by O'Brien and Ward [R.W. O'Brien, D.N. Ward, J. Colloid Interface Sci. 121 (1988) 402] which is valid for κa1 and zero frequency.     

A theoretical study on NH bond dissociation enthalpies of oxo, thio and seleno carbamates and their N-protonated and N-deprotonated species

The effect of N-protonation and N-deprotonation on structure, NH bond dissociation enthalpies (BDEs) and stabilities of radicals formed on H-abstraction from nitrogen atom of carbamates and their thio- and seleno-analogs have been investigated. For those molecules where experimental results are available for comparison, the ROB3LYP/6-311++G(d,p)//B3LYP/6-31+G* theoretical level is in agreement within the estimated experimental uncertainty. The NH BDE of carbamates H₂NC(=X)YCH₃ [X = O; Y = O, S, Se] are higher but lower when X = S, Se and Y = O, S, Se in comparison to NH BDE of NH₃. DFT calculations indicate that the NH bond dissociation enthalpies are decreased by protonation and deprotonation at nitrogen atom; but the effect of deprotonation is rather smaller than the protonation. The variations are analyzed in terms of stabilities of molecules, their protonated and deprotonated species along with their respective radicals. The electron delocalization from nitrogen, X and Y atoms, electrostatic interactions, conjugative interactions and spin delocalization are the important factors affecting the stability. The spin delocalization and shift of radical center to chalcogen X (X = S, Se) are the main determinants for radical stability.     

Dihydrogen, dihydride and in between: NMR and structural properties of iron group complexes

Tabulating the structures and characteristic NMR properties of 17 iron complexes, 98 ruthenium complexes and 70 osmium complexes that contain dihydrogen or compressed dihydride ligands reveals a variety of trends. The HH bond lengths increase from similar Fe(II) to Ru(II) to Os(II) complexes. Iron(II) displays a narrow range of HH distances for stable complexes. Electronegative atoms Cl and O, when attached on the metal trans to the dihydrogen ligand, result in elongation of the HH bond relative to more electropositive atoms H, C, P and N. The family of cyclopentadienyl ligands also causes this elongating effect. The dihydrogen ligands with short HH distances and weak interactions with the metal, especially on iron and ruthenium are in the fast spinning regime. One exception is the biporphyrin complex of ruthenium with the side-on bridging H₂ ligand which has an HH distance of 118 pm but is in the fast spinning regime. There are some ruthenium complexes with HH distances greater than 110 pm that are in the slow motion regime and several complexes of osmium with HH distances greater than 130 pm that are in this regime. The large J_HH due to quantum mechanical exchange coupling are observable for some of these osmium complexes with HH distances in the range of 140–160 pm. The dihydrogen ligands in many complexes appear to have librational motions or other motions that place them in the intermediate motion regime. New equations to correlate J_HD with HH distances for ruthenium dihydrogen complexes and for osmium dihydrogen complexes are introduced here.     

Nucleophilic destruction of organophosphate toxins: A computational investigation

Computed reaction enthalpies, free energies, and activation barriers in vacuo are presented for the nucleophilic detoxification of the organophosphorus compounds (H)(HO)P(O)F, (H)(H₃CO)P(O)F and (H₃C)(CH(CH₃)₂O)P(O)F via the reaction R₁OH + (R₂)(R₃O)P(O)F → (R₂)(R₃O)P(O)(OR₁) + HF for a wide variety of R₁OH nucleophiles. Density functional theory at the B3LYP/6-311++G(d,p) computational level was employed for all the calculations. A multi-step Wright-type reaction mechanism [J. B. Wright, W.E. White, J. Mol. Struct. (THEOCHEM) 454 (1998) 259], which proceeds via a proton transfer from the nucleophile to the fluorine atom through the phosphinyl oxygen atom, was consistently found to have a lower activation barrier in the gas-phase than for the corresponding mechanism that operates via a proton transfer from the nucleophile directly to the fluorine atom. Of the nucleophilic agents investigated, peroxybenzoic acid and o-iodosobenzoic acid had the lowest classical activation barrier for the Wright-type mechanism.     

Atmospheric chemistry of CF3CFCH2: Products and mechanisms of Cl atom and OH radical initiated oxidation

The products of Cl atom and OH radical initiated oxidation of CF₃CFCH₂ were studied in 700 Torr of N₂/O₂ diluent at 296 ± 1 K. The reactions of Cl atoms and OH radicals with CF₃CFCH₂ proceed via electrophilic addition to the double bond. The reaction with chlorine atoms proceeds 56 ± 5% via addition to the central carbon. The chlorine atom initiated oxidation of CF₃CFCH₂ gives CF₃C(O)F in a molar yield which is indistinguishable from 100% and independent of [O₂], and HC(O)Cl in a molar yield which increased from 30% to 59% as [O₂] was increased from 3 to 700 Torr. The OH radical initiated oxidation of CF₃CFCH₂ gives CF₃C(O)F as major product in a yield of 91 ± 6%. The results are discussed with respect to the atmospheric chemistry and environmental impact of CF₃CFCH₂.     

DFT study on the intermolecular interactions between Aun (n = 2–4) and thymine

Density functional B3LYP method with 6-31++G** basis set is applied to optimize the geometries of the luteolin, water and luteolin–(H₂O)_n complexes. The vibrational frequencies are also studied at the same level to analyze these complexes. We obtained four steady luteolin–H₂O, nine steady luteolin–(H₂O)₂ and ten steady luteolin–(H₂O)₃, respectively. Theories of atoms in molecules (AIM) and natural bond orbital (NBO) are used to investigate the hydrogen bonds involved in all the systems. The interaction energies of all the complexes corrected by basis set superposition error, are within −13.7 to −82.5 kJ/mol. The strong hydrogen bonding mainly contribute to the interaction energies, Natural bond orbital analysis is performed to reveal the origin of the interaction. All calculations also indicate that there are strong hydrogen bonding interactions in luteolin–(H₂O)_n complexes. The OH stretching modes of complexes are red-shifted relative to those of the monomer.     

Formal chromium–chromium triple bonds and bent rings in the binuclear cycloheptatrienylchromium carbonyls (C7H7)2Cr2(CO)n (n = 6, 5, 4, 3, 2, 1, 0): A density functional theory study

Binuclear cycloheptatrienylchromium carbonyls of the type (C₇H₇)₂Cr₂(CO)_n (n = 6, 5, 4, 3, 2, 1, 0) have been investigated by density functional theory. Energetically competitive structures with fully bonded heptahapto η⁷-C₇H₇ rings are not found for (C₇H₇)₂Cr₂(CO)_n structures having two or more carbonyl groups. This result stands in contrast to the related (C_nH_n)₂M₂(CO)_n (M = Mn, n = 6; M = Fe, n = 5; M = Co, n = 4) systems. Most of the predicted (C₇H₇)₂Cr₂(CO)_n structures have bent trihapto or pentahapto C₇H₇ rings and CrCr distances in the range 2.4–2.5 Å suggesting formal triple bonds. In some cases rearrangement of the heptagonal C₇H₇ ring to a tridentate cyclopropyldivinyl or tridentate bis(carbene)alkyl ligand is observed. In addition structures with CO insertion into the C₇H₇–Cr bond are predicted for (C₇H₇)₂Cr₂(CO)_n (n = 6, 4, 2). The global minima found for the (C₇H₇)₂Cr₂(CO)_n derivatives for n = 6, 5, and 4 are (η⁵-C₇H₇)(OC)₂CrCr(CO)₄(η¹-C₇H₇), (η³-C₇H₇)(OC)₂CrCr(CO)₃(η^2,1- C₇H₇), and (η⁵-C₇H₇)₂Cr₂(CO)₄, respectively. The global minima for (C₇H₇)₂Cr₂(CO)_n (n = 3, 2) have rearranged C₇H₇ groups. Singlet and triplet structures with heptahapto η⁷-C₇H₇ rings are found for the dimetallocenes (η⁷-C₇H₇)₂Cr₂(CO) and (η⁷-C₇H₇)₂Cr₂, with the singlet structures being of much lower energies in both cases.     

DFT study on second-order nonlinear optical properties of the derivatives of 7-vertex cobalt–carborane metallocenyl

Density functional theory (DFT) B3LYP method was employed to calculate electron properties and the second-order nonlinear optical (NLO) responses of the derivatives which were formed by (C₅H₅)Co(C₂B₄H₆) and CHCHC₆H₄NO₂, CHCHC₆H₄NH₂. The results show: when H atom of (C₅H₅)Co(C₂B₄H₆) is substituted by CHCHC₆H₄NO₂, the β_tot values of isomers are all slightly smaller than that of ferrocene (Fc) derivative (FcCHCHC₆H₄NO₂). However, when H atom of (C₅H₅)Co(C₂B₄H₆) is substituted by CHCHC₆H₄NH₂, the β_tot values of isomers are close to that of ferrocene (Fc) derivative (FcCHCHC₆H₄NH₂). It indicates that (C₅H₅)Co(C₂B₄H₆) can be either a donator or an acceptor.     

Molecular structure and conformation of diethylmethylamine determined by gas electron diffraction and vibrational spectroscopy combined with theoretical calculations

We have investigated the molecular structure and conformation of diethylmethylamine, C(4)H₃C(2)H₂N(1)[CH₃]C(3)H₂C(5)H₃, by gas electron diffraction and vibrational spectroscopy with the aid of theoretical calculations. Diffraction data are consistent with a conformational mixture of 35(14)% tt + 27(14)% g⁺t + 20(17)% g⁻t + 18(23)% g⁺g⁺ where the numbers in parentheses denote three times the standard errors (3σ). Normal-coordinate analysis based on B3LYP/6-311+G^** calculations supports the existence of the four conformers. The dihedral angle ₁(C4C2N1C3) (= −₂(C5C3N1C2)) of the tt conformer was 170(4)° whereas the ₁ and ₂ values of the other conformers were fixed at the B3LYP/6-311++G(2df,p) values: 72.4° and −163.3° for the g⁺t, −66.0° and −158.2° for the g⁻t, and 60.3° and 63.5° for the g⁺g⁺. Average values of the structural parameters (r_g/Å and _α/°) with 3σ are: r(N–C) = 1.462(2), r(C–C) = 1.523(3), r(C–H) = 1.113(2), CNC = 111.6(5), NCC = 114.5(5), NCH/CCH_Me = 110.6(5).     

Facile hydrosilylation of norbornadiene by silanes R3SiH and R2SiH2 with molybdenum catalysts

Photochemically activated [Mo(CO)₆] and [Mo(CO)₄(η⁴-nbd)] have been demonstrated to be very effective catalysts for hydrosilylation of norbornadiene (nbd) by tertiary (Et₃SiH, Cl₃SiH) and secondary (Et₂SiH₂ and Ph₂SiH₂) silanes to give 5-silyl-2-norbornene, which under the same reaction conditions transform in ring-opening metathesis polymerization (ROMP) to unsaturated polymers and to a double hydrosilylation product, 2,6-bis(silyl)norbornane. The yield of a particular reaction depends very strongly on the kind of silane involved. The reaction products were identified by means of chromatography (GC–MS) and ¹H and ¹³C NMR spectroscopy. In photochemical reaction of [Mo(CO)₄(η⁴-nbd)] and Ph₂SiH₂ in cyclohexane-d₁₂, η²-coordination of the SiH bond to the molybdenum atom is supported by ¹H NMR spectroscopy due to the detection of two equal-intensity doublets with ²J_HH = 5.4 Hz at δ 6.12 and −5.86 ppm.     

H NMR Dynamic study of thermal Z/E isomerization of 5-substituted 2-alkylidene-4-oxothiazolidine derivatives: Barriers to rotation about CC bond

The rotational barriers between the configurational isomers of two structurally related push–pull 4-oxothiazolidines, differing in the number of exocyclic CC bonds, have been determined by dynamic ¹H NMR spectroscopy. The equilibrium mixture of (5-ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone (1a) in CDCl₃ at room temperature to 333 K consists of the E- and Z-isomers which are separated by an energy barrier ΔG^# 98.5 kJ/mol (at 298 K). The variable-temperature ¹H NMR data for the isomerization of ethyl (5-ethoxycarbonylmethylidene-4-oxothiazolidin-2-ylidene)ethanoate (2b) in DMSO-d₆, possessing the two exocyclic CC bonds at the C(2)- and C(5)-positions, indicate that the rotational barrier ΔG^# separating the (2E,5Z)-2b and (2Z,5Z)-2b isomers is 100.2 kJ/mol (at 298 K). In a polar solvent-dependent equilibrium the major (2Z,5Z)-form (>90%) is stabilized by the intermolecular resonance-assisted hydrogen bonding and strong 1,5-type S · · · O interactions within the SCCCO entity. The ¹³C NMR Δδ_C(2)C(2′) values, ranging from 58 to 69 ppm in 1a–d and 49-58 ppm in 2a–d, correlate with the degree of the push-pull character of the exocyclic C(2)C(2′) bond, which increases with the electron withdrawing ability of the substituents at the vinylic C(2′) position in the following order: COPh COEt > CONHPh > CONHCH₂CH₂Ph. The decrease of the Δδ_C(2)C(2′) values in 2a–d has been discussed for the first time in terms of an estimation of the electron donor capacity of the S fragment on the polarization of the CC bonds.     

A DFT study on the mechanism of the gas phase reaction of ground-state Y (4d5s,D) with 2-butyne

The reaction of ground-state Y with 2-butyne has been investigated in detail using B3LYP method. Four pathways for elimination of H₂ were identified. Two isomers, Y(HCCC)CH₃ and Y(H₂CCCCH₂) were assigned to the observed product, YC₄H₄. The calculated PESs suggest that the concerted H₂-elimination leading to Y(H₂CCCCH₂) + H₂ product is the most favorable pathway. For the elimination of CH₃, combining the results of this work with our previous study on Y + propyne reaction, a general mechanism for the reactions of Y with 2-alkynes bearing RCCCH₃ structure was established: Y + RCCCH₃ → π-complex → TS(H-migration) → HY(CH₂CC)R → TS (CC insertion) → (CH₂)HYCCR → TS(H-migration) → H₃CYCCR → CH₃ + YC₂R. Such mechanism was found to be always energetically more favorable than the direct sp–sp³ CC bond insertion mechanism. Further, such mechanism can also be applied to the elimination of CH₄ and it can be described as: Y + CH₃CCCH₃ → π-complex → TS (H-migration) → HY(H₂CCC)CH₃ → TS(CC insertion) → (H₂CCC)HYCH₃ → TS(H-migration) → CH₄ + YC₃H₂.     

Heterobimetallic aluminium and zinc complex with N-arylanilido-imine ligand: Synthesis, structure and catalytic property for ring-opening polymerization of ε-caprolactone

Treatment of a N-arylanilido-imine ligand [ortho-C₆H₄(NHAr)CHN]₂CH₂CH₂ (Ar = 2,6-Me₂C₆H₃) (LH₂) with one equiv. of AlMe₃ affords a monometallic complex [C₆H₄(NHAr)–CHN)]CH₂CH₂(C₆H₄(NAr)CHNAlMe₂) (1). The monometallic complex 1 reacts with one equiv. of ZnEt₂ to give a heterobimetallic complex [C₆H₄(NAr)–CHNZnEt]CH₂CH₂[C₆H₄(NAr)–CHNAlMe₂] (2). Both complexes were characterized by ¹H and ¹³C NMR spectroscopy and elemental analyses, and the molecular structures of 1 and 2 were determined by X-ray diffraction analysis. The complexes 1 and 2 both are efficient catalysts for ring-opening polymerization of ε-caprolactone in the presence of benzyl alcohol yielding polymers with narrow polydispersity values and complex 2 initiates the polymerization in a controllable manner.