π-Bonding contribution to restricted internal rotations in saccharides

Extensive semiempirical SCF-MO calculations confirm that the exo-anomeric effect in methyl O-, N- and S-glycosides deals with an interaction of π-character along the C₁(SINGLE BOND)Y₁ bond in a X₅(SINGLE BOND)C₁(SINGLE BOND)Y₁(SINGLE BOND)Me moiety (where X = O, S; Y = O, NH, S). The bond-order between orbitals of pπ symmetry on C₁ and Y₁ serves as a measure of all significant molecular orbital interactions responsible for the exo-anomeric stabilization. The set of simpler compounds X(SINGLE BOND)CH₂(SINGLE BOND)Y (X = OH, SH, SeH, TeH; Y = OH, SH, SeH, TeH, NH₂) on which the anomeric effect has been well studied was also calculated and it is noticeable that the π-bond-orders accord with the results of other analyses of the ab initio wave function accounting for the anomeric effect. Although the AM1 and the PM3 parameterizations of MNDO do not accurately reproduce the anomeric effect energetic, they do reproduce accordingly the expected variations in the molecular conformations of complex carbohydrates, and thus it follows that there are maximal π-bond-orders for the synclinal arrangement around the C₁(SINGLE BOND)Y₁ bond. In addition, the π-bond-orders show the same behavior for conformational preferences around the C₁(SINGLE BOND)C′₁ and the C₅(SINGLE BOND)C₆ bonds in methyl C-glycosides and in the hydroxymethyl group of α-D -glucose, respectively. © 1996 by John Wiley & Sons, Inc.     

G2 ab initio calculations on three-membered rings: Role of hydrogen atoms

G2 ab initio calculations on all ABX three-membered rings (TMRs) that can be derived from cyclopropane by systematic substitution of the (SINGLE BOND)CH₂ groups by (SINGLE BOND)NH or (SINGLE BOND)O groups have been performed. Our results show that the decrease in the A(SINGLE BOND)B bond length as the electronegativity of X increases is significantly larger than that found for the corresponding acyclic analogs. In general, a systematic substitution of the (SINGLE BOND)CH₂ groups of cyclopropane by (SINGLE BOND)NH or (SINGLE BOND)O groups implies significant geometric changes that are not reflected in a parallel change of the corresponding conventional ring strain energy (CRSE). When the electronegativity of the groups forming the TMR increases the effect on the CRSE of the system is small, although the charge delocalization inside the ring decreases. The near constancy of the CRSE along the series can be explained in terms of the charge redistribution of the system where the (SINGLE BOND)CH₂ groups play a crucial role. There are, however, significant changes in the hydrogenation energies of the TMR investigated; our results show that, when in an ABX three-membered ring, the electronegativity of X increases the hydrogenation energy of A(SINGLE BOND)B bond decreases and vice versa. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1072–1086, 1998     

Synthesis and properties of polycarbosilanes with the meta-linkage bending unit by hydrosilylation polymerization

The polycarbosilanes (PCS) with meta-linkage bending unit ((SINGLE BOND)Me₂Si(SINGLE BOND)m(SINGLE BOND)C₆H₄(SINGLE BOND)Me₂Si(SINGLE BOND)CH₂CH₂(SINGLE BOND)) were successfully synthesized in mild conditions by hydrosilylation in the presence of [Pt{(CH₂(DOUBLE BOND)CHSiMe₂)₂O}₂]. The PCS obtained were soluble in various solvents owing to the lowering of the crystallinity. These properties are well compared with those of the PCS [(SINGLE BOND)Me₂Si(SINGLE BOND)p(SINGLE BOND)C₆H₄(SINGLE BOND)Me₂Si(SINGLE BOND)CH₂CH₂(SINGLE BOND)]_n. © 1996 John Wiley & Sons, Inc.     

C3(SINGLE BOND)Mα Bond contribution to polarizability tensor and 3J(C1Mα) NMR coupling constant in 1-X-3-M-bicyclo[1.1.1]pentanes

In the present work, the relationship between the large substituent effects on ³J(C₁H) in 1-X-3-M-bicyclo[1.1.1]pentanes, I , and the polarizability of the bridgehead C₃(SINGLE BOND)M_α bond is investigated. The existence of such a relationship is suggested by the finding that the effect of an electronegative substituent X on ³J(C₁M_α) couplings in I (M=H) is due to a distortion of the C₃(SINGLE BOND)H bond toward the C₁ center, which enhances the Fermi contact interaction. If such distortion originates in an electrostatic effect, then in other members of this series it can be expected that the substituent effects on ³J(C₁M_α) couplings should depend strongly on the C₃(SINGLE BOND)M_α bond polarizability. Two approaches are followed. First, the ab initio CLOPPA-IPPP method is applied to study the C₃(SINGLE BOND)M_α bond contribution to the molecular static polarizability tensor in I (M=H, F, CH₃). Such bond polarizabilities are found to follow the same trend as calculated as well as experimentally determined substituent effects on ³J(C₁M_α) couplings, which were measured as part of this work in I [X=H, Cl; M=F, CH₃ and X=OCH₃; M=Sn(CH₃)₃]. Second, ³J(C₁M_α) couplings (M=H, CH₃) are calculated at an ab initio level for X=H, F, and they are compared with those obtained in the parent compound (X=H) if the calculation is carried out in the presence of an inhomogeneous electric field. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 181–188, 1998     

Benzazole-N(SINGLE BOND)BH3 adducts. Reductive transposition of 2-benzimidazole, 2-benzothiazole,and 2-benzoxazole N(SINGLE BOND)BH3 adducts to 1,3,2-benzimidazaborole, 1,3,2-benzoxaborole,and 1,3,2-benzothiazaborole

1,3,2-Benzimidazaborole, 1,3,2-benzoxaborole, and 1,3,2-benzothiazaborole were synthesized from the corresponding 2-benzazole N(SINGLE BOND)BH₃ and 2-benzazole S(SINGLE BOND)BH₃ adducts through a reductive transposition from the isolobal fragment X(SINGLE BOND)C(sp²) (DOUBLE BOND) N(sp²) (SINGLE BOND) B(sp³) (X (DOUBLE BOND) N, O, S) to the fragment X(SINGLE BOND)B(sp²) (DOUBLE BOND) N(sp²) (SINGLE BOND) C(sp³). N(SINGLE BOND)BH₃ substitution shifts to lower frequencies 4-H, C-3a, and C-7a resonances. The X-ray diffraction analysis of 2-(o-methoxyphenyl)benzothiazole N(SINGLE BOND)BH₃ adduct is reported. Two new tetracyclic boron-bridged compounds were observed as by-products (6,9-(ethyl)-diaza-2-oxa-1-bora[3,4,7,8]-dibenzobycyclo[4.3.0]-nona-3,7-diene, 6d, and 8-aza-9-oxa-2-thia-1-bora-[3,4,7,8]dibenzobycyclo[3.4.0]nona-3,7-diene, 15d, when 2-(o-methoxyphenyl)-1-ethylbenzimidazole-BH₃ 6b and 2-(o-methoxyphenyl)-benzothiazole-BH₃ 15b adducts were heated. © 1996 John Wiley & Sons, Inc.     

The hydroxyl radical reaction rate constant and products of ethyl 3-ethoxypropionate

The relative rate technique has been used to measure the hydroxyl radical (OH) reaction rate constant of ethyl 3-ethoxypropionate (EEP, CH₃CH₂(SINGLE BOND)O(SINGLE BOND)CH₂CH₂C(O)O(SINGLE BOND)CH₂CH₃). EEP reacts with OH with a bimolecular rate constant of (22.9±7.4)×10⁻¹² cm³ molecule⁻¹s⁻¹ at 297±3 K and 1 atmosphere total pressure. In order to more clearly define EEP's atmospheric reaction mechanism, an investigation into the OH+EEP reaction products was also conducted. The OH+EEP reaction products and yields observed were: ethyl glyoxate (EG, 25±1% HC((DOUBLE BOND)O)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH₂CH₃), ethyl (2-formyl) acetate (EFA, 4.86±0.2%, HC((DOUBLE BOND)O)(SINGLE BOND)CH₂(SINGLE BOND)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH₂CH₃), ethyl (3-formyloxy) propionate (EFP, 30±1%, HC((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH₂CH₂(SINGLE BOND)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH₂CH₃), ethyl formate (EF, 37±1%, HC((DOUBLE BOND)O)O(SINGLE BOND)CH₂CH₃), and acetaldehyde (4.9±0.2%, HC((DOUBLE BOND)O)CH₃). Neither the EEP's OH rate constant nor the OH/EEP reaction products have been previously reported. The products' formation pathways are discussed in light of current understanding of oxygenated hydrocarbon atmospheric chemistry. © 1997 John Wiley & Sons, Inc.     

Chain transfer by addition-fragmentation mechanism. VII. Radical polymerization of vinyl monomers in the presence of ethyl 2-[1-(trimethylsilylperoxy)ethyl]propenoate and related compounds

Ethyl 2-[1-(trimethylsilylperoxy)ethyl]propenoate 1 , ethyl 2-[1-(dimethylvinylsilylperoxy)-ethyl]propenoate 2 , ethyl 2-[1-(1-(2-ethoxycarbonyl-1-methyl-2-propenylperoxysilyl)-1-methylethylperoxy)ethyl]propenoate 3 , and 2-phenyl-2-trimethylsilylperoxypropane 4 were synthesized and added to the free radical polymerization of vinylic monomers. 1 and 2 were found to show no homopolymerizability but act as effective chain transfer reagents in radical polymerizations of methyl methacrylate (MMA), styrene (St), and n-butyl acrylate (BA). The estimated chain transfer constants (C_tr) are as follows: C_tr ( 1 ) = 0.15 for MMA, 0.90 for St, and 2.03 for BA at 60°C; C_tr ( 2 ) = 0.12 for MMA, 1.16 for St, and 1.9 for BA at 60°C. ¹H–NMR spectra of poly(St) formed in the presence of 1 is consistent with the view that the polymers bear an oxirane at one terminal and an trimethylsilyloxy fragment at the other end. Moreover, peroxysilane 4 showed very low transfer properties by direct homolytic substitution (SH₂). These findings indicate that the ethyl 2-[1-(substituted dimethylsilylperoxy)ethyl]-propenoates 1–3 undergo chain transfer reaction via a intramolecular homolytic substitution (SHi) following an addition process. Preparation of poly(styrene) up to high conversion in the presence of 3 yielded to the formation of the corresponding polymeric structures bearing hydrolysable C(SINGLE BOND)O(SINGLE BOND)Si(SINGLE BOND)O(SINGLE BOND)C bonds. © 1996 John Wiley & Sons, Inc.     

Ab initio studies of three-membered ring formation through intramolecular nucleophilic substitution

Three-membered ring (3MR) forming processes of ⁻X(SINGLE BOND)CH₂(SINGLE BOND)CH₂(SINGLE BOND)F and ⁻CH₂(SINGLE BOND)C((SINGLE BOND)Y)(SINGLE BOND)CH₂(SINGLE BOND)F (X(DOUBLE BOND)CH₂, O, or S and Y(DOUBLE BOND)0 or S) through a gas phase neighboring group mechanism (S_Ni) are studied theoretically using the ab initio molecular orbital method with the 6–31+G* basis set. When electron correlation effects are considered, the activation (ΔG^≠) and reaction energies (ΔG⁰) are lowered by ca. 10 kcal mol⁻¹, indicating the importance of the electron correlation effect in these reactions. The contribution of entropy of activation (−TΔS^≠) at 298 K to ΔG^≠ is very small, and the reactions are enthalpy controlled. The ΔG^≠ and ΔG⁰ values for these ring closure processes largely depend on the stabilities of the reactants and the heteroatom acting as a nucleophilic center. The Bell–Evans–Polanyi principle applies well to all these reaction series. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1773–1784, 1997     

A density functional study of Fe(SINGLE BOND)N2, Fe(SINGLE BOND)N+2, and Fe(SINGLE BOND)N−2

The interaction of an iron atom with molecular nitrogen was studied using density functional theory. Calculations were of the all-electron type and both conventional local and gradient-dependent models were used. A ground state of linear structure was found for Fe(SINGLE BOND)N₂, with 2S + 1 = 3, whereas the triangular Fe(SINGLE BOND)N₂ geometry, of C_2v symmetry, was located 2.1 kcal/mol higher in energy, at least for the gradient-dependent model. The reversed order was found using the conventional local approximation. In Fe(SINGLE BOND)N₂, the N(SINGLE BOND)N bond is strongly perturbed by the iron atom: It has a bond order of 2.4, a vibrational frequency of 1886 cm⁻¹, and an equilibrium bond length of 1.16 Å: These values are 3.0, 2359 cm⁻¹, and 1.095 Å, respectively, for the free N₂ molecule. With the gradient-dependent model and corrections for nonsphericity of the Fe atom, a very small binding energy, 8.8 kcal/mol, was calculated for Fe(SINGLE BOND)N₂. Quartet ground states were found for both Fe(SINGLE BOND)N⁺₂ and Fe(SINGLE BOND)N⁻₂. The adiabatic ionization potential, electron affinity, and electronegativity were also computed; the predicted values are 7.2, 1.22, and 4.2 eV, respectively. © 1997 John Wiley & Sons, Inc.     

Calculation of solvation free energy using RISM theory for peptide in salt solution

We developed a robust, highly efficient algorithm for solving the full reference interaction site model (RISM) equations for salt solutions near a solute molecule with many atomic sites. It was obtained as an extension of our previously reported algorithm for pure water near the solute molecule. The algorithm is a judicious hybrid of the Newton–Raphson and Picard methods. The most striking advantage is that the Jacobian matrix is just part of the input data and need not be recalculated at all. To illustrate the algorithm, we solved the full RISM equations for a dipeptide (NH₂(SINGLE BOND)CHCH₃(SINGLE BOND)CONH(SINGLE BOND)CHCH₃(SINGLE BOND)COOH) in a 1 M NaCl solution. The extended simple point charge (SPC/E) model was employed for water molecules. Two different conformations of the dipeptide were considered. It was assumed for each conformation that the dipeptide was present either as an un-ionized form or as a zwitterion. The structure of the salt solution near the dipeptide and salt effects on the solvation free energy were also discussed. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1724–1735, 1998     

An original route to poly[silylene-co-(2,4-disilapentane-2,4-diyl)] oligomers,efficient precursors of silicon carbide-based ceramics with variable C/Si ratios

Novel oligomers possessing a backbone formed of ((TRIPLE BOND)Si(SINGLE BOND)CH₂(SINGLE BOND)Si(TRIPLE BOND)) and (SINGLE BOND)Si(SINGLE BOND)_n units were prepared by the copolycondensation of bis(chlorosilyl)methanes and various dichlorosilanes in the presence of sodium, in refluxing toluene. The effect of the respective molar ratios of comonomers on the yields and the structure of the copolymers was investigated. The role of substituents on silicon atoms in the ability of these materials to provide convenient ceramic precursors upon pyrolysis was examined. When (TRIPLE BOND)Si(SINGLE BOND)H bonds were present, thermal cross-linking was readily performed and ceramics possessing variable C/Si ratios were prepared.     

Cyanide adsorbed on coinage metal electrodes: A relativistic density functional investigation

The adsorptive properties of cyanide (CN⁻) on coinage metal (M) electrodes (M=Cu, Ag, Au) have been investigated using a relativistic density functional method. The way to model the electrochemical potential applied to the electrodes is to consider the systems in the presence of a perturbative external field F. The field-perturbative approach is proven to be a suitable method in interpreting the observed spectral shifts with electrode potential. The calculated potential-dependent shifts of ω_{M(SINGLE BOND)CN} and ω_{C(SINGLE BOND)M} are similar for the three metals, in agreement with experiment observations. The relativistic effects are required to account for the similarity in the frequency shifts of ω_{M(SINGLE BOND)CN}. The calculated vibrational tuning rates dω_{C(SINGLE BOND)N}/dF are 6.61×10⁻⁷, 6.61×10⁻⁷, and 5.64×10⁻⁷ cm⁻¹/(V/cm) for M=Cu, Ag, and Au, respectively. The coupling of the M(SINGLE BOND)CN and C(SINGLE BOND)N internal modes contributes significantly (about 25%) to the size of the frequency shifts Δω_{C(SINGLE BOND)N} of the ligand. The effect of electric fields on the metal(SINGLE BOND)CN⁻ bonding is also investigated. It is shown that changes in the magnitude of CN⁻ to the metal donation and M(SINGLE BOND)CN bond strength occur under the influence of the electric field. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 175–185, 1998     

Molybdenum (VI) dioxodihalides: Agreement with experiment and prediction of unknown properties through density functional theory

Density functional calculations are reported for the molecular structures, harmonic vibrational frequencies, UV/visible spectra, and oxo-transferability of MoO₂X₂ (X = F, Cl, Br, I). Available experimental data have been used to check the validity of the theoretical calculations. Given the good agreement between theory and gas-phase experiment, predictions have been issued for the less studied members of this family of compounds. Furthermore, electronic spectra of the full series have been computed for the first time. For all transitions studied, excitation energies decrease in the order F > Cl > Br > I. Finally, the labilization of Mo(SINGLE BOND)O bonds generated by the HOMO(SINGLE BOND)LUMO transition, which is related to the oxygen-atom transfer reaction in the active site of molybdenum oxidoreductases, was also investigated. For MoO₂Cl₂ and MoO₂Br₂ compounds, the HOMO-LUMO transition yields a considerable lengthening of the Mo(SINGLE BOND)O bond, yet not requiring a large excitation energy. © 1997 John Wiley & Sons, Inc.     

Internal conrotation and disrotation in H2BCH2BH2 and diborylmethane 1,3 H exchange

The paths of correlated internal disrotation (barrier less than 0.4 kcal/mol) and conrotation (barrier around 1.9 kcal/mol) of the two BH₂ groups in H₂BCH₂BH₂ have been computed employing ab initio [MP2(full)/6–31G**] and density functional theory (Becke3LYP/6–311+G**) methods. Two B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative interactions stabilize the C_s symmetric H₂BCH₂BH₂ isomer ( 1 ). The B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative stabilization, evaluated by homodesmotic reactions and using the orbital deletion procedure (which “deactivates” the “vacant” born p orbital), is less than 6 kcal/mol in diborylmethane. The B(SINGLE BOND)C(DOTTED BOND)B(p) stabilization is shown to be remarkably large in C₄B₆H₁₀ (Td). At MP2(fu)/6–31G**, disproportionation into 1 and methane is only 5.6 kcal/mol exothermic. The 1,3 H exchange in diborylmethane is an asynchronous process and proceeds via a doubly bridged cyclic intermediate with 9.3 kcal/mol barrier. Structures with “planar tetracoordinate” carbon are stabilized considerably by BH₂ substituents, but they are still high in energy. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1792–1803, 1997     

Estimates of the structure and dimerization energy of polyacetylene from ab initio calculations on finite polyenes

Ab initio calculations at the Hartree-Fock (HF) and the second-order Møller-Plesset (MP2) levels are performed for finite polyenes C_2nH_2n+2 to estimate the structure and dimerization energy (E_dim) of polyacetylene. The effect of electron correlation on the structure of finite polyenes is analyzed in detail. The MP3/6–31G* C(DOUBLE BOND)C and C(SINGLE BOND)C bond lengths in polyacetylene are estimated by a simple extrapolation method using empirical corrections for the MP2 deficiencies, yielding values [C(DOUBLE BOND)C(MP3) ∼ 1.36 Å and C(SINGLE BOND)C(MP3) ∼ 1.44 Å] that are in a good agreement with experiment (C(DOUBLE BOND)C (DOUBLE BOND) 1.36 Å and C(SINGLE BOND)C (DOUBLE BOND) 1.44–1.45 Å). Comparison is also made with other theoretical estimates of polyacetylene structure. E_dim is approximated by the energy difference between the equilibrium and hypothetical polyenic structures. It is estimated that E_dim is ∼ 1.4–1.5 kcal/mol (0.06–0.07 eV) per carbon-carbon bond at the HF level with 4–21G and 6–31G* basis sets and ∼ 0.3–0.5 kcal/mol (0.013–0.022 eV) at the MP2 level with the 6–31G* basis set. It is concluded that E_dim is very sensitive to the level of approximation employed so that a proper treatment of electron correlation is essential to obtain a reliable estimate of the dimerization energy. © 1997 John Wiley & Sons, Inc.     

Density functional calculations on Jahn-Teller effect of tetrachloromethane cation

Density functional (DF) calculations of the tetrachloromethane cation and its most important competitive process, the formation of CCl⁺₃, were carried out to explain the possible stability of CCl⁺₄. From results obtained with B-LYP and B-P86 methods, it is possible to produce a slight Jahn-Teller (JT) effect for a C_s planar structure of the cation type CCl₂(SINGLE BOND)Cl(SINGLE BOND)Cl⁺ compatible with the experimental data obtained by electron-spin resonance spectroscopy. A complex of C_3v structure CCl⁺₃(SINGLE BOND)Cl which is similar to the previous one found in CF⁺₄ appears when symmetry-broken wave functions are used in HF-LYP and HF-P86 methods. Depending of the DF method employed, either one of the minima [C_s (planar) and C_3v] is the most stable and competes with the dissociation of the molecular ion to give CCl⁺₃. The JT stabilization energy is smaller when the JT active coordinates are considered. © 1997 John Wiley & Sons, Inc.     

Molecular structure and binding energies of monosubstituted hexacarbonyls of chromium,molybdenum, and tungsten: Relativistic density functional study

Relativistic density functional calculations have been carried out for the group VI transition metal carbonyls M(CO)₅L (M=Cr, Mo, W; L=OH₂, NH₃, PH₃, PMe₃, N₂, CO, OC (isocarbonyl), CS, CH₂, CF₂, CCl₂, NO⁺). The optimized molecular structures and M(SINGLE BOND)L bond dissociation energies, as well as the metal–carbonyl bond energy of the trans CO group, have been calculated. Besides the marked dependence of the trans M(SINGLE BOND)CO bond length on the type of ligand L, such an effect on the that bond energy is also observed. For the chromium compounds, the trans Cr(SINGLE BOND)CO bond length varies from 184 to 199 pm and its bond energy from 242 to 150 kJ/mol. For the molybdenum compounds, the range is 197 to 216 pm and 253 to 128 kJ/mol and, for tungsten, 198 to 214 pm and 293 to 159 kJ/mol. The observed trends can be explained with the π acceptor strength of the L ligand. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1985–1992, 1997     

Cis–trans isomerism in a square‐planar platinum(II) complex bearing bulky fluorinated phosphane ligands

Transition‐metal complexes bearing fluorinated phosphane and thiolate ligands has been an area of study in recent years and the chemical context of the current work is related to the metal‐assisted functionalization of fluorinated derivatives. The cis and trans isomers of the square‐planar complex bis[(pentafluorophenyl)diphenylphosphane‐κP]bis(2,3,5,6‐tetrafluorobenzenethiolato‐κS)platinum(II), [Pt(C₆HF₄S)₂{P(C₆H₅)₂(C₆F₅)}₂], have been crystallized from a single chromatographic fraction and characterized by X‐ray diffraction analysis. The stabilization of the cis isomer results from weak intramolecular π‐stacking interactions and possibly from the formation of a C—F…Pt contact, characterized by an F…Pt separation of 2.957 (6) Å. The natural bond orbital analysis (NBO) for this isomer confirms that the corresponding F → Pt charge transfer accounts for 6.92 kcal mol⁻¹ in the isomer stabilization. Such interactions are not present in the centrosymmetric trans isomer.     

Investigations on organoantimony compounds : XVII. Preparation and properties of heterocyclic trans-dichloro-β-diketonato-cis-diorganoantimony(V) compounds. Influence of stereochemistry on β-diketonate ligand exchange reactions in octahedral dichloro-β-diketonatodiorganoantimony(V) compounds

A series of heterocyclic trans-dichloro-β-diketonato-cis-diorganoantimony(V) compounds of the type R₂SbCl₂X (R₂ = (CH₂)₄, (CH₂)₅, o,o′−C₆H₄C₆H₄, o,o′−C₆H₄CH₂C₆H₄; X = Acac, Dpm) has been synthesized. The stereochemistry of these compounds has been deduced from PMR spectroscopic and molecular dipole moment data. Since the cis-dichloro-β-diketonato-trans-diorganoantimony(V) compounds R₂SbCl₂Acac (R = Me, Et, Ph) were known previously, a set of both cis- and trans-diorgano main group organometallic complexes has thus been made available, which allows a comparative study of the influence of stereochemistry on the strength of metal—ligand interactions in this type of octahedral d¹⁰ metal complex. β-Diketonate—ligand exchange reactions have been studied by PMR spectroscopy, and a marked influence of stereochemistry observed. trans-Dichloro-β-diketonato-cis-diorganoantimony(V) compounds undergo ligand exchange only slowly, if at all, whereas cis-dichloro-β-diketonato-trans-diorganoantimony(V) compounds react instantaneously. Both PMR chemical shift data and IR spectroscopic data point to the occurrence of a stronger antimony-β-diketonate interaction in trans-dichloro-β-diketonato-cis-diorganoantimony than in cis-dichloro-β-diketonato-trans-diorganoantimony compounds. This can be understood in terms of the hybridization of the antimony valence orbitals. The results are in line with the assumption that Sb---O bond rupture is the rate-determining step in β-diketonate ligand exchange.     

Impurity levels and nonlinear optical properties of doped BaTiO3 from extended cluster LDA calculations

The electronic structures of some transition-metal ions doping the ferroelectric oxide BaTiO₃ are calculated through the density functional theory framework (LCAO-LSD ADF method) on extended clusters [XO₆Ba₈Ti₆] embedded in a punctual charge set depicting the crystalline environment. The transition ions X are Feⁿ⁺(n = 2, 3, 4, 5), Ti³⁺, Nb⁴⁺, and Nb⁵⁺. Some related defects like Fe(SINGLE BOND)V_o, where V_o stands for a lacunar oxygen site, are also investigated through a similar process. The positions of impurity levels insides the O_2p(SINGLE BOND)Ti_3d band gap are obtained from eigenvalues and related optical transition or ionization energies are calculated using excited states. This allows us to discuss the photorefractive effect present in such doped materials. The EPR fine-structure parameters a and D are also derived for Fe³⁺ in cubic and tetragonal symmetries from crystal field calculations using parameters drawn from molecular orbital results. © 1997 John Wiley & Sons, Inc.