NMR and ab initio studies of Mg,Ca, Zn,Cu alizarin complexes

Complexation of Mg²⁺, Ca²⁺, Zn²⁺ and Cu²⁺ with alizarin, characterized by NMR and theoretical studies (DFT, B3LYP/6-31G(d,p) PCM and NMR GIAO B3LYP/IGLOIII), indicated that the metal ion is attached to two hydroxyl groups in a bidentate mode. The alternative complexation in the bidentate mode, with only one hydroxyl group and a carbonyl oxygen, is less possible as it has a higher energy.     

The electronic states of buta-1,3-diene studied by ab initio configuration interaction and DFT methods,and electron energy loss spectroscopy

The electronic vertical excitation energies for singlet and triplet valence, and Rydberg states of trans-buta-1,3-diene have been computed using ab initio multi-reference multi-root CI procedures with a [4s3p3d3f] set of Rydberg functions. Close numerical agreement between theory and experiment was found for a number of low-lying electronic states.     

Interproton coupling constant variations in 3-membered ring heterocycles. Separation of lone pair and inductive effects

In order to separate inductive and lone pair effects on geminal and vicinal coupling constants in a stereochemically well-defined system, the ¹H NMR spectra of phenylcyclopropane (1), N-methyl-2-phenylaziridine (2), styrene oxide (3) and 1,1-dimethyl-2-phenylaziridinium fluorosulfonate (4) were compared. In D₂O the heterocyclic ring protons of 4 were split into an ABX pattern which gave J(cis) = 8.5, J(trans) = 7.4 and J(gem) = ?4.8 Hz (signs consistent with INDOR results). From the small solvent effects on J(vic) determined from 4-d₁, it was concluded that J(gem) is ?5.0 ± 1.0 Hz in methylene chloride. The absolute values for the coupling constants for 1 and 4 provide a measure of the inductive effect of the ring hetero group on J. Values of J(gem) for 2 and 3 deviated from those predicted on the basis of the above inductive effect, suggesting lone pair contributions to J(gem) of c. +5.5 Hz per lone pair. With this estimate it was possible to predict accurately the J(gem) values for 2-t-butyloxaziridine and 1-t-butyldiaziridine. The values of J(cis) and J(trans) for 2 and 3 likewise suggested a contribution of ?2.5 Hz to J(cis) and ?2.7 Hz to J(trans) per lone pair. The present results suggest that the major factors causing positive J(gem) values in epoxides and aziridines are increased s character to the C? H bonds and lone pair effects, while the so-called electronegativity effect actually operates in the opposite direction to decrease J(gem). Also, the unusually low J(vic) values of epoxides relative to cyclopropanes are now seen to be due more to negative lone pair contributions than to the electron withdrawing ability of oxygen.     

An ab initio molecular orbital study of Li2O2 and LiO2H

An ab initio molecular orbital study, using a Gaussian basis set, of the geometries of Li₂O₂ and LiO₂H is presented. Consistent with the experimental data available, Li₂O₂ is found to have a rhombic structure; the O-Li-O configuration in LiO₂H is found to form an acute triangle. The bonding in these species is discussed to terms of Mulliken population analyses.     

Structural phase transition and 5f-electrons localization of PuSe explored by ab initio calculations

An investigation into the structural phase transformation, electronic and optical properties of PuSe under high pressure was conducted by using the full potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method, in the presence and in the absence of spin-orbit coupling (SOC). Our results demonstrate that there exists a structural phase transition from rocksalt (B 1) structure to CsCl-type (B 2) structure at the transition pressure of 36.3 GPa (without SOC) and 51.3 GPa (with SOC). The electronic density of states (DOS) for PuSe show that the f-electrons of Pu are more localized and concentrated in a narrow peak near the Fermi level, which is consistent with the experimental studies. The band structure shows that B 1-PuSe is metallic. A pseudogap appears around the Fermi level of the total density of states of B 1 phase PuSe, which may contribute to its stability. The calculated reflectivity R(ω) shows agreement with the available experimental results. Furthermore, the absorption spectrum, refractive index, extinction coefficient, energy-loss spectrum and dielectric function were calculated. The origin of the spectral peaks was interpreted based on the electronic structures.     

Potentiometric and ab initio studies of acid-base interactions of substituted 4-halo(Cl, Br)pyridine N-oxide systems

By using the potentiometric method, acidity constants have been determined in systems of tri- and tetra-substituted pyridine N-oxides. The potentiometric measurements in systems of four 4-chloropyridine N-oxide derivatives containing the chlorine atom at position 4 to the NO₂ group and four bromine counterparts were carried out in polar non-aqueous solvents, viz. amphiprotic methanol (MeOH) and aprotic protophilic dimethyl sulfoxide (DMSO). It was found that in all the systems studied the pK_a values were readily determinable (as indicated by small standard deviations) in MeOH, whereas in DMSO large standard deviations were obtained making the pK_a values either hardly determinable or indeterminable from potentiometric measurements. Furthermore, it was demonstrated that the acidity constants of protonated N-oxides studied in MeOH changed according to the sequence of their acidity constants in water. It was also found that in the polar solvents studied, i.e. in the amphiprotic methanol and the highly basic aprotic dimethyl sulfoxide, the cationic homo-conjugation equlibrium constants could not be determined using potentiometric method. Also, by using ab initio methods at the RHF and MP2 levels and the PCM model, utilizing the Gaussian 6-31++G^∗∗ basis set, energies and Gibbs free energies of the protonation reactions of the N-oxides have been determined. The energy parameters have been compared with acidity constants of the protonated N-oxides determined by potentiometric titration in methanol to establish a correlation between these approaches.     

The conformational stability of gaseous 1-butene studied by femtosecond nonlinear spectroscopy and ab initio calculations

1-Butene was investigated by rotational femtosecond degenerate four-wave mixing spectroscopy (fs DFWM) under supersonic expansion conditions as well as by quantum chemical calculations. Fs DFWM is a time-resolved rotational Raman spectroscopy, which allows for precise determination of rotational constants. The experimental fs DFWM spectrum was successfully reproduced by a fitted simulation using a single structure ascribed to the gauche conformer of 1-butene. The obtained rotational constants A = 22.6 ± 1.7 GHz, B = 4.1554 ± 0.0004 GHz and C = 4.0550 ± 0.0004 GHz are in excellent agreement with the ones from microwave spectroscopy. The cis conformer was not observed in the fs DFWM spectrum recorded under the low-temperature conditions of a supersonic expansion. The absence of the cis form along with simulations of a fs DFWM spectrum indicate that the cis conformer is at most equally but most likely less stable than the gauche rotamer. The experimental findings were supplemented by high level quantum chemical calculations, which predict the gauche form to have lower total energy than the cis structure.     

Inversion,internal rotation,and nitrogen nuclear quadrupole coupling of p-toluidine as obtained from microwave spectroscopy and ab initio calculations

A highly accurate combined experimental and theoretical investigation has been conducted on p-toluidine (4-methylaniline) in its vibrational and electronic ground state.     

A study of gaseous 3-membered ring oxonium and sulphonium ions

The 1-methyl-1-thionia cyclopropane 3 and 1-phenyl-1-thionia cyclopropane 4 ions are stable, with lifetimes greater than 10^?5 sec, and can be identified from their collisional activation spectra. Their metastable counterparts (lifetime window 10^?6–10^?5 sec) have undergone ring opening to the isomeric structures CH₃S⁺CHCH₃9 and C₆H₅S⁺CHCH₃11 prior to decomposition. The 1-methyl-1-oxonia cyclopropane 1 and 1-phenyl-1-oxonia cyclopropane 2 ions could not be generated: instead acyclic structures CH₃O⁺CHCH₃5 and C₆H₅O⁺CHCH₃7 were found for both metastable and long living species. Loss of a phenoxy radical from C₆H₅OCH₂CH₂OCH₃ is shown to be preceded by a reciprocal hydrogen transfer and is not due to a SN_i-type reaction.     

Formosalides A and B, cytotoxic 17-membered ring macrolides from a marine dinoflagellate Prorocentrum sp.

Formosalides A (1) and B (2), two novel cytotoxic 17-membered ring macrolides with all-cis tetraenes, a tetrahydropyran ring, and a tetrahydrofuran ring, were isolated from the cultured marine dinoflagellate Prorocentrum sp. Their gross structures, including local relative stereostructures, were elucidated by extensive spectroscopic studies.     

Synthesis of the (S,S,S)-diastereomer of the 15-membered biaryl ring system of RP 66453

The synthesis of the 15-membered biaryl ring system constituting an appropriately functionalized AB ring system of RP 66453 is detailed.     

Synthesis, dynamic behavior, and reactivity of new unsaturated heterotrinuclear 46 valence electron complexes

The reaction of [NBu(4)](2)[(C(6)F(5))(2)Pt(μ-PPh(2))(2)Pd(μ-PPh(2))(2)Pt(C(6)F(5))(2)] (1a) with [AgPPh(3)](+) results in the oxidation of two bridging diphenylphosphanides to give the 46e species [(PPh(3))(C(6)F(5))(2)Pt(2)(μ-P(2)Ph(2))Pd(μ-PPh(2))(μ-Ph(2)P(4)-P(3)Ph(2))Pt(1)(C(6)F(5))(2)] (3). Complex 3 displays two tetracoordinated terminal platinum centers and a central Pd atom that is bonded to three P atoms and that completes its coordination sphere by a rather long (3.237 ?) dative Pt(2) → Pd bond. Complex 3 is also obtained when [(R(F))(2)Pt(μ-PPh(2))Pd(μ-PPh(2))(μ-Ph(2)P-PPh(2))Pt(R(F))(2)] (2) is reacted with PPh(3). Analogously, the addition of PPh(2)Et, CO or pyridine to 2 affords the 46e complexes of general formula [(L)(C(6)F(5))(2)Pt(2)(μ-P(2)Ph(2))Pd(μ-PPh(2))(μ-Ph(2)P(4)-P(3)Ph(2))Pt(1)(C(6)F(5))(2)] (L = PPh(2)Et, 4; L = CO, 6; L = pyridine, 7). The geometry around Pt(2) is determined by the bulkiness of L bonded to Pt. Thus, in complexes 3 (L = PPh(3)) and 4 (L = PPh(2)Et), the ligand L occupies the trans position with respect to μ-P(2), and in 6 (L = CO), the ligand L occupies the cis position with respect to μ-P(2). Interestingly, for 7 (L = py), both isomers 7-trans and 7-cis, could be isolated. Although 4 did not react with an excess of PPh(2)Et, the reaction with the less sterically demanding CH(3)CN ligand resulted in the opening of the Pt(2)-P(2)-Pd cycle with formation of the saturated 48e species [(PPh(2)Et)(C(6)F(5))(2)Pt(μ-PPh(2))Pd(MeCN)(μ-PPh(2))(μ-Ph(2)P-PPh(2))Pt(C(6)F(5))(2)] (8). The saturated 48e complex [(CO)(C(6)F(5))(2)Pt(μ-PPh(2))Pd(MeCN)(μ-PPh(2))(μ-Ph(2)P-PPh(2))Pt(C(6)F(5))(2)] (9) was obtained by acetonitrile addition to 6. Beside the hindered rotation of the pentafluorophenyl groups and a flip-flop motion of the Pd-P-Pt(1)-P-P ring observed at low T, a rotation about the Pt(2)-P(2) bond and a P-C oxidative addition/reductive elimination process occur for 3 and 4 at room temperature. A "through-space" (19)F-(31)P spin-spin coupling between an ortho-F and the P(4) is observed for complexes 3 and 4, having the C(6)F(5) groups bonded to Pt(2) in mutually trans position. The XRD structures of complexes 3, 6, 7-trans, 7-cis, 8, and 9 are described.     

An ab initio molecular orbital study of structures and energies of spirocompounds: spiro[3.3]heptane and spiro[3.3]hepta-1,5-diene

Ab initio molecular orbital theory with the STO-3G and 4-31G basis sets is used to determine the equilibrium geometries, enthalpies of formation, strain energies and spiro-interactions for spiro[3.3]heptane and spiro[3.3]hepta -1,5 - diene. For spiro[3.3]heptane, molecular mechanics calculations suggest that the component cyclobutane rings are puckered to a greater extent than in cyclobutane itself. For spiro[3.3]hepta - 1,5 - diene, STO-3G calculations predict that the component cyclobutene rings deviate slightly from an orthogonal arrangement. Spiro-interactions in spiro[3.3]hepta - 1,5 - diene are revealed by comparing the calculated structural parameters and strain energies with those of appropriate reference systems. The π-orbitals in spiro[3.3]hepta -1,5 -diene are predicted to be split by about 0.4 eV.     

Conformational study of lanthanide(III) complexes of N-(2-salicylaldiminatobenzyl)-1-aza-18-crown-6 by using X-ray and ab initio methods

A structural study of lanthanide complexes with the deprotonated form of the monobracchial lariat ether N-2-salicylaldiminatobenzyl-aza-18-crown-6 (L⁴) (Ln = La(III)–Tb(III)) is presented. Attempts to isolate complexes of the heaviest members of the lanthanide series were unsuccessful. The X-ray crystal structures of [Pr(L⁴)(H₂O)](ClO₄)₂ · H₂O · C₃H₈O and [Sm(L⁴)(H₂O)](ClO₄)₂ · C₃H₈O show the metal ion being bound to the eight donor atoms of the ligand backbone. Coordination number nine is completed by the oxygen atom of an inner-sphere water molecule. Two different conformations of the crown moiety (labelled as A and B) are observed in the solid state structure of the Pr(III) complex, while for the Sm(III) complex only conformation A is observed. The complexes were also characterized by means of theoretical calculations performed in vacuo at the HF level, by using the 3-21G^∗ basis set for the ligand atoms and a 46 + 4fⁿ effective core potential for lanthanides. The optimized geometries of the Pr(III) and Sm(III) complexes show an excellent agreement with the experimental structures obtained from X-ray diffraction studies. The calculated relative energies of the A and B conformations for the different [Ln(L⁴)(H₂O)]²⁺ complexes (Ln = La, Pr, Sm, Ho or Lu) indicate a progressive stabilization of the A conformation with respect to the B one upon decreasing the ionic radius of the Ln(III) ion. For the [Ln(L⁴)(H₂O)]²⁺ systems, most of the calculated bond distances between the metal ion and the coordinated donor atoms decrease along the lanthanide series, as usually observed for Ln(III) complexes. However, our ab initio calculations provide geometries in which the Ln–O(5) bond distance [O(5) is an oxygen atom of the crown moiety] increases across the lanthanide series from Sm(III) to Lu(III).     

Ab initio study of phase transition and bulk modulus of NaH

The phase transition of NaH from NaCl- to CsCl-type structure is investigated by an ab initio plane-wave pseudopotential density functional theory method with the norm-conserving pseudopotential scheme in the frame of the generalized gradient approximation correction; the isothermal bulk modulus and its first and second pressure derivatives of the NaCl- and CsCl-type structures under high pressure and temperature are obtained through the quasi-harmonic Debye model. The phase transition obtained from the usual condition of equal enthalpies occurs at the pressure of 32 GPa, which is consistent with the experimental and other calculated values. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of cell volume V and lattice constant a on temperature T at zero pressure, the isothermal bulk modulus B₀ and its pressure derivatives B₀′and B₀″ on pressure P along isotherms 0, 300, and 600 K, are also successfully obtained.     

Dependence of the lone pair of bismuth on coordination environment and pressure: An ab initio study on Cu4Bi5S10 and Bi2S3

DFT calculations have been carried out for Cu₄Bi₅S₁₀ and Bi₂S₃ to provide an analysis of the relation between electronic structure, lone electron pairs and the local geometry. The effect of pressure is considered in Bi₂S₃ and the results are compared to published experimental data. Bi³⁺ in Cu₄Bi₅S₁₀ is found at both symmetrically and asymmetrically coordinated sites, whereas the coordination environments of Bi in Bi₂S₃ are asymmetric at room conditions and get more regular with increasing pressure. The charge density maps of the asymmetric sites show the lone pairs as lobes of non-shared charge. These lobes are related to an effective Bi s-Bi p hybridization resulting from coupling to S p orbitals, supporting the modern view of the origin of the stereochemically active lone pair. No effective Bi s-p hybridization is seen for the symmetric site in Cu₄Bi₅S₁₀, whereas Bi s-p hybridization coexists with a much reduced lone pair in Bi₂S₃ at high pressure.     

Structure and crystal chemistry of fluorite-related Bi38Mo7O78 from single crystal X-ray diffraction and ab initio calculations

The floating-zone furnace method was used to synthesize single crystals of the fluorite-related δ-Bi₂O₃-type phase Bi₃₈Mo₇O₇₈ for the first time. Single crystal synchrotron X-ray diffraction data, in conjunction with ab initio (density functional theory) calculations, were used to solve, optimize, and refine the 5×3×3 commensurate superstructure of fluorite-type δ-Bi₂O₃ in Pbcn (a=28.7058(11) Å, b=16.8493(7) Å and c=16.9376(6) Å, Z=4, R_F=11.26%, wR_I=21.67%). The structure contains stepped channels of Mo⁶⁺ in tetrahedral environments along the b axis and chains of Mo⁶⁺ in octahedral environments along the ac plane. The role of the stepped channels in oxide ion conduction is discussed. The simultaneous presence of both tetrahedral and octahedral coordination environments for Mo⁶⁺, something not previously observed in Mo⁶⁺-doped δ-Bi₂O₃-type phases, is supported by charge balance considerations in addition to the results of crystallographic and ab initio analysis.