A laser desorption fourier transform mass spectrometric study of dimethyl 8-acetyl-3,7,12,17-tetramethylporphyrin-2,18-dipropanoate

Laser desorption Fourier transform ion cyclotron resonance positive- and negative-ion mass spectra are presented for dimethyl 8-acetyl-3,7,12,17-tetramethylporphyrin-2,18-dipropanoate. The 248-nm laser ionization thresholds for both positive and negative ions are observed to be about 2.5 MW cm^?2. The M⁺˙ molecular ion is assigned to the base peak in the low-power spectra whereas it is the M^?˙ ion for the corresponding anion spectra. Increased intensities of [M + H]⁺ and [M ? H]^? are observed with increased laser fluences of up to 38 MW cm^?2. At high laser powers the negative-ion results reveal that a series of carbon-nitrogen cumulene and polyacetylene cluster ions are formed. Laser evaporation/multiphoton ionization/ and thermal evaporation/electron impact ionization/collision-induced dissociation experiments carried out on the porphyrin M⁺˙ and [M + H]⁺ ions over a range of translational kinetic energies and delay times after acceleration are compared and used to obtain mechanistic and structural information. In contrast to the electron impact experiments, which show only side-chain cleavage, the laser-based collision-induced dissociation experiments reveal that, in addition to side-chain cleavage, it is possible to cleave the porphyrin ring to various extents depending on the ion translational energy selected.     

The Bioconversion of (3RS,E)- and (3RS,Z)-Nerolidol into Oxygenated Products by Streptomyces cinnamonensis. Possible Implications for the Biosynthesis of the Polyether Antibiotic Monensin A??

The ability of the monensin-producing organism Streptomyces cinnamonensis to bioconvert the (E)-and (Z)-isomers of nerolidol (= 3,7,1 1-trimethyldodeca-1,6,10-trien-3-ol) into new oxygenated products has been investigated. When a ³H-labelled racemic form of each sesquiterpene was added to fermentations of S. cinnamonensis, several new ³H-labelled products could be detected. Two products were isolated from bioconversion of (E)-nerolidol, the amide 8 and the 9 (Scheme 2), whereas four products were isolated from the bioconversion of (Z)-nerolidol, the epoxydiol 10 , triols 11 and 12 , and the tetrahydrofuryl alcohol 13 (Scheme 4). Products 9 – 13 were obtained as a 1 : 1 mixture of diastereoisomers, and 12 was shown to arise by the overall anti addition of two OH groups to the trisubstituted (Z)-double bond of (Z)-nerolidol. Both isomers of nerolidol as well as the acetylene 7 are inhibitors of monensin production in shake cultures of S. cinnamonensis.     

The crystal structure of Mn(NO)3P(C6H5)3: The first x-ray analysis of a mononuclear metal trinitrosyl complex

The structure of Mn(NO)₃PPh₃ has been analyzed by single-crystal X-ray diffraction. It shows a tetrahedral geometry with essentially linear nitrosyl groups, and an eclipsed configuration around the MnP bond. Average distances and angles are: MnN 1.686(7) Å, MnP 2.315(2) Å, NO 1.165(10) Å, PC 1.815(4) Å, MnNO 177.2(7)°, PMnN 103.6(2)°, NMnN 114.7(4)°. Final R factor 7.3% for 2064 non-zero reflections. The structure of the five-coordinate nitrito complex Mn(NO)₂(ONO)(PEt₃)₂ is also mentioned briefly.     

Synthesis of digitoxigenin by remote functionalization

Treatment of a 21-hydroxy-20-keto steroid with the mixed anhydride of trifluoroacetic acid—diethylphosphonoacetic acid leads directly to cardenolides by an intramolecular Horner-Emmons reaction. Photolysis of 3β-acetoxy-5β,14α-card-20(22)-enolide and iodobenzenedichloride in benzene solution then yields 3β-acetoxy-5β-carda-14,20(22)-dienolide (β-anhydrodigitoxigenin acetate).     

Complete ozonolysis of alkyl substituted ethenes at -60 degrees C: distributions of ozonide and oligomeric products

The distribution of ozonide and oligomeric structures formed on complete ozonolysis of alkenes in a non-participating solvent at -60 degrees C is governed by the alkyl substitution around the carbon-carbon double bond. The ozonolysis of a 1,1-alkyl substituted ethene generally favours the formation of an ozonide (a 1,2,4-trioxolane). Whereas the ozonolysis of a 1,1,2-alkyl substituted ethene also produces ozonide, a considerable amount of the ozonised products are oligomeric in nature. For example, the ozonolysis of 3-methylpent-2-ene in solution to high conversion in pentane yields oligomers with structural units derived from the fragmentation products of the primary ozonide (a 1,2,3-trioxolane) which are namely butanone carbonyl oxide and acetaldehyde; these can be characterised by electrospray ionisation mass spectroscopy (ESI-MS) under soft ionisation conditions. The predominant oligomers formed are rich in carbonyl oxide units (80 + mol%) and are cyclic in nature. A small proportion of the oligomers formed are open chain compounds with end groups that suggest that chain termination is brought about either by water or by hydrogen peroxide. Residual water in the solvent will react with the carbonyl oxides to produce 2-methoxybut-2-yl hydroperoxide, which we propose readily decomposes generating hydrogen peroxide. A significant yield of oligomers also is obtained from the ozonolysis of a 1,2-alkyl substituted ethene. The ozonolysis of trans-hex-2-ene in pentane yields oligomers containing up to four structural units and are predicted to be mainly cyclic.     

Mechanism of the mild functionalization of arenes by diboron reagents catalyzed by iridium complexes. Intermediacy and chemistry of bipyridine-ligated iridium trisboryl complexes

This paper describes mechanistic studies on the functionalization of arenes with the diboron reagent B(2)pin(2) (bis-pinacolato diborane(4)) catalyzed by the combination of 4,4'-di-tert-butylbipyridine (dtbpy) and olefin-ligated iridium halide or olefin-ligated iridium alkoxide complexes. This work identifies the catalyst resting state as [Ir(dtbpy)(COE)(Bpin)(3)] (COE = cyclooctene, Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl). [Ir(dtbpy)(COE)(Bpin)(3)] was prepared by independent synthesis in high yield from [Ir(COD)(OMe)](2), dtbpy, COE, and HBpin. This complex is formed in low yield from [Ir(COD)(OMe)](2), dtbpy, COE, and B(2)pin(2). Kinetic studies show that this complex reacts with arenes after reversible dissociation of COE. An alternative mechanism in which the arene reacts with the Ir(I) complex [Ir(dtbpy)Bpin] after dissociation of COE and reductive elimination of B(2)pin(2) does not occur to a measurable extent. The reaction of [Ir(dtbpy)(COE)(Bpin)(3)] with arenes and the catalytic reaction of B(2)pin(2) with arenes catalyzed by [Ir(COD)(OMe)](2) and dtbpy occur faster with electron-poor arenes than with electron-rich arenes. However, both the stoichiometric and catalytic reactions also occur faster with the electron-rich heteroarenes thiophene and furan than with arenes, perhaps because eta(2)-heteroarene complexes are more stable than the eta(2)-arene complexes and the eta(2)-heteroarene or arene complexes are intermediates that precede oxidative addition. Kinetic studies on the catalytic reaction show that [Ir(dtbpy)(COE)(Bpin)(3)] enters the catalytic cycle by dissociation of COE, and a comparison of the kinetic isotope effects of the catalytic and stoichiometric reactions shows that the reactive intermediate [Ir(dtbpy)(Bpin)(3)] cleaves the arene C-H bond. The barriers for ligand exchange and C-H activation allow an experimental assessment of several conclusions drawn from computational work. Most generally, our results corroborate the conclusion that C-H bond cleavage is turnover-limiting, but the experimental barrier for this bond cleavage is much lower than the calculated barrier.     

Adsorption of aspartic acid on kaolinite

The interaction of aspartic acid with kaolinite was studied by potentiometric titrations and by adsorption measurements both at constant aspartate concentration (but varying pH) and at a constant pH of 5.5. The temperature was 25 degrees C, and the ionic medium 5 mM KNO3. Aspartic acid dissociation constants estimated from titrations agreed with those from the literature. The adsorption of aspartic acid to kaolinite was weak and varied only slightly with pH; 10-18% of 100 microM aspartic acid adsorbed to kaolinite at 100 m(2)L(-1) between pH 3 and 10. Data from the titrations and adsorption experiments were fitted closely by an extended constant-capacitance surface complexation model, in which monodentate outer-sphere complexes formed between deprotonated aspartic acid molecules and protonated sites on the variable-charge edges of the kaolinite crystals. There appeared to be no adsorption to the permanently charged crystal faces.     

A SIFT ion-molecule study of some reactions in Titan's atmosphere. reactions of N(+), N(2)(+), and HCN(+) with CH(4), C(2)H(2), and C(2)H(4)

The results of a study of the ion-molecule reactions of N(+), N(2)(+), and HCN(+) with methane, acetylene, and ethylene are reported. These studies were performed using the FA-SIFT at the University of Canterbury. The reactions studied here are important to understanding the ion chemistry in Titan's atmosphere. N(+) and N(2)(+) are the primary ions formed by photo-ionization and electron impact in Titan's ionosphere and drive Titan's ion chemistry. It is therefore very important to know how these ions react with the principal trace neutral species in Titan's atmosphere: Methane, acetylene, and ethylene. While these reactions have been studied before the product channels have been difficult to define as several potential isobaric products make a definitive answer difficult. Mass overlap causes difficulties in making unambiguous species assignments in these systems. Two discriminators have been used in this study to resolve the mass overlap problem. They are deuterium labeling and also the differences in reactivities of each isobar with various neutral reactants. Several differences have been found from the products in previous work. The HCN(+) ion is important in both Titan's atmosphere and in the laboratory.     

Curvy-steps approach to constraint-free extended-Lagrangian ab initio molecular dynamics, using atom-centered basis functions: convergence toward Born-Oppenheimer trajectories

A dynamical extension of the "curvy-steps" approach to linear-scaling self-consistent field calculations is presented, which yields an extended-Lagrangian formulation of ab initio molecular dynamics. An exponential parametrization of the one-electron density matrix, expressed in terms of atom-centered Gaussian basis functions, facilitates propagation along the manifold of density matrices in a geometrically correct fashion that automatically enforces idempotency constraints. The extended Lagrangian itself is constraint free, thus neither density matrix purification nor expensive, iterative solution for Lagrange multipliers is required. Propagation is highly efficient, and time steps compare favorably to those used in Car-Parrinello molecular dynamics simulations. The behavior of the method, especially with regard to the maintenance of adiabatic decoupling of nuclei and electrons, is examined for a sequence of diatomic molecules, and comparison is made to trajectories propagated on the converged Born-Oppenheimer surface. Certain claims to the contrary notwithstanding, our results demonstrate that vibrational frequencies may depend on the value of the fictitious mass parameter, even in an atom-centered basis. Light-atom stretching frequencies can be significantly redshifted, even when the nuclear and electronic energy scales are well separated. With a sufficiently small fictitious mass and a short time step, accurate frequencies can be obtained; we characterize appropriate values of these parameters for a wide range of vibrational frequencies.     

Emission band shape probes of the mixed-valence excited state properties of polypyridyl-bridged bis-ruthenium(II) complexes

The absorption spectra and emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that the Ru(II)/Ru(III) electronic coupling is weak in their lowest energy metal to ligand charge transfer (MLCT) excited states. Many of these PP-bridging ligands contain pyrazine moieties and the weak electronic coupling of the excited states contrasts to the strong electronic coupling inferred for the correlated mixed-valence ground states. Although the bimetallic complexes emit at significantly lower energy than their monometallic analogs, the vibronic contributions to their 77 K emission spectra are much stronger than expected based on comparison to the monometallic analogs (around twofold in some complexes) and this feature is characteristic of bimetallic complexes in which the mixed-valence excited states are electronically localized. The weaker excited state than ground state donor/acceptor electronic coupling in this class of complexes is attributed to PP-mediated super-exchange coupling in which the mediating orbital of the bridging ligand (PP-LUMO) is partly occupied in the MLCT excited states, but is unoccupied in the ground states; therefore, the vertical Ru(III)-PP⁻ (MLCT) energy is larger and the mixing coefficient smaller in these excited states than is found for Ru(II)-PP in the corresponding ground states.