Electron transfer and structural change: distinguishing concerted and two-step processes

In electron-transfer reactions accompanied by structural changes, the structural change can be concerted with electron transfer or can occur in a separate reaction either preceding or following the electron-transfer step. In this paper we discuss ways of distinguishing concerted reactions from the latter two-step type. Included are recent examples in which no intermediates have been detected in the reactions, thus precluding the direct assignment to the two-step category. In these cases, other means are used to build support for the two-step mechanism with respect to the concerted process. These include an example of structural change preceding electron transfer, a demonstration that the current models of concerted reactions cannot fit the voltammetric data, and a case in which an independent measure of the inner reorganization energy was used to show that the reaction could not be a concerted electron transfer and structural change.     

Neutral hexa- and pentacoordinate silicon(IV) complexes with SiO(6) and SiO(4)N skeletons

The neutral heteroleptic hexacoordinate silicon(IV) complexes 4 and 5 (SiO(6) skeletons) and the neutral pentacoordinate silicon(IV) complexes 7-9 (SiO(4)N skeletons) were synthesized, starting from the hexacoordinate precursor 2 and the pentacoordinate precursor 6, respectively. In these reactions, two monoanionic cyanato-N ligands are replaced by one dianionic bidentate O,O-chelate ligand. Compounds 4, 5, and 7-9 were characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. The chiral silicon(IV) complexes 4, 5, 7, and 8 were obtained as racemic mixtures, whereas 9 was isolated as a cocrystallizate consisting of the two diastereomers, (C,S)-9 and (A,S)-9 (ratio 1:1). The stereodynamics of 5 and 8 were studied by variable-temperature (1)H NMR experiments.     

Photoelectron spectroscopy and electronic structure calculations of d1 vanadocene compounds with chelated dithiolate ligands: implications for pyranopterin Mo/W enzymes

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the electronic structures of open-shell bent vanadocene compounds with chelating dithiolate ligands, which are minimum molecular models of the active sites of pyranopterin Mo/W enzymes. The compounds Cp2V(dithiolate) [where dithiolate is 1,2-ethenedithiolate (S2C2H2) or 1,2-benzenedithiolate (bdt), and Cp is cyclopentadienyl] provide access to a 17-electron, d1 electron configuration at the metal center. Comparison with previously studied Cp2M(dithiolate) complexes, where M is Ti and Mo (respectively d0 and d2 electron configurations), allows evaluation of d0, d1, and d2 electronic configurations of the metal center that are analogues for the metal oxidation states present throughout the catalytic cycle of these enzymes. A "dithiolate-folding effect" that involves an interaction between the vanadium d orbitals and sulfur p orbitals is shown to stabilize the d1 metal center, allowing the d1 electron configuration and geometry to act as a low-energy electron pathway intermediate between the d0 and d2 electron configurations of the enzyme.     

Electronic spectroscopy of jet-cooled HCP+: molecular structure, phosphorus hyperfine structure, and Renner-Teller analysis

Laser-induced fluorescence spectra of jet-cooled HCP(+) and DCP(+) have been obtained with the pulsed discharge technique using HCPDCP and argon precursor mixtures. Transitions involving all of the excited state vibrations have been observed and a set of vibrational constants has been obtained. High-resolution spectra of the (2)Pi(32) components of the 0(0) (0) bands of both isotopomers have been recorded, and these spectra show resolved phosphorus hyperfine structure which allowed the determination of the excited state Fermi contact parameter. The B values were used to obtain the ground and excited state effective geometric parameters as r(0) (")(CH)=1.077(2) A, r(0) (")(CP)=1.6013(3) A, r(0) (')(CH)=1.082(2) A, and r(0) (')(CP)=1.5331(3) A. A Renner-Teller analysis of the ground state vibrational energy levels obtained from the literature was attempted. All of the observed levels of DCP(+) and the majority of those of HCP(+) were satisfactorily fitted with a standard Renner-Teller model, but three HCP(+) levels showed large systematic deviations which could not be accommodated by reassignments or improvements in the Fermi resonance Hamiltonian. Further improvements in the theory or in the experimental data will be needed to resolve this discrepancy.     

Conformationally restricted (+)-cacospongionolide B analogues. Influence on secretory phospholipase A2 inhibition

A new approach to (+)-cacospongionolide was developed to access conformationally restricted variants of the natural product. The flexible aliphatic region between the decalin and side chain portion of the natural product was replaced with alkenyl and alkynyl linkers to probe the influence of structural rigidity in the inhibition of secretary phospholipase A2 (sPLA2). It was found that when the aliphatic section is replaced with a Z-olefin or an alkyne, sPLA2 inhibitory activity suffered relative to the natural product; however, an E-olefin-containing analogue led to an enhanced activity. These results suggest that preferred sPLA2 binding conformation of the natural product is similar to the geometry of the E-olefin-containing analogue.     

Highly effective contact antimicrobial surfaces via polymer surface modifiers

Contact antimicrobial coatings with poly(alkylammonium) compositions have been a subject of increasing interest in part because of the contribution of biocide release coatings to antibiotic resistance. Herein, a concept for antimicrobial coatings is developed on the basis of the thermodynamically driven surface concentration of soft block side chains. The concept incorporates structural and compositional guidance from naturally occurring antimicrobial proteins and achieves compositional economy via a polymer-surface modifier (PSM). To implement this concept, polyurethanes were prepared having random copolymer 1,3-propylene oxide soft blocks with alkylammonium and either trifluoroethoxy or PEGlyted side chains. Six carbon (C6) and twelve carbon (C12) alkylammonium chain lengths were used. The PSMs were first tested as 100% coatings and were highly effective against aerosol challenges of Gram-negative bacteria (Pseudomonas aeruginosa and Escherichia coli). To evaluate the surface concentration, solutions containing 2 wt % PSM with a conventional polyurethane were evaporatively coated onto glass slides. These 2% PSM coatings were tested against aerosol challenges of Gram-negative (Pseudomonas aeruginosa and Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria (107 CFU/mL/30 min). A copolymer soft block containing trifluorethoxy (89 mol %) and C-12 alkylammonium (11 mol %) side chains gave the highest biocidal effectiveness in 30 min: 2 wt %, Gram(+/-) bacteria, 100% kill, and 3.6-4.4 log reduction. A zone of inhibition test showed no biocide release for PSMs and PSM-modified compositions. Characteristics that contribute to concept validation include good hard block/soft block phase separation, a cation/co-repeat group ratio mimicking natural biocidal proteins, a semifluorinated "chaperone" aiding in alkylammonium surface concentration, and a low Tg for the alkylammonium soft block.     

Mixed-valence states formation in conformationally flexible metal-free 5,10,15,20-tetraferrocenylporphyrin and 5,10-bisferrocenyl-15,20-bisphenylporphyrin

Metal-free 5,10,15,20-tetraferrocenylporphyrin and 5,10-bisferrocenyl-15,20-bisphenylporphyrin have been prepared and characterized by UV-Vis, MCD, (1)H, (13)C, and variable-temperature NMR, APCI- and ESI-MS, and M?ssbauer spectroscopy, while their redox properties were investigated using electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation approaches. The electronic structure calculations at Density Functional Theory level reveal that both compounds adopt saddle conformations and the HOMOs in both complexes are predominantly metal-centered, while the LUMOs predominantly consist of porphyrin pi* orbitals. In spite of the rotational freedom of ferrocenyl substituents at room temperature, both metal-free 5,10,15,20-tetraferrocenylporphyrin and 5,10-bisferrocenyl-15,20-bisphenylporphyrin are able to form mixed-valence states upon the successive ferrocene-based two- and one-electron oxidations, respectively, as confirmed by UV-Vis, MCD, M?ssbauer, electro-, and spectroelectrochemical methods, and thus, the earlier suggested (Boyd et al. Chem. Commun., 1999, 637) requirements for the formation of mixed-valence states in ferrocene-containing porphyrins should be revised.     

4,4'-Bipyridine-N-monoxide. A hybrid ligand for building networks using a combination of metal-ligand and hydrogen-bonding interactions

The ligand 4,4'-bipyridine-N-monoxide, (BIPYMO) coordinates through the pyridine N-donor to Pt(II) and Pd(II) to form square planar [ML(4)](2+) complexes and to Cu(II) and Zn(II) to form octahedral trans-[M(H(2)O)(2)L(4)](2+) complexes. Single crystal X-ray structures show that these individual building blocks are organized via hydrogen bonding through the external N-oxide O-atoms to form 2D and 3D networks.     

The formation and properties of acrylic‐polyurea interpenetrating networks formed by reaction injection moulding (RIM)

Interpenetrating networks (IPNs), comprising a crosslinked acrylic as one component and either a rubbery copoly(ether‐urea) or a glassy copoly(urea‐isocyanurate) as the other component, have been formed by reaction injection moulding (RIM). Reaction kinetics during RIM processing of the IPNs were studied using adiabatic temperature rise (ATR) measurements. The effects of (i) crosslinker concentration in the acrylic component and (ii) the weight fraction of acrylic, on the formation of the IPNs during RIM and on the dynamic mechanical properties of finally‐formed IPNS, were evaluated. The results are interpreted in terms of differences in the rates of polymerisation and in the solubilities of the acrylic‐ and polyurea‐forming components, and of the phase‐separated structures of the IPNs.     

Achieving reliability of calculations for flat potential surfaces in density functional theory: The case of Al4 and Al4+1

Harmonic frequencies obtained by finite-differences from nonlocal density functional calculations are presented for the ground states of Al₄ (neutral and cationic). The effect of varying the step size used in the finite-difference evaluation and the influence of the density convergence threshold are discussed. Potential energy curves along the most important normal coordinates are shown. With these results, we found that for Al₄ the square and the rhombus minima are almost degenerate with each other, while for Al₄⁺¹, the rhombus is more stable and the square is a transition state. © 1997 John Wiley & Sons, Inc.