The appearance of a compensation effect in the thermal decomposition of manganese(II) carbonates, prepared in the presence of other metal ions

In this study, a compensation effect is observed for the thermal decomposition of manganese(II) carbonates, prepared in the presence of Al³⁺ and Na⁺ ions. This compensation effect is described by the equation log A = aE + b, and the parameters are showm to be a = 0.1 and b = −2.9. The mechanism of decomposition was found to follow first order kinetics.

Both A, the pre-exponential function, and E, the energy of activation, depended on the concentration and type of metal ion present in the carbonate preparation, and on the experimental method used to obtain Arrhenius parameters. In the rising temperature experiments, more than one Arrhenius plot was obtained over different temperature ranges.     

Modeling the effect of solvation on solute retention in reversed-phase liquid chromatography

The retention of a homologous series of alkylbenzenes was determined on octyl and octadecyl reversed-phase columns in several polar organic liquids. Free energies of transfer were calculated by the SM5.0R classical solvation model for each organic liquid tested and for several alkanes. The relationships between the measured retention factors and the calculated free energies of transfer were then investigated. Although the natural logarithms of the retention factor and the calculated free energies of transfer were linearly correlated, the obtained free energies of transfer of the solutes did not completely explain the retention behavior of the solutes. Nonetheless, even in these pure organic liquids, the energetics of RPLC retention behaved very similarly to those of partitioning.     

A study of linear versus angled rigid rod polymers for proton conducting membranes using sulfonated polyimides

Linear and angled monomers were incorporated into the main chain of a polyimide in order to investigate the effect of kinked versus linear polymers on membrane properties such as water uptake and proton conductivity. Polymers prepared using linear 4,4′-sulfonyldianiline, SPI1, and using angled 3,4′-sulfonyldianiline, SPI2, were cast into membranes possessing ion exchange capacities that varied from 0.79 to 2.75 meq g⁻¹. Membranes are thermally stable up to 300 °C under air. Proton conductivity of both membranes increases with temperature to values of 0.1-0.2 S cm⁻¹. The conductivity of angled, SPI2 membranes is greater than those prepared from SPI1 for a given IEC but water uptakes are lower. These differences are attributed to increased entanglements of the angled polymers, which limits the degree of swelling and increases the proton concentration. These results may be important in the design of proton conducting membranes from other rigid polyarylenes.     

A transition-metal-catalyzed enediyne cycloaromatization

The pentamethylcyclopentadienyl iron cation, generated from [(eta5-C5Me5)Fe(NCMe)3]PF6, triggers the room temperature cycloaromatization of acyclic and alicyclic enediynes, in the presence of either 1,4-cyclohexadiene or terpinene as the hydrogen-atom donor, to give metal-arene products in good to excellent yields. Photolysis of the metal-arene complexes liberates the arene from the metal in excellent yield. The first demonstration of a transition-metal-catalyzed cycloaromatization of conjugated enediynes has been achieved under photochemical conditions utilizing either [(eta5-C5Me5)Fe(NCMe)3]PF6 or [(eta5-C5Me5)Fe(eta6-1,2-(Prn)2C6H4)]PF6 as the catalyst precursor. The use of a metal and light has led to a convenient method for cycloaromatization of a trans-enediyne.     

Fluorescence decay parameters for pyrene (S1) in micellar and homogeneous liquid dispersions

Relative quantum yields and time constants for the fluorescence from pyrene (S₁) stimulated by UV light have been measured for dispersions of the aromatic in several liquid solvents and aqueous surfactant micelles. Values of the relative radiative decay parameter, k_F, were extracted and its medium dependence tabulated. This parameter was found to vary with medium polarity, being higher in more polar media. This effect, characterized for homogeneous liquid phases, was used to demonstrate that pyrene in surfactant micelles is strongly affected by the polar influences of water molecules which deeply penetrate the micelle in the region of the probe.     

Structural, optical, and photophysical properties of nickel(II) alkylthioporphyrins: insights from experimental and DFT/TDDFT studies

The ground- and excited-state properties of a Ni(II) porphyrin bearing peripheral alkylthio group, NiOMTP (OMTP = 2,3,7,8,12,13,17,18-octakis methylthio porphyrinate) have been investigated by steady-state and time-resolved absorption spectrometry and DFT/TDDFT theoretical methods. Several conformations corresponding to different deformations of the porphyrin core and to different orientations of the alkylthio groups have been theoretically explored. The nearly degenerate, purely ruffled D(2d) and hybrid (ruffled with a modest degree of saddling) D(2) conformations, both characterized by an up-down (ud) orientation of the vicinal methylthio groups are by far the preferred conformations in the "gas phase". In contrast to NiOEP, it is the orientation of the peripheral substituents rather than the type and degree of distortions of the porphyrin core that determines the stability of the NiOMTP conformers. The ground-state electronic absorption spectra of NiOMTP exhibit significant changes compared to its parent NiP and beta-alkylated analogues, such as NiOEP, resulting in a considerable red shift of the B and the Q bands, intensification and broadening of the Q band, and additional weak absorptions in the region between the Q and B bands. These spectral changes can be understood in terms of the electronic effects of the methylthio groups with nonplanar distortions of the porphyrin ring playing a very minor role. Transient absorption measurements with sub-picosecond resolution performed in toluene and TDDFT calculations reveal that following photoexcitation, NiOMTP deactivates by the pathway 1(pi,pi) --> 3(d(z2),d(x2-y2))--> ground state. The (d,d) state exhibits complex spectral evolution over ca. 8 ps, interpreted in terms of vibrational relaxation and cooling. The cold ligand-field excited state decays with a lifetime of 320 ps. At variance with the highly distorted nickel porphyrins but similar to the planar analogues, the (d,d) spectrum of NiOMTP has transient absorption bands immediately to the red of the bleaching of the ground-state Q and B bands.     

Excited state relaxation dynamics of the zinc(II) tetraphenylporphine cation radical

A new technique employed to study the photophysical properties of the zinc(II) tetraphenylporphine cation radical is reported. It employs a combination of controlled potential coulometry and femtosecond absorption spectrometry. The fast transient lifetime of 17 ps of the pi-cation species originates in very efficient mixing of the a(2u) HOMO cation orbital that places electronic density mainly on pyrrolic nitrogens and metal d-orbitals. That explains the lack of any emission of the cationic species. This nonradiative decay process might elucidate the processes taking place in photosynthetic systems when an electron is removed from the porphyrinic moiety and the hole is produced.     

A LASER FLASH KINETIC SPECTROPHOTOMETRIC EXAMINATION OF THE DYNAMICS OF SINGLET OXYGEN IN UNILAMMELLAR VESICLES

Abstract— The kinetic properties of O₂(₁Δ_g) have been examined in unilamellar vesicle dispersions composed of didodecyldimethylammonium bromide, di- n -octadecyl phosphate and egg lecithin. Light absorbing sensitizers 2-acetonaphthone, methylene blue and a methylene blue derivative of enhanced water-solubility were used. The rate parameters for singlet oxygen were monitored by observing the time profile of the bleaching of the reactive substrates diphenylisobenzofuran and anthracene dipropionate. The natural lifetime of O₂(₁Δ_g) in D₂O-based suspensions was shown to be 46/JS in good agreement with that found earlier for D₂O alone and D₂O-based micelle systems. The bimolecular rate constants for reaction with diphenylisobenzofuran and dimethylindole (both lipid-bound) and histidine (water-bound) were also in close conformity with the values found earlier in micellar media. Kinetic spectrophotometry has been shown to be a useful technique for examining rate parameters in these heterogeneous media.