Showing papers in "The Journal of Physical Chemistry in 1998"

Cobalt-Porphyrin Catalyzed Reduction of CO2: Radiation Chemical, Photochemical, and Electrochemical Studies

Abstract: Several cobalt porphyrins (CoP) have been reduced by radiation chemical, photochemical, and electrochemical methods, in aqueous and organic solvents. In aqueous solutions, the CoIP state is stable at high pH but is shorter lived in neutral and acidic solutions. Stable CoIP is also observed in organic solvents and is unreactive toward CO2. One-electron reduction of CoIP leads to formation of a species that is observed as a transient intermediate by pulse radiolysis in aqueous solutions and as a stable product following reduction by Na in tetrahydrofuran solutions. The spectrum of this species is not the characteristic spectrum of a metalloporphyrin π-radical anion and is ascribed to Co0P. This species binds and reduces CO2. Catalytic formation of CO and HCO2- is confirmed by photochemical experiments in acetonitrile solutions containing triethylamine as a reductive quencher. Catalytic reduction of CO2 is also confirmed by cyclic voltammetry in acetonitrile and butyronitrile solutions and is shown to occur ...

Ultraviolet Absorption Spectrum and Cross Sections of Vinyl [C2H3] Radical in the 225-238 nm Region

Abstract: The room-temperature gas-phase ultraviolet absorption spectrum and cross-sections of vinyl (C 2 H 3 ) radicals have been determined in the spectral range 225–238 nm, employing cavity ring-down absorption spectroscopy. Vinyl radicals in these experiments were produced from the 193 nm excimer laser photolysis of methyl vinyl ketone (CH 3 COC 2 H 3 ) and vinyl bromide (C 2 H 3 Br). The spectra obtained from the two systems were nearly identical. The observed spectrum exhibits a relatively broad and featureless absorption with a cross-section of 5.3×10 −18 cm 2 molecule −1 at 230 nm. A combined uncertainty of ∼25% for cross-section values has been assessed. The electronic transitions in the vinyl radical have been calculated by ab initio quantum chemical methods at the CIS, EOM-CCSD, CASSCF and CASPT2 levels of theories which assign the new observed band to the highly allowed and `in plane' π * (2a″) ← π(1a″) transition.

Hardness and Chemical Potential Profiles for Some Open-Shell HAB → HBA Type Reactions. Ab Initio and Density Functional Study

Abstract: The electronic structure, hardness (η), and chemical potential (μ) for the 1A‘ and 3A‘‘ states of HNO−HON and the 2A‘‘ state of HSO−HOS have been calculated using HF/6-311++G** and B3LYP/6-311++G** methods. The η and μ profiles of the 1A‘ state of HNO−HON and those of HSO−HOS are obtained in agreement with the salient features of the maximum hardness principle (MHP). However, a quite erratic η profile is predicted for the 3A‘‘ state of HNO−HON. This can be attributed to the nature of the variation in the energy difference of the two states along the reaction path. The relative energies, ionization potentials (I), and electron affinities (A) are calculated at the stationary points of the B3LYP surface using B3LYP and MPn (Full) methods. Most of these values are obtained in very good agreement with the available experimental data. The η values based on these I and A identify the most stable species correctly but do not follow the expected trend with regard to the relative stability of the transition state. ...

Coductivity,NMR Measurements and Phase Diagram of the K2S2O7-V2O5 System

Abstract: The phase diagram of the catalytically important K2S2O7−V2O5 system has been investigated by means of conductometric and NMR spectroscopic methods up to 500 °C. From the marked change of the specific conductivity by change of phase, found for 14 different compositions of the K2S2O7−V2O5 binary system, the phase transition temperatures have been obtained. In the V2O5-rich region, where glass formation is pronounced, 39K NMR spectra of six different compositions showed a marked change of the line width at the liquidus temperatures. The phase diagram based on the combined results exhibits three maxima corresponding to the formation of compounds with the stoichiometry 3K2S2O7·V2O5, 2K2S2O7·V2O5, and 1K2S2O7·V2O5 and melting temperatures of 352, 398, and 425 °C, respectively. Three eutectics were found at the compositions XV2O5 = 0.17, 0.27, and 0.39 with melting temperatures of 314, 348, and 366 °C, respectively. The measured conductivities in the liquid region of the 14 different compositions have been fitte...

Solvent effect on intramolecular long-range electron-transfer reactions between porphyrin and benzoquinone in acetonitrile solution : molecular dynamics calculations of reaction rate constants

Abstract: The reaction mechanism of long-range intramolecular electron transfer between the porphyrin−benzoquinone donor−acceptor pair linked by an organic spacer in acetonitrile solvent is investigated theoretically. The rate formula is derived on the basis of Fermi's golden rule for reactions induced by the through space and through bond type electronic couplings involving the dynamical effects of solvent fluctuation. Molecular dynamics (MD) calculations are carried out to construct the reaction free energy curves. The reaction rate is estimated on the basis of the results of MD calculations. It is found that there are important contributions from the vibrationally induced couplings by the solvent fluctuation. In order to examine the solvent fluctuation effect, we determine the effective electronic coupling element (ECE) and perform the decomposition analyses. The important electron pathway in determining the solvent-induced ECE is discussed.

Charge separation in a photosynthetic reaction center

Abstract: Structural influences on the direction of electron transfer in the charge separation process of the photosynthetic reaction centers of Rhodopseudomonas viridis and Rhodobacter sphaeroides are studied using quantum chemical models to calculate the electronic factor. Our results support the sequential mechanism for the primary charge separation. Using crystallographic coordinates refined in 1994, we find a larger coupling between the special pair D and the accessory bacteriochlorophyll BA of the A branch than between D and BB of the inactive B branch. We have been able to localize the coupling to the acetyl group of ring I of DB and the methyl group of ring III of BA. The corresponding contact between DA and BB has less coupling, apparently due to a distorting hydrogen bond between the acetyl group on DA and the imidazole side group of HisL168. The coupling between BA and bacteriopheophytin ΦA is accomplished by two methyl groups, directly connected to the conjugated π system of the chromophore.

Electronic Structure and Thermodynamic Properties of the Molecule GeC from All-Electron ab initio Calculations and Knudsen Effusion Mass Spectrometric Measurements

Abstract: All-electron ab initio multiconfiguration self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) calculations have been carried out to determine the low-lying states of the molecule GeC. The electronic ground state is predicted to be 3Π. Based on the results of the MRCI calculations, the equilibrium distance for the 3Π ground state has been computed as 1.842 A and the vibrational frequency as 827 cm-1. The ground state is separated from the excited states 3Σ-, 1Σ+, 1Π, and 1∆ by 3552, 5768, 7322, and 8303 cm-1, respectively. The chemical bond in the 3Π electronic ground state has double-bond character. It is polar with charge transfer from Ge to C giving rise to a dipole moment of 1.68 D at 1.84 A. Thermal functions as derived from the theoretical molecular parameters were used in the calculation of the thermodynamic properties of the GeC molecule from new mass spectrometric equilibrium data. Also, the literature value for the dissociation energy of GeC has been reevaluated. The recommended dissociation energy, D°0(GeC), and enthalpy of formation, ∆fH°298.15(GeC), in kJ mol-1 are 455.7 ( 11 and 630.4 ( 11, respectively.

Kinetics and Mechanism of the Reaction of NH2 with O2 in Aqueous Solutions

Abstract: The reaction of NH3 with •OH or SO4•- radicals produces the aminyl radical, •NH2. Pulse radiolysis and laser flash photolysis techniques were utilized to study the formation of this radical, its absorption spectrum, its reaction with O2, and the mechanism of formation of subsequent intermediates and the main final product, peroxynitrite. The rates of formation of •NH2 and its absorption spectrum are in agreement with previous reports. The reaction of •NH2 with O2, however, was observed to take place much more rapidly than reported before and to involve an equilibrium of these reactants with the aminylperoxyl radical, NH2O2•. The equilibrium is shifted toward completion of the reaction via catalyzed decomposition of this peroxyl radical, and this decomposition affects the observed rate of reaction of •NH2 with O2. The peroxyl radical deprotonates and isomerizes and finally forms NO. In the presence of O2•-, NO is converted rapidly to peroxynitrite, ONO2-. This product, which is stable at alkaline pH, was c...