Oxidation potentials, E1/2(ox) of alpha-hydroxyalkyl radicals of the type R(1)R(2)C(*)OH (denoted K(1)H(*)) have been obtained in acetonitrile by photomodulated voltammetry. The values of E1/2(ox) increase as the R(1) and R(2) groups are changed from alkyl to aryl and, in particular, strong electron-withdrawing functionalities such as CN and CF3. Using rate data available in the literature for the pinacol photoexchange reaction K + K(1)H(*) --> KH(*) + K(1), it is found that as the difference in the standard potential of the ketone K, EK degrees and the oxidation potential of K(1)H(*), E1/2(ox), increases there is a modest increase in the exchange rate constant, k(ex). This indicates that even if some charge transfer may occur between the hydroxyalkyl radical and the ketone in the transition state, it is certainly not to the extent of a complete electron transfer. If the exchange reaction is treated as a simple hydrogen atom transfer process within the Marcus model, the intrinsic barrier is found to be 8-13 kcal mol(-1) due to the changes occurring in bonds, hybridizations, and bond angles. Finally, acid dissociation constants for K(1)H(*) are provided by means of a thermochemical cycle.
Synthetically flexible, rigid, tetrad molecules are shown to closely mimic structural and photochemical properties of the bacterial photosynthetic reaction centre.
Scanning tunneling microscopy (STM) images of 1,10-phenanthroline (PHEN) and dipyrido[3,2-a:2',3'-c]phenazine (DPPZ) on Au(111) are recorded using both in situ and ex situ techniques. The images of PHEN depict regimes of physisorption and chemisorption, whereas DPPZ is only physisorbed. All physisorbed structures are not pitted and fluctuate dynamically, involving aligned (4 × 4) surface domains with short-range (ca. 20 molecules) order for PHEN but unaligned chains with medium-range (ca. 100 molecules) order for DPPZ. In contrast, the chemisorbed PHEN monolayers remain stable for days, are associated with surface pitting, and form a (4 × √13)R46° lattice with long-range order. The density of pitted atoms on large gold terraces is shown to match the density of chemisorbed molecules, suggesting that gold adatoms link PHEN to the surface. For PHEN, chemisorbed and physisorbed adsorbate structures are optimized using plane-wave density-functional theory (DFT) calculations for the surface structure. Realistic binding energies are then obtained adding dispersive corrections determined using complete-active-space self-consistent field calculations using second-order perturbation theory (CASPT2) applied to cluster-interaction models. A fine balance between the large adsorbate−adsorbate dispersive forces, adsorbate−surface dispersive forces, gold ligation energy, and surface mining energy is shown to dictate the observed phenomena, leading to high surface mobility and substrate/surface lattice incommensurability. Increasing the magnitude of the dispersive forces through use of DPPZ, rather than PHEN, to disturb this balance produced physisorbed monolayers without pits and/or surface registration but with much longer-range order. Analogies are drawn with similar but poorly understood processes involved in the binding of thiols to Au(111).
Mixed self-assembled monolayers of 2-(mercaptooctyl)hydroquinone (QH2) and alkylthiols were formed on gold electrodes in EtOH and the redox process of the hydroquinone moiety of QH2 was characterized by cyclic voltammetry (CV) in 0.1 M H2SO4. The monolayers were formed at a series of QH2:alkylthiol ratios and the QH2:alkylthiol ratio in solution was compared to the electrochemical response from QH2 in the obtained monolayer. Mixed monolayers of QH2 with hexylthiol, dodecylthiol, and octadecylthiol were studied. The length of the alkylthiol is crucial for the electrochemical response from QH2 in the monolayer. The total concentration of thiols during monolayer formation and incubation times were also studied and low concentrations of <2.5 mM and long incubation times gave rise to lower peak separation, lower peak half widths in the CVs of the mixed monolayers, and lower background current. The stability of a pure QH2 monolayer and a 1:4 QH2:hexylthiol monolayer toward high potentials of up to 1.5 V versus Ag/AgCl was also studied and it was observed that the mixed monolayer is significantly more stable than the pure QH2 monolayer.
The solvation of carbanions in the solvents N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) has been analyzed on the basis of experimental and theoretical data. Experimental solvation energies are obtained from present and previously reported electrochemical measurements of reduction potentials of the corresponding radicals. Theoretical solvation energies are obtained from quantum chemical calculations using the polarizable continuum model (PCM). It is found that the solvation energy is relatively independent of molecular size and structure for the saturated carbanions. This indicates that the negative charge is strongly localized to the anionic carbon. The conjugated carbanions have considerably lower absolute solvation energies ( |) than the saturated carbanions. This is a consequence of the strong delocalization of the negative charge in the former group. The propargyl anion is also found to have a surprisingly low absolute solvation energy. However, high-level quantum chemical calculations show that the electronic structure has large contributions from two different resonance structures, CH⋮CCH2- and -CHCCH2, which results in a significant charge delocalization. There is good agreement between calculated and experimental solvation energies for both the conjugated and nonconjugated primary anions. However, the PCM method consistently underestimates the absolute solvation energies of the secondary and tertiary carbanions. This is attributed to an insufficient treatment of first-layer solvation effects in the method. According to the experimental measurements, the absolute solvation energies are on average 2−3 kcal mol-1 lower in THF than in DMF. The theoretical data indicate a considerably larger solvent effect, 7−10 kcal mol-1. The discrepancy between theory and experiment may partly be attributed to the use of a supporting electrolyte in the measurements, but the main cause seems to be that the short-range interaction tendencies of the solvent cannot be fully characterized by its dielectric constant.
Oxidation (E(1/2)(ox)) and reduction potentials (E(1/2)(red)) of a series of para-substituted phenylthiyl radicals XC(6)H(4)S* generated from the pertinent disulfides or thiophenols have been measured by means of photomodulated voltammetry in acetonitrile. The values of E(1/2)(ox) are of particular interest as they give access to the hitherto unknown thermochemistry of short-lived phenylsulfenium cations in solution. Both E(1/2)(OX) and E(1/2)(red) decrease as the electron-donating power of the substituent raises, resulting in linear correlations with the Hammett substituent coefficient sigma(+) with slopes rho(+) of 4.7 and 6.4, respectively. The finding of a larger substituent effect on than is a consequence of a corresponding development in the electron affinities and ionization potentials of XC(6)H(4)S* as revealed by quantum-chemical calculations. Solvation energies extracted for XC(6)H(4)S(+) and XC(6)H(4)S(-) from thermochemical cycles show the expected substituent dependency; i.e., the absolute values of the solvation energies decrease as the charge becomes more delocalized in the ions. Acetonitrile is better in solvating XC(6)H(4)S(+) than XC(6)H(4)S(-) for most substituents, even if there is a substantial delocalization of the charge in the series of phenylsulfenium cations. The substituent effect on is smaller in aqueous solution than acetonitrile, which is attributed to the ability of water to stabilize in particular localized anions through hydrogen bonding.
The exceptionally long lived charge separation previously observed in a β,β′-pyrrolic-fused ferrocene-porphyrin-fullerene triad (lifetime 630 μs) and related porphyrin-fullerene dyad (lifetime 260 μs) is attributed to the production of triplet charge-separated states. Such molecular excited-state spin polarization maintained over distances of up to 23 Å is unprecedented and offers many technological applications. Electronic absorption and emission spectra, femtosecond and nanosecond time-resolved transient absorption spectra, and cyclic voltammograms of two triads and four dyads are measured and analyzed to yield rate constants, donor–acceptor couplings, free-energy changes, and reorganization energies for charge-separation and charge-recombination processes. Production of long-lived intramolecular triplet states is confirmed by electron-paramagnetic resonance spectra at 77–223 K, as is retention of spin polarization in π-conjugated ferrocenium ions. The observed rate constants were either first predicted (singlet manifold) or later confirmed (triplet manifold) by a priori semiclassical kinetics calculations for all conceivable photochemical processes, parameterized using density-functional theory and complete-active-space self-consistent-field calculations. Identified are both a ps-timescale process attributed to singlet recombination and a μs-timescale process attributed to triplet recombination.
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