Puzzling Aqueous Solubility of Guanine Obscured by the Formation of Nanoparticles.

Dissolution of guanine in neutral solutions was obscured by peculiar behavior of guanine, indicating an apparent dependence of solubility on the amount of solid guanine used. Here, we demonstrate that the problem is caused by the formation of tiny guanine nanoparticles that tend to grow forming stable particles of ca. 800 nm size. This effect can be minimalized by using small quantities of guanine powder for dissolution. We also show that assuming a constant, independent of pH, concentration of neutral form of guanine, at 25 °C equal 25.4 µM, and applying known p Ka values related to its dissociation or protonation, it is possible to calculate the concentrations of all conjugate acids and bases of guanine at the given pH value, and by summing them up, the guanine solubility.

Dissociative electron transfer in polychlorinated aromatics. Reduction potentials from convolution analysis and quantum chemical calculations.

Formal potentials of the first reduction leading to dechlorination in dimethylformamide were obtained from convolution analysis of voltammetric data and confirmed by quantum chemical calculations for a series of polychlorinated benzenes: hexachlorobenzene (-2.02 V vs. Fc(+)/Fc), pentachloroanisole (-2.14 V), and 2,4-dichlorophenoxy- and 2,4,5-trichlorophenoxyacetic acids (-2.35 V and -2.34 V, respectively). The key parameters required to calculate the reduction potential, electron affinity and/or C-Cl bond dissociation energy, were computed at both DFT-D and CCSD(T)-F12 levels. Comparison of the obtained gas-phase energies and redox potentials with experiment enabled us to verify the relative energetics and the performance of various implicit solvent models. Good agreement with the experiment was achieved for redox potentials computed at the DFT-D level, but only for the stepwise mechanism owing to the error compensation. For the concerted electron transfer/C-Cl bond cleavage process, the application of a high level coupled cluster method is required. Quantum chemical calculations have also demonstrated the significant role of the π*ring and σ*C-Cl orbital mixing. It brings about the stabilisation of the non-planar, C2v-symmetric C6Cl6˙(-) radical anion, explains the experimentally observed low energy barrier and the transfer coefficient close to 0.5 for C6Cl5OCH3 in an electron transfer process followed by immediate C-Cl bond cleavage in solution, and an increase in the probability of dechlorination of di- and trichlorophenoxyacetic acids due to substantial population of the vibrational excited states corresponding to the out-of-plane C-Cl bending at ambient temperatures.

Autocatalytic cathodic dehalogenation triggered by dissociative electron transfer through a C-H···O hydrogen bond.

A combined action of the C-H···Oalkoxide hydrogen bonding and Cl···πpyrazolyl dispersive interactions facilitates intramolecular electron transfer (ET) in the transient {Mo(I)(NO)(Tp(Me2))(Oalkoxide)}˙(-)···HCCl3 adduct ([Tp(Me2)](-) = κ(3)-hydrotris(3,5-dimethylpyrazol-1-yl)borate), setting off a radical autocatalytic process, eventually leading to chloroform degradation. In the voltammetric curve, this astonishingly fast process is seen as an almost vertical drop-down. The potential at which it occurs is favorably shifted by ca. 1 V in comparison with uncatalyzed reduction. As predicted by DFT calculations, crucial in the initial step is a close and prolonged contact between the electron donor (Mo(I) 4d-based SOMO) and acceptor (σ(*)(C-Cl)-based LUMO). This occurs owing to the exceptionally short (dH···O = 1.82 Å) and nearly linear C-H···Oalkoxide bonding, which is reflected by a large ΔνC-H red-shift of 380 cm(-1) and a noticeable reorganization of electronic density along the H-bond axis. The advantageous noncovalent interactions inside the cavity formed by two pyrazolyl (pz) rings are strengthened during the C-Cl bond elongation coupled with the ET, giving rise to possible transition state stabilization. After the initial period, the reaction proceeds as a series of consecutive alternating direct or Mo(II/I)-mediated electron and proton transfers. Alcohols inhibit the electrocatalysis by binding with the {Mo(I)-Oalkoxide}˙(-) active site, and olefins by trapping transient radicals. The proximity and stabilization effects, and competitive inhibition in the studied system may be viewed as analogous to those operating in enzymatic catalysis.

Adsorption structures on metals--a thermodynamic description.

Thermodynamic conditions of the adsorption of gaseous molecules on metals are discussed in this work. The equilibria of the formation of surface arrays comprising several atoms, the formation of ordered structures, and the dissolution of gas atoms in the metal have been reviewed. It was found that the heat and the free energy of chemisorption is affected by the change in the number and energy of bonds between metal atoms and the adsorbate in the structures being formed. It was pointed out that the change in the interactions between the surface metal atoms before and after the adsorption must be followed by an energetic effect that would affect the chemisorption energy.