Carbon Isotope Fractionation of Substituted Benzene Analogs during Oxidation with Ozone and Hydroxyl Radicals: How Should Experimental Data Be Interpreted?

Oxidative processes frequently contribute to organic pollutant degradation in natural and engineered systems, such as during the remediation of contaminated sites and in water treatment processes. Because a systematic characterization of abiotic reactions of organic pollutants with oxidants such as ozone or hydroxyl radicals by compound-specific stable isotope analysis (CSIA) is lacking, stable isotope-based approaches have rarely been applied for the elucidation of mechanisms of such transformations. Here, we investigated the carbon isotope fractionation associated with the oxidation of benzene and several methylated and methoxylated analogs, namely, toluene, three xylene isomers, mesitylene, and anisole, and determined their carbon isotope enrichments factors (εC) for reactions with ozone (εC = -3.6 to -4.6 ‰) and hydroxyl radicals (εC = 0.0 to -1.2‰). The differences in isotope fractionation can be used to elucidate the contribution of the reactions with ozone or hydroxyl radicals to overall transformation. Derivation of apparent kinetic isotope effects (AKIEs) for the reaction with ozone, however, was nontrivial due to challenges in assigning reactive positions in the probe compounds for the monodentate attack leading to an ozone adduct. We present several options for this step and compare the outcome to quantum chemical characterizations of ozone adducts. Our data show that a general assignment of reactive positions for reactions of ozone with aromatic carbons in ortho-, meta-, or para-positions is not feasible and that AKIEs of this reaction should be derived on a compound-by-compound basis.

Synthesis of Furan-Annelated BINOL Derivatives: Acid-Catalyzed Cyclization Induces Partial Racemization.

In this account, we describe the synthesis of a series of BINOL-based bis- and trisphosphoric acids 11d/e/f, which commonly feature an unusual phosphoric acid monoester motif. This motif is generated by an acid-catalyzed 5- endo- dig cyclization of the 3-alkynyl-substituted BINOL precursors to give the corresponding Furan-annelated derivatives, followed by phosphorylation of the remaining phenolic alcohols. In the cyclization reaction, we observed an unexpected partial racemization in the bis- and tris-BINOL scaffolds, leading to mixtures of diastereomers that were separated and characterized spectroscopically and by X-ray crystal structure analyses. The cyclization and racemization processes were investigated both experimentally and by DFT-calculations, showing that although the cyclization proceeds faster, the barrier for the acid-catalyzed binaphthyl-racemization is only slightly higher.

DFT calculations suggest a new type of self-protection and self-inhibition mechanism in the mammalian heme enzyme myeloperoxidase: nucleophilic addition of a functional water rather than one-electron reduction.

The mammalian heme enzyme myeloperoxidase (MPO) catalyzes the reaction of Cl(-) to the antimicrobial-effective molecule HOCl. During the catalytic cycle, a reactive intermediate "Compound I" (Cpd I) is generated. Cpd I has the ability to destroy the enzyme. Indeed, in the absence of any substrate, Cpd I decays with a half-life of 100 ms to an intermediate called Compound II (Cpd II), which is typically the one-electron reduced Cpd I. However, the nature of Cpd II, its spectroscopic properties, and the source of the additional electron are only poorly understood. On the basis of DFT and time-dependent (TD)-DFT quantum chemical calculations at the PBE0/6-31G* level, we propose an extended mechanism involving a new intermediate, which allows MPO to protect itself from self-oxidation or self-destruction during the catalytic cycle. Because of its similarity in electronic structure to Cpd II, we named this intermediate Cpd II'. However, the suggested mechanism and our proposed functional structure of Cpd II' are based on the hypothesis that the heme is reduced by charge separation caused by reaction with a water molecule, and not, as is normally assumed, by the transfer of an electron. In the course of this investigation, we found a second intermediate, the reduced enzyme, towards which the new mechanism is equally transferable. In analogy to Cpd II', we named it Fe(II'). The proposed new intermediates Cpd II' and Fe(II') allow the experimental findings, which have been well documented in the literature for decades but not so far understood, to be explained for the first time. These encompass a) the spontaneous decay of Cpd I, b) the unusual (chlorin-like) UV/Vis, circular dichroism (CD), and resonance Raman spectra, c) the inability of reduced MPO to bind CO, d) the fact that MPO-Cpd II reduces SCN(-) but not Cl(-), and e) the experimentally observed auto-oxidation/auto-reduction features of the enzyme. Our new mechanism is also transferable to cytochromes, and could well be viable for heme enzymes in general.

High temperature shock-tube study of the reaction of gallium with ammonia.

The gas-phase reaction of Ga atoms with NH(3) was studied behind reflected shock waves in the temperature range of 1380 to 1870 K at pressures of 1.4 to 4.0 bar. Atomic-resonance-absorption spectroscopy (ARAS) at 403.299 nm was applied for the time-resolved determination of the Ga-atom concentration. Trimethylgallium (Ga(CH(3))(3)) was used as a precursor of Ga atoms. After the initial increase in Ga concentration due to Ga(CH(3))(3) decomposition, the Ga concentration decreases rapidly in the presence of NH(3). For the simulation of the measured Ga-atom concentration profiles from the studied reaction, additional knowledge about the thermal decomposition of Ga(CH(3))(3) is required. The rate coefficient k(4) of the reaction Ga + NH(3) → products (R4) was determined from the Ga-atom concentration profiles under pseudo-first-order assumption and found to be k(4)(T) = 10(14.1±0.4) exp(-11 900 ± 700 K/T) cm(3) mol(-1) s(-1) (error limits at the one standard deviation level). No significant pressure dependence was noticeable within the scatter of the data at the investigated pressure range.

Quantum chemical studies of the adsorption of single acetone molecules on hexagonal ice I(h) and cubic ice I(c).

The interaction energies of free acetone molecules with surfaces of two different ice polymorphs have been investigated by quantum chemical methods. Special emphasis has been given to sites for adsorption on the (0001) surface of hexagonal ice (I(h)) and the (1[combining macron]01) surface of cubic ice (I(c)), respectively. The structural optimisations made use of conventional electronic structure methods including HF and B3LYP using moderate basis sets up to 6-31+G(d) as well as local and ONIOM methods using 2 or 3 layers which were treated at different levels of theory. The adsorption energies at T = 0 K were calculated for the optimised adsorption geometries performing single points at the B3LYP, MP2 and LMP2 level in conjunction with valence triple-zeta basis sets up to 6-311+G(d,p). Including corrections for basis set superposition errors (BSSE) the most extensive calculations provide adsorption energies (T = 0 K) of -39.1 and -57.5 kJ mol(-1) for the energetically most favourable sites for adsorption of a single acetone molecule on ice I(h) and ice I(c), respectively. By vibrational analysis this can be transformed to adsorption enthalpies at around a temperature of 200 K yielding values of -31.5 for adsorption on ice I(h) and -49.9 kJ mol(-1) for adsorption on ice I(c). The current results support experimental observations of Behr et al. (J. Phys. Chem. A, 2006, 110, 8098) in which evidence was presented that acetone adsorbs on ice around 200 K at two different sites; each of which has a different adsorption enthalpy.