Reaction Mechanism of the Reduction of Ozone on Graphite.

The kinetics of the ozonation of graphite with different particle sizes (106 µm, G106; 6.20 µm, G6.2) was studied at several temperatures under a flow of O3 diluted in O2. The reaction was first-order with respect to graphite and to the consumption of ozone. X-ray photoelectron spectrum (XPS) showed that the reactions occurring in the solid under steady-state conditions maintain the original stoichiometry, as predicted by the postulated mechanism for SO2. The deoxygenation reaction occurred along with the ozonation reaction at 100 °C. The rate of oxygen elimination in the flow system has the same rate-determining kinetic barrier as ozone insertion. Ozonation and deoxygenation reactions are sequentially related. Ozonation occurs with the insertion of O3, forming a 1,2,3-trioxolane followed by an oxygen transfer that produces a peroxide valence tautomer in equilibrium with 1,3-dicarbonyl, [peroxide ↔ dicarbonyl], and an oxirene that eliminates atomic oxygen. The decarboxylation reaction was studied at 600 °C from the ozonated G106 (ΔG≠ = 83.60 ± 0.08 kcal·mol-1). Total decarboxylation at 600 °C matched the number of moles of CO2 removed and the oxygen content after ozonation, showing that the reduction of ozone on graphite was essentially a clean reduction with no secondary oxidations. When ozonized graphite was heated to 600 °C, only [peroxide ↔ dicarbonyl] species remained in the matrix. The peroxide tautomer isomerized to dioxirane and eliminated CO2 as a dioxicarbene. Total deoxygenation of decarboxylated graphite G106 was obtained by pyrolysis. There was residual oxygen that arose from the atomic oxygen eliminated from the oxirene, intercalated in graphite layers, and formed basal epoxy groups. Also, incoming O atoms reacted with the intercalated O atoms to produce O2 molecules. Thermal annealing deintercalated molecular oxygen (600-900 °C).

Photo-immobilization of proteins on carbons.

The photofunctionalization of three different carbons with two proteins was studied at room temperature. Water solutions of bovine serum albumin, BSA, and α-amylase, AA, were photolyzed at 21 °C in the presence of graphite microparticles (6.20 µm), MPG, graphene oxide, MPGO, and graphene oxide modified with SO2, mMPGO. The insertion of BSA on carbon matrixes occurred with a deoxygenation reaction, most likely due to a dehydration step of a water molecule. XPS, TOC and TGA, showed that the BSA photo-insertion on MPG was highly efficient with 34.9% of the weight of MPG after photolysis, with an initial concentration of 1 g∙L-1 of BSA. A high yield of AA photoinsertion on the carbons was also obtained. The calculated weight of AA inserted on MPG and MPGO after photolysis was 22.30% and 18.08%, respectively, with respect to the initial weight of carbon, when the initial concentration of AA was 60 mg∙L-1. AA immobilized on MPG was active while the enzyme on MPGO showed a smaller activity, within the experimental error. Although a certain extent of denaturalization of both proteins was observed during photolysis, the molecular weight and composition changed very little during the photolysis, which would produce mainly conformational changes and isomerization reactions.

Photolytic insertion of albumin on activated carbon modified with ozone.

254nm photolyses of bovine serum albumin [BSA] in aqueous solutions, were carried out in the presence of activated carbons modified by reaction with ozone. The photolyses were monitored by fluorescence spectroscopy and UV spectrophotometry, and the products were characterized by elemental analysis, FTIR, TGA, total organic carbon analyses [TOC], and XPS. The ozonation reaction was carried out at room temperature with O3 under dry and wet conditions. The carbon characterization showed that the reaction increased the amount of epoxide and carbonyl groups on the carbon matrix. The activated carbon modified with dry O3 exhibited higher concentration of oxidized groups in its surface, smaller surface area and lower thermal stability. Characterization of the photolysis of ozonized carbons pointed to a small release of carbon organic groups during the reaction with elimination of epoxide groups and increase of carbonyl groups without change of thermal stability. Photolysis of BSA in aqueous solution occurred with fluorescence quenching due to changes of the local microenvironment and/or macromolecular conformational changes. Absorbance increase of the UV spectrum indicated a hyperchromic effect due to albumin structure modifications during photolysis. TGA analysis of the photolysed activated carbons in the presence of BSA suggested that ozonized carbon samples underwent insertion of BSA upon photolysis, in particular the sample ozonized under dry conditions. The changes observed for the FTIR and elemental analysis agreed with this conclusion, which was further supported by 13C SS-NMR, fluorescence emission and XPS.

Photolysis of phenylalanine in the presence of oxidized carbon nanotubes.

Photolyses at 254 nm of phenylalanine (Phe) in aqueous solutions, were carried out in the presence of oxidized carbon nanotubes modified by the reaction with SO2 (mNTO). Kinetics of the photolyses were followed by UV spectrophotometry at 220 nm, and the products were characterized by HPLC, XPS, and (13)C-SSNMR. The ratio of the initial rates of photolysis in the presence and absence of mNTO, k*/ko*, showed a systematic decrease. The photolytic decay of Phe occurs with minor formation of tyrosine. The mass of nanotubes produced an exponential attenuation of the photolytic decomposition of Phe. Total carbon analyses (TCA) showed no inorganic carbon formation after the photolyses. The first-order rate constant of photofunctionalization of mNTO by the insertion of phenylalanine onto the nanotube matrix was calculated from TCA to be kin = 30.1 min(-1). Comparison of the XPS spectra of the mNTO before and after the photolysis, using the atom inventory technique, suggests the insertion of Phe along with the extrusion of a sulfide radical anion ((•)S(-)) which undergo subsequent oxidation to SO4(2-). The obtained results show the effects of mNTO on the photolysis of Phe and provide a new method of photofunctionalization of carbon materials, modified by the intermediates of the reduction of SO2, with an organic moiety.

Selective insertion of sulfur dioxide reduction intermediates on graphene oxide.

Graphite microparticles (d50 6.20 µm) were oxidized by strong acids, and the resultant graphite oxide was thermally exfoliated to graphene oxide sheets (MPGO, C/O 1.53). Graphene oxide was treated with nonthermal plasma under a SO2 atmosphere at room temperature. The XPS spectrum showed that SO2 was inserted only as the oxidized intermediate at 168.7 eV in the S 2p region. Short thermal shocks at 600 and 400 °C, under an Ar atmosphere, produced reduced sulfur and carbon dioxide as shown by the XPS spectrum and TGA analysis coupled to FTIR. MPGO was also submitted to thermal reaction with SO2 at 630 °C, and the XPS spectrum in the S 2p region at 164.0 eV showed that this time only the nonoxidized episulfide intermediate was inserted. Plasma and thermal treatment produced a partial reduction of MPGO. The sequence of thermal reaction followed by plasma treatment inserted both sulfur intermediates. Because oxidized and nonoxidized intermediates have different reactivities, this selective insertion would allow the addition of selective types of organic fragments to the surface of graphene oxide.

Field evaluation of systemic imidacloprid for the management of avocado thrips and avocado lace bug in California avocado groves.

BACKGROUND: The efficacy of systemic applications of imidacloprid for the management of avocado thrips and avocado lace bug was determined in field trials. Following insecticide treatment by chemigation, leaves of appropriate age for each insect were sampled over a 6 month period and used for bioassays. Imidacloprid residues were measured by ELISA in leaves used for bioassays to determine concentrations of insecticide that were toxic to both pests. RESULTS: The uptake of imidacloprid into treated trees was extremely slow, peaking in the current year's leaf flush at only 8 ng cm(-2) leaf tissue after 15 weeks. Avocado thrips mortality in bioassays with young flush leaves, the preferred feeding substrate for this insect, was minimal, indicating that imidacloprid concentrations were below threshold levels needed for effective control. Residues present in older leaves, which are preferred by the avocado lace bug, were higher than in young flush leaves, and provided good control of this pest. Probit analysis of bioassay data showed that the avocado lace bug (LC(50) = 6.1 ng imidacloprid cm(-2) leaf tissue) was more susceptible to imidacloprid than the avocado thrips (LC(50) = 73 ng imidacloprid cm(-2) leaf tissue). CONCLUSIONS: In spite of the slow uptake of imidacloprid into avocado trees, the levels of imidacloprid would be sufficient to control avocado lace bug infestations. In contrast, the slow uptake would be problematic for avocado thrips control because inadequate levels of insecticide accumulate in new flush foliage and would allow avocado thrips populations to build to levels that would subsequently damage developing avocado fruit.

Mechanisms of acid decomposition of dithiocarbamates. 5. Piperidyl dithiocarbamate and analogues.

In this work, the acid cleavage at 25 degrees C in 20% v/v aqueous ethanol of a series of analogues of piperidine dithiocarbamate X(C2H4)2NCS2(-) (X = CH2, CHCH3, NH, NCH3, S, O) was studied. The pH-rate profiles were obtained in the range of H(o)-5 and pH 5. They all presented a dumbell shaped curve with a plateau from which the pH-independent first-order rate constant k(o) (or the specific acid catalysis k(H)) was calculated, in addition to the acid dissociation constant of the free (pKa) and conjugate acid (pK(+)) species of the DTC. LFERs of the kinetically determined pKa and pK(+) versus pKN (pKa of parent amine) were used to characterize the reactive species and the structure of the transition state of the rate-determining step. For X = CH2, CH3CH the values of k(H) agree with those of alkDTCs in the strong base region of the Brønsted plot of log k(H) versus pKN where the transition state is close to a zwitterion formed by intramolecular water-catalyzed S-to-N proton transfer of the dithiocarbamic acid. However, when X = NH, CH 3N, O, S, the reactive species is the DTC anion, which is as reactive as an arylDTC, and similarly, the pK(+) values correspond to a parent amine that is about 3-4 pK units more basic. The solvent isotope effect indicated that the acid decomposition of these dithiocarbamate anions is specifically catalyzed by a Hydron anchimerically assisted by the heteroatom through a boat conformation.

Resistance of avocado thrips (Thysanoptera: Thripidae) to sabadilla, a botanically derived bait.

Bioassays were conducted on four avocado thrips (Scirtothrips perseae Nakahara) populations in southern California that had had limited past exposure to the botanical pesticide sabadilla, with the objective of establishing baseline susceptibility levels for the purpose of resistance monitoring. Reports of avocado thrips resistance in a grove that had received six sabadilla sprays over 2 years were confirmed when a bioassay indicated resistance ratios of 7.6 and 18.8 at the LC50 and LC90, respectively. Owing to the availability of other unrelated pesticides, sabadilla sprays were discontinued at this site, and, after 5.5 years, resistance ratios had dropped to near baseline levels. Avocado thrips baseline susceptibility to cyfluthrin was also obtained from one grove for future resistance monitoring studies. Resistance management principles are discussed in relation to the limited future options available for avocado thrips control.

Baseline susceptibility of persea mite (Acari: Tetranychidae) to abamectin and milbemectin in avocado groves in Southern California.

Persea mite, Oligonychus perseae Tuttle, Baker, and Abatiello, susceptibility to abamectin and milbemectin was evaluated in 2003 to determine baseline susceptibility levels in avocado groves in San Diego and Ventura Counties (California, USA) where more than 70% of the state's avocado production is concentrated. Milbemectin has yet to be used in avocado production in California and abamectin has been available for use since 1999. Baseline susceptibility ratios (in relation to the most susceptible population) of five persea mite field strains to milbemectin varied 2.1- to 2.8-fold at the LC50 and LC90, respectively. The susceptibility of seven field strains to abamectin varied slightly more (2.1- to 3.5-fold) with one strain subjected to seven sprays over the past 4 years showing slight but significant separation of LC50 and LC90's from the most susceptible strain, which is suggestive of the early stages of resistance to this product. Based on these data, baseline susceptibility levels are proposed that might be used to monitor for future persea mite resistance to these chemicals as their use in California avocado production continues.

Evaluation of relative photonic efficiency in heterogeneous photocatalytic reactors.

The evaluation of photonic efficiency in heterogeneous photocatalysis remains elusive because the number of absorbed photons is difficult to assess experimentally. The photonic efficiency of heterogeneous photocatalytic reactors depends on the reactor geometry, irradiation source, and photocatalyst properties. In this work, the relative photonic efficiency of heterogeneous photocatalytic reactors to degrade an azo dye was evaluated using phenol as the standard system. The experimental tests were carried out in a batch reactor under different conditions of pH, catalyst dosage, initial concentration, and ultraviolet (UV) lamps. The kinetics of disappearance of both phenol and azo dye were studied using the initial rate method and were described according to the Langmuir-Hinshelwood (L-H) kinetic model. It was observed that the relative photonic efficiency depends on the adsorption/desorption properties of the photocatalyst.

Reinterpretation of the kinetic data and the non-steady state hypothesis (two-step mechanism) for the S(N)2 reaction between p-nitrophenoxide and methyl iodide in aprotic solvents containing water.

Kinetic data obtained by conventional spectrophotometry for reaction of sodium p-nitrophenoxide with methyl iodide in degassed acetone are reported. The rate constants obtained from the first 10% of reaction do not differ significantly from those obtained over longer reaction times (e.g 50% reaction)--the main criteria of Parker et al. (Org. Biomol. Chem., 2003, 1, 36-38) for a non-steady state two-step mechanism. Reactions are accelerated by crown ether, suggesting a mechanism via a free ion pair. Product studies by high performance liquid chromatography of reactions in aqueous acetonitrile (used by Parker et al.) show that the yield of methylated product is strongly affected by at least two base-neutralising side reactions.

Mechanisms of acid decomposition of dithiocarbamates. 3. Aryldithiocarbamates and the torsional effect.

The acid decomposition of some p-substituted aryldithiocarbamates (arylDTCs) was observed in 20% aqueous ethanol at 25 degrees C, mu = 1.0 (KCl, for pH > 0). The pH-rate profiles showed a dumbell shape with a plateau where the observed first-order rate constant k(obs) was equal to k(o), the rate constant of the decomposition of the dithiocarbamic acid species. The acid dissociation constants of the dithiocarbamic acids (pK(a)) and their conjugate acids (pK(+)) were calculated from the pH-rate profiles. Comparatively, k(o) was more than 10(4)-fold faster than alkyldithiocarbamates (alkDTCs) with similar pK(N) (the acid dissociation constant of the parent amine). It was observed that the values of pK(a) and pK(+)were 5 and 8 units of pK, respectively, higher than the expected values from the pK(N) of alkylDTCs. The higher values were attributed to the inhibition of the delocalization of the nitrogen electron pair into the benzene ring because of the strong electron withdrawal effect of the thiocarbonyl group. Comparison of the activation parameters showed that the rate acceleration was due to a decrease in the enthalpy of activation. Proton inventory indicated the existence of a multiproton transition state, and it was consistent with an S to N proton transfer through a water molecule. There are two hydrogens contributing to a secondary SIE, and there are also two protons that are being transferred at the transition state to form a zwitterion followed by fast C-N bond cleavage. The mechanism could also be a concerted asynchronic process where the N-protonation is more advanced than the C-N bond breakdown. The kinetic barrier is similar to the torsional barrier of thioamides, suggesting that the driving force to reach the transition state is the needed torsion of the C-N bond that inhibits the resonance with the thiocarbonyl group and the aromatic moiety, increasing the basicity of the nitrogen and making the proton transfer thermodynamically favorable.

Mechanisms of acid decomposition of dithiocarbamates. 4. Theoretical calculations on the water-catalyzed reaction.

A theoretical study of the water-catalyzed dithiocarbamic acid cleavage has been performed using N-methyl- (MeDTC) and N-phenyldithiocarbamic acid (PhDTC) as model molecules. Calculations have been carried out within the Density Functional Theory (DFT) formalism, using the B3LYP hybrid functional together with medium-sized basis sets, both in gas phase and by considering solvent effects through dielectric continuum methods. According to the results obtained, both in gas phase and in solution, MeDTC decomposes through a proton-transfer step assisted by a water molecule (this being the rate-determining step), leading to a zwitterionic intermediate, followed by a fast N-C bond-breaking process. In the case of PhDTC, the theoretical results point to a one-step mechanism in which the N-C bond breaking takes place in a concerted manner with the proton transfer. The calculated Delta Delta G(++) of the proton-transfer step for MeDTC and PhDTC is 4.0 kcal mol(-1), which is similar to the experimental values. For both compounds the water-assisted intramolecular proton transfer occurs with a twisting of the CS(2) group that inhibits the resonance of the thiocarbamic group, making the nitrogen more basic and therefore favoring the proton transfer. The difference in the torsional barrier has been calculated to be ca. 5 kcal mol(-1), and it is therefore concluded that most of the activation barrier of the reaction is due to the torsional barrier of the CS(2) group.

Mechanisms of Acid Decomposition of Dithiocarbamates. 2. Efficiency of the Intramolecular General Acid Catalysis.

The acid decomposition of ethylenebis(dithiocarbamate) (EbisDTC) and glycinedithiocarboxylate (glyDTC) was studied in water at 25 degrees C in the range of H(o) -5 to pH 5. The acid dissociation constants of all species involved were calculated from LFER and from the pH-rate profiles. According to the pK(a) of the parent amine of the reactive species, both compounds decompose through the dithiocarbamate anion and a zwitterion intermediate. The intermolecular N-protonation rate constant of the carboxylic conjugate acid of glyDTC anion is 12.6 M(-)(1) s(-)(1), slower than the C-N breakdown. This species also cleaves through an intramolecular general acid-catalyzed mechanism where the rate constant for the N-protonation is (7.1 +/- 4.2) x 10(3) s(-)(1) and the efficiency of the proton-transfer step as measured by the effective molarity is (5.6 +/- 3.3) x 10(2) M. The acid decomposition of the dithiocarbamic conjugate acid of EbisDTC anion proceeds through a fast N-protonation and a slower C-N breakdown. The intramolecular general acid catalysis rate constant is (8.2 +/- 2.8) x 10(6) s(-)(1), but the efficiency of this fast proton transfer is only (14.3 +/- 4.9) M. The intramolecular general acid catalysis of the free acid forms of the carboxylic and dithiocarbamic groups is unfavorable for about 4 kcal mol(-)(1) with respect to the protonation of the external hydron, and consequently, no external buffer catalysis is expected to be observed for dithiocarbamates that decompose through a zwitterion intermediate. The difference between the pK(b) of the proton acceptor and the pK(a) of the donor follows the order of the proton efficiency. Estimation of the strength of the hydrogen bonding in the reagent and product supports the assumption that a thermodynamically favorable change of hydrogen bonding from reagent to product increases the efficiency of proton transfer.