Number Of Methyl Substitutions Research Articles

Electrolytic removal of volatile organic compounds: Keys to understand the process

This paper is focused on the electrochemical degradation of several substituted benzenes as models of volatile organic compounds, as a first step in the degradation of these species during electrochemically assisted absorption process. To do this, the influence of the electrode material (using DSA® and BDD), supporting electrolyte and current intensity is evaluated and intermediates are characterized in order to propose a mechanism that explains the degradation of these species. Results demonstrate that the reactivity of these species increases with the number of methyl substitutions on the benzene ring. Removal efficiencies of BTX compound (for a specific applied electric charge of 7.2 A h dm−3) were higher than 95%, showing that the electrochemical process is functional for this kind of organic compound. On the other hand, removal of TOC was from 65% to 90 %, so it confirms that mineralization is the main pathway in BTX degradation by electro-oxidation. No aromatic intermediates were detected in the electrolysis performed with DSA®, while in the ones performed with BDD were found phenol, quinones and aldehydes, normally detected in electro-oxidation of aromatic compounds. These results are an important step in understanding the mechanisms involved in the electrochemical oxidation of BTX in water matrices.

OH-initiated atmospheric degradation of hydroxyalkyl hydroperoxides: mechanism, kinetics, and structure–activity relationship

Abstract. Hydroxyalkyl hydroperoxides (HHPs), formed in the reactions of Criegee intermediates (CIs) with water vapor, play essential roles in the formation of secondary organic aerosol (SOA) under atmospheric conditions. However, the transformation mechanisms for the OH-initiated oxidation of HHPs remain incompletely understood. Herein, the quantum chemical and kinetics modeling methods are applied to explore the mechanisms of the OH-initiated oxidation of the distinct HHPs (HOCH2OOH, HOCH(CH3)OOH, and HOC(CH3)2OOH) formed from the reactions of CH2OO, anti-CH3CHOO, and (CH3)2COO with water vapor. The calculations show that the dominant pathway is H-abstraction from the -OOH group in the initiation reactions of the OH radical with HOCH2OOH and HOC(CH3)2OOH. H-abstraction from the -CH group is competitive with that from the -OOH group in the reaction of the OH radical with HOCH(CH3)OOH. The barrier of H-abstraction from the -OOH group slightly increases when the number of methyl groups increase. In pristine environments, the self-reaction of the RO2 radical initially produces a tetroxide intermediate via oxygen-to-oxygen coupling, and then it decomposes into propagation and termination products through asymmetric two-step O–O bond scission, in which the rate-limiting step is the first O–O bond cleavage. The barrier height of the reactions of distinct RO2 radicals with the HO2 radical is not affected by the number of methyl substitutions. In urban environments, the reaction with O2 to form formic acid and the HO2 radical is the dominant removal pathway for the HOCH2O radical formed from the reaction of the HOCH2OO radical with NO. The β-site C–C bond scission is the dominant pathway in the dissociation of the HOCH(CH3)O and HOC(CH3)2O radicals formed from the reactions of NO with HOCH(CH3)OO and HOC(CH3)2OO radicals. These new findings deepen our understanding of the photochemical oxidation of hydroperoxides under realistic atmospheric conditions.

Hydrogen elimination from the dissociation of methyl-substituted silanes on tungsten and tantalum surfaces

The elimination of H2 from the dissociation of four methyl-substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS), and tetramethylsilane (TMS), on a heated tungsten or tantalum filament surface has been studied using laser ionization mass spectrometry. Two complementary ionization methods, i.e., single photon ionization (SPI) using a vacuum ultraviolet wavelength at 118 nm (10.5 eV) and a dual ionization source incorporating both 10.5 eV SPI and laser-induced electron ionization, were employed to detect the production of H2. Examination of the intensity of the H2+ peak from the four molecules has shown that it increases with temperature until reaching a plateau at around 2000−2100 °C on both tungsten and tantalum filaments. These methyl-substituted silanes are dissociatively adsorbed on tungsten and tantalum surfaces by Si−H bond cleavage, and as the temperature is raised, by C−H bond rupture. Experiments with the isotopomers of MMS, DMS, and TriMS have shown that the formation of H2 follows the Langmuir−Hinshelwood mechanism where two adsorbed hydrogen atoms on metal surfaces recombine to produce H2. The determined activation energy (Ea) for H2 formation from MMS, DMS, and TriMS, in the range of 58.2−93.4 kJ mol−1, has been found to increase with the number of methyl substitutions in the precursor molecule. Comparison of these Ea values with the reported values of 51.1−78.8 kJ mol−1 for the methyl radical formation from the same three precursor molecules has led to the conclusion that the initial Si−H bond cleavage in the dissociative adsorption of MMS, DMS, and TriMS is the rate-limiting step for the formation of both H2 molecules and ·CH3 radicals.

Reversible sigma C-C bond formation between phenanthroline ligands activated by (C5Me5)2Yb.

The electronic structure and associated magnetic properties of the 1,10-phenanthroline adducts of Cp*2Yb are dramatically different from those of the 2,2'-bipyridine adducts. The monomeric phenanthroline adducts are ground state triplets that are based upon trivalent Yb(III), f(13), and (phen(•-) ) that are only weakly exchange coupled, which is in contrast to the bipyridine adducts whose ground states are multiconfigurational, open-shell singlets in which ytterbium is intermediate valent ( J. Am. Chem. Soc 2009 , 131 , 6480 ; J. Am. Chem. Soc 2010 , 132 , 17537 ). The origin of these different physical properties is traced to the number and symmetry of the LUMO and LUMO+1 of the heterocyclic diimine ligands. The bipy(•-) has only one π*1 orbital of b1 symmetry of accessible energy, but phen(•-) has two π* orbitals of b1 and a2 symmetry that are energetically accessible. The carbon pπ-orbitals have different nodal properties and coefficients and their energies, and therefore their populations change depending on the position and number of methyl substitutions on the ring. A chemical ramification of the change in electronic structure is that Cp*2Yb(phen) is a dimer when crystallized from toluene solution, but a monomer when sublimed at 180-190 °C. When 3,8-Me2phenanthroline is used, the adduct Cp*2Yb(3,8-Me2phen) exists in the solution in a dimer-monomer equilibrium in which ΔG is near zero. The adducts with 3-Me, 4-Me, 5-Me, 3,8-Me2, and 5,6-Me2-phenanthroline are isolated and characterized by solid state X-ray crystallography, magnetic susceptibility and LIII-edge XANES spectroscopy as a function of temperature and variable-temperature (1)H NMR spectroscopy.

Influence of anion on the quadratic nonlinearity and depolarization ratios of scattered second harmonic light from cation-π complexes

We have investigated quadratic nonlinearity (β(HRS)) and linear and circular depolarization ratios (D and D('), respectively) of a series of 1:1 complexes of tropyliumtetrafluoroborate as a cation and methyl-substituted benzenes as π-donors by making polarization resolved hyper-Rayleigh scattering measurements in solution. The measured D and D(') values are much lower than the values expected from a typical sandwich or a T-shaped geometry of a complex. In the cation-π complexes studied here, the D value varies from 1.36 to 1.46 and D(') from 1.62 to 1.72 depending on the number of methyl substitutions on the benzene ring. In order to probe it further, β, D and D(') were computed using the Zerner intermediate neglect of differential overlap-correction vector self-consistent reaction field technique including single and double configuration interactions in the absence and presence of BF(4) (-) anion. In the absence of the anion, the calculated value of D varies from 4.20 to 4.60 and that of D(') from 2.45 to 2.72 which disagree with experimental values. However, by arranging three cation-π BF(4)(-) complexes in a trigonal symmetry, the computed values are brought to agreement with experiments. When such an arrangement was not considered, the calculated β values were lower than the experimental values by more than a factor of two. This unprecedented influence of the otherwise "unimportant" anion in solution on the β value and depolarization ratios of these cation-π complexes is highlighted and emphasized in this paper.

Microstructure evolution in aqueous solutions of cetyl trimethylammonium bromide (CTAB) and phenol derivatives

Surfactant aggregate microstructure changes induced by the addition of a series of phenolic organic dopants (phenol, cresol, 4-ethyl phenol and 4- n- and i-butyl phenol) to a micellar solution of cetyl trimethyl ammonium bromide (CTAB) have been studied using cryogenic transmission electron microscopy (cryo-TEM). As the concentration of each of the dopants is increased, there is a systematic reduction in the curvature of the observed microstructures. For phenol and cresol the microstructure changes from globular micelles to rod-like micelles and then to long worm-like micelles. For 4-ethyl phenol and 4-butyl phenol, the microstructure transforms from globular micelles to worm-like micelles to unilamellar vesicles and then bilamellar vesicles. The concentrations at which these morphological transitions take place reduce as the number of methyl substitutions on the phenolic ring is increased. These microstructure transitions are attributed to changes in the packing parameter of the surfactant complex as dopants penetrate to different degrees in the surfactant palisades layer, resulting from a balance of interaction between the surfactant tails and the dopant aromatic chain and the hydrogen bonding of the hydroxyl group with surrounding water. The alignment of the hydroxyl dipoles reduces the electrostatic field around the CTAB head groups, decreases interlayer repulsive interactions and allows membrane fluctuations to stabilize the bilamellar vesicles. These highly tunable microstructures can be exploited for templated materials synthesis, and have implications for the micellar-enhanced ultrafiltration process.