Experimental and computational study of diethyl sulfide pyrolysis and mechanism

The pyrolysis of diethyl sulfide (C₂H₅SC₂H₅), a simulant for mustard chemical warfare agents, was studied in a turbulent flow reactor with extractive gas composition analysis by GC/MS and FTIR. Experiments were performed at approximately atmospheric pressure for four different temperatures between 630 and 740 °C with maximum residence times between 0.06 and 0.08 s. Temperature and species profiles were obtained on the centerline of the reactor. The mixing characteristics in the reactor were determined by using carbon monoxide as a tracer. 80% destruction of diethyl sulfide was observed for the experiment at the temperature of 740 °C and the residence time of 0.06 s. The following species were quantified: diethyl sulfide, ethylene, methane, ethane, acetylene, carbon disulfide, and thiophene. In addition, ethanethiol, methyl thiirane CH₃-(Cy-CH-CH₂-S), ethyl methyl disulfide, and diethyl disulfide were identified but not quantified. A light yellow solid containing sulfur condensed in sampling probes. Thermochemical properties for all species and a detailed mechanism were developed for modeling the reaction system. Thermodynamic and kinetic parameters were based on density functional theory and ab initio calculations using isodesmic work reactions for enthalpies. Kinetic parameters for chemical activation and unimolecular dissociation reactions were determined with multi frequency quantum RRK analysis for k(E) and master equation for fall-off. Important reactions were identified by sensitivity analysis and reaction pathway analysis of the mechanism. Model predictions show overall good agreement with experiment.     

The Raman and Infrared Spectra and the Structures of Dimethyl,Methyl Germyl and Digermyl Sulfide

Abstract

The Raman and infrared spectra of dimethyl sulfide (CH₃?S-CH₃) have been assigned in terms of a D_3d model¹. The structurally related compounds, digermyl sulfide (GeH₃?S-GeH₃) and methyl germyl sulfide(CH₃?S-GeH₃) are prepared according to the literature^2–4, and their vibrational spectra are measured. The analysis of these spectra comparing with those of CH₃?S-CH₃ allows us to determine the molecular structures to be D_3d for GeH₃?S-GeH₃ and a C_3v for CH₃?S-GeH₃ with a linear—S—.     

Evidence for non-thermalized vibrationally excited molecule production during azomethane pyrolysis in methane desorption from Cu(001)

A Cu(001) surface was exposed to products of an azomethane pyrolysis doser at varying temperatures. In addition to methyl radical adsorption, for certain doser conditions one or more doser emergent species can undergo an activated adsorption on the copper face. Directly after exposures, temperature programmed desorption between 170 K and 500 K was used to indicate the relative concentrations of adsorbed atomic hydrogen and methyl species, and thermally induced surface reactions. Two methane desorption features were invariably observed, indicating the presence of adsorbed methyl groups (CH₃) and transient adsorbed atomic hydrogen. The deduced relative surface concentrations levels of both H and CH₃ depend on the total exposures and the operating temperatures of the azomethane pyrolysis doser. The initial H concentrations apparent at surface temperatures between 275 K and 375 K are shown to arise from defect-related methyl decomposition and, at high operating doser temperatures, from the initial adsorption of one or more activated Cu incident species. It is proposed that the distributions of vibrational energies of emergent molecular hydrogen or methane species from higher temperature dosers are non-thermal. Hence, with doser temperatures of 800 °C or above, the effects of subsequent dissociative molecular adsorption on the copper surface can dominate over Cu defect chemistries.     

A spectroscopic study of the adsorption and reactions of methanol,formaldehyde and methyl formate on clean and oxygenated Cu(110) surfaces

The adsorption and reaction of methanoi (CH₃OH), methyl formate (CH₃OCHO) and formaldehyde (H₂CO) on clean and oxygen-covered Cu(110) surfaces has been studied with EELS, UPS and thermal desorption spectroscopy (TDS). The clean surface is relatively unreactive but adsorbed oxygen readily attacks the hydroxyl proton and formyl carbon atoms to generate the intermediate methoxy (CH₃O) and formate (HCOO). Methyl formate is split into two intermediates, methoxy and formate. By correlating the three techniques we analyse (a) the condensed multilayer at 90 K; (b) the weakly bound molecular monolayer states prior to dissociation or reaction and (c) the reactive intermediates at higher temperatures. Formaldehyde forms the surface polymer polyoxymethylene [(CH₂O)_n] in the monolayer on Cu(110) which subsequently reacts with oxygen to generate formate. No molecular formaldehyde was observed above 120 K. Correlation of the EELS and UPS results for polyoxymethylene shows that an earlier interpretation by Rubloff et al. [Phys. Rev. B14 (1976) 1450] of anomalous shifts in the formaldehyde UPS spectrum on surfaces is incorrect, and due simply to the new polymeric structure of surface formaldehyde. Methyl formate coordinates to copper via the carbonyl lone pair orbital and methanol via the oxygen lone pair orbital. No evidence was found for methyl formate synthesis by dimerization of formaldehyde (the Tischenko reaction) or dehydrogenation of methanol on the clean Cu(110) surface. These latter reactions are facile over copper catalysts at atmospheric pressure. The success of the oxidation experiments and the failure of the synthesis reactions in UHV is a consequence of the pressure dependence of the equilibrium constants for the different reactions. As found previously in Fischer-Tropsch studies, condensation reaction equilibria are pressure dependent and product formation is considerably suppressed at UHV pressures.     

TDS and EELS observations for CO,O2, and CH3OH bound to Ru(0001)/Cu

The electronic properties of monolayers of copper atoms adsorbed onto a Ru(0001) single crystal surface have been studied with thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (EELS) utilizing carbon monoxide (CO), dioxygen (O₂), methanol (CH₃OH), and to some extent water (H₂O) as chemical probes. Whereas a three-monolayer-thick film exhibits most properties of a Cu(111) crystal distinct deviations are found at lower Cu coverages. TDS as well as EELS show a weakened RuCO bond and a strengthened CuCO bond as a result of metal-metal interaction. The stronger CuCO bond is accompanied by a higher probability for O₂ dissociation. The mobilities of copper and oxygen atoms are such that annealing to 650 K produces an overlayer structure which is independent of adsorption sequence: Cu/O₂ or O₂/Cu, but where RuO as well as CuO vibrations can be identified. Methanol adsorbs reversibly on a monolayer of copper atoms. Metal bound methoxy species are formed in the presence of oxygen atoms. The decomposition paths of such methoxy intermediates alter towards more formaldehyde (CH₂O) relative to CO with increasing copper and methoxy coverages.     

Reinvestigation of microwave spectrum of dimethyl ether and rs structures of analogous molecules

Microwave spectra of dimethyl ether and its sixteen isotopic species have been measured. For species with singlet spectra, a least-squares analysis of observed transition frequencies gave rotational and five quartic centrifugal distortion constants. For species with multiplet spectra due to the methyl internal rotation, a least-squares analysis of observed multiplet frequencies gave not only unperturbed rotational and five quartic centrifugal distortion constants but also the quantities related to the methyl internal rotation. The r_s structures from (CH₃)₂O, CH₃OCD₃, and (CD₃)₂O species as the parent species, respectively, were compared with one another. The proposed r_s structure has been established from all the species measured and was compared with the r_s-like structure obtained by a diagnostic least-squares method and with the reported structure. The r_s structure of the present molecule was compared with the reported structures of dimethyl sulfide and dimethyl silane in relation to the tilt phenomenon. The r_s structure of dimethyl sulfide was revised based on the present comparison.     

Thermal chemistry of diiodomethane on Ni(1 1 0) surfaces I. Clean and hydrogen-covered

The thermal chemistry of diiodomethane on Ni(1 1 0) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Diiodomethane was chosen as a precursor for the formation of methylene surface species. I 3d and C 1s XPS data indicated that, indeed, adsorbed diiodomethane undergoes the C-I bond dissociations needed for that transformation, and detection of iodomethane production in TPD experiments pointed to the stepwise nature of those reactions. Significant amounts of methane are produced from further thermal activation of the chemisorbed methylene groups. This involves surface hydrogen, both coadsorbed from background gases and produced by dehydrogenation of some of the adsorbed diiodomethane, and can be induced at temperatures as low as about 160 K, right after the C-I bond breaking steps. Unique to this system is the detection of significant amounts, up to 10% of the total CH₂I₂ adsorbed, of heavier hydrocarbons, including ethene, ethane, propene, propane, and butene. Deuterium labeling experiments were used to provide support for a mechanism where the initial hydrogenation of some adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates. Facile subsequent β-hydride elimination and reductive elimination with coadsorbed hydrogen account for the formation of ethene and ethane, respectively, while a second and third methylene insertions lead to C₃ and C₄ production. Based on the final product distribution, the methylene insertion was estimated to be approximately 20 times slower than the following hydrogenation-dehydrogenation reactions. Normal kinetic isotope effects were observed for most of the hydrogenation and dehydrogenation reactions involved.     

Single-file motion of polyatomic molecules in one-dimensional nanoporous materials

In a continuation of the study of the mobility of fluids adsorbed in nanoporous materials, molecular dynamics simulations are used to investigate the behaviour of polyatomic ethane molecules adsorbed in AlPO₄-5. The current work is based on the use of the united atom approach as a better model than the single-centre ethane used to date. Ethane molecules are modelled as rigid diatoms, and as a result the molecules have more degrees of freedom in the form of the rotational components that are absent in the single-centre ethane model. This represents a more sophisticated model for ethane and is used in the simulations to test earlier findings. Simulations with binary mixtures of methane and ethane also have been conducted with three mixture compositions. The transition from ordinary diffusion to single-file motion for a finite residence time is found to occur at a methyl group diameter of 4.75 Å. This is identical to the ethane diameter in the earlier study. Thus, only the minimum dimension determines the transition size. Also it is shown that the diatomic molecules undergo free rotation within the channel even when they are in the single-file mode of motion. In the case of binary mixtures, the methane molecules still undergo ordinary diffusion. Ethane molecules exhibit single-file motion at a methyl group diameter of 4.75 Å. The diffusion coefficient of methane decreases with increasing ethane size, while the trends in the single-file mobility of ethane as a function of methyl group diameter are nonlinear.     

The effects of dimethyl ether and ethanol on benzene and soot formation in ethylene nonpremixed flames

Soot volume fractions, C1-C12 hydrocarbon concentrations, and gas temperature were measured in ethylene/air nonpremixed flames with up to 10% dimethyl ether (CH₃OCH₃) or ethanol (CH₃CH₂OH) added to the fuel. The measurement techniques were laser-induced incandescence, photoionization mass spectroscopy, and thermocouples. Oxygenated hydrocarbons have been proposed as soot-reducing fuel additives, and nonpremixed flames are good laboratory-scale models of the fuel-rich reaction zones where soot forms in many full-scale combustion devices. However, addition of both dimethyl ether and ethanol increased the maximum soot volume fractions in the ethylene flames studied here, even though ethylene is a much sootier fuel than either oxygenate. Furthermore, dimethyl ether produced a larger increase in soot even though neat dimethyl ether flames produce less soot than neat ethanol flames. The detailed species measurements suggest that the oxygenates increase soot concentrations because they decompose to methyl radical, which promotes the formation of propargyl radical (C₃H₃) through C1 + C2 addition reactions and consequently the formation of benzene through propargyl self-reaction. Dimethyl ether has a stronger effect than ethanol because it decomposes more completely to methyl radical. Ethylene does not decompose to methyl, so its flames are particularly sensitive to this mechanism; the alkane-based fuels used in most practical fuels do decompose to methyl radical, so the mechanism will be much less important for practical devices.     

The surface chemistry of methyl and methylene radicals adsorbed on Cu(111)

The interactions of methyl and methylene radicals on Cu(111) were investigated with XPS, AES and HREELS under various exposure conditions. The CH₂ and CH₃ radicals are generated through a hot nozzle source with ketene and azomethane gases. It is shown that with substrate at 300 K, the impinging CH₃ radicals are trapped mainly as CH₃(ads), while a part of the adsorbate decomposes to form CH₂(ads) and H(ads). H atoms are found to desorb at about 380 K, while the chemisorbed hydrocarbon adspecies desorb at about 420 K. In drastic contrast, exposing the clean Cu surface to methylene radicals results not only in the trapping of CH₂(ads), but also in the formation of complex aromatic species. The adlayer is sensitive to annealing at elevated temperatures. Desorption and partial conversion to methylidyne take place at around 420 K. The CH(ads) species can survive up to 700 K and then decomposes to form residual carbon above 800 K. In both radical-Cu(111) systems, surface coverage appears to saturate near one monolayer. The relative concentrations of different surface species in the adlayer, however, depend on the amount of radical exposure. The reaction properties of the two systems are compared and discussed.     

Adiabatic and vertical carbon 1s ionization energies in representative small molecules

Adiabatic and vertical carbon 1s ionization energies are reported for methane (CH₄), ethane (CH₃CH₃), ethene (CH₂CH₂), ethyne (HCCH), carbon monoxide (CO), carbon dioxide (CO₂), fluoromethane (CH₃F), trifluoromethane (CHF₃), and tetrafluoromethane (CF₄) with an absolute accuracy of about 0.03 eV. The results are in good agreement with earlier values but are measured with higher resolution and accuracy than has previously been available.     

Neutron scattering study of the structure and dynamics of butane adsorbed on graphite2

Inelastic and elastic neutron scattering have been used to study the dynamics and structure of monolayer films of butane (CH₃(CH₂)₂CH₃) adsorbed on a graphitized carbon powder at 77 K. In addition to the intramolecular torsional modes found in the bulk solid, the inelastic spectra of the films contain new excitations associated with coupling of the molecular motion to the substrate. Model calculations are described which show the monolayer excitation spectrum to be sensitive to the orientation of the adsorbed butane molecule and the location and strength of the bonds to the substrate. For butane we infer that the molecule is adsorbed with the plane of the carbon skeleton parallel to the graphite layers. We have also used elastic neutron diffraction to investigate the possibility of long-range order in the butane films. Although we have not found Bragg peaks indicative of an ordered two-dimensional structure in a 1.5 layer film at 81 K, a large modulation of the graphite 002 Bragg reflection is observed. The experimental approach discussed here would seem to be applicable to the study of the dynamics, molecular orientation, and bonding of other hydrogenous adsorbates as a function of film thickness and temperature. Measurements are presently being extended to propane (CH₃CH₂CH₃) and ethane (CH₃CH₃) adsorbed on graphite.     

Thermal reactions of 2-iodoethanol on Cu(1 0 0)

Temperature-programmed reaction/desorption, X-ray photoelectron spectroscopy, and reflection-absorption infrared spectroscopy have been employed to investigate the reactions of ICH₂CH₂OH on Cu(1 0 0) under ultrahigh-vacuum conditions. ICH₂CH₂OH can dissociate on Cu(1 0 0) at 100 K, forming a -CH₂CH₂OH surface intermediate. Density functional theory calculations predict that the -CH₂CH₂OH is most probably adsorbed on atop site. -CH₂CH₂OH on Cu(1 0 0) further decomposes to yield C₂H₄ below 270 K. No evidence shows the formation of -CH₂CH₂O- intermediate in the reactions of ICH₂CH₂OH on Cu(1 0 0) in contrast to the decomposition of BrCH₂CH₂OH on Cu(1 0 0) and ICH₂CH₂OH on Ag(1 1 1) and Ag(1 1 0), exhibiting the effects of carbon-halogen bonds and metal surfaces.     

Microwave spectrum of two top molecules in the excited states dimethyl ether, dimethyl sulfide, and dimethyl silane

Microwave spectra of dimethyl ether, dimethyl sulfide, and dimethyl silane in the torsionally excited states have been measured. The methyl internal rotations of these molecules were analyzed from the observed multiplets and from the reported multiplets of transitions. The method developed for ethyl silane in the previous paper was extended to equivalent two top molecules. For equivalent two top molecules, apparent barriers of methyl internal rotations obtained from the experiments were corrected by the kinetic and potential cross terms. They are V₃=2645.8±10.0, 2632.4 ± 42.9, 2146.0 ± 13.8, 1651.5 ± 10.1, 1648.0 ± 13.7, and 1649.9 ± 11.8 cal/mol for (CH₃)₂O, (CD₃)₂O, (CH₃)₂S, (CH₃)₂SiH₂, (CH₃)₂SiD₂, and (CH₃)₂SiHD, respectively. The potential cross terms, V₁₂(1−cos3α₁)(1−cos3α₂) terms are negligible for the three molecules, while V^′₁₂sin3α₁sin3α₂ terms are also very close to zero except those for (CH₃)₂O and (CD₃)₂O which are small but not negligible (V^′₁₂=−124.4,−158.0 cal/mol). The investigations were extended to those of non-equivalent two top species and the corrected barriers of the methyl tops, V₃, are obtained to be 2615.6 ± 8.6 and 2155.0 ± 15.2 cal/mol for CH₃OCD₃ and CH₃SCD₃. The corrected barrier, V₃(CD₃) of CH₃OCD₃, is obtained to be 2634.4 ± 7.1 cal/mol, while that of CH₃SCD₃ cannot be solved due to the lack of the data available.     

Novel Method for the Oxidation of Aliphatic Hydrocarbons to Alcohols

A novel method for the conversion of hydrocarbons to alcohols using a reaction of gas-phase oxidation by oxygen in the presence of boron trichloride has been developed and described in detail. The reaction represents radical long-chain alkoxylation of boron trichloride. It proceeds at moderate temperatures of 150–180°C and atmospheric pressures of less than one atmosphere, resulting in methane conversion to (CH₃O)_3–nBCl_n (n = 0–2) and ethane conversion to (CH₃CH₂O)_3–nBCl_n (n = 0–2). The hydrolysis of the reaction products generates CH₃OH and C₂H₅OH, respectively. The yield of methanol reaches up to 55% at the conversion of methane of ~15% at the early stages of the reaction. The yield of ethanol is at least 65% of the reacted ethane nearly to the end of the reaction.     

Thermal stability of adsorbed CH3NCO on Cu{110} AND Pt{110} from vibrational and photoelectron spectroscopies

The thermal stability of CH₃NCO adsorbed on Cu{110} and Pt{110} is investigated using HREELS, TPD, and ARUPS. CH₃NCO desorbs largely without fragmentation from Cu{110}, but on Pt{110} only about 20% of the adsorbed CH₃NCO desorbs intact, with 80% decomposing on the surface at T > 200 K into CO(a), H(a), CH_x(a), N(a) and NH_y(a) fragments. The kinetics of the surface decomposition were characterised for 220 < T < 300 K by HREELS and the activation energy for CH₃NCO decomposition was found to vary strongly as a function of coverage.     

Shock-tube and modeling study of ethyl methyl ether pyrolysis and oxidation

Pyrolysis and oxidation of ethyl methyl ether (EME) were studied behind reflected shock waves in the temperature range 900-1750 K at total pressures between 0.9 and 3.1 atm. The study was carried out using following methods, (1) time-resolved IR-laser absorption at 3.39 μm for EME decay and CH-compound formation rates, (2) time-resolved UV absorption at 216 nm for mainly CH₃ radical formation rate, (3) time-resolved UV absorption at 306.7 nm for OH radical formation rate, (4) time-resolved IR emission at 4.24 μm for CO₂ formation rate and (5) a single-pulse technique for product yields. The pyrolysis and oxidation of EME were modeled using a reaction mechanism including the sub-mechanisms for methane, acetylene, ethylene, ethane, formaldehyde, acetaldehyde and ketene oxidation. The reaction mechanism used in this study could reproduce almost all of experimental results. The sub-mechanisms of methane, ethylene, ethane, formaldehyde, and acetaldehyde were found to play an important role in EME oxidation.     

Measurement and modeling of the sooting propensity of binary fuel mixtures

The sooting behaviour of binary fuel mixtures was evaluated both experimentally and through computer simulations. The soot volume fraction in laminar diffusion flames of mixtures of ethylene/propane, methane/ethylene, methane/propane, methane/ethane, methane/butane, ethane/propane and ethane/ethylene fuels was measured using 2-dimensional line of sight attenuation. A synergistic effect was observed for the ethylene/propane, methane/ethylene, methane/ethane and ethane/ethylene mixtures. The synergistic effect translated into a higher soot concentration for a mixture fraction than could be yielded by the added contribution of both pure fuels. Such an effect was not observed for the methane/propane, methane/butane and ethane/propane mixtures. Through experiments in which the flame temperature was kept constant, it was determined that the synergistic effect in the methane/ethylene mixture is very temperature dependent whereas, that in the ethylene/propane mixture is not. This phenomenon was further studied through the modeling of the ethylene/propane mixture. Numerical simulations were carried out using two different soot models. The simulations confirmed the presence of a synergistic effect. It was found that the effect could be directly correlated to a synergistic effect in the concentration of n-C₄H₅ and n-C₄H₃, which could be traced back to an interaction between ethylene and methyl radical species. These results yield further insight into the pathways to soot formation and highlight the importance of further analyzing binary fuel mixtures as a means of understanding soot formation in practical devices using industrial fuels.     

The Raman,Infrared and Proton NMR Spectra and the Structures of Dimethyl Chalcogenides

The vibrational bands of dimethyl chalcogenides (CH₃-X-CH₃) (X = O,S,Se,Te) have been assigned on the basis of C_2V symmetry by most of previous workers.^1–13) On the other hand, some ones have assigned D^3d to these molecules ^14–15) and to structurally related compounds; SiH₃-O-SiH₃ ¹⁶), CH₃-Hg-CH₃ ¹⁷), CF₃-Hg-CF₃ ¹⁸) and Cl₃Si-O-SiCl₃ ¹⁹). The symmetry of ethane(CH₃-CH₃) is obviously D_3d ²⁰⁾, because the vibrational spectra which indicate the lack of coincidence between Raman and infrared frequencies can be explained based upon D_3d symmetry, and the bent c c should theoretically be impossible. The CH₃-X-CH₃ molecules are considered as a derivative in which CH₃ group is connected to CH₃ in opposite direction along the z axis and x atom is inserted between the two CH₃ groups. Consequently, for a linear C-X-C model, the vibrational spectra of CH₃-X-CH₃ ^14,15) should be similar to those of CH₃-CH₃, except being added C-O-C stretching and bending bands. The obtained spectra of CH₃-X-CH₃ ^14,15) show a marked correspndence to that of ethane, and indicate that the rule of mutual exclusion holds between Raman and infrared frequencies. Therefore the symmetry of CH₃-X-CH₃ Seems to be D_3d, But mst of the previom papers ^1–13) have assigned C_2v to CH₃-X-CH₃ as mentioned above.     

Adsorption and dissociation kinetics of alkanes on CaO(100)

The adsorption kinetics of ethane, butane, pentane, and hexane on CaO(100) have been studied by multi-mass thermal desorption (TDS) spectroscopy. The sample cleanliness was checked by Auger electron spectroscopy. A molecular and dissociative adsorption pathway was evident for the alkanes, except for ethane, which does not undergo bond activation. Two TDS peaks appeared when recording the parent mass, which are assigned to different adsorption sites/configurations of the molecularly adsorbed alkanes. Bond activation leads to desorption of hydrogen and several alkane fragments assigned to methane and ethylene formation. Only one TDS feature is seen in this case. Formation of carbon residuals was absent.