Initial Decomposition Products Research Articles

Predicting decomposition temperatures and carbonization feasibility of heterocycles with their decomposition mechanism

Pyrolysis of heterocyclic compounds such as Meldrum’s acid (MA) derivatives has been studied with focus on two key mechanisms analysing their energy barrier diagrams using the hybrid density functional theory method M06-2X. The feasibility of these decomposition reactions was calculated specifically for MA, monomethyl Meldrum’s acid (MMA), dimethyl Meldrum’s acid (DMMA), methylene Meldrum’s acid (MeMA) and (dimethylamino)methylene Meldrum’s acid (DMAMeMA), based on previous experimental analysis. The barrier to initial decomposition products (ketene or substituted ketenes with CO2 and acetone) were found to be endothermic requiring activation energies (∼180 – 230 kJ/mol) at decomposition temperatures of 280, 299, 301, 503 and 414 K respectively. Transition state theory calculations show MeMA and DMAMeMA are thermally more stable than other derivatives with low values for the rate constants and high energy barriers to produce initial decomposition products, suggesting they can produce fine carbon materials during carbonization. DMMA is found the best option for obtaining heteroatoms-doped carbon material formation.

Oxygen functionalized carbon obtained from pyrolysis of heterocyclic compounds with their decomposition mechanism

In this study, two heterocyclic compounds, 2,2-dimethyl-1,3-dioxane-4,6‑dione (DMDO) and 2,2,5-trimethyl-1,3-dioxane-4,6‑dione (5-DMDO), were thermally decomposed in an inert atmosphere of nitrogen to obtain oxygen functionalized carbon. The decomposition of these compounds was investigated by thermal gravimetric analysis (TGA) and gas chromatography-mass spectroscopy (GCMS) as well as hybrid-density functional theory (h-DFT). DMDO was found to have better thermal stability compared to 5-DMDO and thus gave a higher yield of carbon after decomposition at 1000 °C. This experimental observation was supported by the h-DFT analysis of the energy barriers of the two decomposition mechanisms proposed from the initial decomposition products detected above 100 °C with GCMS analysis and the thermodynamic spontaneity of the final product (solid carbon) at 800 to 1000 °C with TGA. X-ray photoelectron spectroscopy, scanning electron microscopy / energy dispersion spectroscopy and cyclic voltammetry were used to characterize the carbon and evidence was found to suggest that the electrochemical activity of the material towards the [Fe(CN)6]4−/[Fe(CN)6]3- redox couple was dependent on the oxygen content of the carbon.

Experimental insight into interaction mechanism of guanidinium-5-aminotetrazole (GA) and nitrocellulose

5-Amino-1H-tetrazole (5-ATZ), an environmentally friendly energy-containing material for new propellants, is gradually applied in high-energy propellants and its ionic salt compounds have received wide attention. This study focused on investigating the interaction mechanism of guanidinium-5-aminotetrazole (GA) and nitrocellulose (NC) by kinetic methods and TG-DSC-FTIR analysis. DSC and TG experiments indicated that there was a certain interaction during the pyrolysis process of NC and GA. Comparing the kinetic models of NC and peak 3 in NC-GA, it can be found that the addition of GA mainly affected the m-value and contribution in the second step B→C, and the mechanism function changed from Fn to Cn,m. TG-DSC-FTIR experiments indicated that the initial decomposition product of NC, NO2, participated in the thermal decomposition process of GA, and the two reacted chemically to form CO, N2O, which made NO2 unable to completely oxidize the skeleton of NC to produce CH2O, HCOOH.

Experimental insight into interaction mechanism of 1H-tetrazole and nitrocellulose by kinetics methods and TG-DSC-FTIR analysis

Nitrogen heterocycles as Nitrogen-rich energetic material has been widely popularized due to their desirable properties like low sensitivities, excellent stability, and lower environmental impact, less erosive and less toxic combustion gases, especially in gun propellant application areas. The focus of this study is on investigating interaction mechanism of 1 H-tetrazole (1 H-TZ) and nitrocellulose (NC) by kinetics methods and TG-DSC-FTIR analysis. The mixture (NC-TZ) between 1 H-TZ and NC was prepared by means of physical blending. DSC experiments indicated that the open and closed environment can generate different and important influence on the chemical reaction between 1 H-TZ and NC. Combined with the change of activation energy, it illustrated that the gas 2 H-tetrazole, the isomer of 1 H-TZ, had a significant influence on the thermal decomposition of NC. In comparison with kinetic models of NC and NC-TZ, it can be found that the addition of 1 H-TZ mainly affected the contribution factor and m value in the first stage. The value of m decreased from 1.00 to 0.15, meanwhile, the contribution factor was decreased from 0.50 to 0.12. It showed that NO2, the initial decomposition product of NC, had taken part in the thermal decomposition process of 1 H-TZ. TG-DSC-FTIR experiments indicated that there was a chemical reaction between 2 H-tetrazole and the initial decomposition product of NC, NO2, to form CO, N2O and HN3.

Decomposition mechanisms of insensitive 2D energetic polymer TAGP using ReaxFF molecular dynamics simulation combined with Pyro-GC/MS experiments

The two-dimensional energetic polymer triaminoguanidine–glyoxal (TAGP) has been used in modification of EMs to formation of various hybrid materials for higher density and better stability due to its excellent capability to desensitize high energetic crystals without any energy loss. In this paper, theoretical calculations based on ReaxFF-MD were performed to demonstrate the thermal decomposition mechanisms of TAGP, which has been further proved by experimental results from Pyro-GC/MS analysis. The results show that there are three dominant pathways for the initial decomposition of TAGP, i.e., the breakage of C-N bond with production of C2H3ON2 (most likely to happen), the scission of N-N bond with release of C2H2ON, and the rupture of N-N bond accompanied with the proton transfer to release C2H3ON. Therefore, the C2H3ON2, C2H2ON and C2H3ON are the dominant initial decomposition products of TAGP, which may gradually transform to the final gaseous products. The experimental results confirmed the presences of these substances. The major final gaseous products of TAGP include N2, H2, H2O, NH3, NH4, NH2, N2H, N2H2, HCN and CO2, among which the concentration of N2 is the highest, relatively higher than H2, H2O and NH3. Besides, the initial decomposition rate of TAGP largely depends on its MW. The activation energies for formation of C2H3ON2 and N2 under isothermal conditions has been obtained to further illustrate the effect of MW on the decomposition rate.

Thermal stability and exothermic behaviour of imidazole ionic liquids with different anion types under oxidising and inert atmospheres

Ionic liquids (ILs), as a type of salt that is liquid at room temperature, break the entrenched views on salt when it was proved that some of them could be ignited. In this research, an innovative method was used to evaluate the thermal stability of such special salts and speculated their ignition mechanism based upon gas analysis. Four typical representative imidazole ILs (BMIMBF4, BMIMDCN, BMIMNO3, and BMIMOAc) were selected, and their thermal decomposition characteristics and thermal effect were obtained by simultaneous thermogravimetric analyser (STA) and differential scanning calorimetry (DSC), respectively. Furthermore, their decomposition products were investigated by thermogravimetry coupled with Fourier-transform infrared spectroscopy (TG-FTIR). It is noteworthy that they behaved diversely in DSC experiments and exhibited a radically different thermal effect in their decomposition. In addition, there exists positive feedback in the mechanism of the exothermic phenomenon of selected ILs in the air. Nevertheless, TG curves of four selected ILs were almost the same under air or nitrogen conditions. Series TG-FTIR experiments confirmed that the initial decomposition products of BMIMBF4 and BMIMDCN under air or nitrogen are extremely different. The decomposition products of BMIMNO3 and BMIMOAc in the air atmosphere have more oxidative products than nitrogen. The results denoted that the thermal decomposition mechanism of the four selected ILs in the air were the same as under nitrogen. The reducibility of the decomposition products induced the peculiar thermal effects of these imidazole ILs under the different gas environments. This study was innovative in researching the thermal effect and mechanism of imidazole ILs’ decomposition and provided certain safety guidance for the process safety and loss prevention of imidazole ILs.

Experimental and ReaxFF‐based molecular dynamics studies of the reaction of oxygen with DR‐2 as a low global warming potential working fluid

AbstractThe cis‐1,1,1,4,4,4‐hexafluoro‐2‐butene (DR‐2 or HFO‐1336mzz(Z)) is a novel environmentally friendly working fluid with appropriate physicochemical characteristics. The present work firstly investigated the decomposition mechanism and thermal stability of DR‐2 in a oxygen (O2)‐containing atmosphere at high temperature experimentally. The oxidative degradation features of DR‐2 were explored at the temperature of 473–1073 K and the products were monitored by GC–MS and IC. The experimental and ReaxFF‐based molecular dynamics results demonstrated the promotion effects of O2 on the DR‐2 decomposition. The participation of O2 molecules was supposed to lower the initial thermal decomposition temperature by 240 K approximately and increase the number of products such as hydrogen fluoride (HF), but the enhancement effect was weakened by the increasing reaction temperature. The reasonable Arrhenius parameters were calculated from the first‐order kinetic analyses‐based reactive molecular dynamics (RMD) simulations. Combined with the density functional theory, the pathways of initial oxidation decomposition product firstly observed in the experiment and RMD simulations were proposed in this study. These results may pave the way for further study of DR‐2 as a working fluid with lower global warming potential.

Microscopic pyrolysis mechanism on the octyphenylsiloxane flame retarded polycarbonate by reactive molecular dynamics

Pyrolysis mechanisms and flame retardant properties of polycarbonate (PC) and PC/ octyphenylsiloxane (OPS) composites at different temperatures (2000–4000 K with an interval of 500 K) were studied using reactive force field (ReaxFF) molecular dynamics simulation. The initial decomposition mechanism, products distribution, and main products generation and consumption paths of PC and PC/OPS composites were analyzed. Results showed that decomposition of PC molecular chain was caused by the rupture of the CO bond, and the decomposition of OPS molecule was caused by the rupture of the C-Si bond. The breaking of partial CSi bond made OPS from cage structure into a lamellar-like or reticular structure, which has been proven this structure was a key factor to improve the flame retardance of PC by suppressing the transfer of heat, flammable volatile gaseous products. From the potential energy and decomposition time of PC and PC/OPS composites, the addition of OPS slowed down the decomposition rate of the PC molecular chain and improved the thermal stability of the composites. These have overall good agreement with the reported results. The amounts of CO2, CO, C6H5OH and their derivatives in PC/OPS composites were lower than those in PC at the later stage of reaction, which was because unsaturated Si atoms in OPS adsorbed small oxygen-containing molecules to suppress these harmful materials. Moreover, OPS promoted the combination of molecular products into carbon clusters and large thermally stable organic structures. Thus, the ReaxFF method could be effectively used to study the pyrolysis mechanism of PC/OPS composites, and provide a new method for the prediction and analysis of flame retardance.

Reactive molecular dynamics simulation on thermal decomposition of n-heptane and methylcyclohexane initiated by nitroethane

Normal alkanes and cycloalkanes are important components of endothermic hydrocarbon fuels. The present work conducts the reactive molecular dynamics (RMD) simulations based on reactive force field to compare the thermal cracking of n-heptane and methylcyclohexane (MCH) with the same carbon atom number initiated by nitroethane, focusing on initial decomposition temperature, intermediates and product distribution, and the related kinetics. From the RMD simulations, we find nitroethane can accelerate both n-heptane and MCH decompositions, since the C–NO2 bond rupture of nitroethane can proceed at relatively lower temperatures to produce the reactive radicals compared to the C–C bond cleavage of n-heptane and MCH to start the free radical chain reactions, but the promoting effect becomes weaker after it reaches a certain level with temperature. Reasonable Arrhenius parameters estimated from RMD simulations based on first-order kinetic analysis compare well with experimental results. The observed difference of pyrolysis behaviors and kinetic parameters between n-heptane and MCH can be ascribed to their molecular structure, coinciding with experimental observations. In the presence of nitroethane, reductions of initial temperature and activation energy for n-heptane decomposition are larger than those for MCH decomposition. More alkenes are observed in n-heptane decomposition compared to MCH decomposition. Due to the endothermic capacities of fuel being sensitive to the yields of alkenes, more heat could be absorbed by the n-heptane decomposition. These results indicate that RMD simulations can provide important insights at the atomistic level on the difference of decomposition behaviors between normal alkanes and cycloalkanes initiated by nitroethane under supercritical conditions.

Effects of C Addition on the Microstructures of As-Cast Cu-Fe-P Alloys.

Effects of C addition on the microstructures of as-cast Cu–Fe–P (mass fraction) alloys were systematically investigated. The results show that C addition can refine the matrix microstructure and make Fe particles finer. The Fe particles observed in both the non-C-alloyed and C-alloyed specimens are α-Fe particles, which possess a body-centered cubic (bcc) structure with a Nishiyama–Wassermann orientation relationship with the matrix. C is reported to be an γ-Fe stabilizer in the literature. The reason for the difference between the phases of Fe particles observed in this study, and that reported in the literature, are finally discussed. Additionally, C addition facilitates the decomposition of the supersaturated solid solution which occurs by the simultaneous precipitation of very fine Fe particles. Such initial decomposition product has an face-centered cubic (fcc) structure with a cube-on-cube orientation relationship with the matrix.

Insight into the catalytic thermal decomposition mechanism of ammonium perchlorate

Copper oxide (CuO) is an attractive burn rate modifier for composite solid propellant based on ammonium perchlorate (AP). However, the mechanism of catalytic decomposition of AP in the presence of CuO is still uncertain. Amount of gaseous products at various decomposition temperatures of AP in the presence and absence of CuO is lacking. Herein, a systematic study using thermogravimetry-mass spectroscopy (TG-MS) was carried out to evaluate the effect of CuO on AP decomposition mechanism. A novel hydrothermal method was reported for the preparation of nanosized copper oxide. The conversion of precursor Cu(NO3)2·3H2O to intermediate Cu2(NO3)(OH)3 was accomplished by in situ carbonization of small quantity of cotton fiber in the hydrothermal reaction chamber. pH of the reaction medium was controlled by utilizing the nitric acid in the reaction medium by in situ carbonization of cotton. This synthetic method aimed to minimize the shape-controlling agents and strong alkali for controlling the pH of the reaction medium for the preparation of catalyst grade CuO. Evolved gas analysis of AP decomposition products by TG-MS showed increased oxygen, chlorine and nitrogen evolution and decreased ammonia evolution in the presence of CuO, compared with that of AP without catalyst. This confirmed the enhanced interaction of ammonia and perchloric acid; the initial decomposition products of AP; and their further reaction in the presence of CuO. The change in composition of volatile products indicates change in mechanism of AP decomposition from proton transfer to electron transfer in the presence of CuO.

Non-enzymatic conversion of primary oxidation products of Docosahexaenoic acid into less toxic acid molecules

Docosahexaenoic acid (DHA) is long chain omega-3 fatty acid with known health benefits and clinical significance. However, 4-hydroxy hexenal (HHE), an enzymatic oxidation product of DHA has recently been reported to have health-damaging effects. This conflict raises major concern on the long-term clinical use of these fatty acids. Even though the enzymatic and non-enzymatic conversion of HHE to nontoxic acid molecules is possible by the aldehyde detoxification systems, it has not yet studied. To address this, primary oxidation products of DHA in lipoxidase system were subjected to non-enzymatic conversion at physiological temperature over a period of 1 week. The reaction was monitored using HPLC, IR spectroscopy and biochemical assays (based on the loss of conjugated dienes, lipid peroxides aldehydes). Short term and long term cytotoxicity of the compounds generated at various time points were analyzed. IR and HPLC spectra revealed that the level of aldehydes in the primary oxidation products reduced over time, generating acids and acid derivatives within a week period. In short term and long term cytotoxicity analysis, initial decomposition products were found more toxic than the 1-week decomposition products. Further, when primary oxidation products were subjected to aldehyde dehydrogenase mediated oxidation, it generated products that are also less toxic. The study suggests the possible non-enzymatic conversion of primary oxidation products of DHA to less cytotoxic acid molecules. Exploration of the physiological roles of these acid molecules may explain the biological potential of omega-3 fatty acids.

A Theoretical Study of Pyrolysis of exo-Tetrahydrodicyclopentadiene and Its Primary and Secondary Unimolecular Decomposition Products.

Theoretical calculations of the rate constants and product branching ratios in the pyrolysis of exo-tetrahydrodicyclopentadiene (JP-10) and its initial decomposition products at combustion-relevant pressures and temperatures were performed and compared to the experimental results from the recently reported molecular beam photoionization mass spectrometry study of the pyrolysis of JP-10 ( Zhao et al. Phys. Chem. Chem. Phys. 2017 , 19 , 15780 - 15807 ). The results allow us to quantitatively assess the decomposition mechanisms of JP-10 by a direct comparison with the nascent product distribution-including radicals and thermally labile closed-shell species-detected in the short-residence-time molecular beam photoionization mass spectrometry experiment. In accord with the experimental data, the major products predicted by the theoretical modeling include methyl radical (CH3), acetylene (C2H2), vinyl radical (C2H3), ethyl radical (C2H5), ethylene (C2H4), allyl radical (C3H5), 1,3-butadiene (C4H6), cyclopentadienyl radical (C5H5), cyclopentadiene (C5H6), cyclopentenyl radical (C5H7), cyclopentene (C5H8), fulvene (C6H6), benzene (C6H6), toluene (C7H8), and 5-methylene-1,3-cyclohexadiene (C7H8). We found that ethylene, allyl radical, cyclopentadiene, and cyclopentenyl radical are significant products at all combustion-relevant conditions, whereas the relative yields of the other products depend on temperature. The most significant temperature trends predicted are increasing yields of the C2 and C4 species and decreasing yields of the C1, C6, and C7 groups with increasing temperature. The calculated pressure effect on the rate constant for the decomposition of JP-10 via initial C-H bond cleavages becomes significant at temperatures above 1500 K. The initially produced radicals will react away to form closed-shell molecules, such as ethylene, propene, cyclopentadiene, cyclopentene, fulvene, and benzene, which were observed as the predominant fragments in the long-residence-time experiment. The calculated rate constants and product branching ratios should prove useful to improve the existing kinetic models for the JP-10 pyrolysis.

Reaction Mechanisms in the Thermal Decomposition of CL-20 Revealed by ReaxFF Molecular Dynamics Simulations

The thermal decomposition of condensed CL-20 was investigated using reactive force field molecular dynamics (ReaxFF MD) simulations of a super cell containing 128 CL-20 molecules at 800-3000 K. The VARxMD code previously developed by our group is used for detailed reaction analysis. Various intermediates and comprehensive reaction pathways in the thermal decomposition of CL-20 were obtained. Nitrogen oxides are the major initial decomposition products, generated in a sequence of NO2, NO3, NO, and N2O. NO2 is the most abundant primary product and is gradually consumed in subsequent secondary reactions to form other nitrogen oxides. NO3 is the second most abundant intermediate in the early stages of CL-20 thermolysis. However, it is unstable and quickly decomposes at high temperatures, while other nitrogen oxides remain. At all temperatures, the unimolecular pathways of N-NO2 bond cleavage and ring-opening C-N bond scission dominate the initial decomposition of condensed CL-20. The cleavage of the N-NO2 bond is greatly enhanced at high temperatures, but scission of the C-N bond is not as favorable. A bimolecular pathway of oxygen-abstraction by NO2 to generate NO3 is observed in the initial decomposition steps of CL-20, which should be considered as one of the major pathways for CL-20 decomposition at low temperatures. After the initiation of CL-20 decomposition, fragments with different ring structures are formed from a series of bond-breaking reactions. Analysis of the ring structure evolution indicates that the pyrazine derivatives of fused tricycles and bicycles are early intermediates in the decomposition process, which further decompose to single ring pyrazine. Pyrazine is the most stable ring structure obtained in the simulations of CL-20 thermolysis, supporting the proposed existence of pyrazine in Py-GC/MS experiments. The single imidazole ring is unstable and decomposes quickly in the early stages of CL-20 thermolysis. Many C-4 and C-2 intermediates are observed after the initial fragmentation, but eventually convert into stable products. The distribution of the final products (N-2, H2O, CO2, and H-2) obtained in ReaxFF MD simulation of CL-20 thermolysis at 3000 K quantitatively agrees with the results of the CL-20 detonation experiment. The comprehensive understanding of CL-20 thermolysis obtained through this study suggests that ReaxFF MD simulation, combined with the reaction analysis capability of VARxMD, would be a promising method for obtaining deeper insight into the complex chemistry of energetic materials exposed to thermal stimuli.

In situ synchrotron XAS study of the decomposition kinetics of ZDDP triboreactive interfaces.

One of the major obstacles in replacing the widely used zinc dialkyldithiophosphate (ZDDP) antiwear additive with a more environmentally friendly one is the difficulty of time-resolving the surface species resulting from its decomposition mechanism under high contact pressure and temperature. To tackle this issue, a newly developed miniature pin-on-disc tribotester was coupled with synchrotron X-ray absorption spectroscopy (XAS) to perform in situ tribological tests while examining the composition of the formed triboreactive films. The results showed that in the case of bare steel surfaces the initial decomposition products are mainly zinc sulfate species, which with further shearing and heating are reduced to zinc sulfide mixed with metal oxides. The mixed base layer seems to enhance the tenacity of the subsequently formed zinc phosphate layers composing the main bulk of the protective triboreactive film. This base layer was not observed in the case of coated substrates with hydrogenated diamond-like carbon (a-C:H DLC) coating, which results in the formation of less durable films of small volume barely covering the contacting surfaces and readily removed by shear. Comprehensive decomposition pathways and kinetics for the ZDDP triboreactive films are proposed, which enable the control and modification of the ZDDP triboreactive films.

Structures, electronic properties and reaction paths from Fe(CO)5 molecule to small Fe clusters

ABSTRACTThe geometries, electrical characters and reaction paths from Fe(CO)5 molecule to small Fe clusters were investigated by using all-electron density functional theory. The results show that in the decomposition process of pentacarbonyl-iron, Fe(CO)5 molecule prefers to remove a carbon monoxide and adsorb another Fe(CO)5 molecule to produce nonacarbonyldiiron Fe2(CO)9 then Fe2(CO)9 gradually removes carbon monoxide to produce small Fe clusters. As It can be seen from the highest occupied molecule orbital–lowest unoccupied molecule orbital gap curves, the Fe(CO)n=3, and 5 and Fe2(CO)n=3, 7 and 9 intermediates have higher chemical stability than their neighbors. The local magnetic moment of the carbon monoxide is aligning anti-ferromagnetic. The effect of external magnetic field to the initial decomposition products of Fe(CO)5 can be ignored.

Shock response of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX): The C-N bond scission studied by molecular dynamics simulations

The shock response has a great influence on the design, synthesis, and application of energetic materials in both industrial and military areas. Therefore, the initial decomposition mechanism of bond scission at the atomistic level of condensed-phase α-RDX under shock loading has been studied based on quantum molecular dynamics simulations in combination with a multi-scale shock technique. First, based on the frontier molecular orbital theory, our calculated result shows that the N-NO2 bond is the weakest bond in the α-RDX molecule in the ground state, which may be the initial bond for pyrolysis. Second, the changes of bonds under shock loading are investigated by the changes of structures, kinetic bond lengths, and Laplacian bond orders during the simulation. Also, the variation of thermodynamic properties with time in shocked α-RDX at 10 km/s along the lattice vector a for a timescale of up to 3.5 ps is presented. By analyzing the detailed structural changes of RDX under shock loading, we find that the shocked RDX crystal undergoes a process of compression and rotation, which leads to the C-N bond initial rupture. The time variation of dynamic bond lengths in a shocked RDX crystal is calculated, and the result indicates that the C-N bond is easier to rupture than other bonds. The Laplacian bond orders are used to predict the molecular reactivity and stability. The values of the calculated bond orders show that the C-N bonds are more sensitive than other bonds under shock loading. In a word, the C-N bond scission has been validated as the initial decomposition in a RDX crystal shocked at 10 km/s. Finally, the bond-length criterion has been used to identify individual molecules in the simulation. The distance thresholds up to which two particles are considered direct neighbors and assigned to the same cluster have been tested. The species and density numbers of the initial decomposition products are collected according to the trajectory.

Modeling Oil Shale Pyrolysis: High-Temperature Unimolecular Decomposition Pathways for Thiophene.

The thermal decomposition mechanism of thiophene has been investigated both experimentally and theoretically. Thermal decomposition experiments were done using a 1 mm × 3 cm pulsed silicon carbide microtubular reactor, C4H4S + Δ → Products. Unlike previous studies these experiments were able to identify the initial thiophene decomposition products. Thiophene was entrained in either Ar, Ne, or He carrier gas, passed through a heated (300-1700 K) SiC microtubular reactor (roughly ≤100 μs residence time), and exited into a vacuum chamber. The resultant molecular beam was probed by photoionization mass spectroscopy and IR spectroscopy. The pyrolysis mechanisms of thiophene were also investigated with the CBS-QB3 method using UB3LYP/6-311++G(2d,p) optimized geometries. In particular, these electronic structure methods were used to explore pathways for the formation of elemental sulfur as well as for the formation of H2S and 1,3-butadiyne. Thiophene was found to undergo unimolecular decomposition by five pathways: C4H4S → (1) S═C═CH2 + HCCH, (2) CS + HCCCH3, (3) HCS + HCCCH2, (4) H2S + HCC-CCH, and (5) S + HCC-CH═CH2. The experimental and theoretical findings are in excellent agreement.

Thermal Decomposition Mechanism for Ethanethiol.

The thermal decomposition of ethanethiol was studied using a 1 mm × 2 cm pulsed silicon carbide microtubular reactor, CH3CH2SH + Δ → Products. Unlike previous studies these experiments were able to identify the initial ethanethiol decomposition products. Ethanethiol was entrained in either an Ar or a He carrier gas, passed through a heated (300-1700 K) SiC microtubular reactor (roughly ≤100 μs residence time) and exited into a vacuum chamber. Within one reactor diameter the gas cools to less than 50 K rotationally, and all reactions cease. The resultant molecular beam was probed by photoionization mass spectroscopy and IR spectroscopy. Ethanethiol was found to undergo unimolecular decomposition by three pathways: CH3CH2SH → (1) CH3CH2 + SH, (2) CH3 + H2C═S, and (3) H2C═CH2 + H2S. The experimental findings are in good agreement with electronic structure calculations.

Structure and Dissociation Pathways of Protonated Tetralin (1,2,3,4-Tetrahydronaphthalene).

The infrared multiple-photon dissociation (IRMPD) spectrum of protonated tetralin (1,2,3,4-tetrahydronaphthalene, THN) has been recorded using an infrared free electron laser coupled to a Fourier transform ion cyclotron mass spectrometer. IR-induced fragmentation of the protonated parent [THN + H] +, m/z 133, yielded a single fragment ion at m/z 91. No evidence for fragment ions at m/z 131 or 132 was observed, indicating that protonated THN ejects neither atomic H nor molecular H2. Comparison of the experimental spectrum with density functional calculations (B3LYP/6-311++G(d,p)) of the two possible protonated isomers identifies a preference for the position of protonation. Possible decomposition pathways starting from both [THN + H(5)]+ and [THN + H(6)]+ are investigated. The potential energy profiles computed for these decomposition routes reveal that (1) the m/z 91 ionic product resembles the benzylium ion, but with the extra hydrogen and the methylene substituents in various ortho, meta, and para conformations around the aromatic ring and that (2) the decomposition process involving the [THN + H(6)]+ isomer is predominant, while the one involving the [THN + H(5)]+ may play a smaller role. Potential energy pathways from the initial decomposition product(s) to the benzylium and tropylium ions have also been computed. Given the relatively low barriers to these ions, it is concluded that the benzylium ion and, with sufficient activation, the tropylium ion plus neutral propene are the final products.