Gas-phase Formation Research Articles

Mechanism and Direct Kinetic Study of the Polychlorinated Dibenzo-p-dioxin and Dibenzofuran Formations from the Radical/Radical Cross-Condensation of 2,4-Dichlorophenoxy with 2-Chlorophenoxy and 2,4,6-Trichlorophenoxy

A direct density functional theory (DFT) kinetic calculation is carried out for the homogeneous gas-phase formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from the cross-condensation of 2,4-dichlorophenoxy radical (2,4-DCPR) with 2-chlorophenoxy radical (2-CPR) and 2,4,6-trichlorophenoxy radical (2,4,6-TCPR). The possible formation mechanism is investigated and compared with the PCDD/F formation mechanism from the self-condensation of 2,4-DCPR, 2-CPR, and 2,4,6-TCPR. The rate constants and their temperature dependence of the crucial elementary reactions are computed by the canonical variational transition-state theory with the small curvature tunneling contribution over the temperature range of 600-1200 K. This study shows that the multichlorine substitutions suppress the PCDD/F formations. Because of a lack of experimental kinetic data, the present theoretical results are expected to be useful and reasonable to estimate the kinetic properties, such as the pre-exponential factors, the activation energies, and the rate constants, of the elementary reactions involved in the formation of PCDD/Fs.

Mechanism and modeling of silicon carbide formation and engulfment in industrial silicon directional solidification growth

This paper proposes a mechanism of carbon species formation and transport in the gas phase and silicon carbide particle formation and engulfment in the liquid phase. A numerical model considering various forces acting on silicon carbide particles is developed to quantify particle transport and particle engulfment. Numerical simulations are conducted to study fluid flow and temperature distribution in an industrial directional solidification system and particle distribution in the solidified silicon. Strategies to reduce carbon contamination and improve ingot quality are proposed.

Thin Film Deposition and Photodissociation Mechanisms for Lanthanide Oxide Production from Tris(2,2,6,6-tetramethyl-3,5-heptanedionato)Ln(III) in Laser-Assisted MOCVD

Photoionization mass spectrometry reveals details of the multistep unimolecular mechanism, whereby the 2,2,6,6-tetramethyl-3,5-heptanedionato (thd−) anionic ligand decomposes, while still bound in the metal complex, to yield a gas-phase metal oxide product in metal−organic chemical vapor deposition (MOCVD) of lanthanide oxides from Ln(thd)3 precursors. The decomposition occurs with stepwise elimination of small closed-shell hydrocarbon fragments and carbon monoxide up to a penultimate Ln(OC2H) ethyneoxide, from which both LnO (dominant) and LnC2 (minor) products are derived. Formation of the metal oxide and carbide occurs in competition with a previously described mechanism(1-3) wherein sequential dissociation of ligand radicals produces the reduced metal Ln0. Evidence for gas-phase formation of a Ln2(thd)6 dimer as a result of expansion-cooling in the precursor source assembly is also given. Laser-assisted MOCVD of Eu(thd)3 on silica, with subsequent exposure to atmosphere, produces amorphous Eu2O3 with small areas of crystallinity attributed to reaction of the oxide with atmospheric carbon dioxide and water.

N-Nitrosodimethylamine Occurrence, Formation and Cycling in Clouds and Fogs

The occurrence, source, and sink processes of N-nitrosodimethylamine (NDMA) have been explored by means of combined laboratory, field, and model studies. Observations have shown the occurrence of NDMA in fogs and clouds at substantial concentrations (7.5-397 ng L(-1)). Laboratory studies were conducted to investigate the formation of NDMA from nitrous acid and dimethylamine in the homogeneous aqueous phase. While NDMA was produced in the aqueous phase, the low yields (<1%) observed could not explain observational concentrations. Therefore gaseous formation of NDMA with partitioning to droplets likely dominates aqueous NDMA formation. Box-model calculations confirmed the predominant contributions from gas phase formation followed by partitioning into the cloud droplets. Measurements and model calculations showed that while NDMA is eventually photolyzed, it might persist in the atmosphere for hours after sunrise and before sunset since the photolysis in the aqueous phase might be much less efficient than in the gas phase.

Gas Phase Computational Studies on the Competition between Nitrile and Water Ligands in Uranyl Complexes†

The gas phase formation of uranyl dicationic complexes containing water and nitrile (acetonitrile, propionitrile, and benzonitrile) ligands, [UO(2)(H(2)O)(m)(RCN)(n)](2+), has been studied using density functional theory with a relativistic effective core potential to account for scalar relativistic effects on uranium. It is shown that nitrile addition is favored over the addition of water ligands. Decomposition of these complexes to [UO(2)OH(H(2)O)(m)(RCN)(n)](+) by the loss of either H(3)O(+) or (RCN + H)(+) is also examined. It is found that this reaction is competitive with the ligand addition when the coordination sphere of uranyl is unsaturated. Additionally, this reaction is influenced by the size of the nitrile ligand with reactions involving acetonitrile being the most prevalent. Finally, ligand addition to the monocation shows trends similar to that of the dication with energetic differences being smaller for the addition to the monocation.

Thermochemical Properties of exo-Tricyclo[5.2.1.02,6]decane (JP-10 Jet Fuel) and Derived Tricyclodecyl Radicals

exo-Tricyclo[5.2.1.0(2,6)]decane (TCD) or exo-tetrahydrodicyclopentadiene is the principal component of the high-energy density hydrocarbon fuel commonly identified as JP-10. Thermodynamic parameters for the parent TCD molecule and of all the tricyclodecyl radicals corresponding to the loss of hydrogen atoms from different carbons sites (TCD-Ri with i indicating the given carbon center) are determined using several density functional theory and G3MP2B3 and CBS-QB3 higher level composite computational chemistry methods. Five isodesmic work reactions, three involving bridged hydrocarbon reference molecules with similar ring strains, are employed to produce a cancelation of systematic calculation errors in evaluation of standard, gas-phase formation enthalpies at 298 K. Delta(f)H degrees (298) for TCD is found to be -19.5 +/- 1.3 kcal mol(-1), which is several kcal mol(-1) lower than the commonly used values. C(i)-H bond energies for corresponding TCD carbon sites are evaluated as follows: TCD-R1, 107.2; TCD-R2, 100.1; TCD-R3, 98.0; TCD-R4, 98.5; TCD-R9, 98.7; TCD-R10, 104.1 kcal mol(-1). Results from use of five different DFT methods are in very good agreement with composite level values for all work reactions used for the radicals. The exo and endo isomers of TCD are both determined to have chair and boat conformers.

Dioxin Formations from the Radical/Radical Cross-Condensation of Phenoxy Radicals with 2-Chlorophenoxy Radicals and 2,4,6-Trichlorophenoxy Radicals

It is important to understand the role of phenol in the dioxin formations because it is present in the high amount in municipal waste incinerators (MWIs). The formation mechanism of dioxins from the cross-condensation of PhRs with 2-CPRs and 2,4,6-TCPRs was investigated by using hybrid density functional theory (DFT) and compared with the dioxin formation mechanism from the self-condensation of single chlorophenol precursors. The geometrical parameters were optimized at the MPWB1K level with the 6-31+G(d,p) basis set without symmetry constraints. Single-point energy calculations were carried out at the MPWB1K/6-311+G(3df,2p) level of theory. The rate constants were deduced by using canonical variational transition-state (CVT) theory with small curvature tunneling (SCT) contribution over the temperature range of 600-1200 K. The Arrhenius formulas were reported for the first time. Results show that phenol is responsible for the formation of dioxin congeners. This work, together with results already published from our group, provides a comprehensive investigation of the homogeneous gas-phase formation of dioxins from (chloro)phenol precursors and should help to clarify the formation mechanism of dioxins in real waste combustion and to develop more effective control strategies.

Dehydrogenation of propane with selective hydrogen combustion: A mechanistic study by transient analysis of products

Abstract The role of lattice and adsorbed oxygen species in propane dehydrogenation in a perovskite hollow fiber membrane reactor containing a Pt–Sn dehydrogenation catalyst was elucidated by transient analysis of products with a sub-millisecond time resolution. Propane is mainly dehydrogenated non-oxidatively to propene and hydrogen over the catalyst, while lattice oxygen of the perovskite oxidizes preferentially hydrogen to water. For achieving high propene selectivity at high propane conversions, the formation of gas phase O 2 on the shell side of the membrane reactor should be avoided. Otherwise, oxygen species adsorbed over the Pt–Sn catalyst participate in non-selective C 3 H 8 /C 3 H 6 transformations to C 2 H 4 and CO x .

TIMASSS: the IRAS16293-2422 millimeter and submillimeter spectral survey: tentative detection of deuterated methyl formate (DCOOCH3)

High deuterium fractionation is observed in various types of environment such as prestellar cores, hot cores and hot corinos. It has proven to be an efficient probe to study the physical and chemical conditions of these environments. The study of the deuteration of different molecules helps us to understand their formation. This is especially interesting for complex molecules such as methanol and bigger molecules for which it may allow to differentiate between gas-phase and solid-state formation pathways. Methanol exhibits a high deuterium fractionation in hot corinos. Since CH3OH is thought to be a precursor of methyl formate we expect that deuterated methyl formate is produced in such environments. We have searched for the singly-deuterated isotopologue of methyl formate, DCOOCH3, in IRAS 16293-2422, a hot corino well-known for its high degree of methanol deuteration. We have used the IRAM/JCMT unbiased spectral survey of IRAS 16293-2422 which allows us to search for the DCOOCH3 rotational transitions within the survey spectral range (80-280 GHz, 328-366 GHz). The expected emission of deuterated methyl formate is modelled at LTE and compared with the observations.} We have tentatively detected DCOOCH3 in the protostar IRAS 16293-2422. We assign eight lines detected in the IRAM survey to DCOOCH3. Three of these lines are affected by blending problems and one line is affected by calibration uncertainties, nevertheless the LTE emission model is compatible with the observations. A simple LTE modelling of the two cores in IRAS 16293-2422, based on a previous interferometric study of HCOOCH3, allows us to estimate the amount of DCOOCH3 in IRAS 16293-2422. Adopting an excitation temperature of 100 K and a source size of 2\arcsec and 1\farcs5 for the A and B cores, respectively, we find that N(A,DCOOCH3) = N(B,DCOOCH3) ~ 6.10^14 /cm2. The derived deuterium fractionation is ~ 15%, consistent with values for other deuterated species in this source and much greater than that expected from the deuterium cosmic abundance. DCOOCH3, if its tentative detection is confirmed, should now be considered in theoretical models that study complex molecule formation and their deuteration mechanisms. Experimental work is also needed to investigate the different chemical routes leading to the formation of deuterated methyl formate.

Formation and Decomposition of Chemically Activated and Stabilized Hydrazine

Recombination of two amidogen radicals, NH(2) (X(2)B1), is relevant to hydrazine formation, ammonia oxidation and pyrolysis, nitrogen reduction (fixation), and a variety of other N/H/X combustion, environmental, and interstellar processes. We have performed a comprehensive analysis of the N(2)H(4) potential energy surface, using a variety of theoretical methods, with thermochemical kinetic analysis and master equation simulations used to treat branching to different product sets in the chemically activated NH(2) + NH(2) process. For the first time, iminoammonium ylide (NH(3)NH), the less stable isomer of hydrazine, is involved in the kinetic modeling of N(2)H(4). A new, low-energy pathway is identified for the formation of NH(3) plus triplet NH, via initial production of NH(3)NH followed by singlet-triplet intersystem crossing. This new reaction channel results in the formation of dissociated products at a relatively rapid rate at even moderate temperatures and above. A further novel pathway is described for the decomposition of activated N(2)H(4), which eventually leads to the formation of the simple products N(2) + 2H(2), via H(2) elimination to cis-N(2)H(2). This process, termed as "dihydrogen catalysis", may have significant implications in the formation and decomposition chemistry of hydrazine and ammonia in diverse environments. In this mechanism, stereoselective attack of cis-N(2)H(2) by molecular hydrogen results in decomposition to N(2) with a fairly low barrier. The reverse termolecular reaction leading to the gas-phase formation of cis-N(2)H(2) + H(2) achieves non-heterogeneous catalytic nitrogen fixation with a relatively low activation barrier (77 kcal mol(-1)), much lower than the 125 kcal mol(-1) barrier recently reported for bimolecular addition of H(2) to N(2). This termolecular reaction is an entropically disfavored path, but it does describe a new means of activating the notoriously unreactive N(2). We design heterogeneous analogues of this reaction using the model compound (CH(3))(2)FeH(2) as a source of the H(2) catalyst and apply it to the decomposition of cis-diazene. The reaction is seen to proceed via a topologically similar transition state, suggesting that our newly described mechanism is general in nature.

Gas-phase formation and reactions of radical cations of guanosine, deoxyguanosine and their homodimers and heterodimers

Electrospray ionisation of methanolic solutions containing a mixture of the nucleoside deoxyguanosine, dG, incubated with Cu(NO3)2 resulted in the formation of a range of ions, including doubly charged copper nucleoside complexes [CuIIdGn]2+, with n ranging from 2 to 10. Collision-induced dissociation of these complexes proceeds via a number of different pathways that depend on the size of the cluster, n. When n=3, monomeric radical cations are formed via redox processes. When n=4, dimeric radical cations are formed. Related complexes are formed for the nucleoside guanosine, Gs, and these [CuIIGsn]2+ complexes fragment in similar fashions to their [CuIIdGn]2+ counterparts. A key finding is that the radical cations of dG and Gs have fragmentation patterns that depend on the way they are formed. Thus radical cations, dG+ and Gs+, formed directly in the electrospray ionisation source or via collision-induced dissociation of [CuIIdG3]2+ and [CuIIGs3]2+ complexes fragment in the same way, giving the radical cation of the guanine base at m/z 151 via cleavage of the N-glycosidic bond. In contrast, the collision-induced dissociation spectra of radical cations formed via the sequences [CuIIdG4]2+→dG2+→dG+ and [CuIIGs4]2+→Gs2+→Gs+ are dominated by the loss of CH2O and further loss of C2H3O2 from the sugar moiety. These different fragmentation reactions are attributed to different tautomeric structures of the radical cations. Quantum chemical calculations were carried out on possible structures of the radical cation dimer of the model 9-methylguanine. Three low energy structures were found. Two of these represent base pairs of the kind found in supramolecular motifs of guanine derivatives, and one of these possesses a novel tautomeric structure that may have important biological implications.

Quantum Chemical and Kinetic Study on Dioxin Formation from the 2,4,6-TCP and 2,4-DCP Precursors

This study focuses on the homogeneous gas-phase formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) from the 2,4,6-trichlorophenol (2,4,6-TCP) and 2,4-dichlorophenol (2,4-DCP) precursors, which were found both in the gas phase and in the fly ash samples as the dominating chlorophenol congeners in municipal waste incinerators (MWIs). Molecular orbital theory calculations have been performed for the formation mechanism. The geometrical parameters and vibrational frequencies of all the stationary points were calculated at the MPWB1K level with the 6-31+G(d,p) basis set. Single-point energy calculations were carried out at the MPWB1K/6-311+G(3df,2p) level of theory. Canonical variational transition-state (CVT) theory with small curvature tunneling (SCT) contribution was used to predict the rate constants of crucial elementary steps over the temperature range of interest (600-1200 K). The rate-temperature formulas were fitted for the first time. The pre-exponential factor, the activation energy, and the rate constants are reported. This study shows that at least one chlorine substituent in the ortho position is needed for the formation of PCDDs from the condensation of chlorophenols. The results presented here should help to clarify and detail the formation mechanism of PCDD/PCDFs (PCDD/Fs for short) from chlorophenol precursors in real waste combustion.

The abundance of HNCO and its use as a diagnostic of environment

We aim to investigate the chemistry and gas phase abundance of HNCO and the variation of the HNCO/CS abundance ratio as a diagnostic of the physics and chemistry in regions of massive star formation. A numerical-chemical model has been developed which self-consistently follows the chemical evolution of a hot core. The model comprises of two distinct stages. An initial collapse phase is immediately followed by an increase in temperature which represents the switch on of a central massive star and the subsequent evolution of the chemistry in a hot, dense gas cloud (the hot core). During the collapse phase, gas species are allowed to accrete on to grain surfaces where they can participate in further reactions. During the hot core phase surface species thermally desorb back in to the ambient gas and further chemical evolution takes place. For comparison, the chemical network was also used to model a simple dark cloud and photodissociation regions. Our investigation reveals that HNCO is inefficiently formed when only gas-phase formation pathways are considered in the chemical network with reaction rates consistent with existing laboratory data. Using currently measured gas phase reaction rates, obtaining the observed HNCO abundances requires its formation on grain surfaces. However our model shows that the gas phase HNCO in hot cores is not a simple direct product of the evaporation of grain mantles. We also show that the HNCO/CS abundance ratio varies as a function of time in hot cores and can match the range of values observed. This ratio is not unambiguously related to the ambient UV field as been suggested - our results are inconsistent with the hypothesis of Martin et al (2008). In addition, our results show that this ratio is extremely sensitive to the initial sulphur abundance.

Molecular energetics of alkyl substituted pyridine N-oxides

The standard (p° = 0.1 MPa) energies of combustion in oxygen, at T = 298.15 K, for the solid compounds 2-methylpyridine-N-oxide (2-MePyNO), 3-methylpyridine-N-oxide (3-MePyNO) and 3,5-dimethylpyridine-N-oxide (3,5-DMePyNO) were measured by static-bomb calorimetry, from which the respective standard molar enthalpies of formation in the condensed phase were derived. The standard molar enthalpies of sublimation, at the same temperature, were measured by Calvet microcalorimetry. From the standard molar enthalpy of formation in gaseous phase, the molar dissociation enthalpies of the N–O bonds were derived, and compared with values of the dissociation enthalpies of other N–O bonds available for other pyridine-N-oxide derivatives.

Experimental and computational study of the energetics of methoxycoumarins

The structure and energetics of the methoxy-derivatives of coumarin with the substituent either on the benzenic ring (5-, 6-, 7- and 8-methoxy coumarin) or on the pyrone ring (3- and 4-methoxycoumarin) is addressed on the basis of some well documented computational methods. Estimates of the enthalpies of formation are obtained using appropriate homodesmotic reactions and also atomization reactions. The condensed phase standard ( p 0 = 0.1 MPa) molar enthalpy of formation for 7-methoxycoumarin was derived from the standard molar enthalpy of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpy of sublimation, at T = 298.15 K, was measured by Calvet microcalorimetry. Combining these values, the enthalpy of formation in gas-phase, at T = 298.15 K, was derived: −(321.6 ± 2.8) kJ mol −1. The temperature of fusion, T fusion, and fusion enthalpies, at T = T fusion, are also reported. Good agreement between experimental and computational data is achieved. The aromaticity of these systems has been assessed through the evaluation and analysis of the Nucleus Independent Chemical Shifts (NICS) and their most significant components. NICS scan analyses have also been conducted to help clarifying the aromatic nature of the pyrone rings of these systems.

The gas‐phase ligand exchange of copper and nickel acetylacetonate, hexafluoroacetylacetonate and trifluorotrimethylacetylacetonate complexes

The gas-phase ligand-exchange reactions between Cu(II) and Ni(II) complexes containing the acetylacetonate (acac), hexafluoroacetylacetonate (hfac), and trifluorotrimethylacetylacetonate (tftm) ligands were investigated using a triple quadrupole mass spectrometer. The gas-phase mixed-ligand products of [Cu(acac)(tftm)](+), [Ni(acac)(tftm)](+), [Cu(hfac)(tftm)](+), and [Ni(hfac)(tftm)](+) were formed following the co-sublimation of either homo-metal or hetero-metal precursors. The gas-phase formation of [Cu(acac)(tftm)](+), [Cu(hfac)(tftm)](+), [Ni(acac)(tftm)](+), and [Ni(hfac)(tftm)](+) complexes is reported herein for the first time. The corresponding fragmentation patterns of these species along with those of Cu(tftm)(2) and Ni(tftm)(2) are also presented. Mass-selected ion-neutral reactions were investigated.

The fragmentation of negative ions of fullerene C60 trifluoromethyl derivatives

The formation and decay of gas-phase negative ions of trifluoromethylated fullerenes C60(CF3)n(n = 2–10) were studied. The resonance electron capture mass spectra were measured to find that the main fragmentation channel of negative ions was the detachment of trifluoromethyl groups. The degree of fragmentation directly depended on the energy of electrons and reached the complete splitting off of all the CF3 addends with the formation of C60− ions. The observed metastable ion signals were analyzed to determine the scheme of sequential fragmentation of negative ions.

Fragmentation pathways of 2,3‐dimethyl‐2,3‐dinitrobutane cations in the gas phase

2,3-Dimethyl-2,3-dinitrobutane (DMNB) is an explosive taggant added to plastic explosives during manufacture making them more susceptible to vapour-phase detection systems. In this study, the formation and detection of gas-phase [M+H](+), [M+Li](+), [M+NH(4)](+) and [M+Na](+) adducts of DMNB was achieved using electrospray ionisation on a triple quadrupole mass spectrometer. The [M+H](+) ion abundance was found to have a strong dependence on ion source temperature, decreasing markedly at source temperatures above 50 degrees C. In contrast, the [M+Na](+) ion demonstrated increasing ion abundance at source temperatures up to 105 degrees C. The relative susceptibility of DMNB adduct ions toward dissociation was investigated by collision-induced dissociation. Probable structures of product ions and mechanisms for unimolecular dissociation have been inferred based on fragmentation patterns from tandem mass (MS/MS) spectra of source-formed ions of normal and isotopically labelled DMNB, and quantum chemical calculations. Both thermal and collisional activation studies suggest that the [M+Na](+) adduct ions are significantly more stable toward dissociation than their protonated analogues and, as a consequence, the former provide attractive targets for detection by contemporary rapid screening methods such as desorption electrospray ionisation mass spectrometry.

A Second Metal Center Enhances the Reactivity of an Organomagnesate: Comparison of the Gas-Phase Reactions of Water with [RCCMgCl2]− and [RCCMg2Cl4]− (R = H, Ph)

The gas-phase formation and hydrolysis reactions of the mononuclear and binuclear organomagnesate ions [RCCMgnCl2n]− (where R = H, Ph and n = 1, 2) were studied using a combination of ion trap mass spectrometry based experiments and DFT calculations. The organomagnesates were formed via decarboxylation of the carboxylates [RCCCO2MgnCl2n]−. The binuclear organomagnesates [RCCMg2Cl4]− were found to be at least 5 times more reactive than the mononuclear organomagnesates [RCCMgCl2]− (where R = H, Ph). The DFT calculations highlight the role that a second magnesium center can have on both the formation and reaction of an organomagnesate. Thus, DFT calculations reveal that while decarboxylation of the mononuclear carboxylate [HCCCO2MgCl2]− proceeds via only one pathway involving a four-centered transition state, there are two different decarboxylation pathways possible for the isomeric binuclear carboxylates [HCCCO2Mg2Cl4]−: (i) a related four-centered transition state involving a single Mg, resulting in the or...

Atomic layer deposition of GaN using GaCl3 and NH3

GaN films were grown on Si(100) substrate by atomic layer deposition (ALD) using GaCl3 and NH3. Growth conditions were identified for which the growth rate exhibited a plateau at ∼2.0Å∕cycle, consistent with self-limiting adsorption. A relatively wide temperature window (500–750°C) for ALD growth mode was also established for one flow sequence schedule. In this limit, both the (0002) and (101¯1) orientations of GaN were evident, which was attributed to the competition between vertical and lateral growths. Cl incorporation was detected by x-ray photoelectron spectroscopy for samples prepared with long GaCl3 exposure time. It is postulated that gas phase formation of (ClGaNH)n with n=1–3 [Kovács, Inorg. Chem. 41, 3067 (2002)] results in higher Cl content.