Decomposition Channels Research Articles

A Systematic Density Functional Theory Study of VxOy+ and VxOY (X = 2−4, Y = 2−10) Systems

A theoretical study on geometrical, thermodynamic and electronic properties of VxOy+ and VxOy (x = 2−4, y = 2−10) systems in different electronic states has been carried out. Analytical gradient techniques with a hybrid spin unrestricted and spin restricted Hartree−Fock density functional method (B3LYP) have been used. An analysis of potential energy surfaces at ground and some excited states renders the most stable structures an their corresponding vibrational frequencies. The reaction energies of the possible decomposition channels are reported and the results indicate that the formation of larger clusters is energetically favorable from the smaller VO+ (3∑), VO (4∑), VO2 (2A1), VO3 (2A‘),V2O4 (3A‘ ‘) and V2O5 (1A‘) systems and O2 (3∑g). A comparative discussion of the bonding in these species has been carried out on the basis of the topological analysis of the electron localization function. Numerical results are analyzed and confronted with available experimental data.

Soil microbial community patterns related to the history and intensity of grazing in sub-montane ecosystems

Long-term variations in the frequency and intensity of sheep ( Ovis aries) grazing have led to the development of ubiquitous plant successional transitions in sub-montane regions of the UK. In this study, we measured a range of soil microbial properties across these successional transitions in three biogeographic regions of the UK, to establish how gradients of grazing-influence (in terms of the history and intensity of sheep grazing) alter the biomass, activity, and structure of soil microbial communities. We also measured soil physicochemical variables to relate changes in soil microbial community arrangement along these grazing-related successional transitions to key soil properties. Our results from three locations show that microbial communities of soils display some consistent and ‘broad-scale’ trends along successional transitions that are related to the history and intensity of grazing. We show that microbial biomass of soil is maximal at low-to-intermediate levels of grazing influence and that the phenotypic evenness (a component of diversity) of the microbial community declines as the intensity of grazing increases. We also provide evidence that soil microbial communities of heavily grazed sites are dominated by bacterial-based energy channels of decomposition, whereas in systems that are less intensively grazed, or completely unmanaged, fungi have a proportionally greater role. Further studies are needed to establish the significance of these changes in relation to soil-level ecosystem processes of decomposition and nutrient cycling. The data show that human disturbances can have profound effects on the biomass and structure of the soil communities that regulate soil processes in these ecosystems and that these effects are consistent across sites.

1,2‐Pentadiene decomposition

Abstract1,2‐Pentadiene has been decomposed in single pulse shock tube experiments. There appear to be a large number of parallel decomposition and isomerization channels. The following rate expressions for isomerization in the temperature range 1100–1250 K and pressures between 200 and 300kPa have been obtainedk(1,2‐pentadiene → 1‐pentyne) = 1.0 × 1013exp(−32300 ± 900 K/T) s−1k(1,2‐pentadiene → cis/trans‐1,3‐pentadiene = 2.2 × 1014exp(−33900 ± 900 K/T) s−1k(1,2‐pentadiene → cyclopentene) = 1.8 × 1011exp(−26080 ± 900 K/T) s−1Overall, we estimate the 2σ uncertainty in the absolute rate constants to be about 30%. From detailed balance the rate expression for 1,3 to 1,2‐pentadiene isomerization has been found to bek(1,3‐pentadiene → 1,2‐pentadiene) = 1.2 × 1015exp(−42070 ± 900 K/T) s−1The significance of the 1,3 to 1,2 isomerization process as a bridge between the two‐carbon and three‐carbon species in high temperature hydrocarbon decomposition systems is discussed.The decomposition processes involve bond breaking and retroene reactions. The rate expression for the latter has been found to bek(1,2‐pentadiene → propyne + ethylene) = 6.6 × 1012exp(−29240 ± 900 K/T) s−1The rate constants for fission of the C3C4and C4C5bonds appear to be close to each other. Our results indicate that the resonance energy of the 1,3‐butadiene‐2‐yl radical is smaller than that of allyl radical by an amount equal to the π bond conjugation energy of butadiene. © 2001 John Wiley & Sons, Inc.*This article is a US Government work and, as such, is in the public domain in the United States of America.Int J Chem Kinet 33: 755–767, 2001

Pressure dependence of the photooxidation of selected carbonyl compounds in air: n-butanal and n-pentanal

Dilute mixtures of n-butanal and n-pentanal in synthetic air (up to 100ppm) were photolyzed with fluorescent UV lamps (275–380nm) in air at 298K. The main photooxidation products, identified and quantitatively analyzed by FT-IR spectroscopy, were ethene (n-butanal) or propene (n-pentanal), CO, vinylalcohol and ethanal. The photolysis rates and the absolute quantum yields Φ of both compounds were found to be slightly dependent on the total pressure. At 100Torr, in the photolysis of n-butanal and n-pentanal Φ100=0.48±0.02 and 0.40±0.04, whereas at 700Torr, the total quantum yields were Φ700=0.38±0.02 and 0.32±0.01, respectively. These results could be explained by the collisional deactivation of photoexcited molecules. Two decomposition channels were identified in both cases: the radical channel RCHO→R+HCO, and the molecular channel CxHyCHO→Cx−1Hy−3+CH2CHOH, having the relative yields of 68±3 and 32±3% (n-butanal), and 20±4 and 80±4% (n-pentanal) at 700Torr. The product, CH2CHOH, tautomerizes to ethanal. UV absorption cross-sections were measured prior to photolysis. For the comparison, the products were also analyzed after photolysis of both compounds in teflon bags using direct sunlight.

A framework for soil food web diagnostics: extension of the nematode faunal analysis concept

Nematodes, the earth’s most abundant metazoa, are ubiquitous in the soil environment. They are sufficiently large to be identifiable by light microscopy and sufficiently small to inhabit water films surrounding soil particles. They aggregate around and in food sources. They include component taxa of the soil food web at several trophic levels. They can be categorized into functional guilds whose members respond similarly to food web enrichment and to environmental perturbation and recovery. Indices derived through nematode faunal analysis provide bioindicators for disturbance of the soil environment and condition of the soil food web. We enhance the resolution of faunal analyses by providing a weighting system for the indicator importance of the presence and abundance of each functional guild in relation to enrichment and structure of the food web. Graphical representations of food web structure, based on nematode faunal analyses, allow diagnostic interpretation of its condition. Simple ratios of the weighted abundance of representatives of specific functional guilds provide useful indicators of food web structure, enrichment, and decomposition channels.

Photooxidation of n-Hexanal in Air

Dilute mixtures of hexanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) in air at 298 K. The main photooxidation products, identified and quantitatively analyzed by FTIR spectroscopy, were butene, CO, vinylalcohol and ethanal. The photolysis rates and the absolute quantum yield Φ were found to be slightly dependent on the total pressure. At 100 Torr, Φ100 = 0.43 ± 0.02, whereas at 700 Torr the total quantum yield was Φ700 = 0.38 ± 0.02. These results may be explained by the collisional deactivations of photoexcited molecules. Two decomposition channels were identified: the radical channel C5H11CHO → C5H11 + HCO, and the molecular channel C5H11CHO → C4H8 + CH2=CHOH, having the relative yields of 27 and 73 % at 700 Torr. The product CH2=CHOH tautomerizes to ethanal.

Mechanism for Unimolecular Decomposition of HMX (1,3,5,7-Tetranitro-1,3,5,7-tetrazocine), an ab Initio Study

To improve the mechanistic understanding of the possible decomposition in the gas phase of the energetic material HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), we used ab initio calculations to determine the various unimolecular decomposition channels. We find three distinct mechanisms: (i) homolytic cleavage of N−N bond to form NO2 (M = 46) and HMR (M = 250) which subsequently decomposes to form various products; (ii) successive HONO eliminations to give four HONO (M = 47) plus a stable intermediate (M = 108); (iii) O-migration from one of the NO2 groups of HMX to neighboring C atom followed by the decomposition of intermediate (M = 296) to INT222 (a ring-opened RDX structure) and MN-oring (M = 74), which can undergo dissociation to smaller mass fragments. The decomposition scheme for HMX is similar to that for RDX presented earlier (J. Phys. Chem. A 2000, 104, 2261), except that concerted decomposition of HMX to four MN (M = 74) molecules is not a favorable decomposition pathway, whereas this pathway was found in RDX decomposition (both experimentally and theoretically). The formation of RDR-o in the N−N homolysis pathway 1 or the formation of INT222 in pathways 1 and 3 presents an unified mechanistic scheme for the decomposition of both of these nitramines. The HMX decomposition mechanism correlates with available condensed phase experimental results, but detailed comparison of the predicted gas phase energetics is not possible.

The pyrolysis of cyclopentadiene: quantum chemical and kinetic modelling studies of the acetylene plus propyne/allene decomposition channels

Download options Please wait... Supplementary files Tables DOC (192K) Article information DOI https://doi.org/10.1039/B101317N Article type Paper Submitted 09 Feb 2001 Accepted 19 Apr 2001 First published 22 May 2001 Download Citation Phys. Chem. Chem. Phys., 2001,3, 2467-2473 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions The pyrolysis of cyclopentadiene: quantum chemical and kinetic modelling studies of the acetylene plus propyne/allene decomposition channels G. B. Bacskay and J. C. Mackie, Phys. Chem. Chem. Phys., 2001, 3, 2467 DOI: 10.1039/B101317N To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author George B. Bacskay John C. Mackie

A theoretical study on the structure, energetics and bonding of VO x+ and VO x ( x=1–4) systems

A B3LYP/6-31G ∗ study on geometrical, thermodynamic and electronic properties of the VO x + and VO x ( x=1–4) systems in the ground and some excited electronic states has been carried out to obtain the most stable structures, vibrational frequencies and reaction energies of the possible decomposition channels. The most energetically favorable conformations of VO + 2 ( 1A 1) and VO 2 ( 2A 1) are bent structures while VO + 3 ( 1 A ′ ) and VO 3 ( 1 A ′ ) arrangements have pyramidal geometries with C s symmetry. VO + 4 ( 3A) and VO 4 ( 2B 2) ground state render C 1 and C 2v symmetries, respectively. Electron localization function (ELF) has been used to discuss and analyze the nature of the bonding.

Experimental and Modelling Study of 1-Pentene Combustion at Fuel-Rich Conditions

A fuel-rich, non-sooting (C/O = 0.773) 1-pentene/oxygen/argon flame is studied experimentally at 50 mbar. The results are compared to computations. The calculations show satisfactory agreement with the data obtained from the measurements. The main channels for 1-pentene decomposition are presented and discussed in view of previous data for a propene flame at similar experimental conditions. Special emphasis is directed towards the formation of the first aromatic ring and the further growth of small aromatic hydrocarbons. By reaction flow analysis, it is found that the major reaction channel for benzene formation results from the recombination of propargyl radicals. Furthermore, the major reaction channels for naphthalene formation are presented.

Trajectory Investigations of the Dissociation Dynamics of Vinyl Bromide on an ab Initio Potential-Energy Surface

The reaction dynamics of vibrationally excited vinyl bromide have been investigated using classical trajectory methods on a global, analytic potential-energy hypersurface that is developed primarily by least-squares fitting of appropriately chosen functional forms to the results of ab initio electronic structure calculations. These calculations are carried out at the MP4 level of theory with all single, double, and triple excitations included. A 6-31G(d,p) basis set is employed for the carbon and hydrogen atoms. Huzinaga's (4333/433/4) basis set augmented with split outer s and p orbitals (43321/4321/4) and a polarization f orbital with an exponent of 0.5 is used for the bromine atom. The present calculations focus upon the determination of the dependence of the potential upon the stretching coordinates for the bonded atoms in vinyl bromide, the C−C−H and C−C−Br bending coordinates and the dihedral angles. The couplings between these coordinates are also investigated. The total ab initio database is obtained by combining these results with previously reported studies of the vinylidene−acetylene system and our previous calculations of saddle-point geometries and energies for the various decomposition channels, reactant and product equilibrium structures, and vibrational frequencies. The analytic surface fitted to this data base (PES1) is then modified by adjustment of the potential curvatures at equilibrium to provide a better fit to the measured IR and Raman vibrational frequencies of vinyl bromide while simultaneously holding all other topographical features of the surface constant to the maximum extent possible. The surface so developed is labeled PES2. Finally, we have arbitrarily altered the reaction coordinate curvatures for three-center HBr and H2 elimination to produce a third potential surface (PES3). The dissociation dynamics of vinyl bromide on each of these potential surfaces are investigated at several excitation energies in the range 4.5 to 6.44 eV. Total decomposition rate coefficients and product branching ratios are computed as a function of excitation energy. The HBr vibrational-state distribution is computed and found to be Boltzmann with an effective vibrational temperature of 7084 K on PES1. These results are in virtually exact agreement with recently reported measurements of this distribution. Finally, we have investigated the dissociation mechanisms for three-center H2 and HBr elimination reactions. The results show that the dynamics are very similar on PES1, PES2, and PES3. Consequently, small variations in potential-energy curvatures at equilibrium and along the reaction coordinates do not exert significant influence upon the dissociation dynamics. However, some large qualitative variations between the dynamics on the ab initio surface and the more empirical surface previously employed are found to exist. We conclude that great care must be exercised when such surfaces are used to study reaction dynamics in polyatomic systems.

Migration of a Proton as a Function of Solvation within {ROH}n{H2O}H+ Cluster Ions: Experiment and Theory

Metastable and collision-induced decompositions of mass-selected {ROH}n{H2O}H+ cluster ions (where R ≡ CH3−, CH3CH2−, CH3CH2CH2−, and (CH3)2CH−) were observed to exhibit distinct size-dependent behavior. We observe that loss of a water molecule is dominant for n ≤ 8, whereas loss of multiple ROH molecules is the favored decomposition channel for n ≥ 9, resulting in the eventual formation of a stable {ROH}9{H2O}H+ cluster ion. We believe this is evidence for two distinct cluster geometries which explicitly depend on the number of ROH molecules present. That is, below a certain critical size the proton resides on the molecule with the highest proton affinity, the ROH. However, above that critical cluster size the proton will now preferentially reside on the water molecule, if there are sufficient alcohols to completely and symmetrically solvate the central H3O+. The structural implications of these results will be discussed in light of new theoretical calculations which have been performed on this system.

Energy analysis of possible channels for decomposition of a charged drop into two parts

Based on numerical analysis of the mathematical expression for potential energy of an isolated charged spheroidal drop and two approximate spheroidal daughter drops, mechanisms for decomposition of a multiply charged drop in two nearly equal parts were studied taking into account the electrostatic interaction of the daughter drops. It was shown that, as the distance between the daughter drops increased, both the probability of spontaneous decomposition of a multiply charged drop in two daughter drops and the decomposition symmetry degree increase at the moment of breaking the connection between the daughter drops.

Trimethylsilyl group migration in the Criegee intermediate of gas-phase ozonolysis of trimethylsilylethenes

GC, GC/MS and GC/FTIR analyses of the final products of ozonolysis of trimethylsilylethene and trans-1,3-bis(trimethylsilyl)ethene reveal that the ester channel of decomposition of the Criegee intermediate takes place exclusively via migration of Me 3Si group.

Ab initio/Rice–Ramsperger–Kassel–Marcus approach to carbon nitride formation: CH 3NH 2 decomposition

An approach of combination of ab initio calculation with Rice-Ramsperger-Kassel-Marcus theory has been applied to the study of carbon nitride formation. The energetic characteristics and decomposition of one important intermediate CH 3NH 2 used in chemical vapor deposition (CVD) of carbon nitride thin film have been studied over a wide range of pressure and temperature. The calculated rate constants for three possible decomposition channels of the CH 3NH 2 depends strongly on the reaction temperature and pressure. The high rate constants for the CH 3NH 2 decomposition at high temperature might be responsible for the HCN or CNH formation in CVD of carbon nitride thin film.

Mechanism and rate constants for 1,3-butadiene decomposition

Data on the decomposition of 1,3-butadiene have been analyzed. The numerous isomerization processes that have recently been proposed as additional channels for decomposition have been considered. Energy transfer effects have been taken into account through the solution of the time-dependent master equation. We confirm recent supposition that direct formation of ethylene and acetylene is a major reaction channel during the thermal decomposition of 1,3-butadiene. The isomerization to 1,2-butadiene and its subsequent decomposition to form propargyl and methyl is the next most important process. Almost no vinyl radicals are produced. The formation of the isomerization products, in contrast to those from the initial reactant, is generally characterized by long transients before the attainment of a steady state. Standard methods of treating processes where such reactions are important must be carefully analyzed. The calculations are calibrated with experimental results. It proved not to be possible to fit simultaneously all the experimental observations with the model. Rate expressions for the major decomposition and isomerization channels are presented.

Exploring old and new benzene formation pathways in low-pressure premixed flames of aliphatic fuels

A modeling study of benzene and phenyl radical formation is performed for three low-pressure premixed laminar flat flames having an unsaturated C{sub 2} or C{sub 3} hydrocarbon fuel (acetylene, ethylene, and propene). Predictions using three published detailed elementary-step chemical kinetics mechanisms are tested against MBMS species profile data for all three flames. The differences between the three mechanisms predictive capabilities are explored, with an emphasis on benzene formation pathways. A new chemical kinetics mechanism is created combining features of all three published mechanisms. Included in the mechanism are several novel benzene formation reactions involving combinations of radicals such as C{sub 2}H+C{sub 4}H{sub 5}, and C{sub 5}H{sub 3}+CH{sub 3}. Reactions forming fulvene (a benzene isomer) are included, such as C{sub 3}H{sub 3}+C{sub 3}H{sub 5},as well as fulvene-to-benzene reactions. Predictions using the new mechanism show virtually all of the benzene and phenyl radical to be formed by reactions of either C{sub 3}H{sub 3}+C{sub 3}H{sub 3} or C{sub 3}H{sub 3}+C{sub 3}H{sub 5}, with the relative importance being strongly dependent upon the fuel. C{sub 5}H{sub 3}+CH{sub 3} plays a minor role in fulvene formation in the acetylene flame. The C{sub 2}H{sub x}+C{sub 4}H{sub 4} reactions do not contribute noticeably to benzene or phenyl radical formation in these flames, sometimes being a major decomposition channel for either fulvene or phenyl radical. The formation pathways for C{sub 3}H{sub 3} and C{sub 3}H{sub 5}are delineated for the three flames; while the key reactions differ from flame to flame, CH{sub 2}+C{sub 2}H{sub 2} {Longleftrightarrow} C{sub 3}H{sub 3}+H is important for all three flames.

Dissociation Pathways of Peroxyacetyl Nitrate (PAN)

The principal unimolecular dissociation pathways for PAN (peroxyacetyl nitrate, CH3C(O)OONO2) have been studied using a variety of theoretical methods. Reaction enthalpies calculated with the complete basis set (CBS) method were evaluated against a set of atmospheric free radical reactions for which the experimental thermochemistry is well defined. The validation procedure, which included two decomposition channels for HOONO2, demonstrated that the CBS-Q method reproduced the experimental Δ values with a rms error of 5.7 kJ mol-1. We report new Δ values for PAN, CH3C(O)O2, and CH3C(O)O of −240.1, −154.4 and −192.5 kJ mol-1, respectively. Accurate structural calculations for PAN, HOONO2, CH3C(O)O2, and CH3C(O)O augment the thermochemical calculations and show that the B3LYP density functional method describes the chemical bonding in the −OONO2 linkage quite well. The implications of this study for the atmospheric decomposition of PAN are discussed.

Classical Dynamics Study of the Unimolecular Decomposition of CH3SH+

The dynamics of three decomposition channels of the mercapto cation (CH3SH+) were investigated by classical trajectories and RRKM formalisms. The three channels are (I) CH bond dissociation through a “tight” transition state, (II) CS bond cleavage, and (III) SH bond scission. These calculations were performed with an analytical potential energy surface constructed from theoretical and experimental data available in the literature. The relative yields of CH3+ and CH2SH+ products are in qualitative agreement with charge-exchange experiments. The dynamical calculations revealed that the system is intrinsically non-Rice−Ramsperger−Kassel−Marcus (RRKM) at the energies selected in this study. Under nonrandom initial conditions, the system showed strong mode specificity, which may be rationalized by weak couplings between the low- and high-frequency modes, particularly the CH3 stretching normal modes, which is consistent with collisional activation studies. The classical trajectory calculations revealed an inverse isotope effect for both the CS and SH scission channels and a normal isotope effect for the CH bond dissociation process. Finally, we have found that molecular rotation decreases the mercapto cation decomposition rate and that orbital angular momentum dramatically modifies the relative yields of CH3+ and CH2SH+ products.

Unimolecular decay of the thiomethoxy cation, CH3S+: A computational study on the detailed mechanistic aspects

The unimolecular decay of the triplet thiomethoxy cation CH3S+, ion 1, has been investigated by density functional theory, ab initio, and Phase–space/Rice Ramsperger Kassel Marcus (PST/RRKM) calculations. We have first located on the singlet and triplet B3LYP/6-311+G(d,p) [C,H3,S]+ potential energy surfaces the energy minima and transition structures involved in the lowest energy decompositions of 1, including the loss of H, H2, and S. We have subsequently located the minimum energy points lying on the B3LYP/6-311+G(d,p) hyperline of intersection between the singlet and triplet surfaces, using a recently described steepest descent-based method [Theor. Chem. Acc. 99, 95 (1998)]. The total energies of all these species were refined by CCSD(T)/cc-pVTZ single-point calculations. The obtained potential energy surface has been used to outline the full kinetic scheme for the unimolecular decay of ion 1. The rate constants of the various elementary steps have been calculated by the PST and the RRKM theory. We used a nonadiabatic version of the latter to evaluate the rate constants of the elementary steps which involve a change in the total spin multiplicity. We found that the two kinetically favored decomposition channels are the loss of atomic hydrogen, with formation of 2CH2S+⋅, and molecular hydrogen, with formation of 1HCS+. The former process is predicted to prevail for ions 1 in the lowest rotational states and with an internal energy content of at least 60 kcal mol−1. The loss of H2 was found to be by far the prevailing process in the time scale of ca. 10−5 to ca. 10−6 s from the formation of 1. This is fully consistent with the experimentally observed exclusive loss of H2 by the CH3S+ ions which decompose in the “metastable” time window of the mass spectrometer. The loss of H2 from ion 1 with formation of 1HCS+ may occur by two distinct “spin-forbidden” paths, i.e., a simple concerted 1,1 H2 elimination or a 1,2 H shift followed by a 1,2 H2 elimination from the singlet mercaptomethyl ion 2. In the metastable time window, these two mechanisms may occur alternatively, depending on the degree of rotational excitation of 1.