Decomposition Path Research Articles

Quest for IR-Pumped Reactions in Dihydrogen-Bonded Complexes

As possible substrates for one-photon infrared-pumped reaction (IRPR), the structures and decomposition paths of complexes between AlH4- and three proton donors, H2O, HF, and HCl, have been studied by ab initio methods. In each case only one transition state was found for the proton-transfer and H2-loss process. For each cluster, the geometry and energy characteristics of reactants, complex, transition state, and products were analyzed with [AlH4···HCl]- emerging as the best IRPR candidate. The MP2//6-311++G**-calculated intrinsic reaction coordinate (IRC) confirmed the one-step proton transfer and H2 loss with no intermediate. Classical trajectories were calculated on the ab initio potential-energy surface, beginning from a large number of initial conditions. With zero-point vibrational energies (ZPVE = 1/2hνi) assigned to all normal modes, based on their calculated harmonic frequencies, νi, one or more additional excitation quanta were added to modes associated with Cl−H and Al−H stretching. Proton transfer from HCl and loss of H2 were calculated to occur on the femtosecond time scale when stretching modes involving the dihydrogen-bonded hydrogens were excited. However, many vibrational oscillations take place before H2 release. Analysis of the dynamics in terms of the complex's normal modes indicates that the excitation in the reaction-relevant modes remains localized on a time scale >1 ps.

Selective Electrochemical Aromatic Fluorination of Acetophenone and Benzophenone.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Theoretical Study of the Dissociation of N2O in a Transition Metal Ion-Catalyzed Reaction

Models of Fe−zeolites have been used to study the dissociation of N2O, with and without the presence of ammonia. From Density Functional Theory calculations, the mechanism and energetics of N2O decomposition paths are presented and discussed. The influence of ammonia is analyzed. The results obtained for a single metal ion and for models including a zeolite cluster are compared. The approximate activation barrier calculated with this cluster model is in good agreement with experiment. The presence of two ammonia ligands reduces the calculated activation barrier, leading to the conclusion that ammonia plays a role in both steps of the N2O decomposition, i.e., the breaking of the ON2 bond and the reduction of the iron-oxo compounds.

Kinetic study of the aminolysis and pyridinolysis of O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates. A remarkable leaving group effect.

The reactions of a series of secondary alicyclic (SA) amines with O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates (1 and 2, respectively) and of a series of pyridines with the former substrate are subjected to a kinetic investigation in water, at 25.0 degrees C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, all the reactions obey pseudo-first-order kinetics and are first-order in amine. The Brönsted-type plots are biphasic, with slopes (at high pK(a)) of beta(1) = 0.20 for the reactions of SA amines with 1 and 2 and beta(1) = 0.10 for the pyridinolysis of 1 and with slopes (at low pK(a)) of beta(2) = 0.80 for the reactions of SA amines with 1 and 2 and beta(2) = 1.0 for the pyridinolysis of 1. The pK(a) values at the curvature center (pK(a)(0)) are 7.7, 7.0, and 7.0, respectively. These results are consistent with the existence of a zwitterionic tetrahedral intermediate (T++) and a change in the rate-determining step with the variation of amine basicity. The larger pK(a)(0) value for the pyridinolysis of 1 compared to that for 2 (pK(a)(0) = 6.8) and the larger pK(a)(0) value for the reactions of SA amines with 1 relative to 2 are explained by the greater inductive electron withdrawal of PhO compared to EtO. The larger pK(a)(0) values for the reactions of SA amines with 1 and 2, relative to their corresponding pyridinolysis, are attributed to the greater nucleofugalities of SA amines compared to isobasic pyridines. The smaller pK(a)(0) value for the reactions of SA amines with 2 than with O-ethyl S-(2,4-dinitrophenyl) dithiocarbonate (pK(a)(0) = 9.2) is explained by the greater nucleofugality from T(++) of 2,4-dinitrophenoxide (DNPO(-)) relative to the thio derivative. The stepwise reactions of SA amines with 1 and 2, in contrast to the concerted mechanisms for the reactions of the same amines with the corresponding carbonates, is attributed to stabilization of T(++) by the change of O(-) to S(-). The simple mechanism for the SA aminolysis of 2 (only one tetrahedral intermediate, T(++)) is in contrast to the more complex mechanism (two tetrahedral intermediates, T(++) and T(-), the latter formed by deprotonation of T(++) by the amine) for the same aminolysis of the analogous thionocarbonate with 4-nitrophenoxide (NPO(-)) as nucleofuge. To our knowledge, this is the first example of a remarkable change in the decomposition path of a tetrahedral intermediate T by replacement of NPO(-) with DNPO(-) as the leaving group of the substrate. This is explained by (i) the greater leaving ability from T(++) of DNPO(-) than NPO(-) and (ii) the similar rates of deprotonation of both T(++) (formed with DNPO and NPO).

Selective Electrochemical Aromatic Fluorination of Acetophenone and Benzophenone

Electrochemical fluorination of acetofenone and benzophenone was studied in anhydrous HF and in solutions. The electrochemical fluorination of acetophenone in HF occurred exclusively in the ring and furnished ortho- and meta-isomers of fluoroacetophenone, 2,5-difluoroacetophenone, and 1-(3,3,6,6-tetrafluoro-1,4-cyclohexadienyl)-1-ethanone. The fluorination of benzophenone in anhydrous HF furnished predominantly m-fluorobenzophenone, whereas in the presence of chloroform only chlorination products were obtained. The electrochemical fluorination of acetophenone in acetonitrile gave rise only to mono- and difluorinated products. The reasons for readily occurring oxidative fluorination of aromatic compounds into polyfluoro-1,4-cyclohexadienes were discussed, and the decomposition paths of fluorinated products under electron impact were considered.

Solid-state 27Al NMR investigation of thermal decomposition of LiAlH 4

Solid-state nuclear magnetic resonance is used to study the thermal decomposition of lithium tetrahydroaluminate into metallic aluminum, hydrogen and trilithium hexahydroaluminate. Aluminum sites in LiAlH 4 and Li 3AlH 6 were characterized using static, magic angle spinning (MAS) and multiple-quantum MAS NMR. By applying the in situ NMR method, it has been demonstrated that melting is not a prerequisite for the decomposition of LiAlH 4. Based on the observed data, a decomposition path has been established that is consistent with the concentrations of observed Al species at various stages of the thermally induced reaction.

Gaussian-3 Molecular Orbital Study of the Reaction of Boron with Methylamine and the Decomposition Paths of the Reaction Products

G3 molecular orbital calculations predict that the insertions of boron into the CH, NH, and CN bonds of methylamine to form CH2(BH)NH2, CH3N(BH)H, and CH3BNH2 are exothermic by 57.6, 93.9, and 111.8 kcal mol-1, respectively. However, the addition of atomic boron to the N site of methylamine yeilds a weak additon complex CH3NH2··B, which is lower in energy than the reactants by only 17.2 kcal mol-1. The dissociation of CH3N(BH)H to CH3 and HNBH has a low activation energy of 28.7 kcal mol-1 and is the most energetically favored one amomg its three decomposition paths. One of the other two paths yielding the final product CH3NB has a rate-determining step with a high endothermicity of 111.4 kcal mol-1, but the other leading to CNB has one with a high activation barrier of 105.7 kcal mol-1 instead. The conversions of CH3BNH2 to the final products CH2BNH and CH2BNH2 are respectively controlled by their endothermicities of 96.9 and 47.7 kcal mol-1.

Decomposition of 2-hexyne on clean Ru( [formula omitted]) studied by RAIRS

The decomposition of 2-hexyne on clean Ru(0 0 1), under ultra-high-vacuum, was studied by reflection–absorption infrared spectroscopy (RAIRS). The spectra obtained at 100 K are compatible with the formation of a non-dissociative di-σ/π complex. Evidence was obtained for two decomposition paths. By annealing the low temperature complex above 110 K, the weakened C–C bonds adjacent to the anchor atoms (C 1–C 2 and C 3–C 4 bonds) break, eventually yielding methylidyne (RuC–H) and ethylidyne (RuC–CH 3), at 220 K. By direct adsorption of 2-hexyne at 130 K or higher temperatures, the C–C triple bond is the first to break, ethylidyne being the major surface species identified by RAIRS. These results are compared to those obtained for 1- and 3-hexyne isomers.

Preparation and Characterization of Ce<sub>0.8</sub>Gd<sub>0.2</sub>O<sub>1.9</sub> Solid Electrolyte by Polymeric Precursor Techniques

Ce 0.8 Gd 0.2 O 1.9 solid ionic conductors have been prepared by techniques involving polymeric precursors: the Pechini and the amorphous citrate techniques. The aim of this work is to determine among these techniques the most suitable to obtain solid solutions with high chemical homogeneity and high sinterability. The main results show different paths for decomposition of the polymeric precursors. Calcined powders presented similar particle size distribution curves although they are related with the distribution of agglomerated particles. This result explains the difference in average particle size values calculated from results obtained by laser scattering and gas adsorption analysis. X-ray diffraction patterns show the main peaks of the cubic phase for calcinations at 600 °C. Agglomerate morphology is also quite different as observed by scanning electron microscopy.

Investigation of the early stages of decomposition of Cu–0.7at.% Fe with the tomographic atom probe

The early stages of precipitation of the Fe-rich phase in supersaturated Cu–Fe alloy were studied with the field ion microscope and the tomographic atom probe. The specimens of nominal composition Cu–0.7at.% Fe were homogenized at 1173 K and immediately aged in a salt bath for different times at 803 K to follow the decomposition path at this reaction temperature. In this work, preliminary results on the temporal evolution of the decomposition are presented. In particular, the particle radii and particle number density were determined as a function of the ageing time. At the initial stages of the decomposition concentration, fluctuations were observed, which could be quantitatively characterized. The decomposition sequence is discussed in terms of the nucleation and growth theory.

In-situ X-ray powder diffraction studies of hydrothermal and thermal decomposition reactions of basic bismuth(iii) nitrates in the temperature range 20–650 °C

The syntheses of basic bismuth(III) nitrates under hydrothermal conditions and their dry decomposition to α-Bi2O3 in solid-state reactions was investigated by in-situ synchrotron X-ray radiation powder diffraction. Three basic bismuth(III) nitrates with known structures are: [Bi6O4(OH)4](NO3)6·4H2O, C, [Bi6O4(OH)4](NO3)6·H2O, D, and [Bi6O5(OH)3](NO3)5·3H2O, A. They contain the complex ions [Bi6O4(OH)4]6+ and{[Bi6O5(OH)3]5+}2, respectively. A fourth basic bismuth(III) nitrate, X, apparently has the composition [Bi6O4.5(OH)3.5](NO3)5.5·H2O according to chemical analysis. The transformation of C via D to X, or of C directly to X was established in the in-situ hydrothermal synthesis. Similarities in the decomposition paths of C, D and X in the solid-state in-situ investigations support the above composition indicated for X. The compound A has a decomposition path different from those of C, D and X. This is consistent with the complex ion {[Bi6O5(OH)3]5+}2 in A, being different from the complex ion [Bi6O4(OH)4]6+ in the other two basic bismuth(III) nitrates. All four compounds have α-Bi2O3 as the final decomposition product.

Preparation and properties of triosmium propyl and propylidene complexes

Treatment of HOs 3(CO) 10(CHCHCH 3) in THF with LiBHEt 3 gave [HOs 3(CO) 10(CHCH 2CH 3)] −, isolated as the NEt 4 + or NMe 4 + salt. Protonation at −70 °C with HBF 4 formed HOs 3(CO) 10(CH 2CH 2CH 3), which was isolated as a yellow solid stable only at low temperatures. The kinetics of both its equilibration with H 2Os 3(CO) 10(CHCH 2CH 3) at −25 °C and of its decomposition to H 2Os 3(CO) 10 and propene at 19 °C have been determined. Treatment of HOs 3(CO) 10(CHCH 2) with CH 2PPh 3 in THF gave the zwitterionic complex HOs 3(CO) 10(CHCH 2CH 2PPh 3), isolated in 60% yield as a yellow solid. Protonation with HX (X=BF 4 − , CF 3SO 3 − , CF 3CO 2 − ) at −78 °C in dichloromethane occurred at an osmiumcarbon bond to yield [HOs 3(CO) 10(CH 2CH 2CH 2PPh 3)] +, containing a bridging alkyl with one C⋯H⋯Os agostic interaction. At approximately −50 °C partial (X=CF 3SO 3 − ) or complete (CF 3CO 2 − ) isomerization to a terminal alkyl form with coordination of the anion was observed. When the bridging alkyl was the major form, β-elimination occurred readily above −25 °C with formation of [PPh 3CH 2CHCH 2] + and H 2Os 3(CO) 10. The terminal alkyl complex was found to be more stable thermally, and in the case of HOs 3(CO) 10(O 2CCF 3)(CH 2CH 2CH 2PPh 3), the primary path of decomposition at 10 °C was reductive elimination to form [PPh 3CH 2CH 2CH 3] + and HOs 3(CO) 10(O 2CCF 3).

Chlorinated poly(vinyl chloride) and plasticized chlorinated poly(vinyl chloride)—thermal decomposition studies

AbstractThe thermal decomposition of chlorinated poly(vinyl chloride) and three plasticized chlorinated poly(vinyl chloride) systems has been investigated. The routes of decomposition of these systems have been elucidated by investigating char formation and by using a combination of thermogravimetric analysis (TGA) and prolysis/gas chromatography/mass spectroscopy methods (Py/GC/MS). The effects of the charforming/smoke‐suppressing iron(III) compound FeOOH in these polymer systems has also been investigated. The structure of both CPVC polymer and plasticzer determine the path of thermal decomposition and also the quantity and nature of the decomposition compunds formed. Changes in oxygen index and the formation of smoke during burning in these systems have been related to the char that is formed and also to the chemical nature of the decomposition products.

A hybrid density functional theory study of the low‐temperature dimethyl ether combustion pathways. I: Chain‐propagation

AbstractDimethyl ether (DME) has been proposed to be a promising alternative to conventional diesel fuel because of its favorable compression ignition property (high cetane number) and its soot‐free combustion. A radical chain mechanism for hydrocarbon autoignition has been proposed for DME at low temperatures. In this mechanism, the chain initiation step consists of DME undergoing hydrogen abstraction by a highly reactive species (typically ·OH). The CH3O·H2 created in the initiation step then combines with O2; the subsequent CH3OCH2OO· radical is involved in a Lindemann‐type mechanism, which can lead to the production of formaldehyde (CH2 = O) and ·OH. This concludes the chain‐propagating step: the one ·OH produced then sustains the chain‐reaction by creating another CH3O·H2. A relatively stable intermediate (·CH2OCH2OOH), formed via isomerization of CH3OCH2OO· in the chain‐propagation step, can combine with a second O2 to produce a radical (·OOCH2OCH2OOH) that can potentially decompose into two ·OH radical (and other products). This path leads to chain‐branching and an exponential increase in the rate of DME oxidation. We have used spin‐polarized density functional theory with the Becke‐3‐parameter Lee—Parr—Yang exchange‐correlation functional to calculate the structures and energies of key reactants, intermediates, and products involved in (and competing with) the chain‐propagating and chain‐branching steps of low‐temperature DME oxidation. In this article, Part I, we consider only the chain‐propagation mechanism and its competing mechanisms for DME combustion. Here, we show that only certain conformers can undergo the isomerization to ·CH2OCH2OOH. A new transition state has been discovered for the disproportionation reaction ·CH2OCH2OOH → 2CH2O + ·OH in the chain‐propagating step of DME autoignition that is much lower than previous barriers. The key to making this decomposition pathway facile is initial cleavage of the O—O rather than the C—O bond. This renders all transition states along the chain‐propagation potential energy surface below the CH3O·H2 + O2 reactants. In contrast with the more well‐studied CH3·H2 (ethyl radical) + O2 system, the H‐transfer isomerization of CH3OCH2OO· to ·CH2OCH2OOH in low‐temperature DME oxidation has a much lower activation energy. This is most likely due to the larger ring strain of the analogous transition state in ethane oxidation, which is a five‐membered ring opposed to a six‐membered ring in dimethyl ether oxidation. Thus low‐temperature ethane oxidation is much less likely to form the ·ROOH (where R is a generic group) radicals necessary for chain‐branching, which leads to autoignition. Three competing reactions are considered: CH3O·H2 → CH2O + ·CH3; ·CH2OCH2OOH → 1,3‐dioxetane + ·OH; and ·CH2OCH2OOH → ethylene oxide + HOO·. The reaction barriers of all these competing paths are much higher in energy (7–10 kcal/mol) than the reactants CH3O·H2 + O2 and, therefore, are unlikely low‐temperature paths. Interestingly, an analysis of the highest occupied molecular orbital along the CH3O·H2 decomposition path shows that electronically excited (1A2 or 3A2) CH2O can form; this can also be shown for ·CH2OCH2OOH, which forms two formaldehyde molecules. This may explain the luminosity of DME's low‐temperature flames.

Theoretical study of decomposition pathways for HArF and HKrF

To provide theoretical insights into the stability and dynamics of the new rare gas compounds HArF and HKrF, reaction paths for decomposition processes HRgF→Rg+HF and HRgF→H+Rg+F (Rg=Ar, Kr) are calculated using ab initio electronic structure methods. The bending channels, HRgF→Rg+HF, are described by single-configurational MP2 and CCSD(T) electronic structure methods, while the linear decomposition paths, HRgF→H+Rg+F, require the use of multi-configurational wave functions that include dynamic correlation and are size extensive. HArF and HKrF molecules are found to be energetically stable with respect to atomic dissociation products (H + Rg + F) and separated by substantial energy barriers from Rg + HF products, which ensure their kinetic stability. The results are compatible with experimental data on these systems.

The enzymes of glycogen and trehalose catabolism from Hysterothylacium aduncum (Nematoda: Anisakidae)

The activity of alpha-amylase, glucoamylase, maltase, trehalase, glycogen phosphorylase and trehalose phosphorylase was measured in extracts from larval and adult Hysterothylacium aduncum (Rudolphi, 1802), parasitic nematode of marine fish. The content of glycogen and trehalose in the worm's body was also determined. Both the hydrolytic and phosphorolytic paths of sugar decomposition are present in H. aduncum. In the larvae glycogen was utilised mainly via the hydrolytic path. In the adults the activities of phosphorolytic enzymes were higher than in the larvae. In both stages the activity of trehalose phosphorylase is present. In adult nematodes it is uncommonly high. The dominating sugars in the adults were glucose and glycogen, while in the larvae it was trehalose.

Stability of a chemically bound helium compound in high-pressure solid helium

HHeF, a chemically-bound helium compound, has been predicted to be metastable in the gas phase. It decays by tunneling through energy barriers in picosecond timescales into He+HF and H+He+F. This paper studies the stability of HHeF in pressurized solid helium. Using realistic potentials for the HHeF/He interaction, the potential energy along the minimum energy paths for decomposition is evaluated, and tunneling decay times are computed by the WKB approximation. It is found that for pressures above 500 MPa, decomposition into H+He+F is completely suppressed. At 23 GPa, the highest pressure studied, the timescale for HHeF→He+HF is in the millisecond range. At pressures well above 23 GPa, HHeF is thus expected to remain stable indefinitely.

A DFT Study of CHx Chemisorption and Transition States for C−H Activation on the Ru(112̄0) Surface

The thermodynamics of methane decomposition on the ruthenium (1120) surface has been investigated with ab initio periodic calculations. All surface intermediates are more stable than the gas-phase methane even if the last step of the decomposition path: CH → C + H, is highly endothermic. Among all of the surface species, CH2 appears to be the most stable. All of the surface species (CHx, x = 3−1 and H) adsorb on bridge-up sites, while atomic C prefers top-down sites. The transition states of the elementary reactions for the dissociation of methane on the ruthenium (112̄0) surface have been investigated with nudged elastic band method (NEB). The calculated barriers are 56 kJ mol-1 for methane decomposition, 11 kJ mol-1 for methyl decomposition, and 52 kJ mol-1 for methylene decomposition, respectively. The decomposition of CHads requires the highest activation energy from the series with 95 kJ mol-1.

Mass spectral characterization of tetracyclines by electrospray ionization, H/D exchange, and multiple stage mass spectrometry

Electrospray ionization (ESI) and collisionally induced dissociation (CID) mass spectra were obtained for five tetracyclines and the corresponding compounds in which the labile hydrogens were replaced by deuterium by either gas phase or liquid phase exchange. The number of labile hydrogens, x, could easily be determined from a comparison of ESI spectra obtained with N 2 and with ND 3 as the nebulizer gas. CID mass spectra were obtained for [M + H] + and [M − H] − ions and the exchanged analogs, [M(D x ) + D] + and [M(D x ) − D] −, and produced by ESI using a Sciex API-III plus and a Finnigan LCQ ion trap mass spectrometer. Compositions of product ions and mechanisms of decomposition were determined by comparison of the MS N spectra of the un-deuterated and deuterated species. Protonated tetracyclines dissociate initially by loss of H 2O (D 2O) and NH 3 (ND 3) if there is a tertiary OH at C-6. The loss of H 2O (D 2O) is the lower energy process. Tetracyclines without the tertiary OH at C-6 lose only NH 3 (ND 3) initially. MS N experiments showed easily understandable losses of HDO, HN(CH 3) 2, CH 3 − NCH 2, and CO from fragment ions. The major fragment ions do not come from cleavage reactions of the species protonated at the most basic site. Deprotonated tetracyclines had similar CID spectra, with less fragmentation than those observed for the protonated tetracyclines. The lowest energy decomposition paths for the deprotonated tetracyclines are the competitive loss of NH 3 (ND 3) or HNCO (DNCO). Product ions appear to be formed by charge remote decompositions of species de-protonated at the C-10 phenol.

Tetra chlorocopper(II) chemistry: delineation of optical, thermal properties

The thermal decomposition paths of pyridinium tetra chlorocopper(II) ( A) differs from the anilinium tetra chlorocopper(II) ( B). In the case of pyridinium tetra chlorocopper(II) it loses two molecules of pyridinium hydrochloride and CuCl 2 is formed in the temperature range of 140–350 °C. Whereas the anilinium tetra chlorocopper(II) ( B) loses one mole of anilinum hydrochoride along with one molecule of aniline. The compound B has a well define structural change taking place in the region of 70–110 °C which is reflected in DSC as well as resistance measurements. The p-methoxy anilinium tetra chlorocopper(II) monohydrate ( C) shows solvatochromicity. The absorption maximum of p-methoxy anilinium tetra chlorocopper(II)monohydrate in acetonitrile (460 nm) changes to 570 nm on addition of methanol. The absorption at 570 nm can be switched back to 460 nm by suppression of ligand exchange with addition of hydrochloric acid. Thus, chemically driven optical switching properties can be observed.