A computational modeling study of the low-temperature pyrolysis of n-alkanes; mechanisms of propane,n-butane,and n-pentane pyrolyses

The mechanisms of the pyrolyses of the n-alkanes C₃H₈, n-C₄H₁₀, and n-C₅H₁₂ at temperatures between 390 and 560°C have been studied by the construction and evaluation of sets of several hundred reactions. Rate parameter values were assigned using literature data and calculated estimates. Time-dependent numerical solutions were computed for the experimental conditions of several rate and product studies reported in the literature. The comparisons of these a priori computations with experiment show excellent agreement for propane and agreement for butane and pentane within the estimated error limits of the assigned rate parameters. These results demonstrate that the general “state of knowledge” of the mechanism of alkane pyrolysis, namely, the reactions and their rate parameters, is such that reasonable a priori predictions of experimental results can be made. Discussions of the major stepwise processes in the pyrolyses are presented, and the importance of allyl radicals in termination is demonstrated.     

Aminomethylation of Thioureas with N,N-Dimethyl-1-(triethylsiloxy)methanamine,Involving Amino Group Exchange

The O-triethylsilylated hemiaminal Et₃SiOCH₂NMe₂ readily transfers the Me₂NCH₂-group to various thioureas under mild conditions and without catalysts or co-reagents. In the reaction with PhNHC(=S)·NHPh, the initially formed mono-substituted derivative PhNHC(=S)NPhCH₂NMe₂ readily rearranges to produce the unsymmetrical thiourea PhNHC(=S)NMe₂ and hexahydro-1,3,5-triphenyl-1,3,5-triazine.

     

Initial Pyrolysis Reactions in Unmodified and Flame-Retardant Cotton

Abstract

Thermogravimetric data were used to make Arrhenius plots for the vacuum pyrolyses of unmodified cotton and of cotton finished with various add-ons of THPOH-NH3 and THPS-urea-Na₂HPO₄ flame retardants. These plots show that all pyrolyses occurred in consecutive stages: The first and second initial stages, associated with the less ordered regions of the cotton fibers, and the main cellulose pyrolysis reaction, associated with cellulose crystallites. Cotton decrystallized by ball milling showed only the two initial pyrolysis stages. The second stage followed first-order kinetics. The first pyrolysis stage in unmodified cotton was characterized by a moderately low activation energy and by a large negative entropy of activation; the second stage showed a larger activation energy and a less negative entropy of activation. Mechanisms involving cellulose chain scission and chain unzipping are proposed for the first and second stages, respectively. Add-on of the two flame retardants had contrasting effects on the two initial pyrolysis stages. These effects are explained in terms of the way in which the flame retardants are deposited in the less ordered regions of cotton fibers.     

Effects of Chain Ends on Thermal and Mechanical Properties and Recyclability of Poly(γ-butyrolactone)

This work investigates effects of poly(γ-butyrolactone) (PγBL) with different initiation and termination chain ends on five types of materials properties, including thermal stability, thermal transitions, thermal recyclability, hydrolytic degradation, and dynamic mechanical behavior. Four different chain-end-capped polymers with similar molecular weights, BnO-[C(=O)(CH₂)₃O]_n-R, R = C(=O)Me, C(=O)CH=CH₂, C(=O)Ph, and SiMe₂CMe₃, along with a series of uncapped polymers R′O-[C(=O)(CH₂)₃O]_n-H (R′ = Bn, Ph₂CHCH₂) with M_n ranging from low (4.95 kg mol⁻¹) to high (83.2 kg mol⁻¹), have been synthesized. The termination chain end R showed a large effect on polymer decomposition temperature and hydrolytic degradation, relative to H. Overall, for those properties sensitive to the chain ends, chain-end capping renders R-protected linear PγBL behaving much like cyclic PγBL. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2271–2279     

Structural Changes in the Macrocycles of Tetrathioand Dithiodioxo-Substituted 1,8-Dioxa-3,6,10,13-tetraazacyclotetradecane Caused by Complexation with 3<Emphasis Type="Italic">d</Emphasis> M(II) Ions according to Quantum-Chemical DFT Calculation

The bond angles in the macrocycles of tetrathio- and dioxodithio-substituted 1,8-dioxa-3,6,10,13-tetraazacyclotetradecanes and their Cr(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes with the (NNNN) coordination of the ligand donor sites, formed upon the complexation in the ternary systems M(II)–ethanedithioamide H₂N–C(=S)–C(=S)–NH₂ (thiocarbamoylmethanamide H₂N–C(=S)–C(=O)–NH₂)–formaldehyde H₂C(=O) in gelatin-immobilized matrix implants have been calculated by the DFT OPBE/TZVP method with the Gaussian 09 program package. It has been stated that, depending on the nature of M(II) and macrocyclic ligand, the complexation can lead to both the decrease and the increase in the degree of macrocycle distortion (quantatively characterized as the degree of deviation of the macrocycle from coplanarity).     

The pyrolysis of trimethylgermane and trimethylsilane

The number of products and the H₂/CH₄ ratio obtained from the flow pyrolyses of (CH₃)₃GeH and (CH₃)₃SiH were very different. The (CH₃)₃GeH decomposition is consistent with the following mechanism:

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The pyrolysis of (CH₃)₃SiH was found to be much more complex, presumably due to the formation of silicon-carbon double bonded intermediates and the (CH₃)₂Si(H)CH₂ radical. We also present data which supports the presence of a H atom chain sequence during this pyrolysis.     

Mutual stability and molecular structures of asymmetric (555)macrotricyclic 3d-metal chelates formed by self-assembly in M(II) ion-ethanedithioamide-hydrazinomethanethioamide-2-oxopropanal quaternary systems according to density functional theory calculations

The thermodynamic and geometric parameters of asymmetric macrotricyclic Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes with the NSSN-coordination of donor sites of the ligand that can form upon complexation of the corresponding hexacyanoferrates(II) by ethanedithioamide H₂N-C(=S)-C(=S)-NH₂, hydrazinomethanethioamide H₂N-NH-C(=S)-NH₂, and 2-oxopropanal in gelatin-immobilized matrices have been calculated by the OPBE/TZVP nonhybrid DFT method with the use of the Gaussian09 program package.     

[2+4] and [2+2] Cycloaddition Reactions of N‐Sulfinyl Carboxylic Acid Amides with (CF3)2BNMe2. Crystal Structure of cyclo‐(CF3)2B‐NMe2‐S(=O)‐N=C(p‐CH3C6H4)‐O

N‐sulfinylacylamides R‐C(=O)‐N=S=O react with (CF₃)₂BNMe₂ ( 1 ) to form, by [2+4] cycloaddition, six‐membered rings cyclo‐(CF₃)₂B‐NMe₂‐S(=O)‐N=C(R)‐O for R = Me ( 2 ), t‐Bu ( 3 ), C₆H₅ ( 4 ), and p‐CH₃C₆H₄ ( 5 ) while N‐sulfinylcarbamic acid esters R‐O‐C(=O)‐N=S=O react with 1 to yield mixtures of six‐membered (cyclo‐(CF₃)₂B‐NMe₂‐S(=O)‐N=C(OR)‐O) and four‐membered rings (cyclo‐(CF₃)₂B‐NMe₂‐S(=O)‐N(C=O)OR) for R = Me ( 6 and 9 ), Et ( 7 and 10 ), and C₆H₅ ( 8 and 11 ). The structure of 5 has been determined by X‐ray diffraction.     

On the molecular structures of (545)macrotricyclic chelates in the M(II) ion-2,3-butanedione-aminomethanamidine ternary systems

The geometric parameters of the molecular structures of macrotricyclic M(II) complexes with a tetradentate chelating ligand with the (NOON) coordination of donor sites formed by the template reactions in the M(II)-2,3-butanedione (H₃C-C(=O)-C(=O)-CH₃)-aminomethanamidine (H₂N-C(=NH)-NH₂) systems have been calculated by the OPBE/TZVP density functional theory (DFT) method. The bond lengths and bond and torsion angles in the resulting complexes have been reported. The enthalpies and Gibbs energies for the reaction of their formation have also been calculated. A conclusion has been drawn that template synthesis in these systems can be realized only upon complex formation in a gelatin-immobilized matrix.     

NHCs as Neutral Donors towards Polyphosphorus Complexes

The first adducts of NHCs (=N-heterocyclic carbenes) with aromatic polyphosphorus complexes are reported. The reactions of [Cp*Fe(η⁵-P₅)] ( 1 ) (Cp*=pentamethyl-cyclopentadienyl) with IMe (=1,3,4,5-tetramethylimidazolin-2-ylidene), IMes (=1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene) and IDipp (=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene) led to the corresponding neutral adducts which can be isolated in the solid state. However, in solution, they quickly undergo a dissociative equilibrium between the adduct and 1 including the corresponding NHC. The equilibrium is influenced by the bulkiness of the NHC. [Cp′′Ta(CO)₂(η⁴-P₄)] (Cp′′=1,3-di-tert-butylcyclopentadienyl) reacts with IMe under P atom abstraction to give an unprecedented cyclo-P₃-containing anionic tantalum complex. DFT calculations shed light onto the energetics of the reaction pathways.     

Phosphorylformonitrile oxides as spin traps for carbon-centered free radicals: co-formation with radicals in the generation of nitrile oxides from <Emphasis Type="Italic">C</Emphasis>-phosphorylformohydroximoyl halides in the presence of alcohols or ethers

One-electron oxidation of the oximes R₂P(=O)C(=NOH)X (X = Cl or Br) generates the nitrile oxides R₂P(=O)C⁺=NO⁻, which serve as spin traps for unstable carbon-centered radicals.The latter are generated upon addition of PbO₂ to a mixture of formohydroximoyl halide with an alcohol or an ether of the general formula R¹OCHR²R³ under the action of atomic chlorine (bromine) released during the generation of nitrile oxide. This gives rise to new, more persistent C phosphoryliminoxyls R₂P(=O)C(=NO^·)C(OR¹)R²R³ (R¹, R², R³ = H, Alk). When primary alcohols (R¹ = R² = H) are used, acyl radicals generated at the initial step of the reaction are also trapped by nitrile oxides to give C-acyl-C phosphoryl iminoxyl radicals R₂P(O)C(=NO^·)C(=O)R³. Hyperfine coupling constants for more than 20 C-phosphoryl-iminoxyls existing in solutions as mixtures of Z- and E-isomers were determined.The effect of the structure of the primary radical (length of the carbon chain, degree of branching, the presence of a ring, and its size) on the radiospectroscopic characteristics of new C-phosphoryliminoxyl radicals was studied.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 336–341, February, 2005.     

Stability of isomerous chelates in M(II)-thiocarbamoylmethanamide-ethandial systems according to the DFT B3LYP method (M = Mn, Fe, Co, Ni, Cu, Zn)

Calculations of the optimal geometry and standard thermodynamic parameters of Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) isomerous macrotricyclic complexes with MN₂O₂, MN₂S₂, and MN₄ chelate bonds, which can in principle appear as a result of template processes between gelatine-immobilized hexacyanoferrates (II) of corresponding M(II) metal ions, thiocarbamoylmethaneamide (thiooxamide) H₂N-C(=S)-C(=O)-NH₂, and ethanedial HC(=O)-CH(=O), were performed according to the B3LYP hybrid density functional method using a 6-31G(d) basis set with the Gaussian 98 program. It was found that of all of the considered M(II), the most stable are complexes with MN₄ chelate bonds, where the values of a standard enthalpy Δ_f H ₂₉₈^o and a standard Gibbs energy, Δ_f G ^o for all complexes studied are positive.     

Novel Four‐Component Approach for the Synthesis of Polyfunctionalized 1,4‐Dihydropyridines in Aqueous Media

The four‐component reaction of dimethyl acetylenedicarboxylate (=dimethyl but‐2‐ynedioate; DMAD), aromatic aldehydes, and malononitrile (=propanedinitrile) leads to polyfunctionalized 1,4‐dihydropyridine derivatives. The reaction proceeds at room temperature and in the presence of a catalytic amount (20%) of (NH₄)₂HPO₄ as a base in aqueous media.     

A new method for the calculation of vibrational force constants. Application to H2

A new method for the quantum mechanical calculation of vibrational force constants is presented. This method is applied to the calculation of the vibrational force constant of H₂, using a completely optimized wavefunction constructed from a single gaussian orbital. The value of the force constant obtained using this method is k₀ = 0.422341088751 au (= 6.5754 × 10⁵ dyne/cm), compared to the value of k₀ = 0.42234079S380 au (= 6.5754 × 10⁵ dyne/cm) obtained using an analytic method, and the experimental value of k_e = 0.3692 au (= 5.748 × 10⁵ dyne/cm).     

Influences of ClH and BrH on the pyrolyses of neopentane and ethane at small extents of reaction

A symbolic mechanism “μH, YH” has been proposed to account for the homogeneous chain pyrolysis of an organic compound μH in the presence of a hydrogenated additive YH at small extents of reaction. An analysis of this mechanism leads to two limiting cases: the thermal decomposition of neopentane corresponds to the first one (A), that of ethane to the second one (B). Previous experimental work has shown that this mechanism seems to account for a number of experimental observations, especially the inhibition of alkane pyrolyses by alkenes. Experimental investigations were extended by examining the influences oftwo hydrogen halides (ClH and BrH) upon the pyrolyses of neopentane (at 480°C) and ethane (around 540°C). The experiments have been performed in a conventional static Pyrex apparatus and reaction products have been analyzed by gas-liquid chromatography. The study shows that ClH and BrH accelerate the pyrolysis of neopentane (into i-C₄H₈ + CH₄). The experimental results are interpreted by reaction schemes which appear as examples of the mechanism “μH, YH” in the first limiting case (A). The proposed schemes enable one to understand why the accelerating influence of ClH is lower or higher than that of BrH, depending on the concentration of the additive. An evaluation of the rate constant of the elementary steps neo-C₅H₁₁ · → i-C₄H₈ + CH₃ · is discussed. In the case of ethane pyrolysis, BrH inhibits the formation of the majorproducts (C₂H₄ + H₂) and, even more, that of n-butane traces. The experimental results are interpreted by a reaction scheme which appears as an example of the mechanism “μH, YH” in the second limiting case (B). On the contrary, ClH has no noticeable influence on the reaction kinetics. This result inessentially due to the fact that the bond dissociation energy of Cl? H(?103 kcal/mol) is higher than that of C₂H₅—H (?98 kcal/mol), whereas that of Br—H (?88 kcal/mol) is lower.     

Unusual reactions of N-(trifluoromethylsulfonylimino)di-and-trifluoromethanesulfinimidoyl chlorides

N-(Trifluoromethylsulfonylimino)di-and-trifluoromethanesulfinimidoyl chlorides RS(=NSO₂CF₃)Cl (R = CF₃, CHF₂) react with potassium fluoride in 1,2-dimethoxyethane with formation of the corresponding N,N′-bis(trifluoromethylsulfonyl)fluoromethanesulfinimidamide potassium salts RS(=NSO₂CF₃)NSO₂CF₃ ^?K. Analogous methanesulfinimidoyl and fluoromethanesulfinimidoyl chlorides (R = CH₃, CH₂F) fail to react with KF under similar conditions. Treatment of trifluoromethanesulfenamides CF₃SNR₂ with N,N-dichlorotrifluoromethanesulfonamide CF₃SO₂NCl₂ leads to N,N-disubstituted N′-(trifluoromethylsulfonyl)trifluoromethanesul-finimidamides CF₃S(=NSO₂CF₃)NR₂. The reaction of N,N-dimethyl-N′-(trifluoromethylsulfonyl)trifluoromethanesulfinimidamide (R = CH₃) with gaseous hydrogen chloride in diethyl ether gives sulfinimidoyl chloride CF₃S(=NSO₂CF₃)Cl which could not be obtained by imination of CF₃SCl.     

Preparation and Characteristics of Esculin‐Imprinted Polymers

Four molecularly imprinted polymers (MIPs) were prepared in MeOH with esculin (=6,7‐dihydroxycoumarin 6‐(β‐D ‐glucopyranoside)=6‐(β‐D ‐glucopyranosyloxy)‐7‐hydroxy‐2H‐1‐benzopyran‐2‐one) as the imprinted molecule, methacrylic acid (=2‐methylprop‐2‐enoic acid; MAA), acrylamide (=prop‐2‐enamide; AM), 4‐vinylpyridine (=4‐ethenylpyridine; 4‐VP), or 2‐vinylpyridine (=2‐ethenylpyridine; 2‐VP) as the functional monomer, respectively, as well as ethylene glycol dimethacrylate (=2‐methylprop‐2‐enoic acid ethane‐1,2‐diyl ester; EGDMA) as the cross‐linking agent. The interaction between the template and the functional monomers was investigated by fluorescence and UV spectrophotometry, respectively, which revealed the presence of esculin/monomer complexes in the stoichiometric ratio 1 : 2 in the pre‐polymerization mixture. The resultant polymers were studied in equilibrium binding experiments to evaluate the recognition ability and the binding capacity towards esculin. The results showed that MIP₁, prepared with MAA as the functional monomer, exhibited advantageous characteristics of high binding capacity, optimal imprinting effect, and good selectivity towards esculin. The Scatchard analysis indicated that there are two types of binding sites in MIP₁, and its binding parameters including the apparent maximum numbers of binding sites and the dissociation constants were calculated. Finally, by packing an SPE column (SPE=solid‐phase extraction) with MIP₁, the esculin was separated and enriched successfully by this sorbent from samples of Cortex fraxini, and the average recovery was up to 74.7%.     

Mono and Bisphosphonates from Perfluoro Olefins and Polyfunctional Fluoro Ketones. Syntheses,Molecular Structure and Derivatives

Perfluoroalkenyl phosphonates were formed along with Me₃SiF using CF₃CF=CF₂, CF₃CH=CF₂, F₅SCF=CF₂ or F₅SCH=CF₂ and silylated phosphites, (R¹O)₂POSiMe₃ (R¹=Et, SiMe₃). This straightforward method could be extended to perfluorobutadienes CF₂=C(R^F)C(R^F)=CF₂ (R^F F=F, CF₃). The formation of CF₃C(=O)P(=O)(OSiMe₃)₂ and further reactions to yield bisphosphonates will be described. Acetylphosphonates, R²C(=O)P(=O)(OSiMe₃)₂ (R²=CH₃, CF₃) reacted with the ketimine, CH₃C(=NiPr)Ph to give α-hydroxy-γ-imino phosphonates. Trifluoroacetylphenol and 2,6-bis(trifluoracetyl)-4-methyl-phenol have been proven to be versatile precursors for α-and γ-hydroxy phosphonates. Intermediates in these reactions were found to be cyclic λ⁵σ⁵P species.     

The double H‐atom acceptability of the P=O group in new XP(O)(NHCH2C6H4‐2‐Cl)2 phosphoramidates [X = C6H5O– and CF3C(O)NH–]: a database analysis of compounds having a P(O)(NHR) group

In the hydrogen‐bond patterns of phenyl bis(2‐chlorobenzylamido)phosphinate, C₂₀H₁₉Cl₂N₂O₂P, (I), and N,N′‐bis(2‐chlorobenzyl)‐N′′‐(2,2,2‐trifluoroacetyl)phosphoric triamide, C₁₆H₁₅Cl₂F₃N₃O₂P, (II), the O atoms of the related phosphoryl groups act as double H‐atom acceptors, so that the P=O...(H—N)₂ hydrogen bond in (I) and the P=O...(H—N_amide)₂ and C=O...H—N_C(O)NHP(O) hydrogen bonds in (II) are responsible for the aggregation of the molecules in the crystal packing. The presence of a double H‐atom acceptor centre is a result of the involvement of a greater number of H‐atom donor sites with a smaller number of H‐atom acceptor sites in the hydrogen‐bonding interactions. This article also reviews structures having a P(O)NH group, with the aim of finding similar three‐centre hydrogen bonds in the packing of phosphoramidate compounds. This analysis shows that the factors affecting the preference of the above‐mentioned O atom to act as a double H‐atom acceptor are: (i) a higher number of H‐atom donor sites relative to H‐atom acceptor centres in molecules with P(=O)(NH)₃, (N)P(=O)(NH)₂, C(=O)NHP(=O)(NH)₂ and (NH)₂P(=O)OP(=O)(NH)₂ groups, and (ii) the remarkable H‐atom acceptability of this atom relative to the other acceptor centre(s) in molecules containing an OP(=O)(NH)₂ group, with the explanation that the N atom bound to the P atom in almost all of the structures found does not take part in hydrogen bonding as an acceptor. Moreover, the differences in the H‐atom acceptability of the phosphoryl O atom relative to the O atom of the alkoxy or phenoxy groups in amidophosphoric acid esters may be illustrated by considering the molecular packing of compounds having (O)₂P(=O)(NH) and (O)P(=O)(NH)(N)groups, in which the unique N—H unit in the above‐mentioned molecules almost always selects the phosphoryl O atom as a partner in forming hydrogen‐bond interactions. The P atoms in (I) and (II) are in tetrahedral coordination environments, and the phosphoryl and carbonyl groups in (II) are anti with respect to each other (the P and C groups are separated by one N atom). In the crystal structures of (I) and (II), adjacent molecules are linked via the above‐mentioned hydrogen bonds into a linear arrangement parallel to [100] in both cases, in (I) by forming R₂²(8) rings and in (II) through a combination of R₂²(10) and R₂¹(6) rings.     

Quantum chemical calculation of the molecular structures of (666)macrotricyclic chelates of 3D elements in the M(II)-propanedithioamide-formaldehyde systems by the density functional theory method

The nonhybrid OPBE/TZVP density functional theory (DFT) method and the Gaussian09 program package were used to calculate the thermodynamic and geometric parameters of asymmetric macrocyclic M(II) complexes with three six-membered metal rings and (NNNN)-coordination of the donor sites of the ligand. The complexes are formed upon self-assembly (template synthesis) of hexacyanoferrates(II) of the corresponding M(II), propanedithioamide H₂N-C(=S)-CH₂-C(=S)-NH₂, and formaldehyde H₂C(=O) in gelatin-immobilized matrix implants. Note that complexes of this type are formed only for M = Ni, Cu, and Zn, while for M = Mn, Co, and Fe, these compounds are unstable. Bond lengths and bond and torsion angles are presented. In each of these complexes, both the MN₄ chelate units and the N₄ units and all sixmembered metal rings were found to be non-coplanar.