The isokinetic relationship. VI. Equilibrium systems

The isoequilibrium relationship (IER), i.e. the occurrence of a common point of intersection of the van 't Hoff plots of a homologous reaction series, is investigated on the basis of a master equation theory describing forward and reverse reaction rates. The relation between the isoequilibrium temperature and the isokinetic temperatures of forward and reverse reactions is deduced theoretically and compared with experimental results.     

Theoretical study of the pyrolysis of methyltrichlorosilane in the gas phase. 1. Thermodynamics

Structures and energies of the gas-phase species produced during and after the various unimolecular decomposition reactions of methyltrichlorosilane (MTS) with the presence of H2 carrier gas were determined using second-order perturbation theory (MP2). Single point energies were obtained using singles + doubles coupled cluster theory, augmented by perturbative triples, CCSD(T). Partition functions were obtained using the harmonic oscillator-rigid rotor approximation. A 114-reaction mechanism is proposed to account for the gas-phase chemistry of MTS decompositions. Reaction enthalpies, entropies, and Gibbs free energies for these reactions were obtained at 11 temperatures ranging from 0 to 2000 K including room temperature and typical chemical vapor deposition (CVD) temperatures. Calculated and experimental thermodynamic properties such as heat capacities and entropies of various species and reaction enthalpies are compared, and theory is found to provide good agreement with experiment.     

Ab initio study of aluminum chemical vapor deposition from dimethylaluminum hydride: a gas phase reaction mechanism

The effect of preheating of dimethylaluminum hydride (DMAH) as a gas on the epitaxial growth in aluminum chemical vapor deposition (Al-CVD) is studied theoretically. The chemical changes of DMAH in the gas phase such as unimolecular decomposition reactions, bimolecular reactions and polymerizations are treated using ab initio molecular orbital method (MP2/6-31G**) and density functional theory (B3P86/LanL2DZ). The gas phase equilibrium composed of the previous reaction products under the usual experimental conditions for Al-CVD is also investigated in detail as the initial stage of the CVD process. From the energetics point of view, unimolecular decomposition reactions and bimolecular reactions hardly occur, however, polymerizations of DMAH take place readily at the low temperatures found in Al-CVD. A large amount of DMAH-dimer and a small amount of DMAH-monomer and trimer coexist in the equilibrium state.     

Ab initio kinetics of gas phase decomposition reactions

The thermal and kinetic aspects of gas phase decomposition reactions can be extremely complex due to a large number of parameters, a variety of possible intermediates, and an overlap in thermal decomposition traces. The experimental determination of the activation energies is particularly difficult when several possible reaction pathways coexist in the thermal decomposition. Ab initio calculations intended to provide an interpretation of the experiment are often of little help if they produce only the activation barriers and ignore the kinetics of the decomposition process. To overcome this ambiguity, a theoretical study of a complete picture of gas phase thermo-decomposition, including reaction energies, activation barriers, and reaction rates, is illustrated with the example of the β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) molecule by means of quantum-chemical calculations. We study three types of major decomposition reactions characteristic of nitramines: the HONO elimination, the NONO rearrangement, and the N-NO(2) homolysis. The reaction rates were determined using the conventional transition state theory for the HONO and NONO decompositions and the variational transition state theory for the N-NO(2) homolysis. Our calculations show that the HMX decomposition process is more complex than it was previously believed to be and is defined by a combination of reactions at any given temperature. At all temperatures, the direct N-NO(2) homolysis prevails with the activation barrier at 38.1 kcal/mol. The nitro-nitrite isomerization and the HONO elimination, with the activation barriers at 46.3 and 39.4 kcal/mol, respectively, are slow reactions at all temperatures. The obtained conclusions provide a consistent interpretation for the reported experimental data.     

The quaternisation reaction of phosphines and amines in aliphatic alcohols. A similarity analysis using the isokinetic, isosolvent and isoselective relationships

Accurate second-order rate constants were measured at 5 K intervals in the temperature range 298.15-328.15 K for the quaternisation reaction of triethylphosphine with iodoethane in methanol, ethanol, propan-1-ol and butan-1-ol. These data are complemented previously reported rate constants for the quaternisation reaction of triethylamine with iodoethane in the same solvents and at similar temperatures. Each of these two reaction series is analysed in terms of the isokinetic relationship (IKR) with respect to solvent variation and of the isosolvent relationship (ISoR) with respect to temperature variation, using in the latter case five different empirical solvent scales. Statistically validated IKR and ISoR have been found for both reaction series. The resulting isokinetic temperatures of 347 K (phosphine series) and of 730 K (amine series) are discussed in terms of Linert's theory of the isokinetic relationship. The best ISoR correlation is obtained using the Dimroth-Reichardt E(T)(N) solvent scale for the phosphine series and the Kamlet-Taft alpha(KT) solvent scale for the amine series. It is demonstrated that no real solvent can be envisaged as having the characteristics of an isokinetic solvent. The selectivity of the nucleophiles triethylphosphine and triethylamine in the attack on iodoethane is examined by treating together both reaction series in terms of the isoselective relationship (ISeR). The isoselective temperature with respect to solvent is found to be 289 K, which is close to the value of 302 K predicted by Exner and Giese's formula on the basis of the individual isokinetic temperatures. A novel ISeR analysis with respect to temperature is performed. It reveals that the alpha(KT) scale is the most appropriate solvent scale for describing this selectivity series, and that it is feasible to find an isoselective solvent. A new equation is developed for predicting the isoselective solvent parameter from individual isosolvent parameters and is shown to yield realistic values. The present similarity analysis shows that there are significant differences between the courses of these quaternisation reactions. On the basis of the experimentally determined isoparameter values, in liquid alcohols as solvent it is proposed that the reaction between triethylphosphine and iodoethane follows a classic bimolecular nucleophilic substitution pathway, whereas the desolvation of triethylamine molecules has to be taken into account to describe the mechanism of the original Menshutkin reaction.     

Formation of Three-Membered Rings by S(H)i Displacement. Reverse of Cyclopropyl Ring Opening(1)

The general methods, photoinitiated or peroxide-initiated free radical chain additions of halomethanes to olefins, yield 1,2-addition products at temperatures ranging from 20 to 100 degrees C. At lower temperatures, -42 to -104 degrees C, a competitive reaction, subsequent to the addition of CCl(2)X(*), yields alkylcyclopropanes. The reactions of 1-octene or 1-hexene and 1-methylcyclohexene with atomic hydrogen carried out in the presence of several transfer agents (CCl(4), CCl(3)Br, CCl(2)Br(2)) initiate a radical chain addition of CCl(2)X(*) and yield cyclized materials resulting from the S(H)i displacement of halogen by a carbon-centered radical. The radical displacement of a halogen on carbon, the reverse of homolytic displacement on cyclopropyl carbon, is dominant at low temperatures. The rate constants for cyclization (k(c)) vs transfer with halomethane (k(t)) showed isokinetic temperatures of -46 degrees C (CCl(4), 1-hexene); -35 degrees C (CCl(4), 1-methylcyclohexene). The isokinetic temperatures for the reactions of the two substrates carried out in the presence of BrCCl(3) were calculated as -204 degrees C (1-octene) and -109 degrees C (1-methylcyclohexene).     

Microwave-assisted asymmetric organocatalysis. A probe for nonthermal microwave effects and the concept of simultaneous cooling

A series of five known asymmetric organocatalytic reactions was re-evaluated at elevated temperatures applying both microwave dielectric heating and conventional thermal heating in order to probe the existence of specific or nonthermal microwave effects. All transformations were conducted in a dedicated reactor setup that allowed accurate internal reaction temperature measurements using fiber-optic probes. In addition, the concept of simultaneous external cooling while irradiating with microwave power was also applied in all of the studied cases. This method allows a higher level of microwave power to be administered to the reaction mixture and, therefore, enhances any potential microwave effects while continuously removing heat. For all of the five studied (S)-proline-catalyzed asymmetric Mannich- and aldol-type reactions, the observed rate enhancements were a consequence of the increased temperatures attained by microwave dielectric heating and were not related to the presence of the microwave field. In all cases, in contrast to previous literature reports, the results obtained either with microwave irradiation or with microwave irradiation with simultaneous cooling could be reproduced by conventional heating at the same reaction temperature and time in an oil bath. No evidence for specific or nonthermal microwave effects was obtained.     

Thermodynamics and kinetics of initial gas phase reactions in chemical vapor deposition of titanium nitride. Theoretical study of TiCl4 ammonolysis

Thermodynamic equilibrium and kinetics of the gas‐phase reaction between TiCl₄ and NH₃ have been studied computationally using results from recent quantum mechanical calculations of titanium tetrachloride ammonolysis. 1 These calculations were based upon the transition state theory for the direct reactions and RRKM theory for the reactions proceeding via intermediate complex. Rate constants for the barrierless reactions were expressed through the thermodynamic characteristics of the reagents and products using a semiempirical variational method. The kinetic simulation of the gas‐phase steps of CVD was performed within a model of a well‐stirred reactor at temperatures 300–1200 K and residence times between 0.1–2 s. At temperatures below 450 K formation of donor–acceptor complexes between TiCl₄ and NH₃ is the dominating process. At higher temperatures sequential direct ammonolysis takes place. At typical LPCVD conditions the only product of the first step of ammonolysis, TiCl₃NH₂, is formed in substantial amount. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1366–1376, 2001     

Time-resolved,Gas-phase Kinetic and Quantum Chemical Studies of the Reaction of Germylene with Hydrogen Chloride

Time-resolved studies of germylene, GeH₂, generated by laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene at 193 nm and monitored by laser absorption, have been carried out to obtain rate constants for its bimolecular reaction with HCl. The reaction was studied in the gas phase, mainly at a total pressure of 10 Torr (in SF₆ bath gas) at five temperatures in the range 295–558 K. Experiments at other pressures showed that these rate constants were unaffected by pressure. The second-order rate constants at 10 Torr (SF₆ bath gas) fitted the Arrhenius equation: log(k/cm³ molecule⁻¹ s⁻¹)=(−12.06±0.14)+(2.58±1.03 kJ mol⁻¹)/RTln10 where the uncertainties are single standard deviations. Quantum chemical calculations at G4 level support a mechanism in which an initial weakly bound donor-acceptor complex is formed. This can then rearrange and decompose to give H₂ and HGeCl (chlorogermylene). The enthalpy barrier (36 kJ mol⁻¹) is too high to allow rearrangement of the complex to GeH₃Cl (chlorogermane).     

Direct time-resolved study of the gas-phase reactions of germylene with ethyl- and diethylgermane: absolute rate constants, temperature dependences, and mechanism

Time-resolved studies of germylene, GeH2, generated by the 193 nm laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reactions with ethyl- and diethylgermanes in the gas phase. The reactions were studied over the pressure range 1-100 Torr with SF6 as bath gas and at five temperatures in the range 297-564 K. Only slight pressure dependences were found for GeH2 + EtGeH3 (399, 486, and 564 K). The high pressure rate constants gave the following Arrhenius parameters: for GeH2 + EtGeH3, log A = -10.75 +/- 0.08 and Ea = -6.7 +/- 0.6 kJ mol-1; for GeH2 + Et2GeH2, log A = -10.68 +/- 0.11 and Ea = -6.95 +/- 0.80 kJ mol-1. These are consistent with fast, near collision-controlled, association processes at 298 K. RRKM modeling calculations are, for the most part, consistent with the observed pressure dependence of GeH2 + EtGeH3. The ethyl substituent effects have been extracted from these results and are much larger than the analogous methyl substituent effects in the SiH2 + methylsilane reaction series. This is consistent with a mechanistic model for Ge-H insertion in which the intermediate complex has a sizable secondary barrier to rearrangement.     

A unified perspective on the hydrogen atom transfer and proton-coupled electron transfer mechanisms in terms of topographic features of the ground and excited potential energy surfaces as exemplified by the reaction between phenol and radicals

The relation between the hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET) mechanisms is discussed and is illustrated by multiconfigurational electronic structure calculations on the ArOH + R(*) --> ArO(*) + RH reactions. The key topographic features of the Born-Oppenheimer potential energy surfaces that determine the predominant reaction mechanism are the conical intersection seam of the two lowest states and reaction saddle points located on the shoulders of this seam. The saddle point corresponds to a crossing of two interacting valence bond states corresponding to the reactant and product bonding patterns, and the conical intersection corresponds to the noninteracting intersection of the same two diabatic states. The locations of mechanistically relevant conical intersection structures and relevant saddle point structures are presented for the reactions between phenol and the N- and O-centered radicals, (*)NH2 and (*)OOCH3. Points on the conical intersection of the ground doublet D0 and first excited doublet D1 states are found to be in close geometric and energetic proximity to the reaction saddle points. In such systems, either the HAT mechanism or both the HAT mechanism and the proton-coupled electron transfer (PCET) mechanism can take place, depending on the relative energetic accessibility of the reaction saddle points and the D0/D1 conical intersection seams. The discussion shows how the two mechanisms are related and how they blend into each other along intermediate reaction paths. The recognition that the saddle point governing the HAT mechanism is on the shoulder of the conical intersection governing the PCET mechanism is used to provide a unified view of the competition between the two mechanisms (and the blending of the two mechanisms) in terms of the prominent and connected features of the potential energy surface, namely the saddle point and the conical intersection. The character of the dual mechanism may be understood in terms of the dominant valence bond configurations of the intersecting states, which are zero-order approximations to the diabatic states.     

Molecular decomposition of polyolefins: Kinetic parameters and geometry of the transition state

The experimental data on the molecular decomposition of olefins ( 1 -pentens) of various structures to two olefins in a gas phase were analyzed by means of the parabola intersection method. The enthalpies and kinetic parameters characterizing such decomposition were calculated for eighteen reactions. Decomposition of olefins representing two-centered concerted re action was found to be characterized by a very high classic potential barrier of thermoneutral reaction (197.4 kJ/mol). The kinetic parameters (activation energy and velocity constant) of fifteen reverse reactions of formation of olefins from two alkenes were calculated using the parabola-intersection method. The factors affect ing the activation energy of the reactions of olefin decomposition and formation are discussed. Quantum chemical calculations of the transition state energy and geometry for six reactions of olefin decomposition were performed.     

Efficient surface formation route of interstellar hydroxylamine through NO hydrogenation. II. The multilayer regime in interstellar relevant ices

Hydroxylamine (NH(2)OH) is one of the potential precursors of complex pre-biotic species in space. Here, we present a detailed experimental study of hydroxylamine formation through nitric oxide (NO) surface hydrogenation for astronomically relevant conditions. The aim of this work is to investigate hydroxylamine formation efficiencies in polar (water-rich) and non-polar (carbon monoxide-rich) interstellar ice analogues. A complex reaction network involving both final (N(2)O, NH(2)OH) and intermediate (HNO, NH(2)O[middle dot], etc.) products is discussed. The main conclusion is that hydroxyl-amine formation takes place via a fast and barrierless mechanism and it is found to be even more abundantly formed in a water-rich environment at lower temperatures. In parallel, we experimentally verify the non-formation of hydroxylamine upon UV photolysis of NO ice at cryogenic temperatures as well as the non-detection of NC- and NCO-bond bearing species after UV processing of NO in carbon monoxide-rich ices. Our results are implemented into an astrochemical reaction model, which shows that NH(2)OH is abundant in the solid phase under dark molecular cloud conditions. Once NH(2)OH desorbs from the ice grains, it becomes available to form more complex species (e.g., glycine and β-alanine) in gas phase reaction schemes.     

Gas phase kinetic and quantum chemical studies of the reactions of silylene with the methylsilanes. Absolute rate constants, temperature dependences, RRKM modelling and potential energy surfaces

Time resolved studies of silylene, SiH2, generated by the 193 nm laser flash photolysis of phenylsilane, have been carried out to obtain rate coefficients for its bimolecular reactions with methyl-, dimethyl- and trimethyl-silanes in the gas phase. The reactions were studied over the pressure range 3-100 Torr with SF6 as bath gas and at five temperatures in the range 300-625 K. Only slight pressure dependences were found for SiH2+MeSiH3(485 and 602 K) and for SiH2+Me2SiH2(600 K). The high pressure rate constants gave the following Arrhenius parameters: [TABLE: SEE TEXT]. These are consistent with fast, near to collision-controlled, association processes. RRKM modelling calculations are consistent with the observed pressure dependences (and also the lack of them for SiH2+Me3SiH). Ab initio calculations at both second order perturbation theory (MP2) and coupled cluster (CCSD(T)) levels, showed the presence of weakly-bound complexes along the reaction pathways. In the case of SiH2+MeSiH3 two complexes, with different geometries, were obtained consistent with earlier studies of SiH2+SiH4. These complexes were stabilised by methyl substitution in the substrate silane, but all had exceedingly low barriers to rearrangement to product disilanes. Although methyl groups in the substrate silane enhance the intrinsic SiH2 insertion rates, it is doubtful whether the intermediate complexes have a significant effect on the kinetics. A further calculation on the reaction MeSiH+SiH4 shows that the methyl substitution in the silylene should have a much more significant kinetic effect (as observed in other studies).     

Comparative study between gas phase and liquid phase for the production of DMC from methanol and CO2

Direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide over Co1.5PW12O40 in liquid and in gas phase is investigated. The synthesized catalyst has been characterized by means of FTIR and XRD. Liquid phase experiment results showed that high pressures are favorable for the synthesis of DMC. However, DMC formation is limited by the reaction with co-produced water. DMC selectivity is more strongly dependent on the temperature than on the pressure of CO2. As for the reactions in gas phase, it has been found that both CH3OH conversion and DMC selectivity decreased with increasing temperature, owing to the decomposition of DMC at high temperatures. High temperatures and more amount of Co1.5PW12O40 catalyst favor the formation of dimethoxymethane (DMM) and methyl formate (MF).     

甲醇POSR制氢的反应网络热力学分析和有效因子的估算

在Cu/ZnO/Al2O3催化剂上对甲醇部分氧化蒸汽重整制备氢气反应的动力学过程进行了研究。在常压和473 K～1 073 K温度范围内对该反应网络中的甲醇部分氧化、甲醇蒸汽重整、甲醇分解和水煤气反应的化学平衡进行了分析。在对这些反应的催化剂Cu/ZnO/Al2O3动力学研究的基础上，根据有效因子的基本概念，考虑催化剂颗粒内的扩散限制，对每个反应沿反应器床层的有效因子进行了估算。     

Gas-phase activation and reaction dynamics of chiral ion-dipole complexes

A family of enantiomerically pure oxonium ions, that is O-protonated 1-aryl-1-methoxyethanes, has been generated in the gas phase by the (CH(3))(2)Cl(+) methylation of the corresponding 1-arylethanols. Some information on their reaction dynamics was obtained from a detailed kinetic study of their inversion of configuration and dissociation. The activation parameters of the inversion reaction are found to obey two different isokinetic relationships depending upon the nature and the position of the substituents in the oxonium ions. In contrast, the activation parameters of the dissociation reaction obey a single isokinetic relationship. The inversion and dissociation rate constants do not follow simple linear free-energy relationships. This complicated kinetic picture has been rationalized in terms of different activation dynamics in gaseous CH(3)Cl, which, in turn, determine the reaction dynamics of the oxonium ion. When the predominant activation of the oxonium ion involves resonant energy exchange from the 1015 cm(-1) CH(3) rocking mode of unperturbed CH(3)Cl, the inversion reaction proceeds through the dynamically most favored TS, characterized by the unassisted C(alpha)bond;O bond elongation. When, instead, the activation of the oxonium ions requires the formation of an intimate encounter complex with CH(3)Cl, the inversion reaction takes place via the energetically most favored TS, characterized by multiple coordination of the CH(3)OH moiety with the H(alpha) and H(ortho) atoms of the benzylic residue. The activation dynamics operating in the intimate encounter complex with CH(3)Cl is also responsible for the dissociation of most selected oxonium ions.     

Non-isothermal two-membrane reactors for reversible gas phase reactions

The performance of a non-isothermal two-membrane reactor for reversible chemical reactions in gas phase has been analyzed by numerical simulation. The analyzed reactions were of the form: aA = bB + cC. Two membranes, that are permeable to all the components of the reaction mixture, are supposed to be the most permeable to one of the two reaction products, satisfying the condition of reverse products permselectivities. The reactant is taken to be the slowest permeating component. A negative temperature influence on the permeabilities of components has been assumed. Co-current plug flow pattern has been accepted. It has been shown that it is possible to enhance reactant conversion above that of a conventional reactor for both endothermic and exothermic reversible reactions, including adiabatic and non-adiabatic case. By using a two-membrane reactor, considerable lowering of feed temperatures is enabled for an endothermic reaction. For endothermic reactions, there is the optimum feed temperature, whereas for exothermic reactions, the higher the temperature, the lower is the attained conversion. In reactor design, the optimal external heat exchange for both endothermic and exothermic reactions can be determinated.     

Temperature dependence of oxygen isotope acid fractionation for modern and fossil tooth enamels

The oxygen isotope ratio of CO(2) liberated from structural carbonate in tooth enamel apatite was measured at phosphoric acid reaction temperatures of 25 degrees C, 60 degrees C and 90 degrees C, and it was found that apparent acid fractionation factors for pristine enamel, fossilized enamel, and calcite follow different temperature relationships. Using sealed vessel reactions normalized to alpha(25) = 1.01025 (the fractionation factor for calcite at 25 degrees C), the apparent fractionation factor at 90 degrees C (alpha*(90)) for pristine enamel ranged between 1.00771 and 1.00820, and between 1.00695 and 1.00772 for fossilized enamel. Apparent fractionation factors for common acid bath reactions are similar to those for sealed vessel reactions. A significant correlation exists between alpha*(90) and F(-) content, suggesting that change in the acid fractionation factor may be related to the replacement of OH(-) with F(-) during fossilization of bioapatite. These results have important implications for making accurate comparisons between modern and fossil tooth enamel delta(18)O values, and for the uniformity of isotope data produced in different laboratories using different acid reaction temperatures.     

23Na, 29Si, and 13C MAS NMR investigation of glass-forming reactions between Na2CO3 and SiO2

The glass-forming reactions between sodium carbonate (Na2CO3) and silica (SiO2) have been investigated by 23Na, 29Si, and 13C magic-angle spinning (MAS) NMR spectroscopy. The multinuclear MAS NMR approach identifies and quantifies reaction products and intermediates, both glassy and crystalline. A series of powdered batches of initial composition Na2CO3.xSiO2 (x = 1, 2) corresponding to a sodium metasilicate (Na2SiO3) and sodium disilicate (Na2Si2O5) stoichiometry were investigated after periods of isothermal and nonisothermal heat treatments at different temperatures. Analysis of the 23Na quadrupolar coupling parameters has identified the early reaction product in all cases as crystalline Na2SiO3. In the nonisothermal experiment, this reaction is preceded by an early silica-rich melt phase formed around 850 degrees C. The early reactions are controlled by solid-state Na+ diffusion across the reaction zone in the grain interface layer. Crystalline Na2SiO3 precipitates in the interface layer, increasing its thickness between the Na2CO3 and the SiO2 grains and slowing down the rate of Na+ migration. This creates a secondary phase, which is temperature dependent. At low temperatures, where Na+ migration is impaired, the production of Na2SiO3 ceases and silica-richer phases are precipitated. In the case of the sodium disilicate batch, where excess SiO2 is present, a secondary reaction of Na2SiO3 with SiO2 forming a glassy phase is observed. A transient carbon-bearing phase has been identified by 13C NMR as a NaCO3- complex loosely bound to bridging oxygens in the silicate network at the SiO2 grain surface.