Statisticodynamical approach of state distributions in the products of four-atom planar unimolecular reactions. II. Validation and distribution shape analysis in the barrier case

In the first part of this series, we proposed a statisticodynamical approach of state distributions in the products of four-atom planar unimolecular reactions governed by short-range forces. In this second part, the approach is tested against quasiclassical trajectory calculations on an ab initio potential energy surface. The process considered is the fragmentation of isocyanic acid in the first excited singlet electronic state. The study leads to a very good agreement between both methods. In addition to that, we pinpoint in the barrier case the main mechanical parameters governing the shape of rotational state distributions. It appears that these parameters are related to two distinct physical effects. The first one is of the impulsive type. The second, already observed in triatomic processes, is the so-called bending effect.     

Numerical study of the kinetics of unimolecular heterogeneous reactions onto planar surfaces

In this paper we investigate two-dimensional in space mathematical models of the kinetics of unimolecular heterogeneous reactions proceeding onto planar surfaces. The models are based on Langmuir-type kinetics of the adsorption, desorption, and reaction including the surface diffusion of the adsorbate, surface diffusion of the product before its desorption, and slow desorption of the product from the adsorbent. It is also assumed that the reactant diffuses towards an adsorbent from a bounded vessel and the product diffuses from the adsorbent into the same vessel. Diffusivity of all species and kinetic coefficients are constants. The numerical simulation was carried out using the finite difference technique for four models: one model neglects the surface diffusion of the adsorbate and product, the second one includes the surface diffusion of the adsorbate and product, the third of them includes the surface diffusion of the adsorbate and neglects diffusion of the product along the surface, and the last one neglects the surface diffusion of the adsorbate and includes diffusion of the product along the adsorbent. By changing input parameters effects of the surface diffusion of the adsorbate and product and the slow desorption of the product are studied numerically.     

Solution of the master equation for unimolecular reactions

A method is given for computing the rate coefficient of a unimolecular reaction as an eigenvalue solution of an integral master equation, based on Nesbet's algorithm, which overcomes computational difficulties associated with this problem. An illustrative fit to pressure-dependent data on the pyrolysis of azoethane is presented.     

Models for unimolecular reactions in the solid phase and stability prediction of energetic compounds in the condensed state

Russian Chemical Bulletin - The results of analysis and refinement of phenomenological models for unimolecular reactions proceeding homogeneously in the bulk of undistorted crystal lattice or...     

A fitting formula for the falloff curves of unimolecular reactions

A fitting formula is proposed to approximate the falloff curves of the pressure‐ and temperature‐dependent unimolecular reaction rate constants. Compared with the widely used Troe's formula, the present expression has the potential to substantially reduce the computation time in its evaluation because of the mathematical simplicity. Four testing reactions from the VariFlex program package were used to examine the accuracy of the present formula, showing improved performance as compared with previous expressions. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 727–734, 2009     

State-resolved unimolecular dissociation of cis-cis HOONO: Product state distributions and action spectrum in the 2nuOH band region

Nascent OH fragment product state distributions arising from unimolecular dissociation of room temperature HOONO, initiated by excitation in the region of the 2nu(OH) band, are probed using laser-induced fluorescence at sub-Doppler resolution. Phase-space simulations of the measured OH rotational distributions are consistent with the dissociation dynamics being statistical and confirm that all major features in the room temperature action spectrum belong to the cis-cis conformer. The phase-space simulations also allow us to estimate the HO-ONO bond dissociation energy of cis-cis HOONO to be D(0)=19.9+/-0.5 kcal/mol, which when combined with the known heat-of-formation data for the OH and NO(2) fragments gives DeltaH(f) (0)(cis-cis HOONO)=-2.5 kcal/mol. In addition to fragment energy release, spectral features in the cis-cis HOONO action spectrum are examined with respect to their shifts upon (15)N isotope substitution and through ab initio spectral simulation using a two-dimensional dipole surface that takes into account the influence of HOON torsional motion on the OH stretching overtone. The two-dimensional spectral simulations, using CCSD(T)/cc-pVTZ dipole surface, qualitatively reproduces features appearing in the action spectrum and suggest that the strong broad feature occurring approximately 570 cm(-1) to the blue of the cis-cis HOONO 2nu(OH) peak, likely involve excitation of HOON-torsion/OH-stretch combination bands originating from thermally populated excited torsional states. A closer examination of the predictions of the two-dimensional model with experiments also reveals its limitations and suggests that a more elaborate treatment, one which includes several additional modes, will likely be required in order to fully explain the room temperature action spectrum. Ab initio calculations of the HOON torsional potential at the CCSD(T)/cc-pVTZ level of theory are also presented and confirm that cis-perp configuration does not correspond to a bound localized minimum on the HOONO potential energy surface.     

单分子分解成两个自由基反应的半经典统计方法I. 关于CH4分解成CH3.和H.自由基的研究

本文计算了温度范围在300-1500K, CH4→CH3.+H.的分角反应中, 过渡态CH3...H的平均键长、频率因子、活化能及反应速率常数, 计算结果和实验结果符合得很好。     

Thermochemistry of thermal plasma chemical reactions. Part I. General rules for the prediction of products

Equilibrium calculations based on the standard technique of minimization of the Gibbs free energy, with consideration of both gas and condensed phases, are shown to be inadequate for predicting the yield or even the proper composition of the products from thermal plasma reaction systems. This is due to the dominating influence of nucleation kinetics, a nonequilibrium effect.In this paper a modification of the equilibrium approach is proposed, whereby the supersaturation of a phase which may condense is calculated, and species with low supersaturation pressures which are unlikely to precipitate are subsequently removed from consideration.A comparison is made between the former equilibrium predictions and these quasi-equilibrium predictions. When compared with experimental data taken from the extant literature and from the authors' own research, the quasi-equilibrium modification is seen to provide excellent agreement with respect to product composition and yield. Examples are discussed including the thermal plasma production of hydrogen cyanide, ammonia, acetylene, silicon carbide, silicon nitride, and titanium carbide.     

The orientation-averaged rotation-decoupled vibration model for product state distributions in light+heavy-heavy reactions

A new 1D-to-3D transformation scheme for product state distributions applicable to light+heavy-heavy reactions is developed making use of specific dynamical and kinematical properties of such reactions. For the H(D) + F₂ and H(D) + Cl₂ reactions, its results are in full accord with 3D trajectory calculations obtained on the same potential surfaces that were used to generate the input to the 1D-to-3D transformation.     

Emulsion polymerizations. I. A novel approach to the evaluation of the molecular weight distributions

A simple, probabilistic approach describing the number distribution of the molecular weight (MWD) of polymers formed by emulsion polymerization is presented. The radical populations of the reacting system are formally split into infinite, quickly convergent series of particularly tagged radical populations. These populations are characterized by the nature and the number of the state transitions of the associated latex particles. For each kind of single radical or radical pairs a distribution function is then defined. These functions can be specified on the basis of α_i and Q_i, the growth probabilities, within a latex particle in state i, of a single radical and a radical pair respectively. The overall MWDs are then given by summation over all the distributions of the radical populations and over all the allowed states. Only one set of ordinary differential equations (the Smith–Ewart equations) are involved in the mathematical formulation, single distribution functions being obtained by solving simple exponential-type integrals. In this paper, analytic solutions are presented for the zero-one, zero-one-two and general systems. Analogies and differences between our approach and previously reported treatments are critically discussed.     

Laser induced photodissociation of Hn3 at 266 nm. I. Primary products,photofragment energy distributions and reactions of intermediates

A detailed analysis of the primary photodissociation products resulting from the 266 nm laser photolysis of HN₃ is reported. The major primary fragments are N₂(¹Σ_g⁺) and NH(¹Δ). The NH(¹Δ) fragment is formed ? 99.8% in the ν = 0 level with ≈ 900 cm^?1 of rotational energy and ? 5000 cm^?1 of translational energy for the axially scattered fragments. A new chemiluminescent reaction is reported: NH(¹Δ) + HN₃ (¹A′) → NH ₂(²A₁) + N₃(²Π_g), which appears to be a major reaction channel of the primary NH(¹Δ) fragment. A kinetic analysis of this reaction and several other NH(¹Δ) reactions are the subject of the following associated paper. A correlation study of the NH(¹Δ) and N₂(¹Σ_g⁺) products with the dissociating states of HN₃ is made which requires a reassignment of the lower-lying HN₃ transitions.     

H2ClCS单分子分解反应机理的理论研究

用量子化学B3LYP/6 - 311+G(d,p)方法优化了H2ClCS单分子分解反应驻点物种的几何构型,并在相同水平上通过频率计算和内禀反应坐标(IRC)分析对过渡态结构及连接性进行了验证.用QCISD(T)/6-311++G(d,p)方法计算各物种的单点能,并对总能量进行了零点能校正.利用经典过渡态理论(TST)与...     

Theory of multichannel thermal unimolecular reactions. 2. Application to the thermal dissociation of formaldehyde

The thermal dissociation of formaldehyde proceeds on three channels, the molecular-elimination channel H2CO --> H2 + CO (1), the radical-forming bond-fission channel H2CO --> H + HCO (2), and the bond-fission-initiated, intramolecular-hydrogen-abstraction channel H2CO --> H...HCO --> H2 + CO (3) which also forms molecular products. The kinetics of this system in the low-pressure range of the unimolecular reaction is shown to be governed by a subtle superposition of collisional channel coupling to be treated by solving a master equation, of rotational channel switching accessible through ab initio calculations of the potential as well as spectroscopic and photophysical determinations of the threshold energies and channel branching above the threshold energy for radical formation which can be characterized through formaldehyde photolysis quantum yields as well as classical trajectory calculations. On the basis of the available information, the rate coefficients for the formation of molecular and radical fragments are analyzed and extrapolated over wide ranges of conditions. The modeled rate coefficients in the low-pressure range of the reaction (neglecting tunneling) over the range 1400-3200 K in the bath-gas Ar in this way are represented by k0,Mol/[Ar] approximately 9.4 x 10(-9) exp(-33,140 K/T) cm3 molecule(-1) s(-1) and k0,Rad/[Ar] approximately 6.2 x 10(-9) exp(-36,980 K/T) cm3 molecule(-1) s(-1). The corresponding values for the bath-gas Kr, on which the analysis relies in particular, are k0,Mol/[Kr] approximately 7.7 x 10(-9) exp(-33,110 K/T) and k0,Rad/[Kr] approximately 4.1 x 10(-9) exp(-36 910 K/T) cm3 molecule(-1) s(-1). While the threshold energy E0,2 for channels 2 and 3 is taken from spectroscopic measurements, the threshold energy E0,1 for channel 1 is fitted on the basis of experimental ratios k0,Rad/k0,Mol in combination with photolysis quantum yields. The derived value of E0,1(1) = 81.2 (+/-0.9) kcal mol(-1) is in good agreement with results from recent ab initio calculations, 81.9 (+/-0.3) kcal mol(-1), but is higher than earlier results derived from photophysical experiments, 79.2 (+/-0.8) kcal mol(-1). Rate coefficients for the high-pressure limit of the reaction are also modeled. The results of the present work markedly depend on the branching ratio between channels 2 and 3. Expressions of this branching ratio from classical trajectory calculations and from photolysis quantum yield measurements were tested. At the same time, a modeling of the photolysis quantum yields was performed. The formaldehyde system so far presents the best characterized multichannel dissociation reaction. It may serve as a prototype for other multichannel dissociation reactions.     

Determination of the energy distribution in unimolecular reactions under soft collision conditions

The effective energy distribution of activated molecules at the time of reaction under soft collision conditions ?′(E, ω) is pressure dependent and therefore difficult to recover from unimolecular decomposition data obtained at different pressures. We show in this work that the part of this function restricted to the condition that the collision frequency ω has to be equal to the microscopic rate constant k(E) at the energy given ?″[E,ω = k(E)] is a reasonable approximation to the input energy distribution ?(E) for quite soft collisions. This function is not pressure dependent and then recoverable at least in principle and as a matter of fact is not conceptually far from the function that the already reported deconvolution methods based in physical approximations attempt to recover. The deconvolution methods have been checked under soft collision conditions. We have found that the input energy distribution is recovered with reasonable accuracy for energies transferred by collision 〈ΔE〉 above 5 kcal mol^?1, conditions common in polyatomic systems.     

An approach for continuous-flow processing of reactions that involve the in situ formation of organic products

A simple adaptation allows batch protocols developed using microwave heating that involve formation of solid organic products to be scaled up using conventionally-heated flow chemistry with minimal or no re-optimization or modification. The product stream is intercepted with a flow of a suitable organic solvent upon exiting the heated zone, this solubilizing the product and allowing it to pass through the back-pressure regulator without aggregation of particulate material.     

Theoretical investigation into competing unimolecular reactions encountered in the pyrolysis of acetamide

Motivated by the necessity to understand the pyrolysis of alkylated amines, unimolecular decomposition of acetamide is investigated herein as a model compound. Standard heats of formation, entropies, and heat capacities, are calculated for all products and transition structures using several accurate theoretical levels. The potential energy surface is mapped out for all possible channels encountered in the pyrolysis of acetamide. The formation of acetamedic acid and 1-aminoethenol and their subsequent decomposition pathways are found to afford the two most energetically favored pathways. However, RRKM analysis shows that the fate of acetamedic acid and 1-aminoethenol at all temperatures and pressures is to reisomerize to the parent acetamide. 1-Aminoethenol, in particular, is predicted to be a long-lived species enabling its participation in bimolecular reactions that lead to the formation of the major experimental products. Results presented herein reflect the importance of bimolecular reactions involving acetamide and 1-aminoethenol in building a robust model for the pyrolysis of N-alkylated amides.