Structure, dipole moment and large amplitude motions of 1-benzofuran

The rotational spectrum of 1-benzofuran has been investigated by three different rotational spectroscopy techniques: (i) millimeterwave absorption free jet spectroscopy, useful for a fast assignment of the spectrum; (ii) molecular beam Fourier transform microwave spectroscopy, sensitive to detect less abundant isotopic species in natural abundance; (iii) waveguide conventional microwave spectroscopy, useful for the study of intramolecular dynamics, through the rotational spectra of the vibrational satellites of low energy modes. Besides the rotational spectrum of the ground state of the normal species, the spectra of 9 singly substituted 13C and 18O isotopomers in natural abundance, and of 6 vibrational satellites, have been measured. Precise structural parameters for the molecule, as well as information on the potential energy surface of the low energy vibrations, have been obtained. The dipole moment components have been determined to be micro(a)= 0.216 (2) and micro(b)= 0.720 (3) D, respectively.     

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.     

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     

A predictive model for unimolecular reactions of organic ions

The slow unimolecular dissociations of six members of the [C_nH_2n-3]⁺ (n = 3-8) series of unsaturated carbonium ions are explained in terms of a potential surface approach together with some concepts of mechanistic organic chemistry. The occurrence of some dissociations is shown to be precluded because either the reacting configurations or product combinations are inaccessible at energies appropriate to metastable transitions. The approach permits correct predictions to be made concerning the shapes of metastable peaks for dissociations which occur without σ-bond formation in the final step. In particular, the observation of a composite peak, thus indicating two channels for reaction, for C₂H₄ loss from [C₇H₁₁]⁺ is naturally accommodated.     

Anharmonic effect on unimolecular reactions with application to the photodissociation of ethylene

The importance of anharmonic effect on dissociation of molecular systems, especially clusters, has been noted. In this paper, we shall present a theoretical approach that can carry out the first principle calculations of anharmonic canonical and microcanonical rate constants of unimolecular reactions within the framework of transition state theory. In the canonical case, it is essential to calculate the partition function of anharmonic oscillators; for convenience, the Morse oscillator potential will be used for demonstration in this paper. In the microcanical case, which involves the calculation of the total number of states for the activated complex and the density of states for the reactant, we make use of the fact that both the total number of states and the density of states can be expressed in the inverse Laplace transformation of the partition functions and that the inverse Laplace transformation can in turn be carried out by using the saddle-point method. We shall also show that using the theoretical approach presented in this paper the total number of states and density of states can be determined from thermodynamic properties and the difference between the method used in this paper and the thermodynamic model used by Krems and Nordholm will be given. To demonstrate the application of our theoretical approach, we chose the photodissociation of ethylene at 157 and 193 nm as an example.     

Electronic structure and the unimolecular reactions of imine peroixde HNOO

Summary Electronic structure and possible unimolecular reaction paths of a linear four-atom molecule HNOO to be formed by the addition of NH(^3–) toward O₂(³ _g ^– ) are investigated by the SCF and MRD-CI calculations employing the 6–31G** basis functions. HNOO in its ground state (¹ A) is an ozone-like diradicaloid, whose N–O binding energy is only 27 kJ/mol. Geometries and excitation energies of various diradical (excited) states, both singlet and triplet, are examined. The isomerization paths of the ground-state HNOO(¹ A) are traced by a multi-configuration (MC) SCF procedure and the activation barrier heights evaluated by the CI treatment. It has proved that energetically the most favorable is the 1,3-hydrogen migration to give hydroperoxynitrene NOOH(¹ A) with the barrier height of 62 kJ/mol. The nitrene should be extremely unstable; it is liable to be decomposed to NO + OH with virtually no activation barrier.Presented at the 7th International Congress on Quantum Chemistry, Menton, July 1991     

Evaluation of unimolecular rate constants for some outer-sphere electrochemical reactions

Unimolecular rate constants, k_et (s^?1), and transfer coefficients, α_cet, for the outer-sphere electroreduction of several Co(III) ammine and ethylenediamine complexes have been evaluated by electrostatically adsorbing the reactants at silver electrodes coated with chloride or bromide monolayers. Sufficiently strong diffuse-layer adsorption is thereby produced so to enable k_cet and α_cet to be determined by means of linear sweep voltammetry, employing sufficiently fast sweep rates (10–100 V s^?1) and dilute reactant concentrations (≤ 50 μM) so that the bulk solution reactant contributes negligibly to the observed faradaic transients. Comparison with corresponding rate constants and transfer coefficients for the solution reactants enables the influence of precursor-state stability upon the latter rate parameters to be assessed.     

Molecular beam measurements of small specific rate constants for unimolecular reactions

The investigation of unimolecular reactions with small rate constants is difficult owing to competing processes (inelastic collisions and bimolecular reactions) and the diffusion of reactant and product molecules out of the detection volume. For this reason, a new experimental approach for the measurement of specific rate constants in a molecular beam experiment has been exploited; instead of monitoring the temporal change of intensity as in a cell experiment, we monitor the spatial change along the molecular beam axis after laser excitation. For a given particle velocity the flight path between excitation and detection region defines the reaction time. By varying the distance the specific rate constant can be determined directly both from the decrease in the number density of reactant molecules as well as from the increase in product molecules. As a model system, the laser-induced (λ = 193 nm) photodissociation of mesitylene (trimethylbenzene) is studied. Previous experiments on the specific rate constant of mesitylene at this excitation energy differ between each other by about a factor of ten. By combining the new results with measurements at higher excitation energies, rate constants over a range of two orders of magnitude are now available for this reaction. The differences between the various experimental results are discussed within the framework of a statistical theory.     

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...     

On the relation between non adiabatic unimolecular reactions and radiationless processes

In this paper we present the results of a theoretical study of non adiabatic unimolecular dissociation processes with applications to the decomposition of N₂O(¹ ) to yield N₂(¹ ₉ ) and O(³ P). Such unimolecular reactions which involve a change in the electronic state can be handled by the theory of thermally excited intramolecular radiâtionless decay processes in analogy to molecular predissociation and electronic relaxation in the statistical limit. General criteria were advanced for describing the decay probability of a single vibronic level in terms of Fermi's golden rule and for specifying the (high pressure) unimolecular rate constant in terms of a thermally averaged transition probability. The quantum mechanical rate constant for the non adiabatic reaction is characterized by a pre-exponential factor determined by the interstate coupling matrix element and by a temperature dependent activation energy. At low temperatures the activation energy is equal to the continuum onset, and the reaction involves a tunnelling process. In the high temperature limit a general demonstration of the Franck Condon principle for thermal reactions was provided, whereupon the non radiative transition occurs at the intersection of the potential surfaces. Numerical calculations for a one dimensional model system for the thermal decomposition of N₂O were performed utilizing the semiclassical approximation and confirm our general conclusions. A two dimensional linear model has been developed representing the rate constant in terms of a convolution of two generalized line shape functions, which enabled us to study the distribution of vibrational energy among the diatomic N₂ molecules resulting from the thermal decomposition of N₂O. Some predictions concerning the determination of single level decay probabilities and vibrational distribution of the molecular products are presented.

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Zusammenfassung In dieser Arbeit werden die Ergebnisse einer theoretischen Untersuchung nicht adiabatischer unimolekularer Zerfallsreaktionen mitgeteilt und auf den Zerfall von N₂O(¹ ) zu N₂(¹ _g ) und O(³P) angewandt. Solche unimolekularen Reaktionen, bei denen sich der Elektronenzustand ändert, können mit der Theorie thermisch angeregter intramolekularer strahlungsloser Zerfallsprozesse in Analogie zu molekularer Prädissoziation und elektronischer Relaxation im statistischen Limit behandelt werden. Kriterien zur Beschreibung der Zerfallswahrscheinlichkeiten eines einzelnen Vibrationszustands unter Berücksichtigung von Fermis Golden Rule werden entwickelt sowie die unimolekulare Geschwindigkeitskonstante (im Hochdruckbereich), wobei thermisch gemittelte Übergangswahrscheinlichkeiten berücksichtigt werden, mitgeteilt. Die quantenmechanische Geschwindigkeitskonstante für die nicht adiabatische Reaktion wird durch einen präexponentialen Faktor, der durch die Matrixelemente der Kopplung beider Zustände bestimmt ist und durch eine temperaturabhängige Aktivierungsenergie charakterisiert. Bei tiefer Temperatur stimmt die Aktivierungsenergie mit der Energie der Kontinuumsgrenze überein, die Reaktion verläuft über einen Tunneleffekt. Für hohe Temperaturen wurde ein allgemeiner Beweis des Franck-Condon-Prinzips für thermische Reaktionen gegeben, wonach der strahlungslose Übergang beim Schnittpunkt der Potentialflächen auftritt. Rechnungen für ein eindimensionales Modell des N₂O Zerfalls wurden in der semiklassischen Näherung durchgeführt und bestätigen unsere Folgerungen. Ein zweidimensionales Modell wurde entwickelt, das die Geschwindigkeitskonstante als Faltungsintegral zweier verallgemeinerter Linienformintegrale wiedergibt. Dadurch wurde es ermöglicht, die Verteilung der Vibrationsenergie auf die zweiatomigen N₂ Moleküle, die bei dem thermischen Zerfall von N₂O enstehen, zu studieren. Einige Voraussagen über die Bestimmung der Zerfallswahrscheinlichkeiten eines Vibrationszustandes und die Vibrationsverteilung der molekularen Produkte werden mitgeteilt.

     

A predictive model for unimolecular reactions: Reactions of unsaturated carbonium ions

The major slow unimolecular reactions undergone by C₄H₇⁺, C₅H₉⁺ and C₆H⁺₁₁ are discussed in terms of a potential surface approach and the organic chemist's concept of mechanism. It is shown that the observed decompositions which do not involve σ-bond formation in the dissociation step are precisely those expected from the model. Further use of the model correctly predicts the slow reactions of C₇H⁺₁₃ which have not previously been reported. The approach also permits useful limits to be set on the transition state energies for reactions involving σ-bond formation in the dissociation step (H₂,CH₄ loss). It is concluded that stepwise addition of ethylene to the allyl cation is preferred to a concerted 4-electron process which is symmetry forbidden.     

Comparison between hydrogen-transfer and fluoro-transfer reactions on the microcanonical unimolecular rate constants

The microcanonical rate constants for the hydrogen-transfer process of HCCF (reaction 7) and the fluoro-transfer process of FCCF (reaction 8) are carried out with tunneling correction and curvature correction. The results show that the tunneling effects and curvature effects on the rate constant of reaction 7 is quite different from that of reaction 8. The rate constants for different rotational states are also studied for these reactions.     

The mechanism of activation and nonequilibrium distribution functions in unimolecular reactions

Nonequilibrium distribution functions for polyatomic molecules undergoing unimolecular reaction are considered. Two limiting cases of activation by collision are investigated: the mechanism of strong impacts, in which each collision with an activated molecule may be regarded as bringing about deactivation; and the diffusion mechanism in which the transition between states is described by the Fokker-Planck equation. An accurate solution of the diffusion equation for the thermal decomposition of a nonlinear triatomic molecule makes it possible to elucidate the kinetics of various reactions in relation to the activation mechansim.     

Electronic signatures of large amplitude motions: dipole moments of vibrationally excited local-bend and local-stretch states of S0 acetylene

A one-dimensional local bend model is used to describe the variation of electronic properties of acetylene in vibrational levels that embody large amplitude local motions on the S0 potential energy surface. Calculations performed at the CCSD(T) and MR-AQCC levels of theory predict an approximately linear dependence of the dipole moment on the number of quanta in either the local bending or local stretching excitation. In the local mode limit, one quantum of stretching excitation in one CH bond leads to an increase of 0.025 D in the dipole moment, and one quantum of bending vibration in the CCH angle leads to an increase of 0.068 D. The use of a one-dimensional model for the local bend is justified by comparison to the well-established polyad model which reveals a decoupling of the large amplitude bending from other degrees of freedom in the range of Nbend = 14-22. We find that the same one-dimensional large amplitude bending motion emerges from two profoundly different representations, a one-dimensional cut through an ab initio, seven-dimensional Hamiltonian and the three-dimensional (l = 0) pure-bending experimentally parametrized spectroscopic Hamiltonian.     

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.