Unimolecular Reaction Rate Research Articles

Intramolecular energy transfer and mode-specific effects in unimolecular reactions of 1,2-difluoroethane

The unimolecular decomposition reactions of 1,2-difluoroethane upon mode-specific excitation to a total internal energy of 7.5 eV are investigated using classical trajectory methods and a previously formulated empirical potential-energy surface. The decomposition channels for 1,2-difluoroethane are, in order of importance, four-center HF elimination, C–C bond rupture, and hydrogen–atom dissociation. This order is found to be independent of the particular vibrational mode excited. Neither fluorine–atom nor F2 elimination reactions are ever observed even though these dissociation channels are energetically open. For four-center HF elimination, the average fraction of the total energy partitioned into internal HF motion varies between 0.115–0.181 depending upon the particular vibrational mode initially excited. The internal energy of the fluoroethylene product lies in the range 0.716–0.776. Comparison of the present results with those previously obtained for a random distribution of the initial 1,2-difluoroethane internal energy [J. Phys. Chem. 92, 5111 (1988)], shows that numerous mode-specific effects are present in these reactions in spite of the fact that intramolecular energy transfer rates for this system are 5.88–25.5 times faster than any of the unimolecular reaction rates. Mode-specific excitation always leads to a total decomposition rate significantly larger than that obtained for a random distribution of the internal energy. Excitation of different 1,2-difluoroethane vibrational modes is found to produce as much as a 51% change in the total decomposition rate. Mode-specific effects are also seen in the product energy partitioning. The rate coefficients for decomposition into the various channels are very sensitive to the particular mode excited. A comparison of the calculated mode-specific effects with the previously determined mode-to-mode energy transfer rate coefficients [J. Chem. Phys. 89, 5680 (1988)] shows that, to some extent, the presence of mode-specific chemistry is correlated with the magnitude of the energy transfer rate. However, the particular pathways for energy flow seem to be more important than the magnitude of the rate coefficients. It is suggested that the propensity for the energy to remain isolated in small subset of modes, such as the CH2F deformation modes or the rocking modes, is primarily responsible for the observation of mode-specific chemistry. The results clearly demonstrate that an intramolecular energy transfer rate that is fast relative to the unimolecular reaction rate is not a sufficient condition to ensure the absence of mode-specific chemical effects.

The Dependence of Unimolecular Reaction Rates on the Anisotropy of Potential Energy Surfaces

The Dependence of Unimolecular Reaction Rates on the Anisotropy of Potential Energy Surfaces

Application of unimolecular reaction rate theory for highly flexible transition states to the dissociation of NCNO into NC and NO

A recently described method for implementing RRKM theory for unimolecular reactions with highly flexible transition states is applied to the calculation of energy and angular momentum resolved rate constants and rotational–vibrational energy distributions for the reaction NCNO→hν NCNO*→NCNO(vib. hot)→NC+NO. The dissociation rate results are compared to the recent experimental results of Khundkar et al., and the vibrational and rotational distribution results are compared to the experimental values of Nadler et al. Comparison is also made with phase space theory calculations. The calculated rotational distributions at energies below the vibrational threshold of the products are the same as those of PST. At energies (2348, 2875 cm−1) above this threshold energy the rovibrational distribution is in better agreement with the data than is that of PST. The need for obtaining more accurate ab initio potential energy surfaces is noted, particularly for treating reactions at still higher energies.

Reactivity-structure correlations in ion clusters: A study of the unimolecular fragmentation patterns of argon ion clusters, Ar+n, for n in the range 30–200

A high resolution double focusing mass spectrometer in association with a pulsed supersonic nozzle has been used to study the unimolecular fragmentation of Ar+n clusters for n in the range 30–200. All ion clusters within this range are observed to undergo the reaction Ar+n→Ar+n−1+Ar; with some ions also exhibiting further fragmentation involving the loss of two and three argon atoms. A systematic study of the above reaction as a function of n reveals that the relative intensities of the fragment ions fluctuate considerably. In many cases, these fluctuations coincide with depleted or enhanced ion intensities in the normal ion cluster mass spectrum. Through the use of a model based on unimolecular reaction rate theory (RRKM), it has been possible to reproduce many of the features observed in both the fragmentation spectra and the normal mass spectrum. Of the two principle variables involved in the model, reaction symmetry number and dissociation or binding energy, we have been able to show that only the binding energy is important in determining the fragmentation pattern. Through the calculations, the experimental data are interpreted in terms of ‘‘stable’’ and ‘‘unstable’’ ion clusters. Stable ion clusters have enhanced intensities in the normal mass spectrum, whereas unstable ions produce both depletions in the normal mass spectrum, and enhanced fragment ion intensities. It is proposed that unstable argon ion clusters consist of an atom weakly bound to an underlying stable cluster. A series of hard-sphere structures are presented to account for some of the observed stable ion clusters. Each of the structures has icosahedral symmetry and accommodates a central Ar+2 ion. It is concluded that, because the clusters are forced to accommodate a charge-carrying unit, the positive charge appears to have a structural influence which extends well beyond the electrostatic limit.

Unimolecular reaction rate theory for highly flexible transition states. 2. Conventional coordinate formulas for the various possible fragment combinations: miscellaneous topics

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTUnimolecular reaction rate theory for highly flexible transition states. 2. Conventional coordinate formulas for the various possible fragment combinations: miscellaneous topicsStephen J. Klippenstein and R. A. MarcusCite this: J. Phys. Chem. 1988, 92, 19, 5412–5417Publication Date (Print):September 1, 1988Publication History Published online1 May 2002Published inissue 1 September 1988https://doi.org/10.1021/j100330a018RIGHTS & PERMISSIONSArticle Views121Altmetric-Citations33LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (824 KB) Get e-Alerts

Effect of fluctuations in state-specific unimolecular rate constants on the pressure dependence of the average unimolecular reaction rate

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEffect of fluctuations in state-specific unimolecular rate constants on the pressure dependence of the average unimolecular reaction rateWilliam H. MillerCite this: J. Phys. Chem. 1988, 92, 15, 4261–4263Publication Date (Print):July 1, 1988Publication History Published online1 May 2002Published inissue 1 July 1988https://doi.org/10.1021/j100326a003RIGHTS & PERMISSIONSArticle Views45Altmetric-Citations26LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (332 KB) Get e-Alerts

Unimolecular reaction rate theory for highly flexible transition states: use of conventional coordinates

An alternative method for implementing RRKM theory for unimolecular reactions with highly flexible transition states is described using conventional coordinates. The number of available states for motion in the transition state state N_(EJ) is determined via an appropriate average over the absolute space orientations and body-fixed momenta of the two fragments. The results of calculations of N_(EJ) for the C_2H_6 → 2CH_3 reaction (or alternatively for the corresponding recombination reaction) obtained from the present expression are shown to be equivalent numerically to those obtained previously by Wardlaw and Marcus.

General classical variational treatment of the rates of bimolecular exchange and unimolecular reactions involving three bodies

The classical variational theory of chemical reaction rates gives the rate as the equilibrium flux of systems through a trial surface in the phase space of the reaction system. The surface divides the phase space into reactant and product regions and is varied to obtain a least upper bound for the rate of product formation. For bimolecular reactions of the type A+BC→AB+C and the high pressure limit of unimolecular reactions of the type ABC→products, we have derived expressions which give the canonical and microcanonical rate coefficients for the most general dividing surface defined by internal configuration space coordinates. For bimolecular reactions, we have also derived an expression for the energy dependent mean reaction cross section for the most general dividing surface. Expressions for the rate coefficients and mean reaction cross section for any of the more restricted formulations of the dividing surface used in earlier work, and more flexible ones, can be obtained by the substitution of two terms in the appropriate general equation; namely, the partial derivatives of the function that defines the surface with respect to the internal coordinates that define the surface. For example, the flexibility of the surface can be improved systematically by introducing terms in a power series expansion of the surface. The application of a simplex algorithm to determine the coefficients (variational parameters) in an expansion of the surface that includes terms up to the second and third order may give a surface which is close to the best one possible. The minimization procedure corresponds to a search of the potential-energy function for a reaction coordinate that satisfies the variational condition. The variationally determined dividing surface identifies regions of configuration space in which the potential energy must be accurate in order to obtain accurate classical rate coefficients. Thecalculus of variations was applied to the general equations for the rate coefficients to obtain differential equations which give the best dividing surface defined by internal coordinates for the canonical and microcanonical formulations of transition state theory. The corresponding rate coefficients or mean reaction cross section are as close as possible to convergence with the results of classical trajectory calculations. The flux can thus be minimized completely by obtaining a numerical solution for the differential equations which define the best surface or nearly minimized for a series approximation to the best surface. Computational studies are required to determine the more tractable method. By using an appropriate generating function, the procedure described in this work can be extended to reactions involving more than three bodies. The variationally determined dividing surface, reaction coordinate, and reaction surface Hamiltonian could provide a basis for a semiclassical theory of reaction rates.

The Additions and Corrections - Unimolecular Reaction Rate Theory for Transition States of Any Looseness. 3. Application to Methyl Radical Recombination.

ADVERTISEMENT RETURN TO ISSUEPREVArticleThe Additions and Corrections - Unimolecular Reaction Rate Theory for Transition States of Any Looseness. 3. Application to Methyl Radical Recombination.David M. Wardlaw and R. A. MarcusCite this: J. Phys. Chem. 1987, 91, 18, 4864Publication Date (Print):August 27, 1987Publication History Published online8 September 2003Published inissue 27 August 1987https://doi.org/10.1021/j100302a600RIGHTS & PERMISSIONSArticle Views14Altmetric-Citations3LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (107 KB) Get e-Alerts

Ammonia decomposition on Ir and Pt wires

The rates of ammonia decomposition on polycrystalline Ir and Pd wires were measured at temperatures between 900 and 2000 K and at pressures between 50 and 770 Torr. Rates on Ir were fitted with a Langmuir-Hinshelwood unimolecular reaction rate expression. Rates on Pd could not be fitted with a rate expression, although activation energies were obtained.

Unimolecular reaction rate measurements for molecules with excited overtone vibrations

Unimolecular reaction rate constant measurements on systems with well known total energy contents permit detailed comparison with the predictions of statistical theories. Vibrational overtone excitation is a preparation technique that adds a precise energy increment to the reacting molecules and, when applied to a sample cooled in a free jet expansion, produces an ensemble with a known total energy content. Combining this preparation scheme with time resolved product detection yields the unimolecular reaction rate constant for a large molecule, tetramethyldioxetane. These free jet data serve to fix the parameters in an RRKM calculation, which is then averaged over a thermal distribution of initial vibrational energy for comparison to extensive room temperature measurements on the vibrational overtone initiated reaction. Applying this excitation scheme to a small molecule, hydrogen peroxide, in a free jet expansion permits the spectroscopic inference of its fast unimolecular reaction rate. Observation of the width of individual features in the well resolved vibrational overtone excitation spectrum establishes an upper limit to the unimolecular reaction rate constant for these molecules in their few lowest angular momentum states.

The unimolecular reaction of t-BuNO on singlet and triplet surfaces: Spectroscopy, real-time rate measurements, and NO energy distributions

The predissociation of jet-cooled (CH3)3CNO (t-BuNO) following laser excitation in the à 1A″←X̃ 1A′ system has been studied in both the energy (frequency) and time domains. Unlike the smaller nitroso compounds whose predissociation has been already examined, unimolecular reaction is the rate determining step for predissociation near threshold. Consequently, it has been possible to separately measure radiationless transition rates and unimolecular reaction rates in real time. Dissociation on both the ground state (S0) and the first triplet state (T1) has been identified. At threshold, dissociation proceeds only on S0, with lifetimes >3.5 μs, but for E°≥650 cm−1, fast (<10 ns) predissociation via T1 becomes progressively the dominant dissociative route. Nascent NO photofragments have been characterized in detail using one-photon LIF. The rotational and spin-orbit distributions of NO following dissociation on S0 are statistical, depending only on E°. The NO derived from dissociation on the T1 surface is not at all statistical, consistent with a sizable (∼650 cm−1) exit channel barrier on this surface. Most notably, the [NO(2Π3/2)]/[NO(2Π1/2)] ratios are much smaller, and rotational distributions are colder than predicted by a statistical model. In order that some of the predissociating vibrational states could be assigned, the 1A″←1A′ spectrum of t-BuNO has been analyzed and the electronic origin assigned as 13 911 cm−1. The ground state dissociation energy, D0, is found to be 13 930±30 cm−1, i.e., 39.8±0.1 kcal mol−1.

Overtone vibration initiated unimolecular reaction of tetramethyldioxetane in a free jet expansion: A comparison with RRKM theory

Vibrational overtone excitation of tetramethyldioxetane in a free jet expansion creates a nearly monoenergetic ensemble of molecules for which the unimolecular reaction rate is directly measured by monitoring the temporal evolution of the electronically excited acetone formed in the reaction. The measurement for the 5νCH vibrational overtone transition determines the unimolecular reaction rate constant at one energy. It is possible to adjust the parameters in the RRKM calculation of the rate constant to obtain agreement with this nearly monoenergetic measurement and to average the calculated energy dependent rate constant over a thermal distribution of initial vibrational energies for comparison with data on room temperature samples. The calculation reproduces the growth of the chemiluminescence following excitation of two CH stretching transitions 4νCH and 5νCH and of a combination transition involving the methyl deformation 4νCH +νdef.

A comparison of various methods for calculating unimolecular reaction rate constants

AbstractA test of the Kassel quantum expressions for unimolecular rate constants and densities of vibrational states is described. Rate constants are calculated for nine reactions previously used in such tests, over a wide range of temperatures and pressures. The Kassel expression using the geometric mean of the molecular vibration frequencies is generally found to give better results than that using the arithmetic mean.

Unimolecular reaction rate theory for transition states of any looseness. 3. Application to methyl radical recombination

The theory for unimolecular reactions described in part 1 is applied to the recombination of methyl radicals in the high-pressure limit. The model potential energy surface and the methodology are briefly described. Results are presented for the recombination rate constant k_ ∞ at T = 300, 500, 1000, and 2000 K. Canonical and Boltzmann averaged microcanonical values of k_ ∞ are compared, and the influence of a potential energy interpolation parameter and a separation-dependent symmetry correction on k_ ∞ are examined. Earlier theoretical models and extensive experimental results are compared with the present results which are found to have a negative temperature dependence. The present results agree well with some of the available but presently incomplete experimental determinations of the high-pressure recombination rate constant for this reaction over the 300-2000 K temperature range. There is also agreement with a decomposition rate constant for a vibrationally excited ethane molecule produced by chemical activation.

Thermal Decomposition of Cyanogen Measured in C2N2/O2 and C2N2/H2 Reaction Systems by Atomic Resonance Absorption

AbstractThe thermal decomposition of cyanogen has been measured behind reflected shock waves in the temperature range 1900–2650 K at pressures of about 1.8–2.0 bar using Atomic Resonance Absorption Spectroscopy (ARAS). C2N2/O2 and C2N2/H2 mixtures highly diluted in argon were shock heated and O and H atom concentrations were monitored in the post shock reaction zone. Because of the fast secondary reactions of the dissociation product CN, the measured concentrations of O or H were strongly dependent on the rate of dissociation of cyanogen. The experimentally determined rate coefficient data for the dissociation of cyanogen are closely fit by the Arrhenius expression: k = · 2.97 10−7 exp(−53665 K/T)cm3/s. The present rate coefficient data is compared with the results of previous investigators and also with the estimates obtained from the weak collision unimolecular reaction rate theory.

Kinetics of ammonia decomposition of polycrystalline rhodium at pressures between 0.013 and 103 kPa

The rates of ammonia decomposition on polycrystalline Rh wires between 600 and 1800 K and at pressures between 13.3 Pa and 103 kPa were measured and fitted with a Langmuir-Hinshelwood unimolecular reaction rate expression. At high temperatures and pressures the reaction seems to be mass transfer controlled.

Unimolecular reaction rate theories of chlorinated dioxins and related compounds

The importance of reaction-rate theories is outlined and a short review is given over the main theories of unimolecular reactions. A new approach along the lines of the Slater theory is presented. Results of molecular decomposition rates for a chlorinated dioxine and related compounds are presented.

Unimolecular reaction rate theory for transition states of partial looseness. II. Implementation and analysis with applications to NO2 and C2H6 dissociations

Implementation of RRKM theory for unimolecular dissociations having transition states of any degree of looseness is described for reactions involving dissociation into two fragments. The fragments may be atomic, diatomic, or polyatomic species. Action-angle and internal coordinates for the transitional modes of the reaction, transformations to Cartesian coordinates, and other calculational aspects are described. Results for the NO2→NO+O reaction are presented, including the dependence of the microcanonical rate constant on the bond fission and bending potentials for model potential energy surfaces. Illustrative calculations for the C2H6→2CH3 reaction are also given.

Ammonia decomposition on polycrystalline Pt at pressures between 1.3 and 93 kPa

The kinetics of ammonia decomposition on polycrystalline Pt at temperatures between 600 and 1700 K and at pressures between 1.3 and 93 kPa were measured and correlated with a Langmuir-Hinshelwood unimolecular reaction rate expression. This rate expression had previously been shown to fit data in the same temperature range for pressures between 2 and 1400 Pa.