Dynamical Stereochemistry Research Articles

Theoretical study of the stereodynamics of the reactions of [formula omitted

The vector correlations between products and reagents for the chemical reactions He + H 2 + / HD + have been calculated by means of the quasi-classical trajectory (QCT) method at a collision energy of 1.0 eV on the AQUILANTI surface [P. Palmieri, C. Puzzarini, V. Aquilanti, G. Capecchi, S. Cavalli, D. De Fazio, A. Aguilar, X. Gimenez, J.M. Lucas, Mol. Phys. 98 (2000) 1835]. The isotope effect on dynamical stereochemistry of the title reactions is discussed in detail. The pronounced isotopic effect may be ascribed to the difference of the mass factor in the two reactions.

A comparison of the stereodynamics between the reactions H + HH(D, T) and the reactions H − + HH(D, T)

The vector correlation in the H − + HH reaction and its isotopic variants at a collision energy of 35.7 kcal/mol has been studied by using the quasi-classical trajectories on the potential energy surface (PES) constructed by Panda and Sathyamurthy. Though H 3 - system is very similar to H 3 in many ways, for instance, the mass factor, the characteristics of saddle point on the PES for collinear H 3 - and H 3, etc., the remarkable differences are found when compared the rotational polarization of the product HH from the reactions H − + HH(D, T) with that of the product HH from the reactions H + HH(D, T) published by Chen et al. The calculated results also show that the long range interaction may play an important role in dynamical stereochemistry.

Chemical Stereodynamics: Retrospect and Prospect

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Chemical stereodynamics: retrospect and prospect

Thematic aspects of chemical stereodynamics are discussed that offer much scope for further development. Particularly emphasized are opportunities to exploit angular correlations to recover information usually presumed to be inaccessible; means to hybridize rotational angular momentum and thereby create spatially oriented or aligned pendular states in which molecular axes are confined to oscillate about the direction of an external field; the major role of atomic electronegativity differences in producing steric preferences in reactivity, by affecting the location of nodes and asymmetric composition in molecular orbitals; and some newly recognized stereodynamic aspects of DNA replication.

Theoretical study of stereodynamics for the reactions Cl+H2/HD/D2

Studies on the dynamical stereochemistry of the Cl+H2 reaction and its isotopic variants, especially the isotope effect on the product polarization, have been performed at a collision energy of 6.0 kcal/mol on two potential energy surfaces, i.e., G3 surface [T. C. Allison et al., J. Phys. Chem. 100, 13575 (1996)] and BW2 surface [W. Bian and H.-J. Werner, J. Chem. Phys. 112, 220 (2000)]. Quantum mechanical and quasiclassical trajectories calculations of the polarization-dependent differential cross sections for the Cl+H2 reaction have been carried out on the BW2 potential energy surface, and the results indicate that the quasiclassical approximation in general does as good as exact quantum mechanics. Calculations also show that the rotational alignment of the HCl product obtained on the BW2 surface for Cl+H2 reaction is stronger than that calculated on the G3 surface, which implies that the effect of van der Waals force on product polarization is quite weak. The distributions of P(θr) and P(φr) derived from the Cl+H2 and its isotopic reactions indicate that the isotope effect on the product polarization calculated on the G3 potential energy surface is distinct, whereas the isotope effect on the product polarization computed on the BW2 surface is indistinct.

Theoretical studies of product polarization and state distributions of the H + HCl reaction

The angular momentum polarization and rotational state distributions of the H 2 and HCl products from the H + HCl reaction are calculated at a relative translational energy of 1.6 eV by using quasiclassical trajectories on two potential energy surfaces, one from G3 surface [T.C. Allison et al., J. Phys. Chem. 100 (1996) 13575], and the other from BW2 surface [W. Bian, H.-J. Werner, J. Chem. Phys. 112 (2000) 220]. Product rotational distributions obtained on the G3 potential energy surface (PES) are much closer to the experimental results (P.M. Aker et al., J. Chem. Phys. 90 (1989) 4795; J. Chem. Phys. 90 (1989) 4809) than the distributions calculated on the BW2 PES. The distributions of P( φ r) for the H 2 and HCl products obtained on the G3 PES are similar, whereas the rotational alignment effect of the H 2 product is stronger than that of the HCl product. In contrast to the polarization distributions obtained on the G3 PES, the rotational alignment effect of the two products calculated on the BW2 PES is similar. However, the abstraction reaction is dominated by out-of-plane mechanisms, while the exchange reaction is dominated by in-plane mechanisms. The significant difference of the product rotational polarization obtained on the G3 and BW2 PESs implies that the studies of the dynamical stereochemistry can provide a sensitive test for the accuracy of the PES.

Vector correlation in the H+D 2 reaction and its isotopic variants: isotope effect on stereodynamics

The vector correlations in the H+D 2 reaction and its isotopic variants at a collision energy of 35.7 kcal/mol have been studied by using quasiclassical trajectories on two potential energy surfaces (PESs), one from BKMP2, and the other from LSTH. And the isotope effect on dynamical stereochemistry is discussed in detail. The calculations indicate that the distribution of the product angular momentum vector is sensitive to the mass factor. The agreement of the results calculated on both the LSTH and BKMP2 PESs probably implies that the product rotational polarization is mainly controlled by the saddle point properties on PES.

Classical variational rate theory portraits of the dynamical stereochemistry of the F + H2—> FH + H reaction

A general classical variational theory of reaction rates is applied to the F + H2→> FH + H reaction. The variational theory gives the rate as the equilibrium flux of phase points through a trial surface which divides reactants from products and is varied to obtain a least upper bound for the rate. This dividing surface (DS) is defined by a power-series expansion of the H-H internuclear separation (r) in internal coordinates R and θ, i.e., r = F(R, θ) where R is the approach coordinate and θ is the orientation angle. The downhill simplex algorithm is used to search the space of 6 and 10 variational parameters of second- and third-order expansions of the DS and obtain minimum values for the canonical rate constant or, in the microcanonical formulation of the theory, the energy-dependent mean reaction cross section. The presence of angle-dependent terms in the DS makes it possible to describe the dynamical stereochemistry of atom-diatom reactions in a new and useful manner. Portraits of the dynamical stereochemistry are obtained by plotting contours of the density of reaction systems on the DS; such plots are reactivity relief maps of the DS. Reactivity relief maps show how the field of reactivity which surrounds the diatomic reactant molecule expands with increasing temperature and energy. Results are presented here for a new power series formulation of the DS which obeys a condition: δF(R, θ)/δθ = 0 at θ = π/2 which is appropriate for reaction of a homonuclear diatomic molecule. The relationship between reactivity relief maps obtained using quadratic and cubic formulations of the new DS and the locations of angle-dependent energy barriers for reaction is described. Variational and classical mechanical trajectory results are used to show how energy-dependent factors, which correct the variational mean reaction cross section for trajectories which cross and recross the DS, depend on the orientation angle. Key words: variational, transition, rate, dividing, surface.

Application of a general classical variational theory to the F+H2→FH+H reaction

A general classical variational theory of reaction rates [J. Chem. Phys. 87, 5746 (1987)] is applied to the F+H2→FH+H reaction for a series of potential-energy functions (PEFs). The variational theory gives the rate as the equilibrium flux of phase points through a trial surface which divides reactants from products and is varied to obtain a least upper bound for the rate. This dividing surface (DS) is defined by a power-series expansion of the H–H internuclear separation (r) in internal coordinates R and θ where R is the distance between atom F and the center-of-mass of H2 and θ is angle which the H2 internuclear axis makes with a line from the center-of-mass of H2 to atom F. The angle-dependent terms in the DS make it possible to describe the dynamical stereochemistry of atom–diatom reactions in a new and useful manner. The profile of the angle-dependent minimum potential energy for reaction versus orientation angle is varied systematically in the PEF series to define a trend toward a “flatter” angle-dependent barrier. Portraits of the dynamical stereochemistry are obtained for each PEF by plotting contours of the density of variational flux on the DS. These reactivity relief maps show how the accuracy of the variational method depends on the expansion order of the DS and how the field of reactivity which surrounds the diatomic reactant expands with increasing temperature and energy. The accuracy of the variational theory was determined by comparing energy-dependent mean reaction cross sections and incremental (angle-dependent) mean reaction cross sections with results obtained by calculating classical mechanical trajectories. The DS was used to show how the accuracy of the no-recrossing assumption of transition state theory depends on orientation angle. Variational and trajectory results were used to calculate energy-dependent transmission and product coefficients.

Quantum dynamical stereochemistry of atom–diatom reactions

We have used density matrix techniques and angular momentum algebra to obtain quantum–mechanical equations describing the dynamical stereochemistry of the atom–diatom reaction A+BC⇌AB+C. The relative motions of reagents and products are specified by four vectors: rotational angular momenta of diatomic molecules and relative velocities of reagents and products. Our equations show how the correlations between the spatial distributions of these four vectors are related to the scattering matrix determined in quantum scattering calculations. We present three different expressions for the four-vectors correlation. One of them is appropriate to the helicity representation of the scattering matrix, while the others are appropriate to the orbital angular momentum representation with either space-fixed or body-fixed reference frames. The formulation adopted allows for a rigorous comparison between theory and experiment. It takes mixed quantum–mechanical states and unobserved quantum-numbers into account, and all vector distributions are expressed in terms of measurable quantities (scattering angles and polarization moments of rotational angular momenta). Explicit expressions for most of the lower-order vector correlations obtained by direct reduction of the four-vectors correlation formulas are also presented.

Dynamical stereochemistry of bimolecular reactions

Dynamical stereochemistry is the study of how chemical reactivity depends on the angle of approach of reagents to one another: the ‘chemical shape’ of molecules. This chemical shape has profound effects in a number of branches of chemistry, and current experimental and theoretical approaches to understanding dynamical stereochemistry are discussed. The experimental strategies employed have become very diverse in recent years and novel techniques are described, together with the limitations inherent in a number of experimental methods. Simple models for interpreting experimental data are discussed as are the results of more rigorous potential-energy surface and scattering calculations. The level of detail that can be achieved in understanding reaction dynamics and dynamical stereochemistry is illustrated for the reaction of chlorine atoms with methane.

Orientation and Alignment in Reactive Beam Collisions: Recent Progress

The concept of directional axis distributions and orientation-dependent reaction cross sections is used to describe the effect of the mutual orientation of reagents on the outcome of reactive beam collisions. The axis distributions and cross sections are expanded in series of Legendre polynomials and real spherical harmonics, respectively, and characterized by the expansion coefficients (moments). The interrelations between the moments of the cross sections and the directionally dependent experimental data (steric effects) on the one hand and the anisotropic properties of the potential energy surfaces on the other hand are presented. Recent progress in the field of dynamical stereochemistry results from the prep- aration of molecular orientation and alignment by using the novel brute force technique and an optical method, respectively. The theories of both methods are summarized, and typical experimental arrangements are presented. All experimental results based on these techniques are reviewed. Among the most important ones are the first orientation effects observed in a reaction with a (1)Σ: molecule (K + ICl) and the alignment effects found in the bench mark reaction Li + HF → LiF + H.

Preparing Reagents: Time Dependence of HCl(v =1, J) Alignment Following Pulsed Infrared Excitation

AbstractWe discuss the use of photon absorption as a means of aligning reagents for studies of dynamical stereochemistry. A linearly polarized infrared beam from an optical parametric oscillator prepares HCl(v = 1, J = 1), and the alignment is monitored at subsequent times by (2+1) resonance‐enhanced multiphoton ionization. The degree of alignment oscillates in time caused by hyperfine quantum beats. Experimental observations are well matched by calculations that account for the presence of unresolved hyperfine structure. Following reagent preparation by optical pumping, a cumulative hyperfine‐depolarization coefficient can be used to describe the effective reagent alignment in a subsequent chemical reaction.

Trajectory studies of SN2 nucleophilic substitution. III. Dynamical stereochemistry and energy transfer pathways for the Cl−+CH3Cl association and direct substitution reactions

Classical trajectory calculations are performed to determine differences in the microscopic dynamics for two fundamental processes for the Cla−+CH3Clb→ClaCH3+Clb− reactive system: Cla−–CH3Clb complex formation and directly attaining the [Cla–CH3–Clb]− central barrier without first forming the complex. This latter process becomes important when the C–Clb stretch mode is excited in the CH3Clb reactant. The total cross section for complex formation and directly attaining the central barrier increases as nC–Clb is increased. The value for the Cla−—C–Clb angle θ as the reactants interact, the dynamical stereochemistry, is found to be a very important property for distinguishing between the mechanisms for the two fundamental processes. Directly attaining the central barrier requires oriented reactants with θ≊π, while orientation suppresses complex formation. Substantial reactant orientation only occurs for CH3Clb rotational temperatures less than 300 K. The complex is formed by a T→R energy transfer process which involves coupling between the reactant orbital angular momentum and CH3Clb rotational angular momentum. Complex formation does not involve energy transfer to the CH3Clb vibrational modes, which is consistent with an earlier finding that the CH3Clb intramolecular modes are inactive during decomposition of the Cla−–CH3Clb complex. Orientation of CH3Clb enhances coupling between the Cla−+CH3Clb radial motion and the C–Clb stretch mode. This coupling leads to the above direct substitution process and extensive deactivation of the excited C–Clb stretch during direct unreactive collisions. Considerably less deactivation results from Cla−–CH3Clb complex formation followed by dissociation to the reactants. Rotationally exciting CH3Clb eliminates orientation and, thus, suppresses deactivation of the C–Clb stretch.

Measurements of vector correlations in bimolecular reactions by laser-pump and probe techniques

Velocity-aligned, superthermal atoms generated via polarised molecular photodissociation have been used to investigate vector correlations of bimolecular reactions. Doppler-resolved, polarised laser-probe techniques can measure correlations between k (the reagents' relative-velocity vectors), k ′ (the reaction products' velocity vectors) and J ′ (the products' rotational angular momenta using a simple laboratory to centre-of-mass frame transformation. This approach to the study of the dynamical stereo-chemistry of chemical reactions is illustrated by two examples, O( 1D)+N 2O→NO+NO and O( 3P)+CS→CO+S.

The branching ratio in the F+HD reaction: An experimental and computational study

The dynamical stereochemistry of the reaction of hot F atoms with HD is discussed with reference to the measured branching ratio using both exact and model classical trajectory computations. It is argued that the dominant effects are due to the shift of the center of mass from the center of charge. In particular this leads to enhanced reactivity of the D end of the molecule due both to reorientation of the molecule and to the recrossing of the barrier. The observed preference for reaction at the H end is attributed to HD rotational excitation reflecting however the shape of the potential energy surface and not the longer arm of the H atom about the center of mass. Measuring the reaction cross sections for rotationally cold HD will provide a critical test of our understanding of the dynamics.

Kinematic mass effect in the dynamical stereochemistry of activated bimolecular reactions

Kinematic mass effect in the dynamical stereochemistry of activated bimolecular reactions

Dynamical stereochemistry of elementary reactions in solution

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDynamical stereochemistry of elementary reactions in solutionI. Benjamin, Antonio. Liu, Kent R. Wilson, and R. D. LevineCite this: J. Phys. Chem. 1990, 94, 10, 3937–3944Publication Date (Print):May 1, 1990Publication History Published online1 May 2002Published inissue 1 May 1990https://doi.org/10.1021/j100373a014RIGHTS & PERMISSIONSArticle Views39Altmetric-Citations22LEARN 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 (2 MB) Get e-Alerts

Dynamical stereochemistry of activationless bimolecular reactions of ions and neutrals

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDynamical stereochemistry of activationless bimolecular reactions of ions and neutralsR. D. Levine and Richard B. BernsteinCite this: J. Phys. Chem. 1988, 92, 24, 6954–6958Publication Date (Print):December 1, 1988Publication History Published online1 May 2002Published inissue 1 December 1988https://doi.org/10.1021/j100335a023RIGHTS & PERMISSIONSArticle Views45Altmetric-Citations14LEARN 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 (686 KB) Get e-Alerts

ChemInform Abstract: Dynamical Stereochemistry and the Polarization of Reaction Products

ChemInform Abstract: Dynamical Stereochemistry and the Polarization of Reaction Products