Transition-State Models and Hydrogen-Isotope Effects

Weston, Ralph E.

doi:10.1126/science.158.3799.332

Cited by 29 publications

(2 citation statements)

References 33 publications

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“…Issues such as the role of the zero point energy (ZPE) of reactants, products, and transition state, the importance of tunnelling on the reactivity, the energy distribution inside the molecular species involved in the chemical processes and the possibility of inducing changes in the location of the transition state and the minimum energy path can be revealed by means of isotopic substitution (see [1][2][3][4][5][6][7] and references therein). Among all the possible observables accessible to experimental measurement, determination of the kinetic isotope effects (KIE), i.e., the ratio of the rate coefficients corresponding to reactions that differ solely in the isotopologues, has been the preferred target for experimental and theoretical groups devoted to the study of isotopic substitution.…”

Section: Introductionmentioning

confidence: 99%

Understanding the reaction between muonium atoms and hydrogen molecules: zero point energy, tunnelling, and vibrational adiabaticity

et al. 2013

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The advent of very precise measurements of rate coefficients in reactions of muonium (Mu), the lightest hydrogen isotope, with H 2 in its ground and first vibrational state and of kinetic isotope effects with respect to heavier isotopes has triggered a renewed interests in the field of muonic chemistry. The aim of the present article is to review the most recent results about the dynamics and mechanism of the reaction Mu + H 2 to shed light on the importance of quantum effects such as tunnelling, the preservation of the zero point energy, and the vibrational adiabaticity. In addition to accurate quantum mechanical (QM) calculations, quasiclassical trajectories (QCT) have been run in order to check the reliability of this method for this isotopic variant. It has been found that the reaction with H 2 (v=0) is dominated by the high zero point energy (ZPE) of the products and that tunnelling is largely irrelevant. Accordingly, both QCT calculations that preserve the products' ZPE as well as those based on the Ring Polymer Molecular Dynamics methodology can reproduce the QM rate coefficients. However, when the hydrogen molecule is vibrationally excited, QCT calculations fail completely in the prediction of the huge vibrational enhancement of the reactivity. This failure is attributed to tunnelling, which plays a decisive role breaking the vibrational adiabaticity when v=1. By means of the analysis of the results, it can be concluded that the tunnelling takes place through the ν 1 =1 collinear barrier. Somehow, the tunnelling that is missing in the Mu + H 2 (v=0) reaction is found in Mu + H 2 (v=1).

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Section: Introductionmentioning

confidence: 99%

Understanding the reaction between muonium atoms and hydrogen molecules: zero point energy, tunnelling, and vibrational adiabaticity

et al. 2013

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“…Isotopic substitution has long been used in the investigation of the dynamics of chemical reactions and has proven to be a most useful tool for the establishment of basic concepts in this field. [1][2][3][4] In a recent series of works, [5][6][7] the kinetic isotope effect (KIE) has been investigated for isotopic combinations of the prototypic H + H 2 reaction with an extremely large mass ratio. In particular, the studies involved two atypical hydrogen isotopes named as muonium, Mu, and muonic helium, Hem.…”

Section: Introductionmentioning

confidence: 99%

Comparative dynamics of the two channels of the reaction of D + MuH

Aoiz

Aldegunde

Herrero

et al. 2014

Phys. Chem. Chem. Phys.

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The dynamics of the asymmetric D + MuH (Mu = Muonium) reaction leading to Mu exchange, DMu + H, and H abstraction, DH + Mu, channels has been investigated using time-independent quantum mechanical (QM) calculations. Reaction probabilities, cross sections, cumulative reaction probabilities, and rate coefficients were determined for the two exit channels of the reaction. Quasiclassical trajectory (QCT) calculations were also performed in order to check the reliability of the method for this reaction and to discern the genuine quantum effects. Overall, the Mu exchange channel exhibits more structured reaction probabilities and cross sections with much larger rate coefficients than the H abstraction counterpart. Over the 100-1000 K temperature interval considered in this study, the QM rate coefficients for the Mu exchange vary between E5 Â 10 À15 and 2 Â 10 À11 cm 3 s À1 and those for the generation of DH + Mu between 2 Â 10 À18 and 3.5 Â 10 À12 cm 3 s À1 . In common with the rest of the isotopologues of the H + H 2 system, the height of the respective barriers in the collinear (symmetric stretch) vibrationally adiabatic potential energy curves matches the classical total energy threshold very accurately. Indeed, the lower and narrower vibrationally adiabatic collinear barrier as compared with that for the DH + Mu formation determines the preponderance of the DMu + H channel. Comparison of QM and QCT results and their analysis show that tunneling accounts for the reactivity at energies below the height of these barriers and that its effect on the rate coefficients becomes appreciable below 300 K. As expected, with growing temperature the contribution of tunneling to the global reactivity decreases markedly, but the rate coefficients are still much higher for the Mu exchange channel due to the effect of MuH rotational excitation that boosts the formation of DMu while diminishing the H abstraction channel that leads to DH formation. The analysis of the thermal cumulative reaction probabilities of the two channels indicates that at the lowest energies/temperatures the reaction into the DH + Mu channel takes place via 'leakage' from collisions proceeding along the DMu + H reaction path.

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