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

Abstract

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.

Stress Test for Quantum Dynamics Approximations: Deep Tunneling in the Muonium Exchange Reaction D + HMu → DMu + H

“…However, among the two channels, the abstraction reaction D + HMu → DH + Mu is more classical, as shown in ref. 7. The reverse of the abstraction reaction is very similar to the recently studied Mu + H 2 , 34 where it was found that not only RPMD gave the best results to date but also that CVT worked extraordinarily well.…”

Influence of vibration in the reactive scattering of D + MuH: the effect of dynamical bonding

“…For MuH( υ =0), the integral cross sections are negligible until a point where they begin to rise appreciably and show then a smooth tendency to stabilization. As discussed in a previous publication,[36] the onset of appreciable cross section rise is close to the reaction threshold obtained in quasiclassical trajectory (QCT) calculations. The threshold for DMu production is lower than that for the DH formation, but the cross section for this channel becomes larger beyond E tot =1.0 eV ( E coll =0.41 eV).…”

“…As discussed in ref. 36, the predominance of the DH + Mu channel at E coll > 0.4 eV ( E tot =1.0 eV) can be related to the position of the center of mass of MuH, which lies very close to the H atom and it leads to a much larger cone of acceptance for collisions with the hydrogen end of the molecule. In stark contrast, for the D+MuH( υ =1) reaction the cross section for DMu production has no translational barrier.…”

“…Taking into account that the Mu atom is nearly 9 times lighter than H, DMuH might be expected to exhibit some of the properties of “heavy–light–heavy” configurations. In fact, in a previous study by our group [36] it was found that just Mu substitution was enough to induce a well in the (00 0 ) DMuH VAP, which led to a maximum in the evolution of the reaction probability with energy, but had little effect on the integral reaction cross section. Adding vibrational excitation to Mu substitution might lead to a deeper well with appreciable influence on the cross section.…”

How intrinsic nuclear nonadiabaticity affects molecular structure, electronic density, and conformational stability: Insights from the multicomponent DFT calculations of Mu/H isotopologues

“…The thermalization of a positive muon would lead to the formation of muonium (Mu) atom, discovered experimentally by Hughes et al in 1960, which is conceived as a lighter isotope of hydrogen [28][29][30][31][32][33] . This is further established in the light of the comparative chemical kinetic investigations and recent theoretical studies regarding the "independent chemical identity" of the muonium atom with respect to the other members of the Periodic Table [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] .…”