Kinetic Isotope Effects for the Reactions of Muonic Helium and Muonium with H ₂

Abstract

The neutral muonic helium atom may be regarded as the heaviest isotope of the hydrogen atom, with a mass of ~4.1 atomic mass units ((4.1)H), because the negative muon almost perfectly screens one proton charge. We report the reaction rate of (4.1)H with (1)H(2) to produce (4.1)H(1)H + (1)H at 295 to 500 kelvin. The experimental rate constants are compared with the predictions of accurate quantum-mechanical dynamics calculations carried out on an accurate Born-Huang potential energy surface and with previously measured rate constants of (0.11)H (where (0.11)H is shorthand for muonium). Kinetic isotope effects can be compared for the unprecedentedly large mass ratio of 36. The agreement with accurate quantum dynamics is quantitative at 500 kelvin, and variational transition-state theory is used to interpret the extremely low (large inverse) kinetic isotope effects in the 10(-4) to 10(-2) range.

Rate coefficients and kinetic isotope effects of the X + CH4 → CH3 + HX (X = H, D, Mu) reactions from ring polymer molecular dynamics

“…78 Indeed, it was well established that the inclusion of the ZPE results in the shifting and broadening of the vibrationally adiabatic potential, which inhibit tunneling. 61,63,[78][79] Apparently, the same mechanism is operative in this case. Indeed, as shown in Fig.…”

“…29 In other words, we believe that the theoretical rate coefficients are probably more accurate than their experimental counterparts. 79,83 There are several possible sources of errors. The thick barrier and high ZPE of the products shown in Fig.…”

Non-equilibrium phenomena and molecular reaction dynamics: mode space, energy space and conformer space

“…44 Significant challenges in the small-molecule field remain -for example, in understanding non-adiabatic dynamics and testing efficient approaches for obtaining accurate…”

Temporal mapping of photochemical reactions and molecular excited states with carbon specificity

“…Further technical details of the new experimental set-up are summarized more fully in the Supplementary Information. Details on the muon technique, and further references, can be found in several articles 22,23,27,28 .…”

“…22). Mu is thought of as a light pseudo-isotope of the hydrogen atom, in which an electron orbits a muon (μ + ) nucleus, which is 0.11 the mass of H and behaves chemically like a H atom [22][23][24][25][26] . Due to the unsaturated bonds and aromatic rings that are present, muonium can react with TIPS-Pn with almost identical chemistry to a hydrogen atom, creating an electrically neutral radical.…”