Muonium Addition Reactions and Kinetic Isotope Effects in the Gas Phase: k_∞ Rate Constants for Mu + C₂H₂

Abstract

The kinetics of the addition reaction of muonium (Mu) to acetylene have been studied in the gas phase at N2 moderator pressures mainly from ∼800 to 1000 Torr and over the temperature range from 168 to 446 K, but also down to 200 Torr at 168 K and over a much higher range of pressures, from 10 to 44 bar at 295 K, demonstrating pressure-independent rate constants, kMu(T). Even at 200 Torr moderator pressure, the kinetics for Mu + C2H2 addition behave as if effectively in the high-pressure limit, giving k∞ = kMu due to depolarization of the muon spin in the MuC2H2 radical formed in the addition step. The rate constants kMu(T) exhibit modest Arrhenius curvature over the range of measured temperatures. Comparisons with data and with calculations for the corresponding H(D) + C2H2 addition reactions reveal a much faster rate for the Mu reaction at the lowest temperatures, by 2 orders of magnitude, in accord with the propensity of Mu to undergo quantum tunneling. Moreover, isotopic atom exchange, which contributes in a major way to the analogous D atom reaction, forming C2HD + H, is expected to be unimportant in the case of Mu addition, a consequence of the much higher zero-point energy and hence weaker C-Mu bond that would form, meaning that the present report of the Mu + C2H2 reaction is effectively the only experimental study of kinetic isotope effects in the high-pressure limit for H-atom addition to acetylene.

Calculations of quantum tunnelling rates for muonium reactions with methane, ethane and propane

“…the H variant of the reactions have a larger rate compared to the Mu variant), it is known that for addition reactions involving Mu (e.g. Mu + C 2 H 4 , 117 Mu + C 2 H 2 118 and Mu + C 6 H 6 119 ) tunnelling makes much larger contributions.…”

Kinetic isotope effects in chemical and biochemical reactions: physical basis and theoretical methods of calculation

“…It can be observed that the experimental k Mu rate constants are two orders of magnitude higher than the calculated values for k H at the lowest temperatures. The Arrhenius plot for the Mu isotope shows much more curvature than the one for the H isotope (experimental activation energies of 628 cal/mol for Mu versus 2428 cal/mol for H, and versus the calculated value of 2709 cal/mol also for protium). This result is in accordance again to the enhanced tunneling contribution for the lightest isotope, although in this case there are no theoretical calculations to quantify the quantum effect for the Mu isotope addition process.…”

“…Very recently, the muon spin rotation technique has been applied to the Mu + C 2 H 2 reaction, to directly measure the pressure independent (i.e., high‐pressure) addition rate constant . The experimental rates for the Mu + C 2 H 2 reaction are 2–3 times smaller than for the Mu + C 2 H 4 addition process and the Arrhenius plot for the Mu reaction with acetylene shows less curvature (that is, less tunneling) at the lower temperature range than the Mu reaction with ethylene.…”

“…Experimental Arrhenius plot for Mu + C 2 H 2 (experimental values: red squares; fitted line: red line); calculated Arrhenius plots for H + C 2 H 2 (from Ref : dark green; from Ref : light green) and for D + C 2 H 2 (from Ref : magenta line). (Reprinted with permission from Ref . Copyright 2015 American Chemical Society)…”

Muonium Chemistry at Diiron Subsite Analogues of [FeFe]‐Hydrogenase

“…To date, there have been only a very small number of studies on application of μSR to organometallic systems 8. These have been focused on silylenes,9 or cyclopentadienyl or arene (half)sandwich systems;10, 11, 12, 13 in these compounds protonation and proton‐coupled electron transfer are of limited relevance to the key chemical reactions of the metal complexes.…”