Muonium addition reactions in the gas phase: Quantum tunneling in Mu+C2H4 and Mu+C2D4

Abstract

The reaction kinetics for the addition of the muonium (Mu = Jl, + e-) atom to C 2 H 4 and C 2 D 4 have been measured over the temperature range 150-500 K at (N 2 ) moderator pressures near 1 atm. A factor of about 8 variation in moderator pressure was carried out for C 2 H 4 , with no significant change seen in the apparent rate constant k app , which is therefore taken to be at the high pressure limit, yielding the bimolecular rate constant k Mu for the addition step. This is also expected from the nature of the Jl,SR technique employed, which, in favorable cases, gives kapp = k Mu at any pressure. Comparisons with the H atom data of Lightfoot and Pilling, and Sugawara et al. and the D atom data of Sugawara et al. reveal large isotope effects. Only at the highest temperatures, near 500 K, is k Mulk H given by its classical value of2.9, from the mean velocity dependence ofthe collision rate but at the lowest temperatures kMulkH ~ 3011 is seen, reflecting the pronounced tunneling ofthe much lighter Mu atom (mIL = 1/9 m p )' The present Mu results should provide accurate tests of reaction theories on currently available ab initio surfaces.

Reaction Kinetics of Muonium with N2O in the Gas Phase

“…This interpretation is consistent with an expected enhancement in k - a ( T ), from RRKM or even simpler theories of unimolecular dissociation, , but awaits confirmation from specific theoretical calculations of effects of this nature, as does our recent Mu + NO study …”

“…The arguments can be equally applied to other H isotopes and in particular Mu, of interest here, with some adjustment of the numbers, notably the heats of reaction. On the basis of ZPE arguments, ,, Mu reactivity can be expected to be ∼20 kJ mol -1 less exothermic than the corresponding H atom reactions.…”

Addition Kinetics and Spin Exchange in the Gas Phase Reaction of the Ethyl Radical with Oxygen

“…14 Consequently, we conclude that the µSR method appears to be competitive with conventional methods for studying the reaction kinetics of organic radicals in the gas phase. Since the bond to Mu is neither formed nor broken in the reaction of interest, we have to consider only a secondary isotope effect, and these are normally small.…”

“…This leads to MuCH 2 Ċ H 2 formation with the spin label in the methyl group. 14 In addition to its utility for the study of chemical reaction kinetics, the µSR technique is sensitive to spin exchange and spin relaxation phenomena as well. Standard methods of magnetic resonance are not suitable for the investigation of radicals in the gas phase, other than diatomics and simple triatomics, 10 since the resonances are numerous and broadened by collisions, which leads to a low signal-to-noise ratio.…”

Entropic Effects on the Dynamical Bottleneck Location and Tunneling Contributions for C2H4 + H → C2H5: Variable Scaling of External Correlation Energy for Association Reactions

“…The reverse reaction is also important in low-temperature combustion, where it competes with the oxidation of ethyl radical . This reaction has been extensively studied both experimentally − and theoretically, − but is still not completely understood. Overall rate constants for both association and dissociation ( k a and k d , respectively) have been reported in the literature, including their pressure dependence and the temperature dependence of their high-pressure values.…”