Insights Gained by Modeling the Kinetics of Archetypal Hydrogen Atom Transfer Reactions: NH₃ + ·H ⇆ H₂N· + H₂ and CH₄ + ·H ⇆ H₃C· + H₂

Abstract

Experimental measurements of the kinetics of the title reactions extend to temperature ranges of 1360 K for the ammonia-hydrogen reaction and of 1602 K for the methanehydrogen reaction. Curved plots of ln(k) versus 1/T are obtained. Many theoretical calculations modeling these reactions routinely use tunneling corrections to match experiment. The steepness and curvatures of the plots are modeled successfully in this work and are shown to be caused solely by changes in the bond dissociation energies of the bonds involved in the reactions without invoking tunneling or any other adjustable parameters. The conclusion that tunneling does not contribute significantly to the rates in the temperature range of the measurements is in stark contrast with those theoretical calculations invoking large tunneling factors in the experimental temperature range. Support for the conclusion is provided by theoretical calculations of harmonic quantum transition state theory implementing instanton theory. There is direct experimental evidence that significant tunneling occurs in some H atom transfers, as with isotopomers of H 2 + ·H and other H transfers at very low temperatures. However, there is no direct experimental evidence of significant tunneling contributions to the rates of the title reactions in the temperature range of the measurements. Insights are gained into what specific forces must be overcome by the enthalpy of activation for reaction to occur. C 2015 Wiley Periodicals, Inc. Int J Chem

Progress and Prospective of Nitrogen-Based Alternative Fuels

“…Since their results were mostly overlooked in the west, a flawed concensus formed that remained undisturbed up to 1985. While the rate coefficients of post-1985 measurements are in good agreement with each other (Figure a), the modified Arrhenius behavior of 29 was neglected in several of these works. ,, This resulted in errors upon extrapolation to higher and lower temperatures compared to other works. , The kinetic parameters suggested by Zavitas do not follow the latest experimental measurements by Ko et al and Michael et al In contrast, calculated rate coefficients by Nguyen and Stanton (Figure ) follow these experimental values quite closely, lending credibility to their suggested kinetic parameters (Table S2). However, low-temperature validation is still necessary to ensure high-quality predictions at low-temperature ammonia applications. …”

“…319,320,322 This resulted in errors upon extrapolation to higher and lower temperatures compared to other works. 321,323 The kinetic parameters suggested by Zavitas 333 do not follow the latest experimental measurements by Ko et al 323 and Michael et al 321 In contrast, calculated rate coefficients by Nguyen and Stanton (Figure 21) follow these experimental values quite closely, lending credibility to their suggested kinetic parameters (Table S2). However, low-temperature…”

“…This was followed by several flawed works suffering from significant shuffling, incomplete reaction sets, and narrow measurement ranges along with hydrogen impurties. , Only past 1985 were previous lessons implemented, and kinetics for eq were directly measured, resulting in improved values in close agreement with each other. − The measurements by Ko et al, the latest purely experimental work on this reaction, were recommended by Dean and Bozzelli . The following works were almost exclusively theoretical, implementing increasingly advanced computational tools to calculate eq . − These were reviewed by Zavitas in 2015, where he recommended modified Arrhenius kinetic parameters for eq based on both experimental measurements and theoretical results . Finally, Nguyen and Stanton recently applied state-of-the-art HEAT SCTST/VPT2+ level calculations to determine the kinetics of eq…”

“…324−332 These were reviewed by Zavitas in 2015, where he recommended modified Arrhenius kinetic parameters for eq 29 based on both experimental measurements and theoretical results. 333 Finally, Nguyen and Stanton recently applied stateof-the-art HEAT SCTST/VPT2+ level calculations to determine the kinetics of eq 29. 334 The reaction rate coefficients for eq 29 based on the kinetics from the works above are presented in Figure 21, while the kinetic parameters are listed in Table S2.…”