Dynamical barrier and isotope effects in the simplest substitution reaction via Walden inversion mechanism

Abstract

Reactions occurring at a carbon atom through the Walden inversion mechanism are one of the most important and useful classes of reactions in chemistry. Here we report an accurate theoretical study of the simplest reaction of that type: the H+CH4 substitution reaction and its isotope analogues. It is found that the reaction threshold versus collision energy is considerably higher than the barrier height. The reaction exhibits a strong normal secondary isotope effect on the cross-sections measured above the reaction threshold, and a small but reverse secondary kinetic isotope effect at room temperature. Detailed analysis reveals that the reaction proceeds along a path with a higher barrier height instead of the minimum-energy path because the umbrella angle of the non-reacting methyl group cannot change synchronously with the other reaction coordinates during the reaction due to insufficient energy transfer from the translational motion to the umbrella mode.

Unusual KIE and dynamics effects in the Fe-catalyzed hetero-Diels-Alder reaction of unactivated aldehydes and dienes

“…DFT and experimental secondary KIE studies. Kinetic isotope effect (KIE) has been widely employed as a diagnostic tool to help determine the mechanism of many chemical and biochemical reactions, as well as probe the transition-state structures involved [23][24][25][26]43,62,63 . Secondary deuterium (k H /k D ) KIE studies for DA or ODA reactions with RCHO/RCDO should give a smaller KIE value for a later transition state (with a shorter C-C bond formation leading to more C sp3 character and less C sp2 character) 63 .…”

“…Although many computational studies on the mechanism of DA and 1,3-dipolar cycloaddition reactions have been reported 3,4,6,[21][22][23][24][25][26] , computational works on the mechanism of HDA reactions are quite limited [27][28][29] . In addition, Carpenter, Hase, Houk, Singleton, Tantillo, and other groups carried out quasi-classical molecular dynamics (e.g., MD with a density-functional theory (DFT) method) studies to provide several new mechanistic concepts for several uncatalyzed DA reactions and other (almost metal-free) organic and enzymatic reactions (e.g., dynamically concerted and dynamically stepwise mechanisms, energy-dependent reaction selectivity, and dynamically controlled selectivity) [30][31][32][33][34][35][36][37][38][39][40][41][42][43] . These DFT MD studies offered us time-resolved mechanistic insights and helped elucidate the timing of the bond formation in (bio)chemical reactions.…”

Reactive molecular dynamics: From small molecules to proteins

“…The limitation on up to eight atoms is primarily owed to the fact that accurately solving the nuclear SE for systems involving a larger number of nuclei remains an unsolved problem although considerable progress has been made over the past several years. 72,73 One of the earliest efforts to determine outcomes of chemical reactions is the H + H 2 atom exchange reaction. Using classical MD and a modified London-Eyring-Polanyi-Sato (LEPS) surface, 74 the differential cross-sections for the D + H 2 reaction were calculated.…”

“…Specifically in the field of small molecule reactions involving processes such as A + BC→AB+C or AB+CD→A + BCD (or permutations thereof) involving a total number of three to eight atoms, the construction of globally valid PESs is essential to make direct contact between computations and experiments. The limitation on up to eight atoms is primarily owed to the fact that accurately solving the nuclear SE for systems involving a larger number of nuclei remains an unsolved problem although considerable progress has been made over the past several years …”

Machine Learning Potential Energy Surfaces

“…The limitation on up to 8 atoms is primarily owed to the fact that accurately solving the nuclear Schrödinger equation for systems involving a larger number of nuclei remains an unsolved problem although considerable progress has been made over the past several years. 7,8 One of the earliest efforts to determine outcomes of chemical reactions from dynamics studies is the H+H 2 atom exchange reaction. Using classical molecular dynamics (MD) and a modified London-Eyring-Polanyi-Sato (LEPS) surface, [9][10][11] the differential cross sections for the D+H 2 reaction were calculated.…”