Unified Reaction Valley Approach Mechanism of the Reaction CH₃+ H₂→ CH₄+ H

Abstract

A unified reaction valley analysis (URVA) is presented to investigate the mechanism of the reaction CH 3 + H 2 f CH 4 + H at the UMP2/6-31G(d,p) level of theory. URVA is based on the reaction path Hamiltonian, the intrinsic reaction coordinate s, and the characterization of normal modes ω µ (s), reaction path vector η(s), and curvature vector K(s) in terms of generalized adiabatic modes a n g (s) associated with internal parameters that are used to describe the reaction complex. In addition, URVA combines the investigation of the harmonic reaction valley with the analysis of attractive and repulsive forces exerted on the nuclei by analyzing the electron density distribution F(r,s). It is shown that changes in F(r,s) reflect changes in the reaction valley and vice versa. Five reaction phases can be distinguished (reactant, reactant preparation, transition state (TS), product preparation, and product phase), of which the chemically relevant phases are indicated by small (reorganization of electron structure) and large curvature peaks (bond breaking or forming). Relatively large peaks of the adiabatic force constants indicate those positions at which the reaction is accelerated by appropriate electronic structure changes. Position and height of the curvature peaks in the TS region reflect the energetics of the reaction and the nature of the TS in the sense of the Hammond postulate: The reaction is exothermic with an early TS that is shifted by ∆s ) 0.3 amu 1/2 a 0 into the entrance channel.

Finding the Transition State of Quasi-Barrierless Reactions by a Growing String Method for Newton Trajectories: Application to the Dissociation of Methylenecyclopropene and Cyclopropane

“…The question is whether such a change in mechanism can be anticipated from a detailed analysis utilizing URVA [1,[39][40][41][42] for the methylene-ethene reaction. Such an analysis requires a TS as starting point or, in case of a barrierless reaction, another reasonable starting point in a region where the upgoing valley disembogues into the energy plateau being the location of the separated molecules methylene and ethene.…”

“…[1,[39][40][41][42] Here we consider just the changes in the reaction path vector since the complete URVA analysis of the methylene-ethene cycloaddition reaction (referenced against the C 2v -symmetrical approach and compared for changes in substituents and central atom) is beyond the scope of the present paper and therefore a topic of another publication.…”

“…associated with the internal coordinates q n used to describe the reaction complex [1,39] (s: reaction path length; M: mass matrix of the reaction complex) according to…”

“…Konkoli and co-workers [39]. Vectors u n in Eq.s (6) and (7) are the internal modes that characterize the movement along the reaction path.…”

Extended method for adiabatic mode reordering

“…Further on a unified reaction valley analysis may be performed properly, which is a powerful tool for a thorough investigation of a chemical reaction. 11 In addition to that the widely used small curvature tunneling correction is determined with the help of the B i,s (s). 8,10,[12][13][14][15][16] In the easiest case the mode reordering is carried out by symmetry requirements to detect avoided crossings of the i (s) directly.…”