Photodissociation Dynamics of CO-Forming Channels on the Ground-State Surface of Methyl Formate at 248 nm: Direct Dynamics Study and Assessment of Generalized Multicenter Impulsive Models

Abstract

The photodissociation dynamics of methyl formate in the electronic ground state S 0 , initiated by a 248 nm-wavelength laser, is studied by direct dynamics simulations. We analyze five channels, where four of them have as products CH 3 OH + CO, one leading to the formation of three fragments, H 2 CO + H 2 + CO, and a channel characterized by a roaming transition state. The analysis of energy distribution among the degrees of freedom of the product and the comparison with experimental results previously published by other groups provide the ingredients to distinguish the examined dissociation pathways. The interpretation of the results proves that the characterization of dissociation mechanisms must rely on a dynamics approach involving multiple electronic states, including considerations on the features of the S 1 /S 0 conical intersection. Here, we also assess the generalized multicenter impulsive model, GMCIM, that has been designed for dissociation processes with exit barriers, and the energy distribution in the products is predicted on the basis of information from the saddle points and the intrinsic reaction coordinates. Main features, advantages, limits, and future perspectives of the method are reported and discussed.

Photodissociation dynamics of acetaldehyde at 267 nm: A computational study of the CO‐forming channels

“…[2,4,10,22] It was noticed that, however, by utilizing spin-unrestricted density functionals which contain large contribution of exact Hartree-Fock(HF) exchange at long range, it is possible to locate the RSP of methyl formate with acceptable geometry and barrier height. [26] Herein, we investigate the RSP of CH 3 roaming by both UM06-2X and UM11 density functionals. Among the aforementioned methods, Jensen's segmented contracted polarization consistent basis sets (pcseg-n or augpcseg-n) [36] were applied to the density functional calculations.…”

“…Herein, we only review the Generalized Multi-Center Impulsive Model (GMCIM) briefly, the details about the concepts and the algorithm of this kinematic model have been reported in previous works. [25,26,28] GMCIM adopts the idea of minimum energy path (MEP) and the Reaction Path Hamiltonian (RPH) approach, [25,[46][47][48][49] where the molecular dissociation on multidimensional potential energy surfaces can be considered as an effective onedimensional problem (the dissociation coordinate, s) with the 3 N-7 transverse vibrational modes orthogonal to s. Then the sudden approximation is introduced to the dissociation coordinate, by assuming that the motion along the directions of transverse modes is slow and therefore remains frozen during the dissociation. The simulation of GMCIM starts at the saddle point of the dissociation pathway.…”

“…GMCIM does not assume the conventional statistical/impulsive hybrid scheme, [23,50,51] but takes a pure impulsive approach to deal with transverse potential energy. [26] There are two kinds of transverse modes: (i) the conserved modes which correlate with product vibrations at the end of the dissociation; (ii) the transitional modes, which correlate to the translation of products and disappear at the end of the dissociation. The conserved and transitional modes can be identified by examining the mass-weighted Cartesian eigenvector of each transverse mode.…”

“…To solve the issues which are mentioned above, we utilize the direct dynamic simulation of classical trajectory for the CO + CH 4 channel at 267 nm, which are initiated from either the three-center or CH 3 -roaming saddle points, at computational level of various ab initio or density functional methods. With the aid of the generalized multicenter impulsive model (GMCIM), [25][26][27][28] a kinematic model to predict the product states of unimolecular dissociation, this work aims to clarify the dynamic signatures of the conventional TS and CH 3 -roaming pathways in acetaldehyde, and seeks for the theoretical interpretation to the dynamic features of the product states observed in the experiments [17,18] at 267 nm.…”

Pyrolysis of Methyl Formate and the Reaction of Methyl Formate with H Atoms: Shock Tube Experiments and Statistical Rate Theory

“…In addition, important elementary steps within these mechanisms have been characterized in detail. Regarding MF itself, these are in particular the unimolecular decomposition steps − and bimolecular abstraction reactions. ,− …”